AMMA Science

AMMA Science
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118
Theme 1
West African Monsoon and
Global Climate
1.01 – 1.14 (presentations)
1.01P – 1.88P (posters)
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1.01
120
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STRUCTURE TRI-DIMENSIONNELLE ET DYNAMIQUE DES
PERTURBATIONS ASSOCIEES AUX ONDES EST AFRICAINES
La structure moyenne ondes est africaines (AEW) sur l'Afrique de l'ouest et sur l'Atlantique à coté
est isolée par projection des variables dynamiques en provenance des réanalyses et des données de
radiosondages sur l'OLR (Rayonnement sortant longue longueur d’onde) après filtré spatiotemporellement.
Ces résultats sont comparés avec les résultats de modélisation présentés par N. Hall dans un autre
papier. Les structures observées ressemblent beaucoup aux structures des modes propres de ce
modèle pour un état basique d'été sur l'Afrique. Il y a une évolution considérable dans la structure
tridimensionnelle de ces ondes selon leur propagation sur 10N à travers l'Afrique de l'Ouest. A cette
latitude, la convection se trouve dans les courants du nord à l'est du méridien de Greenwich, et
ensuite est déplacée dans le talweg et enfin dans les courants de sud quand les ondes se propagent le
long de la côte et dans l'ITCZ sur l'Atlantique. Par contre, à 15N, au nord du jet d'est africain, la
convection reste dans les courants du sud sur toute la trajectoire de l'onde. Le long de 10N sur
l'Afrique de l'Ouest, la localisation de la convection est en accord avec le forçage dynamique /
adiabatique donné par l'advection des perturbations de vorticité par le vent thermique moyen dans la
direction zonale, ce qui est aussi vu dans le modèle de Hall. Sur la mer, et le long de 15N, la
relation avec la convection et la dynamique est plus compliquée et pas facile à expliquer purement
en terme de forçage dynamique.
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1.02
ETUDE MULTIVARIABLE DE LA DYNAMIQUE DES SYSTEMES
CONVECTIFS EN AFRIQUE DE L’OUEST
Garba ADAMOU (1), Kofivi ABALOVI (1), Diakaria KONE (1),
Arona DIEDHIOU (2), Amadou.Thierno GAYE (3) et G. JENKINS (4)
(1) Ecole Africaine de la Météorologie et de l’Aviation Civile EAMAC, Niamey, Niger
(2) IRD, Niamey, Niger (3) LPASF ENSUT, Dakar Fann, Sénégal
(4) Howard University, Washingtown, USA
En région tropicale, l’absence d’une théorie unifiée à l’instar du concept du géostrophisme
beaucoup plus valable dans les latitudes supérieures, la faiblesse des réseaux de mesures
météorologiques et les faibles valeurs du paramètre de Coriolis, rendent peu aisées la
compréhension et la prédictibilité des systèmes convectifs.
Des travaux réalisés sur l’évolution des systèmes convectifs dans des champs de tourbillon potentiel
et de CAPE ont conduit à des résultats préliminaires particulièrement intéressants, mais ont
concerné l’impact de ces variables prises individuellement sur la dynamique des systèmes
convectifs associés à des lignes de grains(LG). Par ailleurs ces travaux n’ont pas abordé l’influence
du cisaillement de vent sur l’évolution de ces systèmes convectifs.
Le travail que nous présentons ici, associe le tourbillon potentiel, la CAPE et le cisaillement de vent
pour une étude multi variable de la dynamique des systèmes convectifs en Afrique de l’Ouest et
d’analyser l’action combinée de ces trois variables sur l’évolution de la taille des systèmes
convectifs. On fera également une analyse fréquentielle des événements liant la taille des systèmes
convectifs et les variables ci-dessous mentionnées.
Contact :
Garba Adamou - EAMAC, BP 746, Niamey, Niger
Tel : 00227 93 54 33 - Email : [email protected] / [email protected]
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1.03
SEASONAL FORECASTING OF THUNDERSTORMS IN WEST AFRICA
USING SATELLITE DATA
S.O. GBUYIRO and B.N. ORJI
Nigerian Meteorological Agency, Lagos, Nigeria
The dynamics of the interactions of the ocean - atmosphere systems and their implications for the
weather and climate systems in West Africa have been a subject for discussions by many writers. In
most previous writings, there have been problems obtaining data for accurately examining the
characteristics and consequences of patterns of weather systems. With the recently available
satellite data in Nigeria, therefore, it has become a big challenge for meteorologists and
climatologists to use this satellite data and information to critically examine the characteristics of
the weather especially seasonal thunderstorms and climate systems and their implications for
characteristics and consequences of environmental change in the region. But the forecasting of these
thunderstorms had been difficult mainly due to orography, local factors (ITD) and break down of
global models during the transition period from Wet (Dry) to Dry (Wet) in October (March). All
these show the need to examine critically the use of satellite imageries on the development of
weather systems in general and thunderstorms in particular.
It is the purpose of this paper, therefore, to use satellite images and global model charts (850 and
700 hPa charts) for 1998, 1999 and 2000 to: (a) identify the various areas in which thunderstorms
are produced due to the influence of orography and (b) the seasonal latitudinal movements of the
thunderstorms with respect to (a) the beginning of the rainy season (b) the middle of rainy season
(c) the little dry season and (e) the end of the rainy season.
The results of the study showed that (i) The storms in generally move at an average speed of 7
degrees longitude in 24 hours, though faster over the coastal area (8 degree) (ii) Between the
months of March and April and late Sept/Oct most of the storms affecting the coastal areas (Lat. 5 7 degree north) originate from Gabon/Congo. Insitu developments were noticed by late April and
Sept over places like Jos plateau, Adamawa highlands and Oshogbo hills (In Nigeria), Togo
Atakora Mountains and Futa Jallon - Guinea highlands (iii) Between May/ June and Sept, storms
that affect areas between Lat. 10 - 12 degrees north originate over Chad and Central Africa
Republic. (iv) From July - August, most storms originate over Sudan / Ethiopia and move along
Lat. 12 -14 degree north. (v) It was discovered that 70% of the most devastated storms that affected
the coastal areas of Nigeria (which later affected others areas of West Africa) are joint product of
storms that developed over Oshogbo, Akure, Ekiti, (highlands areas of southwest Nigeria) and
western Cameroon mountains merging over Lagos to produce very severe storms.
From the above results, it has become relatively easier to locate and forecast the movements of
storms along specified trajectories in West Africa with an accuracy as high as 80%, thereby
improving the forecasts of thunderstorms. Although the ITD (Inter Tropical Discontinuity) and the
ocean factor are significant for inducing significant changes in the characteristics and patterns of
change in the air-sea interactions. it is now becoming clearer that (a) the influence of topography
and land and sea breezes are the main factors initiating and influencing the origin of the weather
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systems, while the ocean and other factors influence the changes in the patterns of movements of
the weather systems.
Weather forecasting in West Africa has largely depended on the use of climatological techniques
and persistence. But with the advent of global model charts and satellite images, a lot of
improvements and accuracy had been achieved in terms of Nowcasting and Microforecasting.
In this part of the world, line squalls and thunderstorms together contribute at least 80% of the total
annual rainfall (Omotosho, 1984) north of latitude 11o N and 60% south of latitude 10o N. Hence
the socio-economic and political survival of West African countries depend a great deal on these
convective systems.
Existing forecasting charts
The model charts used for operational forecasting are: MSLP charts, 10m winds & 2m Temperature
charts, 925 Hpa charts, 850 Hpa charts, 700 Hpa charts (all from Meteo-france), precipitation charts
(from ECMWF) and Surface Significant Charts (from U.K).
Squall Line / Easterly wave
To a large extent, the West African squall lines are dependent on or controlled by the summer time
monsoon oscillations. They are absent when the monsoon retreats from most parts of west Africa
(Nov - Mar), suppressed if the monsoon is too deep (giving way to monsoon rain) over coastal
regions in June/July and very violent when the monsoon depth is about 1 - 2 km, especially north of
11o N. Other basic conditions determining the occurrence of squall line/thunderstorm are :
(i)
The presence of a deep layer of convective or conditional instability.
(ii)
A release mechanism for the instability usually in the form of low level convergence or
topography.
The forecasting of Line squall is done by watching for an area of organised cumulonimbus clouds
orientated in a north/south direction with reports of thunderstorms accompanied by strong winds on
the surface charts. The above line is expected to move at 30 knots depending on the steering level at
700 Hpa.
Easterly wave form in peculiar region in East/Central Africa (long 25 - 35o E) south of the jet (AEJ)
core where the mean zonal flow is barotropically unstable. The wave structure shows a surface
convergence zone only, a cyclonic vortex at 850 hpa and a distinct wave pattern at 700 hpa. Wave
intensity tends to decrease upwards thereafter (Omotosho 1984). According to Adefolalu (1986)
they have phase speed of 7 - 8 m/s (6 - 7 long/day), a wavelength of about 2000 - 3000m and a 4
day meridional oscillation. Weather occurs to the southeast/southwest of a typically developed
easterly wave. Easterly waves are easily recognized at the 850 hpa level. Their movements follow
the climatological characteristics as enumerated earlier
The Climatological approach / advent of Satellite
The climatological approach to forecasting received wide attention and application, especially in
West Africa because the North/South migrations of the sun could be reasonably associated the
seasons (DRY) during the southward march of the sun in the northern winter and (WET) during the
poleward march in northern summer. Ojo (2001) actually showed that ITD is not enough to explain
the weather and climate of West Africa. From 1970 however, other system like the easterly waves,
line squalls and severe thunderstorms which account for 60 – 80% of the precipitation in West
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Africa were well documented ( Omotosho, (1984)). They rely on surface and upper air data, which
are either sparse or not readily available at the time of observation.
With the installation of the PDUS in 1997 and MDD in 1998 at the central forecast office, a lot of
improvements came into weather forecasting in Nigeria, some of which are:
i)
Satellite data are now used to supplement synoptic analysis, as well as validating the base
chats of the model-forecast charts.
ii)
easy location and intensity of meso-scale systems
iii)
The tracking of meso-scale convective systems as they transverse West African subcontinent.
iv)
It is possible to diagnose weather producing systems and the areas of heavy precipitation
using cloud top temperature contrasts.
References
Adefolalu, (1986): Rainfall trends in Nigeria Arch. For Met. Geophy. and Biokl. Ser. A, 3; 205-209.
Ojo, O et al (2001): Fundamentals of Dynamic and Physical Climatology Published by Sedec Publishers, Lagos
Nigeria
Omotosho, J.B, 1984: Spartial and Seasonal variation of linesqualls over west Africa, Arch. Meteol-Geophics
Bioclimatol Sev. A33; 143 - 150.16
Waters, A.J. (1995 Images in Weather Forecasting-A Practical Guide for interpreting Satellite Radar Imagery.
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PREVISIONS SAISONNIERES D’ORAGES EN AFRIQUE DE L’OUEST
AVEC DES DONNEES SATELLITE
La dynamique des interactions du système océan-atmosphère et leurs implications pour le temps et
le climat en Afrique Ouest ont été un sujet de discussions par beaucoup d'auteurs. Dans la plupart
des articles antérieurs, il y a eu des problèmes d’obtention de données pour examiner correctement
les caractéristiques et les conséquences des types de systèmes météorologiques. Avec les données
satellitaires récemment disponibles au Nigeria, c’est devenu un grand défi pour les météorologistes
et les climatologues de les utiliser pour examiner les caractéristiques du temps, particulièrement les
orages saisonniers, les systèmes du climat et leurs implications pour les caractéristiques et les
conséquences de changement de l'environnement dans la région. Mais la prévision de ces orages a
été difficile principalement en raison de l’orographie, des facteurs locaux (ITD) et des mauvais
comportements des modèles globaux pendant la période des transitions Humide (Sec) à Sec
(Humide) en octobre (mars). Ceci montre le besoin d'examiner l'apport des imageries satellitaires
sur le développement de systèmes du temps en général et les orages en particulier.
C'est le but de ce papier, par conséquent, utiliser des images satellite et des cartes de modèles
globaux (850 et 700 hPa) pour 1998, 1999 et 2000 pour : (a) identifier les diverses régions dans
lesquelles les orages sont produits du fait de la présence d'orographie et (b) les mouvements
latitudinaux saisonniers des orages en ce qui concerne (a) le commencement de la saison pluvieuse
(b) le milieu de saison pluvieuse (c) la petite saison sèche et (e) la fin de la saison pluvieuse.
Les résultats de l'étude ont montré que (i) Les orages se propagent généralement à une vitesse
moyenne de 7 degrés de longitude en 24 heures, quoique plus vite sur la région côtière (8 degré) (ii)
Entre les mois de mars et avril et Sept/Oct tardif la plupart des orages qui affectent les régions
côtières (Lat. 5 - 7 degré nord) proviennent de Gabon/Congo. Les développements in situ ont été
notés fin avril et Sept sur des localisations comme le plateau de Jos, pays montagneux Adamawa et
collines Oshogbo (en Nigeria), Togo Atakora Montagnes et Futa Jallon - pays montagneux de
Guinée (iii) Entre mai / juin et Sept, les orages qui affectent des régions entre Lat. 10 - 12 nord des
degrés provienent du Tchad et de République Centrale d'Afrique. (iv) De juillet - août, la plupart
des orages proviennent du Soudan / Ethiopie et avance le long des Latitudes 12 -14 degré Nord. (v)
Il a été découvert que 70% des orages les plus violents qui ont affecté les régions côtières de
Nigeria (qui plus tard ont affecté d ‘autres régions d'Afrique Ouest) sont liés aux aux orages qui se
sont développés sur Oshogbo, Akure, Ekiti, (régions de pays montagneux de Nigeria sud-ouest) et
montagnes de Cameroun de l'ouest, pour s’associer autour de Lagos pour produire des orages très
violents.
Des résultats précités, il est devenu relativement plus facile de localiser et de prévoir les
mouvements des orages le long de trajectoires spécifiées en Afrique Ouest avec une exactitude aussi
haute que 80%, en améliorant les prévisions d'orages de cette façon. Bien que les facteurs de
l’ITD(Inter Discontinuité Tropique) et de l'océan sont importants pour induire des changements
significatifss dans les caractéristiques et les types de changement dans les interactions air-océan,
c'est maintenant clair clair que (a) l'influence de l’orographie et des brises de mer de terre sont de
principaux facteurs qui initie et influence l'origine des systèmes météororologiques, alors que
l’océan et et d’autres facteurs influence les changements dans les types de mouvements de ces
systèmes.
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1.04
WEST AFRICAN STORM TRACKS AND THEIR RELATIONSHIP
TO ATLANTIC TROPICAL CYCLONES
Susanna B. HOPSCH (1), Chris THORNCROFT (1), Kevin HODGES (2)
and Anantha AIYYER (1)
(1) Department of Earth and Atmospheric Sciences, University at Albany, SUNY, NY, USA
(2) Environmental Systems Science Centre, University of Reading, Reading, UK
Most of the Atlantic Tropical Cyclones form from weather systems originating over West Africa
(e.g. Avila and Pasch, 1992, Pasch and Avila, 1994). The dominating synoptic weather systems
over West Africa are African Easterly Waves (AEWs). Embedded within these AEWs are
mesoscale convective systems (MCSs) that can produce mesoscale vorticity anomalies and also can
provide seedlings for tropical cyclogenesis over the tropical North Atlantic (Carlson,1969).
Previous studies have shown a strong relationship between the variability of West African rainfall
and Atlantic tropical cyclone activity (e.g. Landsea and Gray, 1992, Goldenberg and Shapiro, 1996)
This seasonal to interannual variability may be associated with:
•
Large scale/teleconnections and changes in the environment where storms
tropospheric wind shear)
•
Variability in the West African weather systems
form (e.g.
The focus of this study is to assess the variability in West African weather systems using the
automatic tracking technique used by Thorncroft and Hodges (2001) to identify coherent vorticity
structures at 850hPa over West Africa and the tropical Atlantic in the ECMWF 40-year reanalysis.
Consistent with their study, the presence of two dominant source regions for storm tracks over the
Atlantic was confirmed, see Fig.1, the first south of about 15°N in the rainy zone, and a second
region north of 15°N on the fringes of the Sahara.
Results show that the southern storm track provides about 70% of the systems that reach the main
development region where most tropical cyclones develop, whereas the northern storm tracks play a
much less important role in the tropical Atlantic. Furthermore, the southern storm track is
characterized by marked intraseasonal-to-decadal variability. Evidence also indicates that there
exists a seasonal variability in location and intensity of the storms leaving the West African coast,
which may influence the likelihood of downstream intensification and longevity (see also Hopsch
et.al, 2005).
There exists considerable year-to-year variability in the number of West African storm tracks, both
in numbers over the land and continuing out over the tropical Atlantic Ocean. While the lowfrequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall
(Fig. 2) and SSTs, the interannual variability is found to be uncorrelated. Using the example of two
contrasting years (Fig.3) we examined possible explanations for this apparent poor correlation.
Using the 2-6-day-filtered meridional wind variance, which provides a synoptic-scale view of the
African Easterly Wave activity, a much better relationship with interannual tropical cyclone activity
is found.
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Figure 1: Track density (scaled to number density per unit area (~106 km2 ) per season
(MJJASON) of all storm tracks over the mapped area for the months of May – November,
and years 1958 - 2002. Contour intervals every 0.8, starting at 0.009. Also shown is the
outline of the MDR and latitude/longitude lines for every 10º
Figure 2 : Correlation map of 11 year running mean July – October tracks from southern
storm track area and Hulme precipitation. Shading starts at 95% significance.
Figure 3a : Tropical storm tracks from best track data for MDR storms, formed in 1988
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Figure 3b : storm tracks for July-October,1988, from ERA40 overlaid on SST anomalies
(contours every 0.1ºC, positive anomalies warm colors, negative anomalies cold colors
Figure 3c : same as in a) but for 1989
Figure 3d : same as in b) but for 1989
References
Avila, L. and R.J.Pasch, 1992: Atlantic tropica systems of 1991. Mon. Wea. Rev.,120, 2688-2696
Carlson, T.N., 1969a: Synoptic histories of three disturbances that developed into Atlantic Hurricanes. Mon. Wea.
Rev.,97, 256-276.
Goldenberg, S. and L. Shapiro, 1996: Physical Mechanisms for the Association of El Nino and West African
Rainfall with Atlantic Major Hurricane Activity. J. Climate,9, 1169-1187
Hopsch, S.B., C. Thorncroft, K. Hodges, and A. Aiyyer, 2005: West African Storm tracks and their relationship to
Atlantic tropical cyclones, submitted to J. Climate.
Landsea, C. and W. Gray, 1992: The Strong Association between Western Sahelian Monsoon Rainfall and Intense
Atlantic Hurricanes. J. Climate, 5, 435-453
Pasch, R.J. and L. Avila, 1994: Atlantic Tropical Systems of 1992, Mon. Wea. Rev., 122, 539-548
Thorncroft, C. and K. Hodges, 2001: African Easterly Wave Variability and Its Relationship to Atlantic Tropical
Cyclone Activity. J. Climate,14, 116-1179
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TRAJECTOIRES DE SYSTEMES SYNOPTIQUES SUR L'AFRIQUE DE
L'OUEST ET SES RELATIONS AUX CYCLONES TROPICAUX
DANS L'ATLANTIQUE
Les réanalyses ERA40 ont été utilisées pour identifier des structures cohérentes en vorticité sur
l'Afrique de l'Ouest et l'Atlantique tropical, en se servant de la technique de suivi automatique de
Thorncroft et Hodges (2001). En accord avec cette étude, on trouve deux régions de source des
cyclones sur l'Atlantique. Des statistiques basées sur le niveau 850 hPa sont utilisées pour construire
une climatologie de 45 ans, y compris une description du cycle saisonnier, les caractéristiques
moyennes et la variabilité.
Les résultats montrent que les trajectoires au Sud représentent 70% des systèmes qui atteignent la
région principale de développement des cyclones, alors que les trajectoires au nord moins. De plus,
les trajectoires Sud sont caractérisées par une variabilité intra-saisonnière et décadale plus élevée. Il
y a aussi une évidence d’une variabilité saisonnière dans la localisation et l'intensité des systèmes
qui traversent la côte de l’Afrique de l’Ouest – ce qui peut influencer la probabilité d'une
intensification en aval et une durée de vie plus élevée.
Il y a beaucoup de variabilité d'une année à l'autre sur le nombre de systèmes, sur la surface
continentale et aussi sur la mer. La variabilité à basse fréquence est bien corrélée avec l'activité des
cyclones sur l'Atlantique, la pluie sur l'Afrique de l'ouest et les SSTs. La variabilité inter-annuelle,
par contre, est peu corrélée. En utilisant l'exemple de deux années contrastées, nous recherchons des
explications pour cette faible corrélation. Le vent méridien, filtré à 2-6 jours qui isole les ondes est
africaines, nous permet de trouver une meilleure corrélation.
Contact
Susanna B. Hopsch - Department of Earth and Atmospheric Sciences, ES-351, University at Albany,
Albany, NY 12222, USA - Email: [email protected]
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1.05
THE ATMOSPHERIC DYNAMICS OF THE WEST AFRICAN MONSOON
ONSET AND ITS OCEAN COUNTERPART
B. SULTAN, S. JANICOT, A. LAZAR and C. MENKES
LOCEAN / IPSL, France
Introduction
Precipitation in the Sahel is produced by one rainy season during the northern summer monsoon
over West Africa. The onset of these rains, linked to the northward migration of the Inter-Tropical
Convergence Zone (ITCZ), is an important parameter for a large community of users like
meteorologists, farmers, water resources managers... The aim of this study is to document the onset
stage of the West African monsoon by using OLR data and NCEP / NCEP2 reanalyses and to
describe its ocean counterpart by using TMI SST data and an oceanic simulation.
The onset stage of the West African monsoon
Previous works has described the onset stage of the West African monsoon by using combined
daily rainfall and OLR data on the period 1968-1990 (Sultan and Janicot 2003). The monsoon onset
is characterized by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary
location at 5N in May-June to another quasi-stationary location at 10N in July-August. Composite
analyses based on NCEP reanalyses shows that this northward shift is associated with an enhanced
Saharan heat low dynamics, increasing inland zonal moisture advection. This increase can favorize
the occurrence of strong westerly winds events like the westerly jet previously noticed by Grodsky
et al. (2003). We extend this work by using NCEP2 reanalyses for the 1979-2002 period. We show
that the heat low dynamics around the onset date might impact the coastal upwelling forcing with a
maximum of wind speed and curl at the time of the monsoon onset over the Mauritania-Senegal
coastal area and a sudden decrease after the onset date.
The oceanic counterpart in SST observations
By applying the same composite analysis but to TMI SST data over the 1998-2003 period, we
investigate the SST changes around the monsoon onset. These changes from the 15 days before and
the 15 days after the onset of the monsoon are highlighted by the Figure 1. The SST difference
shows an enhancement of SST in the eastern half of the Atlantic Ocean between 5°N and 20°N
maximum over the Mauritania-Senegal coastal area. This warm SST anomaly is located where the
wind speed (Fig.1) and curl (not shown) decrease is the stronger. The warming might be induced by
the latent heat loss decrease and by ocean stratification changes due to the decrease of the wind curl
in the western part of the Heat low circulation. Figure 1 shows also a cooling in the Guinean Gulf
and in the southern part of the Atlantic Ocean that is not coherent with the atmospheric changes
around the onset date. This meridional SST dipole over the Atlantic with warm anomalies north of
the ITCZ location and cold anomalies southward could be favourable to the northward migration of
the ITCZ.
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Figure 1: SST (shaded) and wind modulus (contour lines) differences
between t0+15 days and t0-15 days
The oceanic counterpart in a numerical simulation
We analyse this SST response in an oceanic general circulation model by using the OPA/ORCA
model of LODYC/IPSL forced by ERS-ECMWF based bulk formula over the 1992-2000 period.
Since the ocean model and the surface bulk fluxes are able to represent the observed SSAT dipole
describing below, we examine the respective contribution of both oceanic and atmospheric
dynamics to this SST dipole through a mixed layer heat budget. The analysis of the mechanisms of
mixed layer temperature change around the onset shows a significant contribution of both
atmospheric and oceanic forcings. Each physical process at play (e.g. upwelling, horizontal
transport, latent heat flux, etc....) is analyzed in terms of positive or negative feedbacks on the
northward migration of the ITCZ. Concerning the oceanic dynamics contribution, the horizontal
processes tend to push the ITCZ poleward while the vertical processes tend to stabilize it.
Concerning the atmospheric forcing, the experiment shows that latent heat fluxes contribute to cold
SST in the Western half of the Atlantic and in the Guinea Cost and solar heating contributes to
warm SST in the Eastern half of the Atlantic and along the Equator. It is also interesting to notice
that the atmospheric forcing is strongly influenced by the mixed layer depth suggesting a feedback
between oceanic and atmospheric processes.
References
Sultan B. and S. Janicot (2003), The West African monsoon dynamics, Part II : The “pre-onset” and the “onset” of
the summer monsoon, Journal of Climate, 16, 3407-3427.
Grodsky, S. A., J. A. Carton, and S. Nigam (2003), Near surface westerly wind jet in the Atlantic ITCZ, Geophys.
Res. Lett., 30(19), 2009, doi:10.1029/2003GL017867
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LA MISE EN PLACE DE LA MOUSSON EN AFRIQUE DE L’OUEST :
LES DYNAMIQUES ATMOSPHERIQUES ET OCEANIQUES
Les pluies au Sahel apparaissent lors d’une unique saison pendant la mousson d’été en Afrique de
l’Ouest. Le démarrage de ces pluies, en liaison avec la migration saisonnière de la Zone de
Convergence InterTropicale (ZCIT) est donc un paramètre important pour l’agriculture et la gestion
des ressources en eau.
En utilisant des données de pluies et d’OLR sur la période 1968-2002, on montre que la migration
saisonnière de la ZCIT est caractérisée par un déplacement rapide d’une première position
d’équilibre à 5N en Mai-Juin à une autre position d’équilibre à 10N en Juillet-Aout. Une analyse
composite des champs de réanalyses NCEP montre que ce déplacement rapide, lié à la mise en
place de la mousson en Afrique soudano-sahélienne, s’accompagne d’un renforcement de la
circulation associée à la dépression thermique saharienne et à un renforcement des advections
zonales d’humidité sur le continent.
En utilisant les données de SST (TMI) on montre que la mise en place de la mousson est
caractérisée par un dipôle thermique sur l’Atlantique qui est favorable à la migration vers le Nord
de la ZCIT. Une simulation du modèle OPA/ORCA du LOCEAN/IPSL forcé par ERS et les
réanalyses ECMWF permet d’analyser les contributions océaniques et atmosphériques de ce dipôle
en décomposant les différents termes du budget de chaleur dans la couche mélangé océanique.
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1.06
NOUVEAU ZONAGE CLIMATIQUE BASE SUR LA VARIABILITE
DE LA PLUVIOMETRIE EN COTE D’IVOIRE ET AU GHANA
A. D. OCHOU, A. AMAN, K. Y. KOUADIO and P. ASSAMOI
Laboratoire de Physique de l’Atmosphère et de Mécanique des Fluides, Université de CocodyAbidjan, 22 BP 582 Abidjan 22, Côte d’Ivoire
Sous l’influence des masses d’air humide (mousson) et sec (harmattan), la zone ouest africaine subit
des fluctuations climatiques marquées par de sévères sécheresses dont celles de 1983, 1994 et par
des pluviométries exceptionnelles telles qu’en 1987 qui ont, à chaque fois, eu un impact socioéconomique négatif. Le lien évident entre le climat et de nombreux secteurs vitaux de
l’environnement nécessite de mieux appréhender la variabilité pluviométrique de cette région
tropicale afin d’en trouver les causes et d’en assurer la prévision nécessaire pour les campagnes
agricoles notamment.
Nous avons donc étudié la variabilité spatio-temporelle de la pluviométrie en Côte d'Ivoire et au
Ghana, deux pays situés à la même latitude en bordure du Golfe de Guinée afin d’actualiser les
zonages climatiques en comparaison de ceux proposés respectivement par l’ASECNA (1979) et
Boateng (1967).
Cette étude est réalisée à l’aide des données mensuelles de la pluviométrie enregistrée de 1964 à
1997 dans 43 stations météorologiques dont 22 en Côte d’Ivoire et 21 au Ghana. En Côte d’Ivoire,
les stations se répartissent sur les trois grandes zones climatiques (Nord, Centre et Sud) définies par
une étude réalisée par l’ASECNA en 1979 sur la base des séries pluviométriques de 1961 à 1975, en
tenant compte de leurs régimes saisonniers. Quant au Ghana, les stations se répartissent sur quatre
zones climatiques notées A, B, C et D identifiées par Boateng (1967) à l’aide d’une base de
données antérieure à 1967.
Appliquée à cette base de données représentées par des séries chronologiques de 34 années de
hauteurs mensuelles de pluie réparties dans les 43 stations de la zone, la méthode de l’Analyse en
Composantes Principales (ACP) a permis d’obtenir un nouveau zonage climatique qui présente, en
comparaison avec l’ancien zonage, les caractéristiques suivantes:
-En Côte d’Ivoire, la zone climatique Sud est réduite et confinée au littoral au profit de la zone
Centre qui, elle-même, connaît une réduction importante au profit de la zone Nord qui se trouve
élargie à l’Ouest et à l’Est.
-Au Ghana, la zone A est réduite au profit de la zone C qui est fortement réduite dans sa partie
supérieure au profit de la zone D. La zone B, quant à elle, est réduite en direction du Sud au profit
de la zone C, tout en s’agrandissant au détriment de C à l’Est.
Bien que le zonage climatique montre des similarités entre zones de la Côte d’Ivoire et du Ghana,
l’étude des anomalies standardisées a permis d’apprécier le degré de variabilité interannuelle de la
pluviométrie de chacune des zones. Ainsi, la zone Nord de la Côte d’Ivoire et, dans une moindre
mesure, les zones B et C du Ghana se caractérisent par une longue série de déficit pluviométrique
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à partir de 1982-1983. Pour chacune des zones climatiques, les déficits pluviométriques importants
ont été obtenus pour la première fois en 1983 (année de sévère sécheresse ayant engendré des
conséquences économiques dramatiques dans la région ouest-africaine). Malgré les particularités
présentées par les anomalies standardisées d’une zone à une autre, on observe une tendance à la
baisse de la pluviométrie sur l’ensemble de ces zones, le passage de l’excédent au déficit s’étant
réalisé quasiment au début des années 80 tant en Côte d’Ivoire qu’au Ghana. De plus, l’étude des
écarts pluviométriques moyens entre la période déficitaire (1980-1997) et la période excédentaire
(1964-1980) montre que la baisse du régime pluviométrique est faible dans la quasi-totalité du
Ghana à l’exception de la zone B alors que la Côte d’Ivoire se caractérise par un déficit plus élevé
dans les zones Sud et Nord mais bien plus faible au Centre.
La présente étude montre ainsi que, la Côte d’Ivoire et le Ghana, bien que situés à la même latitude
se comportent différemment dans la variabilité de la pluviométrie. Une telle situation est, certes,
explicable par les différences de relief et de morphologie des côtes, mais serait aussi et surtout le
fait de l’activité anthropique dont l’impact diffère d’un pays à l’autre selon les stratégies
d’aménagement adoptées.
NEW CLIMATIC ZONING BASED ON RAINFALL VARIABILITY
IN IVORY-COAST AND IN GHANA
Under the influence of humid and dry air masses called monsoon and harmattan respectively, the
west african region undergoes various climatic fluctuations marked by severe drought, of which
those occurred in 1983 and 1994 and by some exceptional precipitations such as in 1987, which
have always had a negative socio-economic impact. The obvious link between the climate and
many environment vital sectors requires to better apprehend the rainfall variability in this tropical
area of west Africa, in order to outline its main causes and to ensure its forecast needed for the food
farming period in particular.
We have then studied the spatio-temporal rainfall variability in Ivory-Coast and Ghana, two
countries located at the same latitude and bordering the Gulf of Guinea, in order to update the
climatic zonings in comparison to those proposed by a study carried out at ASECNA (1979) and by
Boateng (1967).
This study is carried out using monthly rainfall data recorded from 1964 to 1997 in 43 weather
stations of which 22 in Côte d'Ivoire and 21 in Ghana. In Ivory Coast, the stations are distributed
over the three main climatic zones (North, Centre and South) defined by a study carried out at
ASECNA in 1979 using rainfall time series from 1961 to 1975 and by grouping the station having
similar seasonal regime. As for Ghana, the stations are distributed into four noted climatic zones A,
B, C and D identified by Boateng (1967) using a database prior to 1967.
Applied to this database represented by time series of 34 years monthly heights rainfall distributed
over 43 stations, the method of the Principal Components Analysis (PCA) allowed us to obtain a
new climatic zoning which presents the following characteristics when compared to the former
zoning:
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- In Ivory Coast, the South climatic zone is reduced and bordered on the littoral area to the profit of
the Centre zone, which itself is subjected to a significant reduction to the profit of the North zone
which is widened westward and eastward.
- In Ghana, the A zone is reduced to the profit of the C zone, which is also strongly reduced in its
upper part to the profit of the D zone. As for the B zone, it is reduced southward to the profit of the
C zone, while widening to the detriment of the C zone eastward.
Although climatic zoning shows some similarities between certain zones of Ivory Coast and Ghana,
the study of the standardized anomalies allowed us to appreciate the degree of interannual rainfall
variability in each zone. So, the North zone of Ivory Coast, and to a lesser extent, the B and C
zones of Ghana, are characterized by a long series of rainfall deficit sarting at 1982-1983. For each
climatic zone, significant rainfall deficits have occured for the first time in 1983 (year of severe
drought having led to negative impacts on economic activities in the West African region). Despite
some particularities presented by the standardized anomalies from one zone to another, one
observes a trend to a rainfall drop over the whole of the zones, the passage from the excess to the
deficit being achieved at the beginning of the 1980s in Ivory Coast as well as in Ghana. Moreover,
the study of the average rainfall deviations between the periods of deficit (1981-1997) and excess
(1964-1980) shows that the drop of the rainfall regime is weak in almost the whole of Ghana except
the B zone, while Ivory Coast is characterized by a higher deficit in the South and North zones but
much weaker in the Center.
The present study shows that Ivory Coast and Ghana, although located at the same latitude, behave
differently regarding rainfall variablility. Such a situation can indeed be explained by the relief and
coast morphology which are different, but would be more especially the fact of anthropogenic
activity whose impact varies from a country to another according to the management strategies
adopted.
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1.07
EVALUATION DE SIMULATIONS DU CLIMAT PRESENT DE L’AFRIQUE DE
L’OUEST PAR LE MODELE CLIMATIQUE
REGIONAL REGCM3
Abdoulaye SARR (1) et al (2)
(1) Direction de la Météorologie Nationale du Sénégal, Dakar-Yoff, Sénégal
(2) Laboratoire de Physique de l’Atmosphère, ESP/UCAD, Dakar-Fann, Sénégal
Le modèle climatique RegCM3, développé par le groupe PWC de l’ICTP, est utilisé pour une
simulation à haute résolution sur un domaine couvrant l’Afrique de l’Ouest. Les conditions initiales
et aux limites sont issues des réanalyses du NCEP appelées NNRP2. La durée de la simulation est
de 16 ans et va du 1er janvier 1988 au 30 septembre 2003. Les sorties des simulations sont validées
par rapport aux réanalyses et aux observations du CRU. Les résultats préliminaires montrent que le
modèle, à haute résolution, simule raisonnablement les traits majeurs qui caractérisent le climat de
la région, et les structures moyennes de champs simulés comme les précipitations et les
températures sont proches des observations.
Une investigation sur le mouvement de balançoire, entre années sèche et humide, qui caractérise le
régime pluviométrique depuis la fin de la longue période de sécheresse des années 1970 et 1980, est
également faite dans cette étude.
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1.08
INTRASEASONAL AND INTERANNUAL VARIABILITY IN THE TROPICAL
SOUTH EAST ATLANTIC AND WEST AFRICAN RAINFALL
C.J.C. REASON and K. MOREBOTSANE
Dept. of Oceanography, University of Cape Town, South Africa ([email protected])
The tropical South East Atlantic Ocean is a region of substantial intraseasonal and interannual
variability in sea surface temperature (SST) and surface wind (e.g., Florenchie et al., 2003, 2004;
Risien et al., 2004) which impacts on southern African fisheries and rainfall (Rouault et al., 2003).
Here, we consider the links between this region and West African rainfall. Kouadio et al. (2003)
showed that coastal rainfall over Cote d’Ivoire was related not just to SST over the Gulf of Guinea
but also to that off the coast of Angola.
As in Rouault et al. (2003), we define a SST index for the SE Atlantic by averaging over the box
10-20oS, 8oE – coast. Fig. 1 shows this index calculated from the NOAA extended reconstructed
SST (Smith and Reynolds, 2004) along with a time series of West African rainfall from the CRU
data set (New et al., 2000) averaged over the coastal region of 5-10oN, 5oW-5oE for the period
1901-2000. Both series are characterised by substantial interannual variability and appear related to
each other (r = 0.56), particularly after about 1930. Spatial correlations between the SST index and
West African rainfall show that this region is most closely linked to the tropical SE Atlantic with an
out of phase relationship during May (the first bimodal peak in the annual rainfall cycle) and a
stronger in-phase relationship in July-September. The latter shows strongest correlations with SST
in April or May indicating that some predictability, based on SE Atlantic SST, may exist.
An in (out of) phase relationship between May (July-September) sea level pressure and NCEP reanalysis winds over the SE Atlantic also exists which is presumably related to the development of
the cold tongue in boreal summer. A positive correlation between NCEP re-analysis latent heat
fluxes in a region extending off the coast of Angola and rainfall (at 1 and two month lead of the
fluxes) suggests that this region acts as a moisture source for part of the West African monsoon. To
consider the intraseasonal and interannual variability in the SE Atlantic winds in more detail, 19992004 QuikSCAT data are used. Previous analysis of 1999-2000 QuikSCAT data (Risien et al.,
2004) indicated that the coastal waters off Angola are dominated by an intraseasonal peak near 2224 days and a weaker peak near 40 days. These peaks appear to be related to pulses in convection
over West Africa (5-10oN, 5oW-5oE). We use NCEP re-analysis outgoing long wave radiation as a
measure of convection.
Wavelet analysis of the longer 1999-2004 QuikSCAT data shows substantial intraseasonal
variability in the 24-40 day range which appears related to pulses in convection over Angola during
the austral summer and over West Africa in the boreal summer. Enhanced power in the wavelet
spectrum of the winds exists during the 1999/2000 La Niño and 2002/2003 El Niño events relative
to the other years. The relationship between the winds off the Angolan coast and West African
convection indicates a 1-2 month lead of the winds suggesting that the latter result in stronger lowlevel moisture transport from the tropical South East Atlantic towards West Africa where they fuel
the convection.
139
In summary, correlation and wavelet analyses suggests that a link exists between the tropical South
East Atlantic and rainfall over the region of West Africa extending south of 10oN across much of
Cote d’Ivoire, Ghana, Togo, Benin and Nigeria. Warmer SST and stronger winds over this ocean
region appear related to increased July-September rainfall over this region with the opposite
relationship in May. The sign of the SST / wind relationship suggests that the SE Atlantic SST
anomalies may arise through dynamical processes, such as Benguela Niños (Florenchie et al., 2003,
2004), rather than from local air-sea interaction.
References
Florenchie, P., J.R.E. Lutjeharms, C.J.C. Reason, S. Masson and M. Rouault, 2003 : The source of Benguela Niños in
the South Atlantic Ocean. Geophys. Res. Lett., 30 (10), 1505, doi:10.1029/2003GL017172.
Florenchie, P., C.J.C. Reason, J.R.E. Lutjeharms, M. Rouault and C. Roy, 2004: Evolution of interannual warm and
cold events in the south-east Atlantic Ocean. J. Climate, 17, 2318-2334
Kouadio, Y.K., D.A. Ochou and J. Servain, 2003: Tropical Atlantic and rainfall variability in Cote d’Ivoire. Geophys.
Res. Lett., 30, 8005, doi:10.1029/2002GL015290.
New,M., Hulme, M. and P.D., Jones. 2000: Representing Twentieth-Century Space–Time Climate Variability. Part II:
Development of 1901–96 Monthly Grids of Terrestrial Surface Climate. J. Climate: 13, 2217–2238.
Risien, C., C.J.C. Reason, F. Shillington, D.B. Chelton, 2004: Variability in satellite winds over the Benguela upwelling
system during 1999-2000. J. Geophys. Res., 109, C3, C03010, doi:10.1029/2003JC001880.
Rouault, M., P. Florenchie, N. Fauchereau and C.J.C. Reason, 2003: South East Atlantic warm events and southern
African rainfall. Geophys. Res. Lett., 30 (5), 8009, doi:10.1029/2002GL014840.
Smith,T.M. and R.W. Reynolds. 2004: Improved Extended Reconstruction of SST (1854–1997). J.Climate, 17, 2466–
2477.
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Fig. 1. Standardised time series (1901-2000) of February-April SST averaged over 10-20oS, 8oE –
coast and July – September West African rainfall averaged over 5-10oN, 5oW-5oE.
140
VARIABILITE INTRA SAISONNIERE ET INTERANUELLE DE LA MOUSSON
AFRICAINE ET DE L’OCEAN ATLANTIQUE SUD
L'analyse des données de QuikSCAT pour la période 1999-2005 montre que l'océan Atlantique sudest tropical est caractérisé par une variabilité substantielle des vents de surface à des échelles de
temps d'intra saisonnier qui semble être lié à la variabilité de la convection au-dessus de l'Afrique
de l’ouest lors de l'été boréal et à la variabilité du bassin du Congo pendant l'été austral.
Cette région de l'océan Atlantique agit en tant que source d'humidité dans les bas niveaux pour la
mousson d’Afrique de l’Ouest suggérant qu'une meilleure compréhension de sa variabilité puisse
être importante pour comprendre le climat d’Afrique de l’Ouest. La variabilité intra saisonnière des
vents de surface estimée par QuikSCAT pour 1999-2005 au-dessus de l'océan Atlantique sud-est
tropical augmente pendant la Nina (1999/200) et El Nino (2002/2003) par rapport aux années
neutres. Elle est aussi décalée dans le temps impliquant qu'une autre voie peut exister pour
l’influence de ENSO sur le climat l'Afrique de l’Ouest.
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1.09
MECHANISMS OF OCEAN-FORCED SAHEL DROUGHT
Alessandra GIANNINI (1), Michela BIASUTTI (2) and Beate LIEPERT (2)
(1) IRI for Climate and Society, Earth Institute at Columbia University, USA
(2) Lamont-Doherty Earth Obs, Earth Institute at Columbia University, USA
The goal of this paper is to discuss the ”state of the art” in global climate modeling as it pertains to
the simulation of climate variability and change in the West African monsoon, with a focus on the
causes of Sahel drought.
Recently, a ”new wave” of studies, with atmospheric general circulation models forced by the
observed, long-term record of sea surface temperatures (SSTs), has conclusively demonstrated the
dominant role played by the global oceans in causing the persistence of drought in the Sahel during
the 1970s and 1980s (e.g. Giannini et al 2003; Bader and Latif 2003; Lu and Delworth 2005).
Here we first review the results of Giannini et al (2003), that attributed the recent persistence of
drought in the Sahel to global oceanic forcing, and relate intrinsic time scales of oceanic variability
to the time scales of Sahel rainfall variability. We complement the previous statistical results with a
description of possible mechanisms by which an anomalous SST forcing is translated into an
anomalous rainfall response on the African continent in terms of the mean meridional circulation
and deviations from it.
We then discuss the extent to which it is possible to attribute late 20th century climate variability to
global anthropogenic forcing, by comparing the patterns of Sahel climate variability obtained when
forcing atmosphere-only models with the observed record of SST variability to those obtained in the
coupled ocean-atmosphere models made available by the Intergovernmental Panel on Climate
Change in view of the 4th Assessment Report to be published in 2007.
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1.10
IMPACT OF SEA SURFACE TEMPERATURE AND SOIL MOISTURE ON
SEASONAL RAINFALL PREDICTION OVER THE SAHEL
Wassila M. THIAW and Kingtse C. MO
Climate Prediction Center, NCEP/NWS/NOAA, Washington DC, USA
Precipitation (P) forecasts over the Sahel from the NCEP coupled forecast system (CFS) model are
compared to the gauge rainfall analysis. The CFS ensemble forecasts for JAS from initial conditions
in June show a southward shift in the West African rain band. This leaves the Sahel very dry. The
southward shift of the rain band is accompanied by the southward shift of the AEJ.
The CFS forecasts also do not capture the interannual variability in the Sahel rainfall quite
adequately (Fig. 1). The suppressed interannual variability in P suggests the existence of persistent
erroneous forcing. The model simulations and CFS (corrected) have better representation of the
position of the AEJ and the spatial distribution of rainfall across West Africa (not shown). They
also show more realistic interannual rainfall variability.
Part of the P errors comes from the SST systematic errors (not shown). For the forecasts on the
seasonal time scales, SSTs have dominant influence on rainfall over the Sahel. The systematic error
pattern is similar to the decadal SST mode. It shows positive SSTs over the North Pacific and the
North Atlantic and negative errors in the tropical Pacific and the southern oceans.
During the CFS forecasts, the systematic errors are not corrected so they serve as additional forcing.
The persistence of the errors in the SST pattern may cause the errors in rainfall magnitudes and the
suppressed variability. The CFS model does not have realistic ice model as a subcomponent. The
ice information is supplied through the mean monthly climatology and the ocean coupling is limited
to the south of 65N. These model deficiencies may contribute to the warming over the North
Pacific and the North Atlantic.
In addition to the SST errors, the soil moisture feedback mechanism may also contribute to the
southward shift of the AEJ and rainfall. This is demonstrated by the comparison between the AMIP
run and the simulations. Both experiments are forced with the observed SSTs. The main
differences are in soil moisture and surface fluxes. The SIMs are initialized from the R2 in June and
has realistic information on soil moisture and surface fluxes. The AMIP, which is a continuous run,
does not have such information. The AMIP run shows the southward shift of the AEJ, while the
SIMs provide a better representation of the jet location and rainfall spatial pattern.
As expected, the AMIP has less soil moisture over the Sahel and less evaporation (E). E contributes
to P directly, but the largest influence is indirect through the temperature gradients. The radiation
differences are smaller so E is balanced by sensible heat. Less E implies more sensible heat and
indeed the AMIP is warmer over the Sahel than the SIMs. This implies that the largest temperature
gradients over West Africa in the AMIP are located further south than in the SIMs. The temperature
gradients reach the middle troposphere. This serves as a forcing to move the AEJ southward. In
response, The African wave disturbances, which account for much of the rains in the Sahel shift
southward resulting in dryness over the Sahel.
143
As it is well known, the most important contribution to rainfall variability over the Sahel is the
decadal mode. The AMIP forced with the observed SSTs does not capture the decadal changes in
rainfall. The model does not have interactive vegetation fraction and does not use the information
of the leaf area index (LAI). The vegetation fraction is supplied to the model through the monthly
mean vegetation fraction climatology. Therefore, it is not able to simulate the changes of albedo and
surface fluxes due to the greenness of vegetation. Vegetation dynamics is a significant process in
simulating rainfall over the Sahel and it has been found that the decadal variability which is an
essential part of the rainfall variability over the Sahel is better produced when the interactive
vegetation is added to the model. Therefore, a land-surface interaction model coupled with the CFS
will improve precipitation forecasts over the Sahel.
Figure 1 : (a) Seasonal JAS mean precipitation over the Sahel (12.5-17.5°N;17.5°W-20°E)
from the gauge analysis (solid line), CFS forecasts (crosses), CFS (corrected) (open circles)
and SIMs (dark squares); (b) same as (a) but for the AMIP (open circles) and the gauge
analysis (dark circles) from 1950-2001. Standard deviation for July-September (JAS) for
the period 1990 to 2001 from (c) the gauge analysis; (d) CFS forecasts and (e) CFS
(corrected). Contour interval 0.3 mm day-1. Values greater than 1.2 (1.5) mm day-1 are
shaded light (dark).
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1.11
REGIONAL-SCALE CLIMATE CHANGE DETECTION
OVER WEST AFRICA
V. LORANT(1), L. TERRAY (1) and S. JOURDAIN(2)
(1) Cerfacs, Toulouse, France (2) Météo-France, Toulouse, France
As part of the Discendo project, a high resolution database was assembled over West Africa. This
database encompasses new series of homogenized observations data covering the 1950-1980 period
and 4-members ensembles high resolution numerical simulations conducted from 1950 to 2000.
Homogenized observations of monthly mean precipitation, minimum and maximum temperature
are derived from daily measurements collected at 104 observation sites over West Africa.
Ensemble numerical experiments were performed using the Arpege-climat atmospheric global
circulation model. This "variable resolution" model is characterized by a 60km resolution over West
Africa. Two numericals experiments were realized: one uses sea surface temperature, natural and
anthropogenic forcing, and the second uses sea surface temperature only, natural and anthropogenic
forcing being fixed at 1950 values. In the present study advantage is taken of these database to
investigates the impact of anthropogenic forcing on the western African climate. Observation and
model results are analyzed and compared over 1950-2000 to identify potential human-induced
climate change at regional scale. Fingerprint-like detection method accounting for both spatial
pattern and temporal variability is applied to near-surface temperature and precipitation to extract
the signature of the forcing from anthropogenic greenhouse gases and sulfate aerosols concentration
changes. Emphasis of the study is laid on intra-seasonal to inter-annual variability and associated
mechanisms.
Submitted by :
Virginie Lorant - CERFACS, 42 avenue G. Coriolis, 31057 Toulouse Cedex 1, France
Phone : +33 5 61 19 30 76 – Fax : +33 5 61 19 30 00 - E-Mail:[email protected]
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1.12
MINERAL DUST IN SAHELIAN AFRICA:
(I) RATIONALE FOR THE AMMA FIELD EXPERIMENT
P. FORMETNI (1), B. MARTICORENA (1), K. DESBOEUFS (1) and J. L. RAJOT (2)
(1) LISA, CNRS, Universités Paris7/Paris12, Créteil, France
(2) IRD, Niamey, Niger
AMMA offers the unique opportunity to address specific questions related to mineral dust in
western Africa, where it accounts for a large – at times the largest – fraction of the regional aerosol
burden by mass, and where the mineral dust cycle is strongly linked to the alterning
Monsoon/Harmattan regimes. The quantification of mineral dust emissions in the Sahel remains
questionable, especially due to human and climatic disturbances to their natural levels. Such
disturbances are expected to increase in the next future, so their influence on mineral dust emissions
must be assessed right now. Furthermore, the radiative impact of dust emitted from disturbed soils
is considered as an anthropogenic forcing to the natural climate system. Because of its size
distribution ranging from fractions to tenths of microns, dust has multiple impacts on the landocean-atmosphere system, and on human health.
In this presentation, the major features and impacts of mineral dust in western Africa are
illustrated. The specific observational/modelling strategy deployed during AMMA, and expected
scientific outcomes, are described in a related poster.
Content
Mineral dust is the second largest aerosol species by mass at the global scale. To date, it is
estimated that 1 to 5 billions tons of mineral dust are emitted annually from arid and semi-arid areas
[Duce, 1995; IPCC, 2001]. The emission of mineral dust is a natural phenomenon due to aeolian
erosion in arid and semi-arid régions. This is a natural phenomenon, but its intensity can be altered
by changes in soil use (agriculture, pasture…) and in climatic conditions in semi-arid areas [Tegen
et Fung, 1995; Nicholson, 2000]. The contribution of disturbed areas to mineral dust emissions is
not precisely known to date, but this dust is considered of anthropogenic origin, therefore accounts
among the aerosol species exerting a radiative forcing on the atmosphere IPCC [2001].
The above considerations apply to the Sahara/Sahel region, the largest source area in the
world. As a matter of fact, high aerosol concentrations are observed in the Sahelian part of Western
Africa. The aerosol optical thickness in this region exhibit a clear seasonal cycle, with a maximum
in winter, when the “Harmattan”, a northeastern dry cold wind, is responsible for intense dust
emissions and very efficient transport. At contrary, during summer, due to the northern
displacement of the InterTropical Convergence Zone (ITCZ), the Sahel experiences the dry and wet
Monsoon flow from SouthWest. This monsoon flow is responsible precipitations thus for a minimal
aerosol load. This is due to the scavenging of aerosol transported from remote sources and to the
development of the annual vegetation preventing local aeolian erosion. Beside this pronounced
146
seasonal cycle, the mineral dust amount over western Africa is characterized by a high variability
from the daily to the interannual time scale.
On a longer time scale, a continuous increase of the dust load in the Sahelian region has
been observed in correspondence with the successive drought periods of the seventies and the
eighties. At the meteorological station of Gao (16°N, Niger), the annual precipitation was of the
order of 300-400 mm yr–1. at the beginning of the 50th’s, but it dropped down to 100-200 mm yr–1
during the eighties. Simultaneously, the number of days with dust haze has increased from a few
days to about 300 d/yr [N’Tchayi et al., 1994]. Similarly, the mineral dust concentrations measured
between the sixties and the eighties at Barbados in the Carribean Sea, have increased of a factor of 4
[Prospero and Nees, 1986]. These two simultaneous increases (dust haze in the Sahel, concentration
of long range transported dust) have been interpreted as being due to an increase of the local dust
emissions by additional Sahelian sources generated by the decrease of the vegetation cover rate.
The contribution of the Sahelian belt to the mineral dust emission from North Africa has been
further questioned based on numerical simulations of the mineral dust cycle performed with a
global transport model [Tegen and Fung, 1995], showing that a correct simulation of dust
concentrations over the Northern Tropical Atlantic Ocean and of the seasonal pattern of the Saharan
plume requires the inclusion of Sahelian sources with a contribution of 30–50 % of the global dust
emissions. These Sahelian emissions were attributed to regions affected by climatic changes and/or
anthropogenic disturbance. From these results, some authors concluded that the Sahel was the major
source of mineral dust in North Africa [Nicholson, 2000]. However, recent modelling studies tend
to estimate to only 10 to 15% of the total dust emissions the contributions of anthropogenic sources
over the Sahel [Tegen et al., 2004; Yoshioko et al., 2005].
All emitted mineral dust both of natural and anthropogenic origin, affects the radiative
budget of the atmosphere: mineral dust contributes in average to 20% of the aerosol optical depth at
the global scale, reaching up to 90% upwind the major source regions [Li et al., 1996; Chiapello et
al., 1999]. The evaluation of this effect is complex, as, due to their size distribution and
mineralogical composition, mineral dust can scatter and absorb both the solar and the terrestrial
radiations.
In the solar spectrum, mineral dust mainly scatters radiation. Their absorbing power is more
controversial, depending on the wavelength of the incoming radiation and on the mineralogy of the
source region [Sokolik and Toon, 1999; Lafon, 2004]. The net radiative effect is cooling at the
surface and at the top of the atmosphere [IPCC, 2001]. Conversely, absorption dominates in the
infrared spectrum, where dust aerosols absorb the terrestrial radiation and act as a greenhouse gas,
warming the atmosphere. Absorption in the infrared depends on the mineralogical composition and
it is mainly due to super-micron particles. In conclusion, the net radiation budget (solar + infrared)
may be negative or positive (cooling or heating of the atmosphere) depending on particle size and
mineralogical composition [Claquin et al., 1998; Myhre and Stordal, 2001]. As an example, the
sensitivity study by Myhre and Stordal [2001] shows that the mean global radiative forcing (net,
solar + infrared) varies between –0.40 W m–2 et +0.39 W m–2 depending on aerosol properties.
These authors indicates that the spatial distribution, in particular the height of transport, the size
distribution and the mineralogical composition are the most influent parameters in controlling the
direct radiative effects, but also those affected by the largest uncertainties.
Besides affecting the radiative balance of the atmosphere by scattering and absorption,
mineral dust can alter the physical and radiative properties of clouds (their lifetime and reflectance,
respectively), therefore their precipitating capacity, by altering the size distribution of the cloud
condensation nuclei [Levin et al., 1996; Wurzler et al., 2000; Yin et al., 2002].
147
Also, mineral dust can provide the surface for hetereogeneous reactions for some trace gases
such as HNO3, SO2, O3, N2O5 [Dentener et al., 1996]. As a consequence, the presence of mineral
dust can locally alter the trace gas concentrations. Furthermore, is the gas is an aerosol precursor (as
in the case of SO2), the chemical composition and the size distribution of the resulting aerosols will
be modified, and so their optical and radiative properties [Dentener et al., 1996; Li-Jones et al.,
1998]. The measurements of Chiapello [1996] upwind North Africa in the Atlantic Ocean have
shown that up to 100% of the mass of sulfur aerosols in the supermicron fraction result from the
heterogeneous reaction of SO2 and calcium carbonate (CaCO3) of mineral origin. In the absence of
this reaction, the SO2 would have been photoxided and would have produced sulphate aerosols in
the submicron fraction, more efficient in scattering radiating [Li-Jones et al., 1998].
Dust outbreaks also provide a source of nutrients to the ocean, especially iron. Fe is
important because it allows nitrogen fixation supplying nutrients to surface phytoplankton in the
otherwise nutrient starved regions of the subtropical gyres (Mahaffey et al, 2003). Thus, the amount
and location of dust deposition impacts biological productivity and subsequently the global carbon
cycle – of fundamental importance to the climate system. Wet and dry deposition of Saharan dust in
the North Atlantic region ensures that around 48% of the total Fe flux to the global oceans occurs to
the North Atlantic (Gao et al, 2001).
Finally, mineral dust also has impact on human health. Over western Africa, it is suspected
to be involved in the onset of meningococcal meningitis outbreaks. During the Harmattan season,
the warm dry and dusty air may causes damage to the mucous membranes of the respiratory system
and create conditions propitious to trigger meningitis epidemics. However, the quantitative
relationships between the epidemic intensity and onset period and climatic conditions and mineral
dust concentrations remain to be investigated.
The assessment of the different impacts of mineral dust required to precisely document the
variability in space and time of the mineral dust load. This implies in particular (1) a high frequency
sampling over long-time period; (2) a precise documentation of the vertical distribution of mineral
dust and its physico-chemical properties, at least for intensive observation periods; and (3) an
assessment of the budget of mineral dust emission and deposition over western Africa.
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LES POUSSIERES MINERALES EN AFRIQUE SAHELIENNE :
(I) JUSTIFICATION DES EXPERIENCES DE TERRAIN
DANS LE CADRE D’AMMA
Le programme AMMA offre l’opportunité unique de traiter des questions spécifiques liées aux
poussières minérales en Afrique de l’Ouest. Elles y elles représentent une fraction majeure, et
souvent largement majoritaire, de la charge massique atmosphérique en aérosol. Le cycle de
l’aérosol minéral en Afrique de l’Ouest est fortement contrôlé par l’alternance des régimes alternés
d’Harmattan et de mousson. La quantification des émissions de poussières au Sahel pose toujours
question, et notamment la perturbation des émissions que constituent les activités anthropiques et
les variations climatiques. De telles perturbations sont attendues dans le futur proche, aussi leurs
influences doivent être évaluée dès maintenant. De plus, l’impact radiatif des émissions par des sols
perturbés doit être considéré comme un forçage radiatif du système climatique naturel. Du fait de
leur distribution en taille, qui s’étend d’une fraction à une dizaine de microns, les poussières
minérales ont des impacts multiples sur le système terre-océan-atmosphère et sur la santé humaine.
Au cours de cette présentation, les principales caractéristiques et impacts des poussières minérales
en Afrique de l’Ouest seront illustrés. La stratégie d’observation et de modélisation déployée dans
le cadre d’AMMA et les avancées scientifiques attendues sont décrites dans un poster associé.
Contact
Béatrice Marticorena - Phone : +33 (0)1 45176569 – Fax : 33 (0)1 45171564 - Email:
[email protected]
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1.13
PRELIMINARY EVALUATION OF GLOBAL MODEL SIMULATIONS
OF TRACE GAS DISTRIBUTIONS OVER WEST AFRICA
K. LAW (1), V. THOURET (2), M. PHAM (1), I. BOUARAR (1), M.-A. FILIBERTI (3),
F. HOURDIN (3), J.-Y . GRANDPEIX (3), P. NEDELEC (2),B. SAUVAGE (2),
V.-H. PEUCH (4), C. GALY-LACAUX (2), D. HAUGLUSTAINE (5), S. SZOPA (5)
and C. GRANIER (2)
(1) Service d’Aéronomie/IPSL, Université Pierre et Marie Curie/CNRS, Paris, France
(2) Laboratoire d’Aérologie, U. Paul Sabatier, Toulouse, France
(3) Laboratoire Meteorologie et Dynamiques/IPSL, Paris, France
(4) CNRM, Toulouse, France (5) Laboratoire des Sciences du Climat et de l’Environnement/IPSL,
CNRS/CEA, Paris, France
West Africa is a large source of anthropogenic and natural emissions which are important for the
production of tropospheric ozone and other oxidants such as OH in the atmosphere. In particular,
this region has large emissions of biomass burning in the dry season and lightning NOx emissions
in the wet season.
Results from two global chemistry-climate models (LMDz-INCA and MOCAGE) will be presented
that have been run for at least one annual cycle and compared to available data over West Africa.
Model results have been compared with vertical profiles of ozone and CO collected over several
West African cities such as Lagos by the MOZAIC programme. This provides an evaluation of
model treatments of vertical transport of pollutants out of the boundary layer by processes such as
deep convection as well as an indication of the validity of emission inventories used in global
models over this region. Analysis of MOZAIC data has also shown clear signatures of long-range
transport of ozone and its precursors from biomass burning regions over central and southern Africa
into the monsoon region over West Africa during the wet season.
However, it is not clear how well global models are able to simulate this phenomenon. Results from
LMDz-INCA have also been compared to data collected as part of the IDAF network on levels of
soluble gases in rainwater such as nitric acid in a first attempt to evaluate the performance of the
model wet deposition scheme. Sensitivity of these model results to prescribed surface
anthropogenic and biomass burning CO emissions and also lightning NOx emissions will also be
discussed.
Contact
Kathy Law – Email : [email protected]
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1.14
IMPACT OF AFRICAN BIOMASS BURNING EMISSIONS ON COMBUSTION
AEROSOL BURDEN, TRANSPORT AND DEPOSITION
C. LIOUSSE (1), B. GUILLAUME (1), J.M. GRÉGOIRE (2), A. KONARÉ (3),
F. SOLMON (4) and C. JUNKER (1)
(1) Laboratoire d'Aérologie, Toulouse, France
(2) Global Vegetation Monitoring Unit, Inst. for Environment and Sustainability, Joint Research
Centre of the European Commission, Ispra, Italy
(3) Laboratoire de Physique de l’Atmosphère, Univ. d’Abidjan-Cocody, Abidjan, Côte d’Ivoire (4)
ICTP, Trieste, Italie
African biomass burning emission inventories have been developed for the year 2000, a first step
towards the determination of real-time emissions during the EOP-AMMA period. In this context,
new emission factors (EF) have been firstly estimated, especially for black carbon (BC) and organic
compounds. Two datasets are proposed, depending on the need of models. EF for primary organic
carbon particles (EFPPOC) and EF for volatile organic carbon (EFVOC) (based on ground
measurements) are provided for models that include an aerosol-chemistry module allowing for the
formation of secondary organic particles (SOA) from VOC. For models without such an aerosol
module, EFVOC and EF for particulate organic carbon (EFPOC) are provided, which are based on
airplane measurements assumed to integrate the major part of secondary organic aerosols and the
remaining VOC. Secondly, following a relevant methodology to derive burnt biomass maps over
Africa, burnt area maps given by GBA 2000 Spot vegetation satellite product have been used with
the GLC 2000 vegetation map (http:// www-gvm.jrc.it).
Then, these new inventories have been introduced in RegCM3 (a regional climatic model) and in
TM4 (a global CTM model) in order to test the regional and global impact of african biomass
burning emissions on aerosol budget, transport and deposition. Liousse et al., (1996) biomass
burning inventory has been used for the rest of the world whereas the inventory of biofuel and fossil
fuel emissions recently developed by Junker and Liousse, (2005) has been considered. A particular
attention is addressed to the organic carbon budget and formation with TM4 model. Two different
experiments are driven ; the first one considers POC as a tracer in TM4-tracer model whereas the
second one considers both PPOC and secondary organic aerosols obtained from VOC with
ORISAM-TM4 model. Differences on POC budget will be discussed. Figure 1 shows the first
results about the ratios PPOC/POC, globally obtained for August 2000. Finally emission and impact
interannual variabilities will be presented by comparing this work, with previous satellite-based
developments existing for the years 1980-1990.
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Figure 1 : Spatial distribution of PPOC/POC obtained with the ORISAM-TM4 model
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1.01P
PERFORMANCE D’UN MODELE DU CLIMAT REGIONAL TESTE AVEC
DIFERENTES HYPOTHESES DE FERMETURE DU SCHEMA DE CONVECTION
PENDANT LE PIC DE LA MOUSSON
DE L’AFRIQUE DE L’OUEST
Ernest A. AFIESIMAMA (1), S. O. OJO (2) et A. C. ANUFOROM (1)
(1) Nigerian Meteorological Agency, Lagos, Nigeria (2) Univ. of Lagos, Lagos, Nigeria
Cette étude examine et évalue différentes hypothèses de fermeture du schéma de convection
pendant le pic de la mousson d’Afrique de l’ouest comme simulé par le modèle régional de climat
de l’ICTP ( version 3 du modèle RegCM3) pour des études de la variabilité du climat et de son
changement. Le modèle utilise les réanalyses NCEP/ /NCAR comme conditions limites latérales.
Le modèle a été utilisé avec une résolution de 50 km avec le schéma de convection de Grell. La
sensibilité des résultats au maximum de l’été avec les fermetures du schéma de convection de
Fritsch-Chappell (FC) et Arakawa-Schubert (AS) sont examinés dans des simulations du modèle
conduites sur l’Afrique Ouest pendant le pic de la saison pluvieuse (août) pour la période 1996 2000. Des comparaisons directes avec les observations, des intercomparisons du modèle avec les
analyses, montrent que la précipitation moyenne sur la région est bien représentée par le modèle et
démontre la possibilité de reproduire la distribution des précipitations sur la région. Cependant, la
fermeture FC produit légèrement moins de pluie mais beaucoup plus proche des observations en
particulier pour la distribution spatiale alors que la fermeture AS surestime les pluies et conduit à
des résultats moins bons. Les températures de surface indiquent aussi des comportements qui
suggèrent une meilleure performance de FC que AS.
Les résultats montrent que le choix approprié du schéma de convection dans le modèle mésoéchelle permet d’étudier la variabilité du climat et son changement pour le développement socioéconomique et viable à travers des estimations de l'impact sur l’Afrique de l’Ouest.
153
PERFORMANCE OF REGIONAL CLIMATE MODEL TO DIFFERENT
CONVECTIVE CLOSURE ASSUMPTIONS DURING THE PEAK OF THE WEST
AFRICAN MONSOON
Abstract
This study examines and evaluates different convective closure assumptions during the peak
of the West African Monsoon as simulated by the International Centre for Theoretical Physics
(ICTP) Regional Climate Model version 3 (RegCM3) for climate variability and change studies.
The model uses the National Center for Environmental Prediction (NCEP) / National Center for
Atmospheric Research (NCAR) reanalysis data as lateral boundary conditions.
The model was run at a resolution of 50km with the Grell cumulus cloud scheme. The
sensitivity of the peak summertime results to the Fritsch-Chappell (FC) and Arakawa-Schubert (AS)
closure assumptions schemes are examined in model simulations conducted over West Africa
during peak of the rainy season (August) for the period 1996 - 2000. Direct comparisons against
observations, intercomparisons of the spatial pattern of the analyses, show that the average
precipitation over the region is well represented by the model and demonstrates a fair skill in
reproducing the precipitation distribution over the region. However, the FC scheme produces
slightly less rain but much closer to observation particularly in the spatial distribution than the AS
scheme which overestimates the rains and performs less accurate result. Surface temperatures also
indicate patterns that suggest a better performance in the FC than AS.
The results show that the appropriate choice of the convection scheme in the mesoscale
model offers support in climate variability and change studies for socio-economic development and
sustainability through impact assessments over West Africa.
1. Introduction
Although the monsoon returns with remarkable regularity each summer, the seasonal
amount of rainfall shows a large interannual variability. Since the region depends on rainfall for
agriculture, even a moderate decrease in monsoon rainfall or shift in the north – south extent can
have significant socio-economic impacts. Therefore, an understanding of the interannual variability
of the monsoon is important, not only for the people that live in this region, but also due to the
possible consequences for the earth’s climate system.
Regional climate models are useful tools for understanding rainfall variability and how
external forcings can impact the regional climate of West Africa (Jenkins, 1997; Fulakeza and
Druyan, 2000). However, before any conclusions can be drawn from climate models, there is the
need to conduct sensitivity analysis, especially convection schemes, to determine schemes suitable
over the West African region to assess model’s ability to simulate the mean state during the wet
season.
It has been well recognised for over four decades that cumulus convection is one of the major
processes that affects the dynamics and energetics of atmospheric circulation systems. Since then,
many cumulus parameterisation schemes have been developed for numerical models, to account for
the subgrid-scale release of latent heat and mass transport associated with convective clouds.
Preliminary experiments over West Africa demonstrate that the Grell (1993) mass-flux based
cumulus convection scheme gives a superior reproduction of the observed distribution and magnitude
of meteorological variables (Afiesimama et al., 2005). However, the closure assumption needed to
control the intensity of convection requires further investigations. Therefore, this work has designed
two closure types for the study. These are the Fritsch Chappell type closure and the Arakawa
Schubert closure type.
154
This study will then examine the ability of the International Centre for Theoretical Physics
(ICTP) regional climate model version 3 (RegCM3) to simulate the West African climate using
these different convective closure assumptions during the peak of the West African Monsoon.
2. Description of the Numerical Model and Experiments
2.1 Model Description
The study uses the ICTP RegCM3. Dynamics of the model is based on MM4/5 hydrostatic,
lateral boundary conditions relaxation. The Physics in the model includes: SUBEX large-scale
precipitation scheme (Pal et al. 2000); Radiation scheme (Kiehl et al. 1996); BATS land surface
model (Dickinson et al.1993); Zheng Ocean Flux model (Zheng et al. 1998); Non-local planetary
boundary layer scheme (Holtslag et al. 1990). Detail documentation of the model is in Pal et al.
(2005).
In this study, we are using the Grell scheme with Arakawa-Schubert closure assumption
(AS) and Grell scheme with Fritsch and Chappell closure assumption (FC)
2.2 Experiment design
The model’s domain and topography are important issues in Regional Climate Models
(RCM). There are 60 gridpoints in the north-south direction, 60 gridpoints in the east-west and 23
levels in the vertical with a horizontal spacing of 50-km and a model top of 70-hpa. A Mercator
projection is utilised. Most of the prominent topographic features in the domain such as the Jos
Plateau (Nigeria), Cameroon Mountains (Cameroon), Fouta Djalon Highlands (Guinea) are
captured well.
The choice of a domain larger than the West African region is to have lateral boundaries far
from the region of interest. This is important to reduce the influence of the time-dependent
inflow/outflow boundary conditions over West Africa. In addition, such a large domain was
designed to capture the important aspects of extra-tropical systems that appear to influence the West
African climate.
3. Preliminary Results
PRECIP
GRELL + FC
PRECIP
GRELL + AS
PRECIP
CRU
(a)
(b)
(c)
Fig. 1: Simulated Precipitation of August (1996 – 2000) with RegCM3 using (a) Grell+FC; (b)
Grell+AS and (c) CRU analysis
155
SFC TEMP GRELL + FC
(a)
SFC TEMP GRELL + AS
(b)
SFC TEMP
CRU
(c)
Fig. 2: Simulated Surface Temperature of August (1996 – 2000) with RegCM3 using (a) Grell+FC;
(b) Grell+AS and (c) CRU analysis
3.1 Precipitation Pattern
In the absence of gridded observed station dataset, the CRU analysis is used to compare with
the two different closure assumptions used in the simulations. For the FC simulation in Fig.1a, the
spatial distribution is about the same order of magnitude as the CRU except over the mountainous
regions of Fouta Djalon and Cameroon. Where precipitation values are relatively lower. The
northward extension of the precipitation during the peak of the monsoon in the model is much
higher in the FC than the AS (see Fig.1a&b). A major problem experienced by most models used
over the region is too much precipitation over the coast and far less amount around the Sahel region.
The FC has shown an improvement over this problem. The AS though indicated increased
precipitation over the mountainous regions but the values are overestimate to observations. Also,
the distribution and northward extension are not adequate.
3.2 Surface Temperature Distribution
Considering surface temperature field in Fig.2, we find that southern West Africa is colder
in the model than observation field. However, over the Sahel, the values in the model are fairly
comparable with observation.
Examining individual performance of the schemes, the FC (Fig.2a) is nearly 1oC less than
observation field compared to AS (Fig.2b) having between 2 – 3oC colder than the observation. In
both experiments, the temperature gradient in the model over the Sahel is relatively stronger than
over the region in the observation.
4. Conclusion
™ Our study has shown that the Grell cumulus scheme with Fritsch-Chappell closure
assumption performs better especially in the spatial distribution of precipitation than the
combination of Grell cumulus scheme and Arakawa-Schubert closure assumption scheme
™ Although the model results show colder condition over the southern West Africa than the
observation field, individual experiments indicate fairly better temperature values in the FC
(difference of 1oC), than the AS (difference of between 2 – 3oC). Over the Sahel, the model
captured fairly the temperature distribution.
™ Generally, the model offers support for Climate Variability and Change studies relevant for
impact assessment over the West African region.
Acknowledgment
This work was supported by START International under the START/PACOM Award 2005
through a grant from the US NSF (GEO-0203288). The study was carried out at the Regional
Meteorological Training Institute, Nigerian Meteorological Agency, Lagos, Nigeria.
156
References
Afiesimama, E. A; J. Pal, B. J. Abiodun, W. Gutowski and A. Adedoyin, 2005: Simulation of West African Monsoon
using the RegCM3. Part I: Model Validation and Interannual Variability (submitted to the special issue of Journal of
TAC)
Afiesimama, E. A., 2000: An overview of Nigerian climate trend in the last half Century. J. of Nig, Met. Soc. vol 1
No.2 p1 – 6
Arakawa A., and W. H. Schubert, 1974: Interaction of a cumulus cloud ensemble with the large-scale
environment: Part 1. J. Atmos. Sci., 31, 674-701.
Fritsch, J. M., and C. F. Chappell, 1980: Numerical prediction of convectively driven mesoscale pressure systems.
Part1: Convective parameterisation. J. Atmos. Sci., 37, 1722-1733
Fulakeza, M., and L. M. Druyan, 2000: A regional climate model adapted to West Africa. Proc. of the workshop on the
West African Monsoon Variability and Predictability (WAMAP), WMO/TD No. 1003
Grell, G.A., 1993: Prognostic evaluation of assumptions used by cumulus parameterisations. Mon. Wea. Rev. 121, 764787.
Jenkins, G., 1997: The 1988 and 1990 Summer Season simulations for West Africa using a regional climate model. J.
Climate, 10, 1255-1272
Pal, J.S., F. Giorgi, X. Bi, N. Elguindi, F. Solmon, X. Gao, M. Ashfaq, R. Francisco, J. Bell, N. Diffenbaugh, J.
Karmacharya, L. C. Sloan, A. Steiner, J. M. Winter, A. Zakey, 2005: The ICTP RegCM3 and RegCNET: Regional
Climate Modelling for the Developing World. Submitted to BAMS
Contact - correspondences
Ernest A. AFIESIMAMA - Email : [email protected]
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WEST AFRICAN MONSOON VARIABILITY IN MESOSCALE DATA SETS
Matthew FULAKEZA, Leonard DRUYAN and Patrick LONERGAN
NASA / Goddard Institute for Space Studies and the Columbia University Center for Climate
Systems Research
The intra-seasonal and inter-annual variability of the West African summer monsoon climate is
studied in data sets created by a series of mesoscale climate model simulations. These simulations
over a limited area centered over West Africa are driven by synchronous boundary data from either
NCEP reanalysis (mode 1) or a global climate model (mode 2), effectively downscaling rather
coarse resolution global representations to a grid with 0.5° spacing. The RM3 and RM3b versions
are run with either 15 or 28 vertical layers, respectively. Downscaled results for six summers in
mode 1 were compared to TRMM daily estimates of precipitation, CRU (East Anglia University)
gridded monthly mean precipitation and surface temperature and NCEP reanalysis data. Time-space
distributions of precipitation in the RM3 data sets do not resemble reanalysis precipitation
distributions. Rather, they showed a high degree of congruence with corresponding TRMM daily
estimates, albeit with lower frequencies of extreme values at the low and high-end of the total
range. The seasonal mean distributions of RM3 precipitation compared favorably with CRU
distributions, except for some extreme CRU orographic maxima. The timing of RM3 African
easterly waves at 700 mb matches the reanalysis, but amplitudes were damped. On the other hand,
the daily variability of RM3b 700 mb meridional wind more closely matched reanalysis behavior.
Fourier and wavelet analyses of RM3 meridional wind time series detected realistic spectral peaks
related to the waves at 3-5- day periods. Downscaling experiments in mode 2 explore the potential
for weather and climate predictions over West Africa at mesoscale spatial detail.
Submitted by
Dr. Leonard M. Druyan - Columbia University Senior Research Scientist - Center for Climate
Systems Research at NASA/Goddard Institute for Space Studies a unit of the Earth Institute at
Columbia University, Armstrong Hall, 2880 Broadway, New York, NY 10025, USA
Phone : 212 678-5564 – fax : 212 678-5552 – Email : [email protected]
Matthew Fulakeza <[email protected]>, [email protected]
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SIMULATION DU CLIMAT DE L’AFRIQUE CENTRALE ET DE L’OUEST A
L’AIDE D’UN MODELE CLIMATIQUE REGIONAL (REGCM3)
QUELQUES RESULTATS PRELIMINAIRES SUR LES CHOIX DE LA TAILLE DU DOMAINE, DU SCHEMA
CONVECTIF ET DE LA TOPOGRAPHIE EN VUE D’AMELIORER LES SIMULATIONS
DES PRECIPITATIONS
Lucie A.T. DJIOTANG et François MKANKAM KAMGA
LAMEPA, Université de Yaoundé 1, Cameroun
En vue de l’étude d’impacts du changement climatique sur un bassin versant d’intérêt économique,
nous avons entrepris d’utiliser le modèle régional RegCM3 (version ICTP) pour simuler le climat
présent et le climat perturbé. Mais les modèles sont sensibles à un certain nombre de paramètres tels
que la taille du domaine, les conditions aux limites. De plus, l’une des plus grandes limitation dans
leur utilisation est la nécessité de paramétriser un certains phénomènes, entraînant de choix qui
influe sur les résultats.
Le RegCM3 forcé à ses limites par les analyses du NCEP est évalue en Afrique Centrale et de
l’ouest sur la base de simulations des précipitations et des températures, en fonction de divers
facteurs : taille du domaine, taille de la maille, schéma de convectif, etc .
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1.04P
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1.05P
163
164
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STUDY OF SYNOPTIC ACTIVITY SIMULATED OVER WESTERN AFRICA
USING MM5 MODEL
A. SARR.(1, 2), P. DE FELICE (3), H. SAUVAGEOT (4) and A. T. GAYE (2)
(1) Direction de la Météorologie Nationale du Sénégal, Dakar, Sénégal
(3) Laboratoire de Météorologie Dynamique, Paris, France
(4) Laboratoire d’Aérologie, Toulouse, France
(2) Laboratoire de Physique de l’Atmosphère, Dakar, Sénégal
The fifth generation of the mesoscale model jointly developed by the Pennsylvania State University
(PSU) and The National Center for Atmospheric Research (NCAR), MM5, is used in a set of
simulations over Wester Africa and the Atlantic Ocean. Various configurations for sensitivity
studies are investigated. Initial and lateral boundary conditions (IC&LBC) to force the model are
from the National Centers for Environmental Predictions (NCEP) and NCAR Reanalysis Project
(NNRP2) at a space resolution of 2.5X2.5 degrees and six hourly time resolution. The study shows
that MM5 can capture the synoptic systems over West Africa and the Eastern Atlantic Ocean. They
are mainly the well known wave disturbances with a periods of 3-5 days named African Easterly
Waves (AEWs) (which are the smallest dynamical phenomena modulating rain producing systems
in West Africa) and the others with periods of 6-9 days.
ETUDE DE L’ACTIVITE SYNOPTIQUE EN AFRIQUE DE L’OUEST
SIMULEE PAR LE MODELE MM5
La cinquième génération, du modèle de méso échelle, développé conjointement par l’Université de
l’Etat de Pennsylvanie (PSU) et le NCAR, appelé MM5, est utilisé pour une série de simulations sur
domaine couvrant l’Afrique de l’Ouest et une partie Est de l’Océan Atlantique. Plusieurs
configurations pour des tests de sensibilité sont faites. Les conditions initiales et latérales pour
forcer le modèle proviennent des réanalyses du NCEP (NNRP2) qui ont une résolution spatiale de
2.5°X2.5°, et temporelle de six heures. L’étude montre que le modèle MM5 simule les systèmes
synoptiques dans le domaine d’étude. Ces systèmes sont principalement les ondes de périodes 3-5
jours, bien connues et appelées Ondes d’Est Africaines (OEA) (ils sont les plus petits phénomènes
dynamiques qui modulent les systèmes précipitants en Afrique de l’Ouest), et les autres de périodes
6-9 jours.
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MONSOON ONSET OVER SUDAN-SAHEL: SIMULATION BY THE
REGIONAL SCALE MODEL MM5
Pascal ROUCOU, S. SIJIKUMAR and Bernard FONTAINE
Centre de Recherches de Climatologie, CNRS, Université de Bourgogne, Dijon, France.
1. Introduction and experiments
The purpose of this study is to test ability of MM5 to simulate associated regional circulations of
WAM during the onset phase of monsoon over the Sudan-Sahel region. First we examine the mean
climate produced by the model; this is followed by a comparison of the onset date between CMAP,
MM5 and reanalysis and a description of the changes in the circulation around this date. MM5 is a
non hydrostatic, sigma coordinate mesoscale atmospheric model that includes advanced model
physics (Grell et al., 1994). The physical schemes used for the present study are Betts-Miller
convective scheme, Blackadar PBL scheme, stable explicit precipitation moisture scheme, CCM2
radiation scheme and five-layer soil Model (Dudhia et al., 2005). We have selected eight individual
seasons (March to September) of the years 1994 to 2001 for the study, because during this period
intraseasonal variability in precipitation has been well characterized (Louvet et al., 2003). Domain
selected covers West Africa and neighboring areas, roughly 65°W-35°E and 15°S-35°N. The
horizontal resolution of the model domain is 60 km and there are 23 vertical levels from surface to
100 hPa. Initial and lateral boundary conditions are taken from ECMWF Reanalysis project
(ERA40) and NCEP’s optimum interpolated Sea Surface Temperature (SST) data are used as lower
boundary condition. The weekly SST data are linearly interpolated to 12hr data. The model is
initialized at 00Z on 1 March for all years and integrated for next 212 days with a time step of 150
seconds. First five days of simulations are assimilated towards the analysis with Newtonian
relaxation method to get a better balance of the initial model conditions. Boundary conditions
including SST are updated every 12hr and model output saved every 12hr. CPC Merged Analysis of
Precipitation (CMAP) data are used to validate simulated precipitation because the data are very
close to in situ precipitation data over West Africa both in rhythm and amplitude (Louvet et al.,
2003). The spatial resolution is 2.5 degree and time resolution is pentad (Xie and Arkin, 1997).
2. Results
Rainfall over West Africa begins in April-June over Guinean coast with copious amount of rainfall
and migrates northward to Sudan-Sahel region during June-July with north most rainfall maxima in
August. Figure 1 shows the Hovmoller diagram of 8 year mean rainfall averaged over 10°W-10°E
for CMAP, MM5 and ERA. As CMAP values are in pentads, we reorganized the simulated and
reanalyzed rainfall in pentads.
Figure 1 exhibits large differences in rainfall: MM5 (ERA) produces higher (lower) amounts than
CMAP. The higher rainfall amounts can be associated with vertical surface atmosphere fluxes. In
our simulations the atmospheric part of the model is not coupled to a land surface model. Thus, the
absence of coupling with the surface can explain the differences as in Gallée et al. (2004).
Nevertheless it seems that MM5 provides a better information than ERA. After June, the rainfall
activity shifts north of 7.5°N in both CMAP, simulation and reanalysis: this is the signal of the
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monsoon onset. Considering the fact that when Sudan-Sahel precipitation increases, Guinean
rainfall decreases, we develop a basic methodology to find onset dates.
Figure 1 : Time-latitude diagram of 8 year mean CMAP (a), simulated (b) and ERA40
(c) precipitation in mm/day averaged over 10°W -10°E.
First we compute two rainfall indexes, a Guinean index located between 10°W-10°E and 0-7.5°N,
and a Sahelian one between 10°W-10°E and 7.5°N-20°N. A low-pass Butterworth filter (Murakami,
1979) has been applied on the time series to eliminate shortest fluctuations (<15 days). We then
calculated the standardized difference between the standardized Sahelian and Guinean index for
each year. A positive (negative) difference indicates that Sahelian rainfall rate is higher (lower) than
Guinean rate. There is a very good agreement between the CMAP, MM5 and ERA difference
curves showing that the model reproduces well the seasonal difference of rainfall between the two
areas. We select an onset date as the first pentad just above the 0 threshold and followed by at least
4 other successive positive values but results are similar even if we choose 3 successive pentads.
Figure 2. (a) Eight year composite for MM5 of
differences between the two pentad after onset minus
the two pentads before wind and geopotential height
(shade) at 925 hPa. (b) Same as (a) but for ERA40.
Units: meter and m/sec.
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The onset dates computed from simulations are similar to CMAP, except in 1998 and 2001 but it
exists slight differences between CMAP and ERA40 in 1996-97-98 and 2001. The mean date is the
36th pentad (23-27 June) in CMAP data and 35th pentad (18-22 June) in MM5 simulations and
reanalysis. The standard deviation is similar (2 pentads) for both CMAP and simulations but higher
(2.8 pentads) for reanalysis. The low level characteristics of circulation after and before the onset in
simulated data and in reanalysis have to be compared. Figure 1a shows the 8 year composite
difference between the two pentads after onset and the two pentads before (after minus before) for
simulated wind and geopotential height at 925 hPa. It is clear that just after the onset, zonal flow
from west increases between 10°N and 15°N over the continent. This increase is driven by a
pressure gradient created by the enhancement of pressure south of 10°N consistent with a deepening
of the heat low north of 20°N. Thus there is an increase in inflow of moist air from eastern Atlantic
in low levels just after the onset that feeds the ITCZ convection. This is in good agreement with
Grodsky and Carton (2001) who show an increase in zonal flow at the beginning of the Guinean
rainfall season, in May, in association with an enhancement of the pressure gradient force between
ocean and continent. The results show that a main signal of the onset could be the enhancement of
zonal circulation, meaning that the increase of precipitation is linked not only to the moisture from
the Guinea Gulf (Gong and Eltahir, 1996) but also from east Atlantic.
3. Conclusion
It is the first time that MM5 is used to study the monsoon onset over Sudan-Sahel area and
associated features. For robustness of results, simulations are carried out for eight consecutive
seasons from 1994 to 2001. It is shown that MM5 can produce the jump of the rainfall band from
south of 7.5 N to north at the time of onset and a good accuracy with CMAP. Another major result
is that the model produces a very close onset date with CMAP most of the years. The simulations,
in agreement with previous studies, show that the main signals of onset are the deepening of the
heat low and as a consequence, the enhancement of zonal circulation between ocean and continent
around 10°N-15°N. This signal is also found in reanalysis (Figure 4 b) meaning that a part of the
increase in precipitation after the onset is probably linked to the moisture flow from east Atlantic.
These results show that the MM5 RSM can be a useful tool to detail the basic process linked to the
African Monsoon onset. Another perspective is to use the model for dynamical downscaling to
produce high spatio-temporal rainfall resolution data needed for local impact studies.
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1.08P
HIGH RESOLUTION, REGCM3 SIMULATIONS OF THE
WEST AFRICAN CLIMATE SYSTEM
Mouhamadou Bamba SYLLA (1), Gregory S. JENKINS (2)
and Amadou Thierno GAYE (1)
(1) Laboratory for Atmospheric and Oceanic Physics, ESP; Dakar University, Senegal
(2) Howard University Program for Atmospheric Sciences (HUPAS), Washington DC, USA
1. Background
According to the International Panel for Climate Change (IPCC; 2001), Africa is one of the
most vulnerable regions in the world to climate change. Since the late sixties, extreme climate
variability, in form of negative precipitation anomalies, has occurred during a critical portion of
rainy season (June to September) in West Africa. Over the next century, this change in precipitation
patterns are expected to continue and be accompanied by a warming trend, rise in sea level and
increased frequency of extreme weather events. Their impacts on human welfare and the
environment are multiple. Ecosystems, Water resources, Agriculture, Food security and Health will
be highly affected. So there is an imperious need for a better understanding of this climate change
and its variability.
This is possible with climate models. Then, the Regional Climate Models have been
increasingly used for climate research in West Africa. So, in this presentation, we use the Regional
Climate Model (RegCM3) and simulate West African climate conditions.
2. Experiment design
RegCM3 is an augmented version of the National Center for Atmospheric Research Pennsylvania State University Mesoscale Model MM4. It It is based on the concept of one way
nesting, and employed to drive the coarse mesh lateral boundary conditions of global datasets and
produce fine mesh output data. It is describe in detail by (Giorgi et al. 1993a et b). RegCM3 is a
three dimensional limited area model. A dimensionless sigma coordinate is used to define the model
levels. A number of physics parameterizations were incorporated: the Biosphere - Atmosphere
Transfer Scheme (BATS) (Dickinson at al. 1993) for the land surface model, and a detailed
atmospheric radiation transfer scheme (Briegleb, 1992) from the Community Climate Model
(CCM3). RegCM3 includes also the Grell (1993) scheme for precipitation parameterizations.
The National Center for Environ-mental Predictions (NCEP) reanalysis are employed to
drive the model. We use the convection scheme developed by Grell (1993). The simulations begin
on the 1st of January 1993 and end on the 31th of December 2000. Precipitations and Temperature
are compared to the Climate Research Unit (CRU) dataset.
170
3. Some Results
Fig.1 :Observed (CRU) Precipitations (mm/day) during JAS 1993-2000
Fig.2 : Simulated (RegCM) Precipitations (mm/day) during JAS 1993-2000
Figure 1 and 2 show (respectively) the spatial distribution of the observed (CRU) and
simulated(RegCM) precipitations during July, August and September (JAS) from 1993 to 2000.
Compared to the Observations, RegCM3 does a good simulation of precipitations during JAS 19932000. It localises the high rain rates over the orographic zones (but they are understimated) and the
lowest in the north of Africa as the observations of CRU do. Their meridional gradients are also
well represented.
Fig. 3 : Observed (CRU Temperature (°C) during JAS 1993-2000
171
Fig. 4 : Simulated (RegCM) Temperature (°C) during JAS 1993-2000
RegCM3 simulates quite well temperature over West Africa during JAS 1993-2000. As in
the CRU observations (fig. 3), it finds the highest values of temperature in the Sahara desert and
the lowest in the guinea regions (fig.4). But it shows cold biases over Guinea regions and warm
biases over the Sahara.
4. Conclusion
RegCM3 does good simulations of the parameters mentioned above, but has some problems
with precipitations over the orographic zones and shows some biases of temperature. These errors
can be due to parameterizations.
References
Briegleb, B.P., 1992: Delta Eddington approximation for solar radiation in the NCAR Community Climate Model. J.
Geophys. Res., 97, 7603-7612.
Dickinson, R., Henderson-Sellers, A. and Kennedy, P., 1993: Biosphere-atmosphere transfer scheme (bats) version
1e as coupled to the NCAR community climate model, Technical report, National Center for Atmospheric Research.
Giorgi, F., Marinucci, M.R. and Bates, G.T., 1993a: Development of a Second Generation Regional Climate Model
(RegCM2). Part I: Boundary-layer and radiative transfer processes. Mon. Wea. Rev., 121, 2794-2812.
Grell, G., 1993: Prognostic evaluation of assumption used by cumulus parameterizations. Mon. Wea. Rev., 121, 764787.
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1.09P
173
174
1.10P
THE INFLUENCE OF AFRICAN AIR POLLUTION ON THE REGIONAL AND
GLOBAL TROPOSPHERIC CHEMISTRY
ADETUTU M. AGHEDO, MARTIN G. SCHULTZ AND SEBASTIAN RAST
Max Planck Institute for Meteorology, Hamburg, Germany
1. Introduction
Much of the air pollution emitted in Africa comes from four major sources, mainly biomass
burning, natural emission from vegetation and soil, lightning NOx and anthropogenic sources such
as emissions related to industry and transport. Africa contributes a significant amount to the global
emissions from the first three categories, while fossil fuel combustion emissions are only important
on a local and regional scale. Most of the gas-phase species such as CO, CO2, non-methane
hydrocarbons (NMHC), NOx (i.e. NO and NO2), N2O, CH4, H2 are directly emitted into the
atmosphere, others such as O3 or PAN (peroxyacetyl nitrate) are secondary products of atmospheric
photochemical reactions. It has been demonstrated that many of these compounds directly or
indirectly influence the oxidizing capacity and alter the radiation budget of the atmosphere. We
investigate the relative importance of African biomass burning, biogenic volatile organic
compounds (BVOC), lightning and anthropogenic emissions to the tropospheric ozone budget over
Africa and globally. Using a coupled global chemistry climate model, we find that these emissions
contribute 9 Tg (2.4%), 13 Tg (3.4%), 8 Tg (2.1%) and 4 Tg (1%), respectively to the global
tropospheric ozone burden. Further sensitivity studies indicate that African BVOC together with
soil NOX emissions contribute about 17 Tg to the global tropospheric ozone burden. Our model
calculation suggests that only about 30% of the tropospheric ozone produced from emissions in
Africa stays on the continent, thus exerting a noticeable influence on a large part of the tropical
troposphere.
2. Model and Experiments
The 3-D global chemistry climate model MOZECH [Rast et al., in prep.] is part of the Hamburg
Earth System Model (ESM) and consists of the 3-D global general circulation model ECHAM5
[Roeckner et al., 2003] into which the chemical reaction schemes of MOZART [Horowitz et al.,
2003] was implemented. The production of NOx from lightning flashes is parameterized according
to Grewe et al., 2001 with a vertical distribution following Pickering et al., 1998. It also includes
dry deposition [Ganzeveld, 2001] and wet deposition [Stier et al., 2004] schemes. The model
experiments for this study are based on recent simulations performed in the framework of the
international IPCC/ACCENT intercomparison experiment where 24 global chemistry transport and
general circulation models submitted results for a number of prescribed emission and climate
scenarios [Stevenson et al., 2006]. Table 1 shows the global annual CO, NOx and NMHC emission
estimates and the amount contributed by African emissions. The climate conditions (sea surface
temperatures and sea ice fields) were taken from coupled ocean-atmosphere simulations performed
at the Max Planck Institute for Meteorology, Hamburg [Roeckner et al., in prep.]. Present-day
constant concentrations were maintained for CH4, CO2, N2O and CFCs. Each experiment was run
175
for a minimum of 5 years in the T42L31 resolution (approximately 2.8° by 2.8°). The sensitivity
experiments were as follows:
S1: Present-day emissions in present-day climate (this is the reference experiment).
S1a: Same as S1, but all African Biomass burning emissions are set to 0.
S1b: Same as S1, but all African Biogenic VOC emissions are set to 0.
S1c: Same as S1, but Lightning NOx emissions over Africa are set to 0.
S1d: Same as S1, but African anthropogenic emissions are set o 0.
Table 1 : Global and African (in parentheses) trace gas emissions by source used in this study
Sources
Biomass burning
Biogenic
Industrial
Lightning
Aircraft
Ocean
CO (Tg C)
217 (93)
69 (14)
201 (32)
—
—
9 (—)
NOx (Tg N)
10 (5)
8 (2.4)
28 (1.4)
2.7 (0.71)
0.7 (0.03)
—
NMHC (Tg C)
19 (8)
756 (186)
66 (9)
—
—
4.4 (—)
3. Results
Figure 1 shows the 1997-2001 average of surface ozone produced by African biomass burning,
BVOC and anthropogenic emissions (derived as the differences between simulations S1 and S1a,
S1b and S1d, respectively). Lightning NOx emissions have an insignificant effect on surface ozone
but produce the second largest impact on the middle and upper tropospheric ozone concentrations,
after BVOC emissions. African biomass burning emissions have the largest contribution to surface
ozone in Africa. Nigeria, South Africa and Egypt are the major countries affected by African
anthropogenic emissions.
(a)
(b)
(c)
Figure 1: 1997-2001 average surface ozone produced by African (a) Biomass burning,
(b) Biogenic VOC and (c) anthropogenic emissions. Note the difference in the scales.
The monthly mean tropospheric ozone burden over Africa is shown in Figure 2a. The peaks of the
surface ozone produced by African biomass burning emissions clearly reveal the two burning
seasons; which are DJF and JJA for the northern and southern hemispheric part of Africa,
respectively. Apart from these two burning seasons, emissions from vegetation have the largest
contribution to the photochemical ozone burden over Africa. BVOC emissions over Africa yield the
highest contribution to global tropospheric ozone burden (Figure 2b).
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(a)
(b)
Figure 2: The contribution of African emissions to (a) African and (b) global
tropospheric ozone burden. Note the difference in the scales.
References
Ganzeveld, L. (2001), PhD thesis, University of Utrecht, The Netherlands.
Grewe, V. et al., (2001), Atmospheric Environment 35 (2001), 3421-3433.
Horowitz, L. W., et al., (2003), J. Geophys. Res., 108(D24), 4784.
Pickering, K. E. et al., (1998), J. Geophys. Res., 103(D23), 31,203–31,216.
Rast, S. et al., manuscript in preparation.
Roeckner, E. et al., (2003), Max Planck Institute Report 349.
Roeckner, E. et al., manuscript in preparation.
Stevenson, D. S. et al., (2006), J. Geophys. Res., doi:10.1029/2005JD006338.
Stier, P. et al., (2005), Atmospheric Chemisty and Physics, 5, 1125-1156.
Correspondence to: [email protected]
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1.11P
STUDIES ON POLYCYCLIC AROMATIC HYDROCARBONS (PAHS)
IN THE LAGOS LAGOON
R.A. ALANI (1), K. O. OLAYINKA (1) and B. I. ALO
Nigeria National Petroleum Coroporation, R&D Dept, Chemistry dept,
University of Lagos, Akoka, Nigeria
Lagos and its environs are thought to harbour about 75% of the manufacturing industries in Nigeria.
In the city, wastes are openly incinerated, and oil related and fishing activities take place in the
harbour and the lagoon. Open incineration can cause a change in the air quality. Bimonthly
sampling of water and sediment from 12 locations on the lagoon between longitude 3°23" and
3°26"E and latitude 6°26" and 6°36"N was carried out for one year (from February to December
2004). A total of 16 USEPA priority Polycyclic Aromatic Hydrocarbons (PAHs) were analyzed for.
In April, the sediment sample at the harbour contained 12 PAHs, Fluorene, 747ug/kg being the
lowest and Benzo(k) Fluoranthene 147,000ug/kg being the highest. The water sample from the
same point contained 14 PAHs, Fluoranthene, 10.71ug/L (lowest) and Pyrene, 578ug/L (highest). In
the same month, a point in Ikorodu (far removed from suspected sources of PAHs) contained 14
PAHs in the sediment sample, Naphthalene, 807^g/kg being the lowest and Benzo (g,h,i) Perylene,
72,000 jig/kg being the highest. The water sample from the same point contained 15 PAHs,
Anthracene, 7.51ug/L being the lowest and Benzo (g,h,i) Perylene 1300ug/L being the highest. In
June (rainy season) a point at Okobaba, (where sawdust is openly burnt) contained 9 PAHs in the
sediment sample, Naphthalene, 1120747ug/kg (lowest) and Indeno (1,2,3-cd) Pyrene, 31,200ug/kg
(highest). The water sample at the same point contained 13 PAHs, Fluoranthene 16.43ug/L being
the lowest and Pyrene, 753|ug/L being the highest. All results obtained exceed the World Health
Organization (WHO) and USEPA recommended maximum contamination levels.
The results obtained showed that all the 16 USEPA priority PAHs were present at different levels in
all the samples at different times. The PAHs levels in the water samples were significantly lower
than those in the sediment samples collected from the same points. This confirms the fact that PAHs
do not dissolve in the water column but tend to absorb to particles. With the evidence of long-range
transport of these substances to regions where they have never been used or produced, the threats
they pose to the environment are of regional and global scale. This paper therefore calls on the
appropriate environmental bodies such as, the Directorate for Petroleum Resources (DPR), the
Federal Ministry of Environment, etc., to develop regulatory standards and guidelines, and join in
the global fight to protect the people and the entire global environment from the menace of PAHs.
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1.12P
STUDY OF PLUMES MIXING FROM BIOMASS BURNING AND DUST HAZE
OVER WEST AFRICA
F. BOUO-BELLA (1), S. CAUTENET (2) and G. CAUTENET (2)
(1) Université d’Abidjan, Côte d’Ivoire (2) LAMP, Clermont-Ferrand, France
During dry season over West Africa in Guinea savanna, plumes from biomass burning and dust
haze are mixed and transported by the Harmattan flow and spread southwards below 10°N.
Sometimes, meteorological conditions favor deep convection. In this study, real cases are examined
where the precipitation water has been analyzed in the framework of IDAF network in Lamto,
center of Ivory Coast. Ionic concentrations of nitrate, formiate, calcium and magnesium are two
times higher when there are plumes mixings than when there is only a fire plume. A mesoscale
simulation from the meteorological model RAMS coupled online with a condensed chemistry code
in gaseous, aqueous and heterogeneous phases, has been performed. Sensitivity tests for the
accommodation rate of HNO3 uptake onto mineral aerosols surfaces, and for the calcite content
from mineral aerosols have been run. We conclude that the processes: (i) production of HNO3 from
biomass burning, (ii) adsorption of HNO3 by the surface of mineral particles, and (iii) scavenging
of HNO3, either gas or adsorbed, must be taken into account. Model issues agree with
measurements of nitrate found in precipitation. We show that the accommodation rate of HNO3 is
close to 0.1, and the percentage of calcite in mineral aerosol is about 10%.
These researches highlight the important role of mixing plumes which will be studied during the
SOP0 in AMMA experiment.
Submitted by :
Sylvie Cautenet
LaMP, Université Blaise Pascal, CNRS, 24, Avenue des Landais, 63177 Aubière, France
Tel : +33 4 73 40 73 59 – Fax : +33 4 73 40 51 36 - [email protected]
179
1.13P
AN OVERVIEW ON CIRRUS PROPERTIES IN THE TROPICAL UT-LS FROM
PREVIOUS FIELD CAMPAIGN
F. CAIRO (1). F. FIERLI (1) and G. DIDONFRANCESCO (2)
(1) Institute of the Atmospheric Sciences and Climate, CNR, Italy
(2) ENEA-CLIM, Frascati, Italy
We present an overview of the lidar in-situ aerosol observations onboard stratospheric balloons and
the Geophysica aircraft during the last years in the frame of the APE-THESEO, HIBISCUS and
TROCCINOX campaigns, held in the Indian Ocean and in Brazil from 1999 to 2005. We give first
a brief review on the impact of deep convection in the UT-LS chemical and aerosol composition
and a description backscatter sonde and microlidar experimental systems that will also be deployed
during the AMMA campaign.The analysed case studies show that cirrus reveal small scale
structures that are linked to water vapor variability.It appear that water vapor is likely to be
controlled by deep convection but quasi-horizontal transport, driven by synoptic motion in the
tropopause region, can also play also a key role in determining the water vapor content and hence
the cirrus formation potential.Moreover, observations revealed the presence of thin aerosol layers in
the lower stratosphere where deep convection should not inject directly water vapor; nevertheless
mesoscale transport analyses are unable to highlight any features that could explain an excess in
water vapor leading to aerosol formation.
Contact :
F. Cairo : [email protected]
F. Fierli : [email protected]
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1.14P
WEST AFRICAN MONSOON CONVECTIVE SYSTEMS AND OZONE BUDGET
IN THE UPPER TROPOSPHERE :
FROM LOCAL TO GLOBAL SCALE
C. MARI (1), J.L. ATTIE (1), A. BORBON (2), J.P. CHABOUREAU (1), C. DELON (1), H.
HOELLER (3), C. JAMBERT (2), B. JOSSE (4), K. LAW (5), P. MASCART (1),
P. PERROS (2), V.H. PEUCH (4), J.P. PINTY (1), C. REEVES (6), D. SERCA (1),
H. SCHLAGER (3) AND V. THOURET (1)
(1) Laboratoire d'Aérologie, Toulouse, France (2) LISA, Créteil, France
(3) DLR, Germany (4) CNRM, Toulouse, France (5) Service d'Aeronomie, Paris, France
(6) University of East Anglia, UK
The composition of the atmosphere and the state of the global climate depend crucially on processes
in the tropical regions. Mesoscale convective systems are important for the transport of trace
constituents from the boundary layer into the free troposphere, for the source of tropospheric NOx
by lightning, and for the loss of trace constituents by heterogeneous removal processes including
washout. Once emitted to the atmosphere, gases can be rapidly lifted into the free troposphere by
deep convection and transported over large distances away from source regions. As such, emissions
over West Africa can affect atmospheric properties and processes on intercontinental and global
scales. However, the details of these processes still requires further investigation. During the
AMMA-SOP 2 period in July-August 2006, the impact of MCS on ozone precursors and ozone
budget in the upper troposphere will be studied. Dedicated observations will be inferred from
ground-based stations in Djougou (Benin), Lamto (Ivory Coast) and Hombori (Mali);
ozonesoundings in Cotonou (Benin), small balloons fully equiped with chemistry in Niamey
(Niger) and aircrafts (based at Niamey and Ouagadougou). The experimental strategy has been
designed such that the chemical composition of entrainement and detrainement convective fluxes
will be characterized for several chosen MCSs. In particular, ground-based and low-level aircraft
measurements will provide information on the chemical composition of the boundary layer air
masses. Background concentrations of ozone and precurors will be sampled before the MCS
passage. Measurements in the cloud anvils will provide information on the major detrainement
fluxes in the upper troposphere. The stratiform part of the MCS will also be observed. For some
MCSs, high-altitude measurements will be performed up to the stratosphere to document the TTL
region. How can these measurements help improving the parameterizations of convective transport,
scavenging or NOx production in clouds?
Results from previous experiments in tropical regions (e.g. TROCCINOX over Brazil) have shown
that a multi-scale approach is needed from the cloud-scale to the regional and global scales. Cloud
resolving models will be used to simulate one sampled MCS over Niamey or Djougou. At cloud
scale, convection is resolved explicitely, scavenging follows the evolution of the microphysical
reservoirs and lightning branches are simulated in 3D. The dynamical and chemical fields from
these simulations are compared directly with the aircraft, the small balloons and ground-based
measurements (both in-situ chemistry and radar). From this simulation, convective fluxes of ozone,
NOx and others soluble and non-soluble precursors can be derived with no a-priori hypothesis on
the anvil dimensions, or lightning vertical placement. A regional model is then run on the same case
181
at lower resolution with parameterized convection and associated scavenging and NOx produced by
lightning. From these simulations, a budget of convective fluxes can be derived (entrainement,
detrainement, lightning-NOx, scavenging). The quantity of gases released back to the environment
in the upper troposphere is calculated from the detrainement fluxes. These fluxes can be compared
to the fluxes calculated with the cloud-scale model. The results from the regional simulation needs
to be upscaled at continental and global scale based on global clouds and lightning activity
compared to clouds and lightning activity over West Africa. The up-scaling procedure however is
not straightforward and still needs to be refined. After the upscaling procedure, the budget can be
compared with budgets from global models. The differences between the cloud, regional and global
models budgets will point on the missing or ill-represented processes.
Contact :
Céline Mari ([email protected])
182
LES SYSTEMES CONVECTIFS DE LA MOUSSON OUEST AFRICAINE ET LE
BUDGET DE L'OZONE DANS LA HAUTE TROPOSPHERE :
DE L'ECHELLE LOCALE A L'ECHELLE GLOBALE
La composition chimique de l'atmosphère et l'état du climat global dépendent sensiblement des
processus dynamiques et chimiques dans les régions tropicales. Les systèmes convectifs
mésoéchelle jouent un rôle majeur dans le transport des gaz depuis la couche limite vers la
troposphère libre, dans la source de NOx par les éclairs et dans la perte des constituants gazeux par
les processus hétérogènes jusqu'au lessivage par les précipitations. Une fois émis dans l'atmosphère,
les gaz sont soulevés rapidement dans la troposphère libre par la convection profonde et transportés
sur de grandes distances loin des régions sources. Les émissions en Afrique de l'ouest sont ainsi
susceptibles d'affecter les propriétés atmosphériques aux échelles intercontinentales et globales.
Cependant, le détail des processus qui interviennent dans le transport et le vieillissement des masses
d'air natives de l'Afrique de l'ouest est peu connu faute d'observations dédiées. Le traitement de ces
processus par les modèles de chimie-transport n'est pas encore satisfaisant.
L'objectif de cette étude est de proposer une stratégie générale pour améliorer la représentation des
impacts de la convection sur la chimie dans les modèles. Les résultats de campagnes expérimentales
passées (ex. TROCCINOX) ont montré la nécessité d'une approche multi-échelle depuis la
convection explicite vers les échelles régionales et globales.
La stratégie expérimentale qui sera mise en place pendant la phase mature de la mousson africaine
en Août 2006 permettra des avancées importantes. Le présent travail montre comment cette
stratégie expérimentale servira l'approche de modélisation multiéchelle en contraignant par exemple
la composition chimique dans les flux d'entraînement et de détrainement à la fois en amont et en
aval des systèmes convectifs mésoéchelle. Ce travail présente aussi la stratégie en modélisation qui
sera utilisée pour faire le lien entre les flux des précurseurs de l'ozone à l'échelle du nuage,
régionale et globale.
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1.15P
OZONE AND CARBON MONOXIDE OVER WEST AFRICA AS SEEN BY THE
MOZAIC PROGRAM
B. SAUVAGE (1), V. THOURET (1), J-P. CAMMAS (1), A.M. THOMPSON (2,3),
J. WITTE (3), G. ATHIER (1) and P. NEDELEC (1)
(1) Laboratoire d’Aérologie, Toulouse, France, (2) Penn State University, USA
(3) NASA-GSFC, USA
The MOZAIC program provides data of ozone and carbon monoxide over Equatorial Africa, since
April 1997 and December 2001 respectively. This data set is of particular interest (in the frame of
the AMMA LOP) as it fills the gap from the previous available in-situ data over the African region.
Particularly, ozone vertical profiles recorded over 6 years (1997-2003), have lead to the first
tropospheric “climatology” over 3 different equatorial regions, namely Gulf of Guinea, Central and
East Africa. The monthly mean vertical profiles have been systematically analyzed with monthly
mean ECMWF data using a Lagrangian-model (LAGRANTO). We assess the roles played by the
dynamical features of Equatorial Africa and the intense biomass burning sources within the region
in defining the ozone distribution. The lower troposphere exhibits layers of enhanced ozone during
the biomass burning season in each hemisphere (boreal winter in the northern tropics and boreal
summer in the southern tropics). The monthly mean vertical profiles of ozone are clearly influenced
by the local dynamical situation. Over the Gulf of Guinea during boreal winter, the ozone profile is
characterized by systematically high ozone below 650 hPa (see Figure 1). This is due to the high
stability caused by the Harmattan winds in the lower troposphere and the blocking Saharan
anticyclone in the middle troposphere that prevents any efficient vertical mixing. In contrast,
Central African enhancements are not only found in the lower troposphere but throughout the
troposphere. The boreal summer ozone maximum in the lower troposphere of Central Africa
continues up to November in the middle troposphere due to the influx of air masses laden with
biomass burning products from Brazil and Southern Africa. Despite its southern latitude, Central
Africa during the boreal winter is also under the influence of the northern tropical fires.
Moreover, the tropical Atlantic region is also known for its “Ozone Paradox” and “Zonal WaveOne” (Thompson et al., 1999; Thompson et al., 2003). We show how the MOZAIC data, merged to
the SHADOZ network are used to go a step ahead in the understanding of these scientific questions.
Specifically, MOZAIC profiles of ozone over west Africa and the Congo allow evaluation of the
continental ozone latitudinal distribution during the period of the "Atlantic Paradox", a phrase that
refers to a greater tropospheric ozone column amount over the South Atlantic than the North
Atlantic during the west African biomass burning season. During DJF (December-JanuaryFebruary), the lower troposphere over Africa exhibits a higher ozone signal in the burning
hemisphere, i.e. north of the equator, so the apparent "Paradox" does not appear over the African
continent. The examination of the MOZAIC dataset over Africa highlights another component of
the wave-one feature characteristic in the tropospheric ozone mixing ratio viewed in zonal crosssection (see Figure 2). The lower troposphere makes a non-negligible contribution to the regionally
higher ozone column during the biomass burning periods of each hemisphere (DJF) for West Africa
and JJA (June-July-August) as far as the Congo region. Moreover, a southern preference for the
wave-one maximum is confirmed with a stonger maximum in SON (September-October184
November). Finally, both phenomena show the first-order effects of the African continent as a
major source of biomass burning and lightning emissions.
These results have been published in two separate papers.
1) Sauvage B., V. Thouret, J- P. Cammas, F. Gheusi, G. Athier and P. Nédélec, Tropospheric
ozone over Equatorial Africa: regional aspects from the MOZAIC data. Atmos. Chem. Phys., 5,
311-335, 2005.
2) Sauvage B., V. Thouret, A.M. Thompson, J.C. Witte, J- P. Cammas, P. Nédélec, and G.
Athier, enhanced View of the "Tropical Atlantic Ozone Paradox" and "Zonal Wave-one" from the
In-situ MOZAIC and SHADOZ Data, J. Geophys. Res, in press, October 2005.
Contact and presenting author: [email protected]
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1.16P
RAINWATER CHEMISTRY AND WET DEPOSITION OVER THE WET
SAVANNA ECOSYSTEM OF LAMTO (IVORY COAST)
V. YOBOUE (1), C. GALY-LACAUX (2), J.P. LACAUX (2) and S. SILUE (1)
(1) Laboratoire de Physique de l’Atmosphère, Côte d’Ivoire
(2) Laboratoire d’Aérologie, France
New results on rainfall chemistry at the Lamto site (Côte d’Ivoire), representative of wet savannas,
are presented. These results are to be associated with those from other IDAF sites in West Africa, at
Banizoumbou (dry savanna, Niger) and at Zoetele (equatorial forest, Cameroon).
In this IGAC-DEBITS Africa (IDAF) network, data sets on precipitation chemistry collected at the
‘wet savanna ecosystem’ site of Lamto, are analyzed for the whole period 1995-2002, without any
gap. Inorganic (Ca2+, Mg2+, Na+, K+, NH4+, Cl-, SO42-, NO3-) as well as organic (HCOO-,
CH3COO ) ions contents were determined using Ion Chromatography. The analyzed 631 rainfall
events represent 8420.9 mm of rainfall from a total of 9631.1 mm, thus with quite a significant
sampling efficiency (87%) of the precipitation regime at Lamto.
The average rainfall chemical content at Lamto is computed at seasonal, interannual time scales and
for the full 1995-2002 period.
The precipitation chemistry at Lamto is influenced by four main sources: natural biogenic emissions
from savanna soils (NOx and NH3), biomass burning (savanna and domestic fires), terrigeneous
particle emissions from dry savanna soils, and marine compounds embedded in the summer
monsoon. The inter-annual variability of the weighted volume average concentration of chemical
species linked with wet deposition is ~ 20% over the full period, in connection with the variability
of atmospheric sources and rainfall amounts.
Chemical signatures from these gas and particles sources in West Africa are observed at Lamto.
Concentrations of maritime ions (Na+, Cl-, and Mg2+) are comparable to that obtained at other sites
away from the ocean. Ammonium with a VWM of 17.6µeq.l-1 is the most abundant, representative
of the NH3 source, and attributed to domestic animal wastes, fertilizers and biomass burning.
Ammonium concentrations are found to be the highest at Lamto when compared to all other IDAF
sites in the West Africa ecosystems. 70% of total deposited nitrate (4.2kg.ha-1.year-1) is from NH4.
Rainfall concentrations of nssCa2+, nssSO42-, nssK+, and nssMg2+ are linked to terrigeneous sources
due to wind erosion of Sahara-Sahel soils. These particles transported by the Harmattan air mass are
slowly deposited, from dry savanna to wet ones and the equatorial forest. Negative concentration
gradients for nssCa2+ are from 30.8µeq.l-1 in dry savannas to 9.2µeq.l-1 in wet ones at Lamto and
8.9µeq.l-1 in the Cameroon forest. A similar gradient is also obtained from mineral particles rainfall
contents with concentrations relatives up to 80% in dry savannas, to 40% in wet savannas, and 20%
only in equatorial forest southward. This latter result emphasizes the importance of multiphase
processes between gases and particles over West African ecosystems. In particular, acid gasparticles reactive chemistry can explain the acid rains and their acidity gradient along a transect dry
savanna - wet savanna - equatorial forest
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In spite of such high potential acidity of 30.5µeq.l-1 due to NO3-, SO42-, HCOO- and CH3COO-, a
relatively weak acidity of 6.9µeq.l-1 is effectively measured. For a proportion of 40%, this acid
neutralization is explained by the acid gas – alkaline soil particles interactions. The remaining
neutralization results from absorption of gaseous ammonia. When the Lamto results are compared
to those at Banizoumbou (dry savanna) and Zoetele (equatorial forest), a regional coherent
overview for West African wet tropospheric chemistry processes emerges. High concentrations of
the particulate phase in precipitation emphasize the importance of multiphase processes between
gases and particles in the atmospheric chemistry of West Africa ecosystems. Typically, the nss Ca2+
precipitation content, a major indicator for terrigeneous particles, evolves from 30.8µeq.l-1 in dry
savannas to 9.2µeq.l-1 at Lamto and 8.9µeq.l-1 in the Cameroon forest
Contact
(1) LPA, Université de Cocody, 22 BP 50082, Abidjan 22, Côte d’Ivoire
E-mail : [email protected] - Phone : (33) 5 61 33 27 13
(2) LA, 14, Av. Edouard Belin 31 400 Toulouse, France
E-mail: [email protected] -Phone : (33) 5 61 33 27
CHIMIE DES PLUIES ET DEPOTS HUMIDES DANS L’ECOSYSTEME HUMIDE
DE LA SAVANNE DE LAMTO (COTE D’IVOIRE)
De nouveaux résultats sont présentés sur la chimie des pluies au site de Lamto (Côte d’Ivoire), site
représentatif de la savane humide. Dans le cadre du réseau IDAF, ces résultats sont à comparer à
ceux obtenus aux sites de Banizoumbou (Niger, savane sèche) et de Zoétélé (Cameroun, forêt
équatoriale).
Dans ce réseau IGAC-DEBITS Afrique (IDAF) d’Afrique de l’Ouest, les données de chimie des
pluies collectées au site de Lamto (écosystèmes typiques des savanes humides) sont analysées pour
la période 1995 – 2002 sans interruption. Les ions inorganiques (Ca2+, Mg2+, Na+, K+, NH4+, Cl-,
SO42-, NO3-) et organiques (HCOO-, CH3COO-) sont déterminés par Chromatographique ionique.
Les 631 épisodes pluvieux analysés représentent un total de pluie de 8420.9 mm, sur un total de
9631.1 mm, ce qui fournit un taux d’échantillonnage de 87% du régime total de précipitations à
Lamto.
La composition chimique moyenne des pluies à Lamto est calculée sur des bases interannuelles,
saisonnières et annuelles sur toute la période d’étude (1995-2002).
Quatre sources d’émissions principales influencent sur la chimie des pluies à Lamto : les émissions
biogéniques naturelles des sols de savane (NOx et NH3), les feux de biomasse (savanes et feux
domestiques), les émissions terrigènes par les sols en savane sèche, et les espèces d’origine marine
emportées par le flux de Mousson.
La variabilité interannuelle des concentrations moyennes pondérées des espèces chimiques et leur
dépôt humide associé est d’environ 20% pour la période entière, selon l’amplitude des sources
d’émissions et des précipitations.
Les signatures chimiques en particules et gaz typique de ces savanes sont observées à Lamto. Les
concentrations en ions d’origine marine (Na+, Cl-, Mg2+) sont comparables à celles relevées aux
187
sites éloignés de l’océan. L’ammonium, avec une concentration moyenne pondérée de 17.6µeq.l-1
est le plus abondant, il est typique de la source de NH3, liée aux déchets d’animaux, aux fertilisants
et aux feux de biomasse. C’est à Lamto que ces concentrations sont les plus élevées,
comparativement aux autres écosystèmes ouest africains. 70% des nitrates déposés (4.2 kg.ha-1.year1
) provient de l’ion NH4+. Les concentrations non marine de Ca2+, SO42-, K+ et Mg2+ (nssCa2+,
nssSO42-, nssK+ et nssMg2+ ) sont à relier aux sources terrigènes associées à l’érosion éolienne des
sols Sahara-Sahel. Ces particules, transportées par le flux d’Harmattan, se déposent
progressivement le long de leur trajectoire, depuis la savane sèche jusqu’à la savane humide et la
forêt. Des gradients négatifs de concentrations des nssCa2+ sont observés, de 30.8 µeq.l-1 en savane
sèche, à 9.2 µeq.l-1 en savane humide de Lamto et 8.9 µeq.l-1 à Zoétélé dans la forêt du Cameroun.
Un tel gradient s’observe également sur les teneurs des pluies en particules minérales, avec des
concentrations relatives de 80% en savanes sèches, 40% en savanes humides et 20% seulement plus
au sud en zones forestières. Ce résultat met l’accent sur l’importance des processus multiphasiques
entre gaz et particules sur les différents écosystèmes de l’ouest africain. En particulier, la réactivité
chimique entre gaz acides et particules permet d’expliquer l’acidité des pluies et le gradient de leur
acidité le long des transects savane sèche - savane humide - forêt. En dépit d’un fort potentiel acide
de 30.5µeq.l-1 lié à NO3-, SO42-, HCOO- et CH3COO-, une faible acidité de 6.9µeq.l-1 est
effectivement mesurée. Pour 40%, cette neutralisation s’explique par les interactions entre gaz
acides et particules alcalines, le reste de cette neutralisation étant dû à l’absorption d’ammoniac
gazeux dans les gouttes. De la comparaison de tels résultats aux sites contrastés de Lamto,
Banizoumbou et Zoétélé, se dégage une vue d’ensemble cohérente sur les processus physicochimiques en Afrique de l’Ouest. Les fortes concentrations de la phase particulaire dans les
précipitations soulignent toute l’importance des processus multiphasiques gaz-particules sur les
différents écosystèmes ouest africain. Ainsi, typiquement, la teneur en nssCa2+ des précipitations, en
indicateur majeur des espèces terrigènes, s’établit entre 30.8 µeq.l-1 en savane sèche, contre 9.2
µeq.l-1 à Lamto et 8.9 µeq.l-1 à Zoétélé.
Contact
(1) LPA, Université de Cocody, 22 BP 50082, Abidjan 22, Côte d’Ivoire
E-mail : [email protected] - Phone : (33) 5 61 33 27 13
(2) LA, 14, Av. Edouard Belin 31 400 Toulouse, France
E-mail: [email protected] -Phone : (33) 5 61 33 27
188
1.17P
REGIONAL AND GLOBAL ASPECTS OF AEROSOLS IN WESTERN AFRICA:
FROM AIR QUALITY TO CLIMATE
Mian CHIN(1) and Thomas DIEHL (2)
(1) NASA, Goddard Space Flight Center, U.S.A
(2) UMBC/NASA, Goddard Space Flight Center, U.S.A
Western Africa is one of the most important aerosol source regions in the world. Major sources
include: Dust from the Sahara desert, biomass burning over in Sahel during the dry seasons,
pollution emissions from anthropogenic activities, and biogenic emissions from vegetation. In
addition, Western Africa also receives significant amount of aerosols from other continents, for
example, pollution from Europe and dust from Middle East.
We present here the aerosol sources, compositions, and distributions over western Africa during dry
and wet seasons from a global model, the Goddard Chemistry Aerosol Radiation and Transport
(GOCART) model. Our motivation is to use the GOCART model in combination with the satellite
data to support the African Monsoon Multidisciplinary Analysis (AMMA) field observations, to
analyze the AMMA data, and to assess the global impact of African aerosols. The GOCART model
uses assimilated meteorological fields from the Goddard Earth Observing System-Data
Assimilation System (GEOS DAS), and has a horizontal resolution at 1° latitude x 1.25° longitude
or 2° x2.5° resolution with 30-55 vertical layers. The model simulates concentrations and optical
thickness of sulfate, dust, black carbon, organic carbon, sea-salt aerosols as well as total aerosols
from both natural and anthropogenic sources. Processes in the model include emission, chemistry,
dry and wet depositions, gravitational settling, convection, advection, and hygroscopic growth as a
function of ambient relative humidity (Chin et al., 2000, 2002, 2004; Ginoux et al., 2001, 2004).
Figure 1 shows the GOCART model simulated aerosol optical thickness (AOT) at 550 nm and
comparisons with the sun-photometer measurements from the Aerosol Robotic Network
(AERONET) at 10 AERONET sites in or near the AMMA field study area. The model captures the
observed large magnitude of day-to-day aerosol variations, and shows that the dust is the dominant
aerosol type in the AMMA study area through the year except in the dry season (December –
February) at Ilorin when biomass burning contributes to 30-70% of AOT.
To understand the origins of aerosol composition over Africa, we tag the aerosols produced from
major pollution and dust source regions over the world. The pollution source regions include North
America, Europe, and Asia, and the dust source regions include Africa, Middle East, and Asia.
Figure 2 shows the model simulated sulfate and dust aerosol column burden (top panels) in the wet
season (August) and the percentage contributions from different source regions (bottom panels).
While majority of the dust over Africa is from the Sahara desert, Middle East could contribute to 10
– 30% of dust loading in the Sahel region in the wet season. Figure 2 also reveals that Europe
anthropogenic emissions could contribute to 30 – 50% of sulfate loading over northern Africa (20 –
40 % over the AMMA field study area). On the other hand, Figure 2 clearly shows the transport of
aerosols from African continent to the Atlantic Ocean that will exert significant influence in
modifying weather and climate.
189
Finally, we use the model results to estimate the aerosol concentrations at the surface, which are
normally known as PM2.5 or PM10 (particular matter with diameter less than 2.5 µm or 10 µm,
respectively). PM is one of the major components that indicate the surface air quality because they
could induce respiratory diseases and impair the visibility. The PM2.5 and PM10 concentrations
over western Africa persistently exceed 50 and 100 µg m-3 respectively in the dry season (e.g.,
February), with maximum reaching 1000 and 2000 µg m-3 over the desert. In the wet season,
surface PM concentrations are typically more than a factor of 2 lower than that in the dry season.
Considering the US EPA’s air quality standard of the maximum values of 15 and 50 µg m-3 for
annual average PM2.5 and PM10, aerosols over Africa impose serious air quality and health
concerns to the residents there.
To summarize, the GOCART model results are reasonably realistic for African region (and
beyond). The model shows:
- Dust is always there, smoke aerosol is important in dry seasons;
- Aerosols from Africa (dust and smoke) can transport to long distance, exerting global impacts;
- Africa also receives pollution aerosols from Europe and dust aerosols from Middle East;
- There are significant seasonal variations of aerosol distributions;
There are serious air quality and health concerns over western Africa especially in the dry season.
Dust
BC
OC
Sulfate
Sea-salt
AERONET
Figure 1. GOCART model simulated aerosol optical thickness at 550 nm (color shades) and comparisons
with the AERONET measurements (black circles) at 5 AERONET sites over western Africa (left column)
and 5 sites near Africa (right column). Model results and AERONET data are daily average values in 2000.
190
Figure 2. GOCART model simulated dust and sulfate mass column loading (top panels) and percentage contributions
from Africa dust source (bottom left panel) and from European anthropogenic sulfur emissions (bottom right panel) for
August 2001. Black circles are the locations of AERONET sites in Figure 1, dashed line is the AMMA field study area,
and the red crosses are the AMMA super sites.
We hope to have an opportunity to participate in the AMMA field experiments and to make the
model useful for AMMA.
(For more information please contact Mian Chin, [email protected], 1-301-614-6007).
191
1.18P
REGCM SIMULATION OF ANTHROPOGENIC AEROSOLS OVER
SUB-SAHARA AFRICA
Abdourahamane KONARE (1), Fabien SOLMON (2), FilippoGIORGI (2)
Cathy LIOUSSE (3) and Robert ROSSET (3)
(1) Université de Cocody, Abidjan, Côte d’Ivoire (2) PWC-ICTP, Trieste, Italy
(3) LA, Toulouse, France
West and Central Africa are subject every dry season to intense biomass and biofuels emissions of
carbonaceous particles. These particles have deep impact upon West African climate, particularly as
concerns radiation, temperature and precipitation modifications. RegCM has been run for the year
2000 over a African domain comprises between latitude 27 N and 27 S and longitudes 25W and
52E with a 60 km resolution.
A tracer model has now been introduced into RegCM, including sulfates and carbonaceous ( Black
and Organic Carbon) aerosols ( hydrophilic and hydrophobic). First season BC and OC budgets
have been calculated for two boxes, respectively north and south of the equator, for the two dry
(DJF) and wet (JJA) seasons. Then, monthly aerosol optical depths (AOD) have been calculated and
compared both with the surface AERONET and MODIS satellite data. Next, monthly direct
radiative anthropogenic aerosol forcing have been computed and their cooling/warming effects
together with positive/negative aerosol impact upon precipitation are displayed.
These results are discussed with particular emphasis upon West Africa Sahel region and the Congo
basin.
Contact
(1) Abidjan:[email protected]
(2) [email protected]
(3) C. Liousse : [email protected] – R. Rosset ; [email protected].
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1.19P
ESTIMATING AEROSOL PARAMETERS FROM SEAWIFS OCEAN COLOUR
SENSOR OBSERVATIONS BY USING TOPOLOGICAL MAPS
A. NIANG (1, 2), F. BADRAN (1), C. MOULIN (3), M. CREPON (1) and S. THIRIA (1)
(1) LOCEAN (ex-LODyC), Université Pierre et Marie Curie, PARIS, France
(2) LTI/ ESP. University of Dakar, Dakar-Fann, Senegal
(3) IPSL/LSCE, Gif sur Yvette, France
Atmospheric aerosols are small particles of various origins present in the atmosphere. They modify
the radiation balance of the earth by scattering and absorbing solar and long-wave radiative
transmission leading to two opposite effects: they cool the atmosphere by backscattering the solar
radiation to space and they warm it by absorbing the terrestrial radiation in the lower atmosphere
(Brooks & Legrand, 2000; LeLieveld et al., 2002). The balance of these two effects is still
controversial. The largest radiative contribution comes from aerosols with radii in the range 0.1-1
µm. Aerosols have different scattering and absorption properties depending on their origin, and it is
important to identify them in order to better quantify their radiative impact. Among the most
important aerosol types, which affect the radiative budget of the Earth we can mention sea salt,
sulfates, soot and mineral dust. Aerosols have other environmental impacts also: they serve as cloud
condensation nuclei and they fertilize the ocean enhancing phytoplankton blooms (Jickells et al.,
2005; Ridame & Gieux, 2002).
Aerosols can be measured in the solar spectrum from ground stations or from space using passive
sensors. One of the most natural remote sensing techniques is to use multi-spectral radiometers
dedicated to ocean-color remote sensing (i.e., the measurement of a recognized proxy for
phytoplankton content in surface waters through the estimate of the Chlorophyll-a concentration,
Chl-a). In fact, aerosol and phytoplankton retrievals are linked. The light scattered within the
atmosphere by molecules and aerosols contributes to the Top-Of-Atmosphere (TOA) reflectance in
the blue-green part of the visible spectrum, where the spectral signature of phytoplankton is
observable, more than 80%. The critical issue for ocean color is therefore the atmospheric
correction. Such a procedure consists in estimating the aerosol contribution with a good accuracy in
order to remove it from the measured TOA signals to get the actual spectrum of marine reflectance.
The impact of aerosols on the reflectance spectrum depends firstly on the particle concentration in
the atmosphere, but also on their specific optical characteristics of light-scattering and absorption,
which are controlled primarily by the particle size distribution and complex refractive index.
In the present study we investigated the possibility of retrieving these aerosol optical properties by
using an advanced mathematical method that allows an immediate identification and inversion of
the TOA reflectance spectra measured by a multi-spectral sensor such as SeaWiFS. Several
techniques have been proposed to retrieve the aerosol characteristics from such measurements. A
first category has been widely used for atmospheric correction and is based only on the red and
near-infrared measurements to retrieve the aerosol optical thickness and the Ångström coefficient,
which represent the particle concentration and the particle size distribution respectively (e.g.,
Gordon & Wang, 1994; Jamet et al., 2004). The major drawback of such techniques is that they are
not capable of discriminating between absorbing and non-absorbing aerosols (Gordon, 1997).
Another category consists of algorithms developed for the study of a single aerosol type, e.g.,
193
mineral dust (Moulin et al., 2001a) using both visible and near-infrared measurements to quantify
the absorption effect. The main limitation of this approach is that the aerosol type has to be known
a-priori, which is usually not the case. Some recent methods have been proposed to detect the
aerosol type, for example to discriminate between absorbing mineral dust and non-absorbing
aerosols (Nobileau & Antoine, 2005) by taking into account one wavelength in the visible (510 nm)
and wavelengths in the NIR. A classical “atmospheric correction” technique can then be used
considering the suitable aerosol type.
We developed an approach that makes use of the full spectrum of measurements to perform the
aerosol identification. This method aims at increasing the accuracy and the flexibility of the
previous methods, as well as processing satellite imagery at a higher speed. To achieve this, we
used a specific Topological Neural network Algorithm (TNA), the so-called PRobabilistic Self
Organizing Map (PRSOM) to classify TOA reflectance spectra. TNAs are well adapted for this task
and have been used with success by Niang et al. (2003) for classifying TOA reflectances. The
present work continues the latter study by providing a more refined labeling procedure where the
expertise is provided by a synthetic database comprising large sets of radiative-transfer
computations. The development of the method is carried out in two steps. The first step, which is an
unsupervised procedure, classifies the measured TOA spectra from their statistical properties. The
classifier was trained on one year (1999) Mediterranean SeaWifs images using a temporal
homogeneity. In the second step, the groups found in the first step are labeled, i.e., are assigned to
aerosol optical parameters. In order this to be accomplish, the knowledge contained in the
theoretical equations in which all parameters are known is compared with the observations to
determine the aerosol type, the optical thickness and the Ångström coefficient. Moreover,
configurations where the observed spectrum is not always due to a unique aerosol type, but
represents a mixture of different particles can be handled by the proposed methodology, since it
computes the probability of a spectrum to belong to each type of aerosols.
In the experiments presented here, we considered five aerosol types: the four non-absorbing
aerosols (coastal, maritime, tropospheric, oceanic) used operationally to process SeaWiFS data and
an Saharian dust aerosol (absorbing aerosol). It is important to note that the SeaWiFS atmospheric
correction algorithm fails when dust aerosols predominate (Moulin et al., 2001b). This algorithm
was successfully applied to a set of SeaWiFS images (in 1999 and 2000) representative of the
conditions prevailing in the Eastern Mediterranean. The method leads to accurate and coherent
results as shown by the comparison with in situ aerosol measurements provided by the AERONET
station of Lampedusa and by the study of two aerosol events over the Mediterranean. One of the
major advantages of this method is that it enables us to automatically identify the aerosol type
(such as Sharian dust) and to retrieve the aerosol optical thickness (even values larger than 0.35),
with a better accurcy than classical methods such as those used by the SeaWiFS chain.
References :
Brooks, N., & Legrand, M. (2000). Dust variability over northern Africa and rainfall in the Sahel. In S. McLaren and
D. Kniveton (Eds), Linking Climate Change to Land Surface Change (pp 1-25). Dordrecht, Kluwer Academic
Publishers.
Gordon, H. R. (1997). Atmospheric correction of ocean color imagery in the Earth Observing System Era. Journal of
Geophysical Research, 102(D14), 17081-17106.
Gordon, H. R. & Wang, M. (1994). Retrieval of water-leaving radiances and aerosol optical thickness over the
oceans with SeaWiFS: a preliminary algorithm. Applied Optics. 33(3), 443-453.
IPCC (2001) Climate Change 2001. The scientific Basis. Cambridge University Press.
Jamet, C., Moulin, C., and Thiria, S. (2004). Monitoring aerosol optical properties over the Mediterranean from
SeaWiFS images using a neural network inversion, Geophysical Research Letters, 31, L13107,
doi:10.1029/2004GL019951.
Jickells T. D., Z. S. An, K. K. Andersen, A. R. Baker, G. Bergametti, N. Brooks, J. J. Cao, P. W. Boyd, R. A. Duce,
K. A. Hunter, H. Kawahata, N. Kubilay, J. laRoche, P. S. Liss, N. Mahowald, J. M. Prospero, A. J. Ridgwell, I. Tegen,
R. Torres. (2005): Global Iron Connections Between Desert Dust, Ocean Biogeochemistry, and Climate Science, 308,
pp 67-71.
194
Kohonen, T. (2001). Self Organizing Maps (3rd ed.) Berlin Heidelberg: Springer Verlag. 501pp.
Lelieveld, J., Berresheim, H., Bormamnn, S., Crutzen, P.J., Dentener, F.J., Fisher, H., et al. (2002) . Global air
pollution crossroads over the Mediterranean. Sciences, 298, 794-799.
Moulin C., Gordon, H. R., Banzon, V. F. and Evans, R. H. (2001a). Assessment of Saharan dust absorption in the
visible from SeaWiFS imagery, Journal of Geophysical Research, 106, 18239-18250.
Moulin C., Gordon H. R., Chomko R., Banzo V. F. and Evans R. H. (2001b). Atmospheric correction of ocean color
imagery through thick layers of Saharan dust. Geophyisical Research Letters. 28, 5-8.
Niang, A., Gross, L., Thiria, S., Badran, F. & Moulin, C. (2003). Automatic neural classification of ocean colour
reflectance spectra at the top of atmosphere with introduction of expert knowledge. Remote Sensing of Environment 86,
257-271.
Nobileau, D. & Antoine, D. (2005). Detection of blue-absorbing aerosols using near-infrared and visible (ocean
color) remote sensing observations, Remote Sensing of Environment, 95, 368-387
Ridame C. and C. Gieux (2002): Saharan input of phosphate to the oligotrophic water of the open western
Mediterranean Se, Limnol. Oceanogr., V47(3), pp, 856–869.
1.20P
EFFECTS OF AFRICAN DUST ON RADIATIVE FORCING OVER THE
AFRICAN CONTINENT AND THE ATLANTIC OCEAN
R. T. PINKER, B. ZHANG and H. LIU
Department of Atmospheric and Oceanic Science,
University of Maryland, College Park, MD, USA
In the framework of a NOAA/NASA PATHFINDER related activity and an EOS Validation
project in sub-Sahel Africa, an infrastructure was established for assessing the impact of dust on the
radiative fluxes over the African continent and the Atlantic Ocean. Specifically, radiative fluxes at
0.5 degree resolution were derived from the ISCCP DX data, based on METEOSAT and GOES–E
observations.
Time series that cover at least two decade of satellite based estimates of such fluxes are now
available at the 0.5 degree resolution. Independently, global climatologies of aerosol optical depth
based on the MODerate resolution Imaging Spectro-radiometer (MODIS) instrument on the Terra
satellite, measurements from the AErosol RObotic NETwork (AERONET), and outputs from the
Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model output, have been
developed. These climatologies have been used to test the effect of dust on the inferred surface
fluxes. An analysis of the long-term satellite flux estimates over the region of interest to the AMMA
Project will be presented and results of tests to evaluate the dust radiative forcing with realistic dust
properties will be discussed.
195
1.21P
SENSITIVITY OF THE REGCM TO THE
OPTICAL PROPERTIES OF DUSTS
Ibrah SEIDOU SANDA (1), Fabien SOLMON (2) and Filippo GIORGI (2)
(1) Université Abdou Moumouni, Faculté des Sciences, Niamey, Niger
(2) The Abdus Salam ICTP, Physics of Weather and Climate, Trieste, Italy
Abstract
The optical properties of airborne mineral dusts are one of the main sources of uncertainty
for the estimation of the radiative forcing on the Earth's climate.
In this study, the optical properties of dusts are calculated with the Mie theory for 18
wavelength intervals ranging from 0.2 to 5 µm and for four size bins. Different assumptions are
made on the wavelength dependent refractive index, the size distribution, and the microphysical
properties (internal or external mixture). These properties are averaged over the size distribution in
the four bins and tabulated for the different wavelength intervals and embedded in the RegCM
radiative transfer code.
Here, we present the first results. We investigate the radiative forcing due to dust in Western
Africa and the resulting response of the climate. Sensitivity studies are carried out on the
assumption of internal or external mixture of the different aerosol components and on their size
distribution.
Introduction
Recently many studies have shown the non negligible role of aerosols on the climatic
processes (Tegen at al. 1996, IPCC 2001). Dust can cause either a positive or a negative forcing
leading to a warming or cooling of the climate system (Sokolik et al. 1998). The radiative changes
induced by dusts in the atmosphere affect the atmospheric dynamics by changing the temperature
Karyampudi and Carlson (1998) have shown that the forcing by Saharan dust can reduce the
convection in the equatorial zone. Rosenfeld et al. (2001) have shown that dust have a negative
effect on rainfalls.
Recently, some aerosol parameterizations have been coupled to ICTP regional climate
model RegCM. The present version of the model accounts for the main anthropogenic particles
(SO2, SO4 gaseous and aqueous chemistry, carbonaceous aerosols (Solmon et al., 2005). It has an
on line mineral dust emission, transport and deposition scheme (Zakey et al., article in preparation).
We have developed a parameterization of dust optical properties in RegCM and in this study
we use the model to investigate the direct and indirect effects of dusts on the climate of West
Africa.
Methodology
The parameters used in modeling the radiative effect of aerosols are the specific extinction
coefficient k, the single scattering albedo w and the asymmetry parameter. These parameters are
computed with a Mie scattering program. In the external mixing hypothesis, the optical properties
of each aerosol specie is computed separately, and effective optical properties are calculated. In the
196
internal mixing hypothesis, the different species are assumed to be homogenously mixed. An
effective refractive index is computed using a mixing rule, and is used in the Mie program to
calculate the optical parameters.
In this study we consider the optical properties of four species: SO4, black carbon (BC),
organic carbon (OC) and dust. In the internal mixing hypothesis, we assume that the aerosol
particle is a core of internally mixed dust and black carbon coated with a layer of internally mixed
OC and SO4. The effective refractive indices of the core and the layer are computed as the Volume
weighted average of the refractive indices of the different components. For a given particle size
corresponding to a size bin, the optical properties are calculated with a coated sphere Mie program
and tabulated for all the different volume ratios combinations of the species. For the moment the
volume ratios can vary only by steps of 10% for each specie (Eg. Dust 50%, BC 20%, OC 20%,
sulfate 10%).
First Results
We have performed two runs with no feedbacks of the model with external and internal
mixing hypothesis over West Africa for December 2000. As we can see on the figure bellow there
is a significant difference on the radiative forcing at the top of atmosphere. The warming effects are
more pronounced over land in the case of internal mixing hypothesis. This difference can have
significant feedbacks on the metrological parameters of the area and thus on the climate.
Average radiative forcing of aerosols for external and internal mixing hypothesis
(December 2000)
From this study it appears that there is a need to perform longer runs of the model to assess
the impact of the different assumptions on the climate. It is also necessary to generate a higher
resolution table of optical properties and take into account the water vapor.
197
Acknowledgment
We gratefully acknowledge the support of the International Centre for Theoretical Physics
(ICTP). This research was conducted during a short term visit of the first author at the Physics of
Weather and Climate (PWC) group of ICTP.
References
IPCC Third Assessment Report - Climate Change 2001. Climate Change 2001: The Scientific Basis.
Karyampudi, V. M., and T. N. Carlson, 1988: Analysis and numerical simulations of the Saharan air layer and its
effects on easterly wave disturbances. J. Atmos. Sci., 45, 3102–3136.
Rosenfeld, D., Y. Rudich, and R. Lahav. 2001. Desert dust suppressing precipitation: A possible desertification
feedback loop. Proceedings of the National Academy of Sciences 98(May 22):5975
Sokolik I.N., O.B. Toon, and R.W. Bergstrom. Modeling the radiative characteristics of airborne mineral aerosols at
infrared wavelengths. J. Geophys. Res. 103, 8813-8826, 1998.
Solmon F., Giorgi F., Liousse C. Aerosol modeling for regional climate studies: Application to anthropogenic
particles and evaluation over a European/African domain. In press Tellus B, 2005.
Tegen, I., A.A. Lacis, and I. Fung 1996. The influence of mineral aerosols from disturbed soils on the global
radiation budget. Nature 380, 419-422, doi:10.1038/380419a0.
Keywords : Mineral dusts, optical properties, internal mixture, external mixture, single scattering
albedo.
Contact
(1) Université Abdou Moumouni, Faculté des Sciences, BP 10662, Niamey, Niger
(2) The Abdus Salam ICTP, Physics of Weather and Climate, Strada Costiera 11, Trieste 34014,
Italy
SENSIBILITE DES REGCM
AUX PROPRIETES OPTIQUES DES POUSSIERES
Les propriétés optiques des poussières minérales sont une des plus importantes sources
d’incertitudes sur l’estimation de forçage radiatif du climat terrestre.
Dans cette étude, nous avons calculé les propriétés optiques des poussières avec la théorie de Mie
pour 18 intervalles de longueur d’onde allant de 0.2 à 5 m et pour quatre classes de dimensions
des particules. Différentes hypothèses sont faites sur l’indice de réfraction, la granulométrie et les
propriétés microphysiques des particules (mélange interne ou externe). Ces propriétés sont
moyennées pour chaque classe de dimension et pour les différentes longueurs d’onde puis
incorporées au code radiatif du model RegCM.
Nous présentons ici les premiers résultats sur le forçage radiatifs des poussières en Afrique de
l’Ouest et la réponse sur le climat. Nous faisons des études de sensibilité en fonction des différentes
hypothèses de mélange interne ou externe des différentes composantes des aérosols et leur
granulométrie.
Contact : (1) Université Abdou Moumouni, Faculté des Sciences, BP 10662, Niamey, Niger
(2) The Abdus Salam ICTP, Physics of Weather and Climate, Strada Costiera 11, Trieste 34014,
Italy
198
1.22P
DUST AND THE LOW LEVEL CIRCULATION OVER THE BODÉLÉ
DEPRESSION, CHAD: OBSERVATIONS FROM BODEX 2005
Richard WASHINGTON, Martin C. TODD (*), Sebastian ENGELSTAEDTER,
Samuel MBAINAYEL and Fiona MITCHELL
(*) Department of Geography, University College London, London, UK
Dust plays an important role in climate, recognition of which has led to a concentrated research
effort in field campaigns, development and analysis of remotely sensed data and modelling to better
understand dust.
There have, however, been very few direct surface based field measurements from key dust source
regions. The Bodélé, Chad, has been shown to be one of the premier sources of dust in the world.
This paper reports on the Bodélé Field Experiment (BoDEx 2005) which took place during
February and March 2005 and presents the first surface based measurements of the circulation over
the Bodélé. Based on Pilot Balloon and AWS data, we confirm the existence of the Bodélé Low
Level Jet (LLJ) and show that winds undergo a strong diurnal cycle such that strongest surface
winds typically occur in the mid morning when momentum is mixed downwards in turbulence
induced by radiative heating. In contrast, the core of the LLJ, near 500m, peaks during the evening
and is weakest during the day. The LLJ was present on all days during BoDEx 2005, but winds at
the surface reached speeds necessary for large scale dust entrainment on only a few days. The winds
strength during the main dust plume event of BoDEx (10-12 March 2005) was in the bottom third
of March plume events of the last 4 years. Pathways of dust transport from the Bodélé using a
trajectory model show potential advection of dust over the west African coastline within 5 days.
199
1.23P
ROLE OF AFRICAN EASTERLY WAVE IN THE INITIATION AND
PROPAGATION OF WEST AFRICAN MESOSCALE CONVECTIVE
WEATHER SYSTEMS
B. J. ABIODUN and Z. D. ADEYEWA
Department of Meteorology, Federal University of Technology, Akure, Nigeria
Mesoscale convective systems (MCSs) are unique, well-organised convective cloud clusters that are
well known for the production of rigorous weather and abundant rainfall in West Africa.
Some of the characteristics of these systems have been studied through numerical and observational
analysis. However, to the best of our knowledge, previous studies of these systems over West
Africa have not answered the following question: in the absence of AEW, can well-organized MCS
initiate and propagate over the West African complex terrain? If it does, what will be the pattern of
the wind flow around such MCSs? This study attempts to address these questions by numerically
simulating and studying the characteristics of MCS that evolves, in the absence of AEW, over a
simple idealized terrain and also the complex terrain of West Africa. Using a mesoscale numerical
model, 3-D simulations of MCS over both simple and complex terrain in West Africa have been
carried out with the pre-squall sounding obtained during the COPT 81 experiment as the initial
profile. Results of the simulations indicate that well-organised groups of MCSs do initiate over the
mountains, even in the absence of AEW, and consequently propagate over the region. Also
associated with these MCSs is a trough-like feature ahead of the system.
This study has therefore demonstrated the fact that the presence of African Easterly wave is not a
necessary condition for the initiation and development of MCSs, but that the presence may control
the explosive growth of MCS through the associated ridge.
200
1.24P
THE JET2000 EXPERIMENT
R.R. BURTON, A. DIONGUE-NIANG, M. DIOP, R.J. ELLIS, D.J. PARKER*,
C.D. THORNCROFT, C.M. TAYLOR and A.M. TOMPKINS
(*) Institute for Atmospheric Science, University of Leeds, Leeds, UK
JET2000 was a campaign conducted with the UK C-130 aircraft during 25 – 31 August 2000.
The aircraft and dropsonde data were generally of high quality, and have been used for diverse
scientific and operational applications.
The poster will review the main outcomes of the experiment including: observations of landatmosphere coupling; some new perspectives on the AEJ dynamics; observations of mesoscale
structure in the monsoon circulation; data assimilation in operational forecasts; and an evaluation of
bench forecasting techniques.
Corresponding author :
D.J. Parker - Institute for Atmospheric Science, School of Earth and Environment, University of
Leeds, Leeds, UK – Email : [email protected]
201
1.25P
ONDES EST AFRICAINES ET ACTIVITE ANTICYCLONIQUE SUR
L’ATLANTIQUE : ETUDE COMPOSITE ET ETUDE DE CAS
Moctar CAMARA (1, 2), Arona DIEDHIOU (1), Amadou GAYE (2),
Henri LAURENT (1) et Thierry LEBEL (1)
(1) LTHE/INPG, Grenoble, FRANCE (2) LPA-SF, ESP-UCAD, Dakar-Fann, Sénégal
1. Introduction
Les Ondes d’Est Africaines (OEAs) sont une caractéristique importante du climat Ouest
Africain et de l’Atlantique tropical. Les OEAs se propagent vers l’ouest avec une période de 3-5
jours et sont générées par une instabilité combinée barotrope- barocline du Jet d’Est Africain
(Burpee, 1972). Elles modulent la pluviométrie journalière en Afrique et servent de précurseurs à la
plupart des cyclones de l’Atlantique Nord (Burpee 1972; Avila et Pasch 1992). Bien que le nombre
d’OEAs soit presque constant d’une année à l’autre, le pourcentage d’OEAs qui génèrent des
cyclones présente une forte variabilité interannuelle (Avila et al 2000).
Le principal sujet examiné dans cette étude est la mise en évidence des différences entre les
OEAs associées à des cyclones et celles non associées à des cyclones à l’échelle intra saisonnière.
2. Données et méthodes
Les données utilisées dans cette étude sont les réanalyses du NCEP/NCAR, le rayonnement
ondes longues sortant au sommet de l’atmosphère (appelé OLR en Anglais) et les archives du centre
américain des ouragans (NHC) qui résument les dates d’occurrence et les trajectoires des cyclones.
Dans cette étude, seuls les cyclones qui sont nés au large des côtes ouest Africains (Est de
40°W) et associées à des OEAs sont considérées en vue d’étudier l’influence du climat Ouest
Africain sur l’activité cyclonique dans l’Atlantique Nord à l’échelle intra saisonnière.
Les caractéristiques de ces OEAs sont comparées à celles des OEAs qui ne sont pas
associées à des cyclones. Dans chaque cas, une moyenne de 48 OEAs est considérée.
3. Etude composite
Dans le but d’étudier les zones d’instabilités combinées barotropes- baroclines pour les
OEAs associées ou non à des cyclones, le gradient méridien de tourbillon potentiel (PV) à 315K a
été calculé. Burpee (1972) a trouvé que le gradient méridien du PV d’Ertel change de signe en
Afrique vers 700 hPa. Ce changement de signe satisfait aux conditions nécessaires d’instabilité du
flux moyen (Charney et Stern 1962) ; les zones négatives de gradient méridien de PV (∂PV/∂y) sont
favorables au développement et à la croissance des OEAs (Lau and Lau 1990).
Notre étude montre que les OEAs avec cyclones sont associées aux plus fortes valeurs
négatives de ∂PV/∂y (Figure 1). Ces fortes valeurs s’étendent plus loin sur l’Atlantique Nord
suggérant que les OEAs associées à des cyclones sont plus actives que celles non associées à des
cyclones. Ce résultat est cohérent avec les études de Landsea et Gray (1992) qui ont émis
l’hypothèse qu’à l’échelle inter –annuelle une forte activité cyclonique est associée à la propagation
du continent vers l’océan de puissantes OEAs.
202
Sur le plan thermodynamique, le potentiel de convection (PC) utilisée par Gray (1968) pour
mettre en évidence les paramètres qui sont associées à la cyclogenèse dans l’Atlantique Nord est
diagnostiqué. Le PC est la différence entre la température potentielle équivalente à 1000 hPa et celle
à 500 hPa. De fortes valeurs de PC sont notées dans le cas des OEAs associées à des cyclones. En
faisant la différence entre les OEAs associées ou non à des cyclones (Figure 2), on note la présence
d’un axe positif de PC vers 15°N qui s’étend sur l’Atlantique Nord suggérant que l’atmosphère est
plus instable dans le cas des OEAs avec cyclones. Ce positionnement de l’axe positif de PC plus au
Nord correspond à une pénétration plus profonde du flux de mousson qui reste aussi plus fort dans
le cas des OEAs avec cyclones. Ce résultat est cohérent avec ceux de Newell et Kidson (1984) qui
suggèrent que la phase humide du flux de mousson est accompagnée d’une forte activité
ondulatoire. L’OLR montre de faibles valeurs pour les OEAs avec cyclones suggérant la présence
d’une zone de convergence intertropicale (ZCIT) plus forte et plus profond sur l’Afrique et
l’Atlantique Est.
4. Conclusion
Les précédentes études ont montré que le lien entre le nombre d’OEAs et l’activité
cyclonique n’est pas assez satisfaisant pour expliquer toute la variabilité de l’activité cyclonique
dans l’Atlantique Nord (Thorncroft et Hodges 2001). L’objectif de cette étude était de comprendre
les principales différences entre les OEAs associées ou non à des cyclones.
Les OEAs associées à des cyclones présentent de fortes valeurs négatives de ∂PV/∂y sur le
continent et sur l’Atlantique Est suggérant que les OEAs sont plus actives dans le cas ou elles sont
associées à des cyclones. De fortes valeurs de PC sont aussi trouvées pour les OEAs avec cyclones
traduisant le fait que l’atmosphère est plus instable dans ce cas-ci. Ce résultat est cohérent avec la
présence d’un fort et profond flux de mousson sur l’Afrique et d’une forte convection au niveau de
la ZCIT.
Un cas d’étude effectué avec des données de radiosondes a permis de confirmer les résultats
de l’analyse composite.
Références
Avila, L. A., R. J. Pasch, and J. Jiing, 2000 : Atlantic tropical systems of 1996 and 1997 : Years of contrasts. Bull.
Amer. Met. Soc, 128, 3695-3709.
Avila, L. A., and R. J. Pasch, 1992 : Atlantic tropical systems of 1991. Monthly Weather Review, 120, 2688-2696.
Burpee, R. W., 1972 : The origin and structure of easterly waves in the lower troposphere of north Africa. J. Atmos.
Sci., 29, pp.77-90
Charney, J. G., and M. E. Stern, 1962: On the stability of internal baroclinic jets in a rotating atmosphere. J. Atmos.
Sci., 19, 159-172.
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669-700.
Landsea, C.W., and W.M. Gray, 1992: The strong association between Western Sahelian monsoon rainfall and
intense Atlantic hurricanes. J. Climate, 5, 435-453.
Lau, K.-H., and N.-C. Lau, 1990: Observed structure and propagation characteristics of tropical cyclone summertime
disturbances. Mon. Wea. Rev., 118, 1888-1913.
Newell, RE., and JW. Kidson, 1984 : African mean wind changes between Sahelian wet and dry periods. J. Climate,
4, 27-33.
Thorncroft, C., and K. Hodges , 2001 : African Easterly Wave Variability and Its relationship to Atlantic TC
Activity. J. Climate, 14, 1166-1179.
203
Contact
(1) LTHE/INPG, BP 53, 38041, Grenoble Cedex 9, France
(2) LPA-SF, ESP-UCAD, BP 5085, Dakar-Fann, Sénégal
Figure 1: Gradient méridien de tourbillon potentiel (PV) sur l’isentrope 315K pour les OEAs
associées à des cyclones (haut) et pour les OEAs non associées à des cyclones (bas). L’unité est :
10-14 K m s-1 Kg-1.
Figure 2: Anomalie de Potentiel de Convection (OEAs associées à des cyclones – OEAs non
associées à des cyclones). L’unité est le K.
204
AFRICAN EASTERLY WAVES AND CYCLONIC ACTIVITY OVER THE
EASTERN ATLANTIC : COMPOSITE AND CASE STUIES
1. Introduction
African Easterly Waves (AEWs, hereinafter) are important features of the West African and
tropical Atlantic ocean. AEWs propagate westward with a period of 3-5 days and are generated by a
mixed baroclinic - barotropic instability of the African Easterly Jet (Burpee, 1972). They are known
to both modulate the daily rainfall over West Africa and to initiate most tropical cyclones (TCs)
over the North Atlantic (Burpee 1972; Avila and Pasch 1992). Although the number of AEWs in
the tropical Atlantic is fairly constant from year to year, there is a substantial variability in the
fraction of AEWs that develop into tropical cyclones (Avila and Pasch 2000).
The main subjects examined in this study are the differences over the African continent and
the Eastern Atlantic between an AEW associated with a TC and an AEW not associated with a TC.
2. Data and methods
The data used in this study are the NCEP/NCAR daily reanalyses, Outgoing Longwave
Radiation (OLR) data and the National Hurricane Center (NHC) Best Track archives which
summarizes the dates of occurrence and tracks of North Atlantic TCs during the 1989-2003 period.
and the NHC Best Track.
Only cyclones generated off the West African coast (East of 40°W) and associated with
AEWs are considered in the aim to study West African climate features associated with North
Atlantic cyclonic activity at intraseasonal timescale. The characteristics of these AEWs are
compared to those of AEWs not associated with cyclones. In each case, an average of 48 AEWs is
considered.
3. Composite study
With the aim of depicting the areas of barotropic and baroclinic instabilities for AEWs with
and without cyclones, the meridional gradient of Potential Vorticity (PV) at 315K is computed.
Burpee (1972) found that the Ertel meridional gradient of PV changes sign over Africa near 700
hPa. This sign reversal of meridional gradient of PV satisfies a necessary condition for instability of
the mean flow (Charney and Stern 1962); areas where there is a negative meridional gradient of PV
(∂PV/∂y) are favorable to the growth of AEWs (Lau and Lau 1990).
Stronger negative values of ∂PV/∂y are found for AEWs with cyclones (figure 1). These
larger negative values are found to extend farther into the North Atlantic Ocean. Then AEWs
associated with cyclones are more active than those not associated with cyclones. This result is
consistent with the studies of Landsea and Gray (1992) who hypothesized that on an inter-annual
time scale, strong cyclonic activity is associated with a propagation in the North Atlantic Ocean of a
large number of AEWs with strong amplitude emanating from Africa.
From the point of view of thermodynamic, the Potential of convection (PC) used by Gray
(1968) to study the parameters linked to the genesis of North Atlantic TCs is calculated. PCis the
difference between the equivalent potential temperature at 1000 hPa (surface) and 500 hPa (midtroposphere). The difference of PC between AEWs associated with and without cyclones shows an
axis of maximum positive values along 15°N (figure 2). This axis extends well into the North
Atlantic ocean, indicating that the atmosphere is more unstable for the cases of AEWs associated
with cyclones. This northward shift in the peak in PC for AEWs with cyclones corresponds to a
general northward shift in the monsoon trough. We verified that this greater instability is associated
with the presence of a monsoon layer deeper in latitude and more intense in magnitude (figure not
205
shown). This is coherent with results of Newel and Kidson (1984) suggesting that intense AEWs are
associated with a wetter monsoon phase. NOAA OLR data exhibit lower values over Africa for
AEWs associated with cyclones suggesting deeper convection and a northward shift of the Inter
Tropical Convergence Zone (not shown).
4. Conclusion
Previous studies showed that at inter annual timescale, the links between AEWs and North
Atlantic TCs (number and activity) are not significant enough to explain all the variability of
cyclonic activity over the North Atlantic Ocean (Thorncroft and Hodges 2001). The objective of
this work was to understand what are the main differences between AEWs associated with cyclones
and AEWs not associated with cyclones.
AEWs associated with cyclones are associated with the highest negative intensity of the
meridional gradient of the PV over the continent and further over the North Atlantic Ocean. Highest
values of potential of convection also are found for AEWs with cyclones indicating that the
atmosphere is more unstable in this case. This is consistent with the existence of stronger
convection and of a monsoon stronger and deeper in latitude.
A Case study of the characteristics of an AEW associated with a cyclone and an AEW not
associated with a cyclone confirms the composite study results.
Références
Avila, L. A., R. J. Pasch, and J. Jiing, 2000: Atlantic tropical systems of 1996 and 1997 : Years of contrasts. Bull.
Amer. Met. Soc, 128, 3695-3709.
Avila, L. A., and R. J. Pasch, 1992: Atlantic tropical systems of 1991. Monthly Weather Review, 120, 2688-2696.
Burpee, R. W., 1972 : The origin and structure of easterly waves in the lower troposphere of north Africa. J. Atmos.
Sci., 29, pp.77-90
Charney, J. G., and M. E. Stern, 1962: On the stability of internal baroclinic jets in a rotating atmosphere. J. Atmos.
Sci., 19, 159-172.
Gray, W. M., 1968: Global view of the origin of tropical disturbances and storms. Mon. Wea. Rev., 96, 669-700.
Landsea, C.W., and W.M. Gray, 1992: The strong association between Western Sahelian monsoon rainfall and
intense Atlantic hurricanes. J. Climate, 5, 435-453.
Lau, K.-H., and N.-C. Lau, 1990 : Observed structure and propagation characteristics of tropical cyclone
summertime disturbances. Mon. Wea. Rev., 118, 1888-1913.
Newell, RE., and JW. Kidson, 1984 : African mean wind changes between Sahelian wet and dry periods. J. Climate,
4, 27-33.
Thorncroft, C., and K. Hodges , 2001 : African Easterly Wave Variability and Its relationship to Atlantic TC
Activity. J. Climate, 14, 1166-1179.
Contact
(1) LTHE/INPG, BP 53, 38041, Grenoble Cedex 9, France
(2) LPA-SF, ESP-UCAD, BP 5085, Dakar-Fann, Sénégal
206
Figure 1: Meridional gradient of Potential Vorticity at 315-K isentropic surface for AEWs
associated with cyclones (top) and for AEWs not associated with cyclones (bottom). Unit is : 10-14
K m s-1 Kg-1.
Figure 2: Anomaly of Potential of Convection (AEWs with cyclones – AEWs without cyclones).
Unit is K.
207
1.26P
ONDES D’EST AFRICAINES DANS DES SCENARIOS IPCC
EN BASSE ET HAUTE RESOLUTION
Fabrice CHAUVIN
CNRM, Météo-France
Les Ondes d’Est Africaines (OEA) sont une composante essentielle de la Mousson Ouest-Africaine
(MOA). Elles sont proviennent des instabilités barotropes/baroclines du Jet d’Est Africain (JEA).
Au cours de la campagne GATE (pour Global Atmopsheric Research Program (GARP) Atlantic
Tropical Experiment ), les connaissances sur les OEA ont été largement améliorées (Burpee and
Reed 1982), notamment par l’étude de leur structure verticale. Depuis, de nombreuses études leur
ont été consacrées, que ce soit dans un cadre théorique (Thorncroft and Hoskins 1994), par l’étude
des observations et ré-analyses (Diedhiou 1998) ou bien à l’aide de simulations climatiques (Céron
and Guérémy 1999, Moustaoui et al. 2002, Chauvin et al. 2005). De l’ensemble de ces études, on
peut distinguer deux zones privilégiées pour l’activité des OEA : au Nord et au sud du JEA. La
structure verticale des ondes varie selon qu’on se trouve d’un côté ou de l’autre dudit jet, c’est-àdire dans un profil vertical sec ou humide. Bien que les pluies du continent ouest-africains aient leur
propre dynamique, les OEA sont susceptible d’expliquer une part importante de leur variabilité
intra-saisonnière et pourraient ainsi apporter une aide précieuse dans la prévision saisonnière de
celles-ci, étant donné leur caractère prédictible (Chauvin et al. 2005). Néanmoins, il est important
de vérifier dans quelle mesure les modèles climatiques sont capables de simuler de telles instabilités
dynamiques. Etant donnée la faible couverture du continent par des stations de mesure synoptiques
en altitude, nous avons recours aux ré-analyses du Centre Européen de Prévisions à Moyen Terme,
récemment conduites sur une période de 40 ans (ERA40, 1960-1999). L’approche utilisée dans
cette étude est identique à celle employée dans Chauvin et al. (2005), à savoir la décomposition
spectrale de la variance totale en composantes stationnaire et propagative. La variance propagative
peut être elle-même décomposée en deux parties : les ondes à propagation vers l’ouest et les ondes à
propagation vers l’est. Ce sont ces dernières qui nous intéressent dans cette étude, dans une fenêtre
de fréquence de 3 à 6 jours, correspondant aux OEA.
Les diverses études consacrées aux champs dynamiques analysés montrent une structure spatiale
complexe des OEA, avec la partie située au Nord du JEA centrée sur le Sahel, sans extension
océanique de l’activité, et la partie située au Sud du JEA, associée à la convection humide qui
s’étend de l’Afrique guinéenne à l’océan atlantique tropical et donnant naissance à une grande
partie des ouragans cap-verdiens. Plusieurs études ont montré la capacité du modèle ARPEGEClimat, du Centre National de Recherche Météorologique (CNRM), à représenter les OEA et leur
variabilité inter-annuelle de manière satisfaisante, malgré des défauts récurrents dans les diverses
versions du modèle (Céron and Guérémy 1999, Moustaoui et al. 2002, Chauvin et al. 2005). Elles
indiquent, notamment, une structure trop zonale de la variance des OEA et donc peu de
discernement entre les parties Nord et Sud des ondes. Dans le cadre de la contribution du CNRM au
quatrième rapport d’évaluation du Groupe Intergouvernemental sur l’Evolution du Climat (GIEC),
plusieurs simulations couplées océan-atmosphère ont été réalisées, dont une forcée par les
concentrations de gaz à effet de serre depuis 1860 jusqu’à 2000, suivie de plusieurs scénarios
émanant du GIEC, en fonction des diverses hypothèses d’évolution des sociétés, au cours du XXIème
siècle. Nous ne traiterons, ici, que la simulation couvrant le XXème siècle. Le Centre Européen de
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Recherche et Formation en Calcul Scientifique (CERFACS) participe au projet français ACI-FNS
DISCENDO, dont l’objectif est la détection du changement climatique à l’échelle régionale.
Plusieurs simulations ont été réalisées à basse et haute résolution, avec l’Afrique de l’Ouest comme
centre d’intérêt. Pour les expériences à haute résolution, la technologie de la maille basculée/étirée a
été utilisée, avec le pôle sur le Golfe de Guinée, permettant ainsi, à moindre coût, d’obtenir une
résolution de l’ordre de 50 kilomètres sur le continent africain. Cette simulation couvre la période
1948-1999, avec les divers forçages lithosphériques (aérosols) et solaires correspondant à ce qui a
été observé durant cette période. Une simulation équivalente de résolution beaucoup plus faible
(T63) a été réalisée dans les mêmes conditions.
La figure 1 montre la variance propagative d’est moyenne sur la période 1960-99, calculée à partir
du jeu de données homogène ERA40. On peut voir deux zones distinctes d’activité des OEA, une
située au delà de 17°N et l’autre, sur la côte atlantique, vers 12°N. Cette dernière correspond au
maximum de précipitations observées sur cette région.
Figure 1 : Variance propagative d’est moyenne sur la période 1960-1999 calculée à partir de
ERA40. Contours tous les 1.e-11 s-2.
Le même champ est représenté sur la figure 2 pour les 50 ans de la simulation basculée/étirée. On
peut voir que le modèle est capable de représenter correctement les OEA, même si la répartition
spatiale de leur activité présente quelques différences par rapport aux analyses. Le principal défaut
consiste en l’impossibilité de distinguer les deux régions d’activité indiquées dans les analyses. Ce
défaut avait déjà été mentionné dans Chauvin et al. (2005), et l’augmentation de résolution obtenu
par l’étirement de la grille ne semble pas y remédier. On remarque également une sensibilité aux
reliefs qui semble exagérée, même si les réanalyses indiquent bien une activité renforcée sur le
Hoggar, le Tibesti et, dans une moindre mesure, l’Atlas. Enfin, comme conséquence du manque de
discernement des deux zones d’activité, la région où les ondes quittent le continent est située 5° trop
au Nord. Ce défaut est atténué par un infléchissement de l’activité vers le Sud, après l’arrivée sur
l’Atlantique.
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Les développements ultérieurs de cette étude concerneront les simulations forcée et couplée en
basse résolution ainsi que l’étude de la variabilité inter-annuelle à décadale des OEA.
Figure 2 : Variance propagative d’est moyenne sur la période 1948-1999 calculée à partir de la
simulation DISCENDO basculée/étirée. Contours tous les 1.e-11 s-2.
Nous adressons nos vifs remerciements à Virginie Lorant, pour la fourniture des simulations
DISCENDO et Alain Braun pour le désarchivage des ré-analyses du CEPMMT.depuis la base de
donnée MARS.
Références :
Burpee RW , Reed RJ (1982) Synoptic scale motions. The GATE Monograph, GARP Publ., No 25 WMO/ICSU. pp 61120.
Céron JP , Guérémy JF (1999) Validation of the space-time variability of African easterly waves simulated by the
CNRM GCM. J. Climate 12: 2831-2855
Chauvin, F., J.-F. Royer and H. Douville (2005): Interannual variability and predictability of African easterly waves in a
GCM. Climate Dynamics 24: 523-544.
Diedhiou A, Janicot S, Viltard A , de Felice P (1998) Evidence of two regimes of easterly waves over West Africa and
the tropical Arlantic. Geophys. Res. Letters 25: 2805-2808
Moustaoui M, Royer JF , Chauvin F (2002) African easterly wave activity in a variable resolution GCM. Climate
Dynamics 19: 289-301.
Thorncroft CD , Hoskins BJ (1994) An idealized study of African easterly waves. I: A linear view. Quart. J. Roy.
Meteor. Soc. 120: 953-982.
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AFRICAN EASTERLY WAVES IN LOW AND HIGH RESOLUTION
IPCC SCENARIOS
African Easterly Waves (AEW) are an essential component of the West-African Monsoon (WAM)
which come from barotropic/baroclinic instabilities of the African Easterly Jet (AEJ). During the
GATE (for Global Atmopsheric Research Program (GARP) Atlantic Tropical Experiment ),
knowledge of AEW has been increased (Burpee and Reed 1982), in particular due study of the
vertical structure of the waves. Since then, several studies have been dedicated to AEW, in a
theorical frame (Thorncroft and Hoskins 1994), by study of observations and re-analysis
(Diedhiou 1998) or in GCM simulations (Céron and Guérémy 1999, Moustaoui et al. 2002,
Chauvin et al. 2005). From all these studies, two privilieged regions may be distinguished for AEW
activity: to the North and to the South of AEJ. Vertical structure of the waves vary on both sides of
this jet, i.e. in a dry or wet vertical profile. Even African rainfall have their proper dynamics, AEW
may explain an important part of their intraseasonal variability and could be helpful in seasonal
prediction of these, due to the predictable nature of the latter (Chauvin et al. 2005). Nevertheless, it
is important to verify ability of the models to simulate such dynamical instabilities. Given the poor
coverage of altitude synoptic stations, a recent version of European Centre for Medium-range
Weather Forecast re-analyses were used, covering a 40-years period (ERA40, 1960-1999).
Approach in this study is the same as Chauvin et al. (2005), that is spectral decomposition of total
variance in stationary and propagative components. Propagative variance can be decomposed in two
parts: westerly and easterly waves. These latter are the topic of this study, in a 3 to 6 days frequency
window corresponding to AEW.
The studies dedicated to dynamical fields in analyses show a complex structure of AEW, with the
part North of the AEJ which do not present any oceanic extension of the activity and the southern
part, associated with wet convection, extending from Guinea to tropical Atlantic Ocean and leading
to a major part of cap-verdian hurricanes. Many studies showed the ability of the ARPEGE-Climat
GCM, from Centre National de Recherche Météorologique (CNRM), to correctly represent AEW
and their interannual variability, even if some biases persist from through the different versions of
the model (Céron and Guérémy 1999, Moustaoui et al. 2002, Chauvin et al. 2005). These studies
show a structure of the AEW 2D-variance which is too zonal and thus no distinction between
northern and southern part of the waves. In the frame of the CNRM contribution to the fourth
assessment report of the International Panel on Climate Change (IPCC), several coupled
atmosphere-ocean simulations were performed, among which one forced by greenhouse gases
concentrations from 1860 to 2000, followed by several scenarios from IPCC, differing in the
hypotheses in the 21st century society evolution. In this study, only the 20th century simulation will
be assessed. The Centre Européen de Recherche et Formation en Calcul Scientifique (CERFACS)
participates to the French ACI-FNS DISCENDO project, which objective is detection of climate
change to a regional scale. Many simulations were performed in low and high resolution, with
interest centered on West Africa. For the high resolution simulations, rotated/stretched technology
was used, with the pole on the Gulf of Guinea, allowing an equivalent resolution of 50 kilometers
on the African continent at a low cost. This simulation covers covers period 1948-1999, with the
various lithospheric (aerosols) and solar forcings corresponding to observation during this period.
Another simulation was performed in the same conditions but with a much lower resolution (T63).
Figure 1 shows the easterly 2D-variance over the period 1960-99 calculated from the homogeneous
dataset ERA40. Two AEW activity zones can be distinguished, one North to 17°N and the other on
the Atlantic coast, around 12°N. The latter corresponds to the maximum of precipitation observed
over this region.
211
Figure 1: Easterly 2D-variance over the period 1960-1999, calculated from ERA40. Contours are
1.e-11 s-2.
The same field is showed in figure 2 for the 50 years of the rotated/stretched simulation. It can be
seen that the model is able to correctly simulate AEW, even if spatial repartition of their activity
show some differences with analyses. The major drawback is the lack of distinction between the
two regions of AEW activity showed in the re-analyses. It was already mentioned in Chauvin et al.
(2005), and increase in resolution obtained by stretching does not seem to improve this. It can be
also noticed an exaggerated sensitivity of AEW to orography, even if re-analyses effectively show a
strong activity over Hoggar, Tibesti and, in a minor way, Atlas. Moreover, as a consequence of the
lack of distinction between the two activity zones, the region where the waves leave the continent is
located 5° too North. This bias is mitigated by a southward bending of the activity after the waves
reach the Atlantic Ocean.
Forthcoming developments will assess forced and coupled low resolution simulations, as well as
interannual to decadal variability of AEW.
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Figure 2: Easterly 2D-variance over the period 1948-1999, calculated from the rotated/stretched
DISCENDO simulation. Contours are 1.e-11 s-2.
We are grateful to Virginie Lorant, for giving us datas from the DISCENDO simulations and Alain
Braun for collecting the ECMWF re-analyses from MARS database.
References :
Burpee RW , Reed RJ (1982) Synoptic scale motions. The GATE Monograph, GARP Publ., No 25 WMO/ICSU. pp
61-120.
Céron JP , Guérémy JF (1999) Validation of the space-time variability of African easterly waves simulated by the
CNRM GCM. J. Climate 12: 2831-2855
Chauvin, F., J.-F. Royer and H. Douville (2005): Interannual variability and predictability of African easterly waves
in a GCM. Climate Dynamics 24: 523-544.
Diedhiou A, Janicot S, Viltard A , de Felice P (1998) Evidence of two regimes of easterly waves over West Africa
and the tropical Arlantic. Geophys. Res. Letters 25: 2805-2808
Moustaoui M, Royer JF , Chauvin F (2002) African easterly wave activity in a variable resolution GCM. Climate
Dynamics 19: 289-301.
Thorncroft CD , Hoskins BJ (1994) An idealized study of African easterly waves. I: A linear view. Quart. J. Roy.
Meteor. Soc. 120: 953-982.
213
1.27P
AFRICAN EASTERLY WAVES - EASTERLY JET SYSTEM :
MOIST PHYSICS
Rosalind CORNFORTH (1), Brian HOSKINS (1) and Chris THORNCROFT (2)
(1) Department of Meteorology, University of Reading, UK
(2) Department of Earth and Atmospheric, Sciences, University at Albany, SUNY, USA
African easterly waves (AEWs) are important synoptic weather disturbances that form in the
easterlies in the northern hemisphere summer over West Africa. Despite their probable link to the
daily rainfall in such marginal regions as the Sahel, the waves’ evolution, structure and interactions
with the African easterly jet (AEJ) and moist convection remain difficult to unravel from
observations and complex GCMs. This motivates this moist idealised study in which the forcing of
the AEJ, the AEWs that evolve on it and their coupled interactions are examined using Reading’s
Intermediate General Circulation Model (IGCM).
In contrast to previous studies, a realistic jet evolves in response to prescribed surface and moisture
profiles, with the meridional circulation associated with the Saharan heat low contributing most to
its evolution rather than the ITCZ. Strong coupling between the jet and the waves existed although,
counter-intuitively strong waves evolved on a weak jet through appropriate choice of transfer
coefficients, with implications for NWP. In contrast to the dry study, moist waves grew more
rapidly though still dominated by baroclinic energetics. they exhibited a deep vertical structure with
additional layers of complexity related to interactions of the diabatically-generated PV anomalies.
The phase relationship of the convection to the AEW was consistent with observations, varying
with latitude as a function of the wave tilt, the interaction of the diabatic heating with this tilt, and
the magnitude of the associated temperature and moisture perturbations of the AEWS relative to the
ambient environment. Results confirmed previous speculation that rainfall is “organised” by AEWs
but through a process of “natural selection” where condensation heating in the trough produces
positive feedback and wave growth. Ultimately the process is self-limiting as constructive
interference of the PV anomalies results in the rainfall eventually occurring outside the trough
where negative feedback triggers wave decay.
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SYSTÈME ONDES - JET EST AFRICAINS :
PHYSIQUE DE L’HUMIDITÉ
Les ondes d’est africaines sont des perturbations synoptiques importantes qui se forment dans les
courants d’est dans l’hémisphère nord d’été au-dessus de l’Afrique d’ouest. Malgré l’influence
probable des ces ondes sur les précipitations journalières dans des régions marginales comme le
Sahel, leur structure, évolution et interactions avec le courant d’est africain reste un problème
intéressant qui est abordé par les observations et des modèles GCM complexes. Ceci vient motiver
cette étude idéalisée pour laquelle les caractéristiques humides des circulations sont prises en
compte. Le modèle de circulation générale intermédiaire de Reading IGCM est utilisé pour
modéliser le forçage du courant jet, les ondes qui s’y produisent et l’interaction entres ces deux
systèmes.
A l’opposé des études précédentes, le courant jet répond à un forçage par des données de surface et
profils d’humidité. L’évolution de ce courant est aussi plus influencée par la dépression thermique
saharienne que par la zone de convergence intertropicale. Les ondes et le jet sont deux circulations
fortement couplées l’une à l’autre ; cependant, de manière inattendue, pour un choix approprié de
coefficients de transfert, de fortes ondes peuvent apparaître pour un jet de faible intensité, ce qui
constitue une remarque importante pour les modèles de prévision du temps. A l’inverse de l’étude
précédente pour laquelle l’humidité n’était pas prise en compte, les ondes humides grandissent plus
rapidement mais restent cependant dominées par la baroclinicité de l’atmosphère. Ces ondes
montrent une structure verticale étendue avec des couches additionnelles caractérisées par des
anomalies de tourbillon potentiel (engendrées par des processus diabatiques). La relation de phase
entre la convection et les ondes d’est est en accord avec les observations ; cette relation dépend de
la latitude et ce, en fonction de l’inclinaison des ondes, de l’interaction des processus de chauffage
diabatiques et de cette inclinaison et des perturbations associées de température et d’humidité par
rapport au milieu environnant. Les résultats confirment les recherches précédentes, à savoir que la
quantité de précipitations dépend des ondes d’est mais au travers d’un processus de sélection
naturelle pour lequel la chaleur libérée par la condensation dans le thalweg de la dépression
engendre un feedback positif et renforce la croissance des ondes. Finalement, ce processus est limité
si une interférence constructive des anomalies de tourbillon potentiel lors des précipitations apparaît
en dehors du thalweg, auquel cas un feedback négatif engendre la disparition des ondes.
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1.28P
AFRICAN EASTERLY WAVES - EASTERLY JET SYSTEM :
IMPACT OF THE HADLEY CIRCULATION
Rosalind CORNFORTH (1), Brian HOSKINS (1) and Chris THORNCROFT (2)
(1) Department of Meteorology, University of Reading, UK
(2) Department of Earth and Atmospheric, Sciences, University at Albany, SUNY, USA
The hypothesis tested here is that the PV dynamics of West Africa are unique and that although the
ITCZ and heat low circulations are important contributors to maintaining the AEJ and thus the
AEWs, it is the heat low and its PV dynamics that determine their structures and evolution, whilst
the ITCZ and its progressive movement through the season, modulates them. This builds on the
findings of Thorncroft and Blackburn (1999) but contrasts with the findings of Schubert et al.(1991)
and Cook (1999) who both maintained that the positive PV anomaly diabatically-generated at low
levels beneath and on the poleward side of the ITCZ was quite sufficient to maintain the African
easterly jet (AEJ) and the associated waves (AEWs).
Similar to the previous idealised studies, Reading’s Intermediate general Circulation Model is used
to force the AEJ with simple prescribed surface temperature and humidity distributions. However,
unlike the previous dry and moist studies, here the full Hadley circulation is included by imposing a
broad zonally-averaged temperature and humidity distribution on which the additional temperature
and humidity anomalies are superimposed to represent the moist coastal region of the Gulf of
Guinea and the Sahara. The experimental design enables the impacts of the two different meridional
circulations to be assessed singly and together, on the evolution, structure and convective activity of
the moist waves through comparison with the results from the previous dry and moist studies.
Results show that whilst the Inter Tropical Convergence Zone (ITCZ) and Saharan heat low (SHL)
circulations both contribute to the jet’s evolution, it is the SHL that continues to dominate. The
ITCZ minimally modulates the jet’s characteristics through deceleration of the northern hemisphere
(NH) Hadley cell. Similarly, reversal of the 700 mb meridional PV gradient necessary for mixed
barotropic-baroclinic instability and AEW development, occurs only weakly for the ITCZ but
strongly for the SHL, through the intrusion of its low PV boundary layer and, in additional moist
experiments, through the downwards advection of positive PV in the descending branches of the
SHL and the NH Hadley cell. Some of the observed intra-seasonal variability of the jet and waves
can be therefore be attributed to fluctuations in the strengths of these competing circulations and the
strength of the AEWs, with transitions between single or double AEJ maxima, and more northerly
or southerly excursions from its mean location.
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SYSTÈME ONDES - JET EST AFRICAINS :
IMPACT DE LA CIRCULATION DE HADLEY
Le but de l’étude réalisée ici est de montrer que les caractéristiques de la dynamique du tourbillon
potentiel en Afrique d’Ouest sont particulières. Les circulations reliées à la zone de convergence
intertropicale et aux dépressions thermiques contribuent de manière importante à entretenir le jet
d’est africain et le système d’ondes associées ; cependant, la zone de convergence intertropicale n’a
qu’un rôle de modulateur sur ces systèmes alors que l’évolution et la structure de ces circulations
d’est sont principalement contrôlées par la dynamique du tourbillon potentiel lié aux dépressions.
Cette théorie s’appuie sur les résultats de Thorncroft et Blackburn (1999) mais s’oppose à ceux de
Schubert et al. (1991) et Cook (1999). Ces deux dernières études suggèrent en effet que l’anomalie
positive de tourbillon potentiel engendrée, par processus diabatiques, dans les basses couches du
côté polaire de la zone de convergence intertropicale suffit à entretenir la formation du jet d’est
africain et le système d’ondes associées.
De manière similaire aux études idéalisées précédentes, le modèle de circulation générale
intermédiaire de Reading est utilisé pour forcer le jet africain au moyen de simples distributions de
surface de température et d’humidité. Cependant, à l’opposé des études sèches et humides
précédentes, toute la circulation de Hadley est cette fois incluse par un forçage en température et
humidité moyenné zonalement et sur lequel s’ajoutent les anomalies de température et d’humidité
afin de prendre en compte la région humide et côtière du Golfe de Guinée ainsi que le Sahara. Ces
expériences permettent de distinguer les impacts individuels et communs de ces deux circulations
méridiennes sur la structure et l’activité convective des ondes humides par comparaison avec les
résultats des études sèches et humides précédentes.
Les résultats montrent alors que la zone de convergence intertropicale et la dépression saharienne
contribuent toutes deux à l’évolution du jet, c’est plus particulièrement la dépression saharienne qui
domine. La zone de convergence intertropicale a pour effet de moduler de manière minimale les
caractéristiques du jet en causant la décélération de la cellule de l’hémisphère nord de la circulation
de Hadley. De manière similaire, le retournement du gradient méridien de tourbillon potentiel à
700 mb nécessaire au développement des instabilités mixtes barotropes-baroclines apparaît de
manière plus sensible pour la dépression saharienne au travers de l’intrusion de faible tourbillon
potentiel dans la couche limite. Des expériences complémentaires avec des caractéristiques humides
ont révélé que cette sensibilité pouvait aussi être reliée à l’advection de vorticité positive dans les
courants descendants de la dépression saharienne et de la cellule nord de Hadley. Il en résulte que
certaines caractéristiques de la variabilité inter saisonnière observée des jets et ondes peuvent être
attribuées aux fluctuations des composantes de ces deux circulations en compétition relativement à
celle des ondes, entraînant des transitions entre des maxima de jet simples ou doubles et des
déplacements de jet au nord ou au sud par rapport à la position moyenne.
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1.29P
A NEW PERSPECTIVE ON AFRICAN EASTERLY WAVES
PART 2 : DYNAMICS
Nick HALL (1), George KILADIS (2) and Chris THORNCROFT (3)
(1) LTHE, Grenoble, France, (2) NOAA, Boulder CO, USA, (3) SUNY, Albany NY, USA
A primitive equation model is used to study the strucutre and growth rates of linear normal modes
of the African Easterly Jet (AEJ).
Reanalysis data from the summertime mean flow is used to provide realistic basic states. The modes
have longitudinal dependence determined by the jet entrance/exit, with a surface intensified
baroclinic structure upstream and a deep barotropic structure downstream. They look remarkably
similar to the observational composites presented by George Kiladis. The similarity extends to the
phase relationship of vertical velocity with streamfunction, suggesting a dynamical influence on
convection.
Without damping, the mode is unstable and grows at a rate of 0.253 days-1. However, with a
modest amount of low level damping this mode is neutralised. It has a period of 5.5 days and a
wavelength of about 3500 km. Experiments focused on specific active and passive years for easterly
waves (1988 and 1990) do not yield significantly different results for the modes. From this we
conclude that since the system is stable, the intermittency of easterly waves can not be explained by
stability theory. The possibility that the waves need finite amplitude perturbations to initiate them
will be discussed.
Submitted by
Nick Hall
Laboratoire d'étude des Transferts en Hydrologie et Environnement, CNRS, BP53, 1025 Rue de la
Piscine, Domaine Universitaire, 38041 Grenoble Cedex 9, France
phone: +33 476 825108 or +33 671 273529 - fax: +33 476 825286
http://www.lthe.hmg.inpg.fr/~hall/ - http://www.lthe.hmg.inpg.fr/%7Ehall/>
218
UN NOUVEAU PARADIGME POUR LES ONDES EST AFRICAINES,
PARTIE II : LA DYNAMIQUE
Un modèle aux equations primitives est utilisé pour l'étude de la structure et des taux de croissance
des modes propres du jet est Africain (AEJ). Les réanalyses de l'été moyenne sont prises comme
état de base réaliste.
Les modes ont une variation en longitude qui est déterminé par l'existence de rentrée/sortie du jet,
avec une structure peu profond barocline en amont et une structure profond barotrope en aval. Ils
ressemblent fortement aux observations des composites montrés par George Kiladis. La similarité
comprend la relation des phases de la vitesse verticale et la fonction de courant, ce qui suggère une
influence dynamique sur la convection.
Sans amortissement, le mode est instable et croît à 0.253 jour-1. Par contre, avec une quantité assez
conservateur d'amortissement dans les bases couches le mode est neutralisé. Il a une périodicité de
5.5 jours et une longueur d'ondes d'environ 3500 km. Les expériences focalisées sur des années
actives et passives pour les ondes d'est (1988 et 1990) n’aboutissent pas à des résultats différents de
façon significative pour les modes. Nous concluons donc que le système est stable et la nature
intermittente des ondes est ne peut pas être expliquée par la théorie de la stabilité. La possibilité que
les ondes ont besoin de perturbations de grande amplitude pour être initiées sera discutée.
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1.30P
ANALYSE SPECTRALE SINGULIERE MULTI-CANAL ET CARACTERISATION
DES ONDES D’EST
Maâzou KAMAYE (1) et Guy PLAUT (2)
(1) Département de Physique, Université Abdou Moumouni, Niamey, Niger
(2) Institut Non Linéaire de Nice - Sophia Antipolis, Valbonne, France
Les ondes d’est se manifestent par une modulation de la convection et des précipitations durant la
mousson qui se propage d’est en ouest avec des périodicités de quelques jours. Différentes
méthodes ont été utilisées (classification automatique, analyse composite, transformées en
ondelettes,…) pour mettre en évidence l’existence de deux régimes d’ondes d’est : l’un de période 3
à 5 jours et une longueur d’onde horizontale d’environ 3000km, l’autre de période 6 à 9 jours et une
longueur d’onde horizontale d’environ 6000km (Diedhiou et al, 1990.) Mais le développement et la
description précise de la dynamique de certains caractères de type synoptique sont encore mal
compris.
Nous avons pensé, qu’une façon particulièrement intéressante d’aborder ce problème est de
rechercher si des oscillations types ondes d’est ne peuvent être mises en évidence et caractérisées
sur l’Afrique de l’ouest, particulièrement sur le Niger en utilisant l’analyse spectrale singulière
multi-canal (MSSA), méthode objective de recherches de comportements oscillants même
intermittents mise au point par MM. Plaut et Vautard, qui leur a permis de mettre en évidence les
oscillations récurrentes présentes dans un ensemble de données (réanalyses atmosphériques
américaines, les températures et précipitations sur la période 1960-2000) à divers niveaux
troposphériques dans la région Atlantique-Nord et Europe.
En utilisant les précipitations quotidiennes sur la période 1970-1990 ainsi que les réanalyses du
CEPMT et du NCEP pour la période 1968-1998 pour les niveaux de pression 700hPa, 850hPa et
925hPa.
Ainsi sur le domaine 17,5°O-27,5°E et 0°N-20°N que nous avons appelé North Tropical Africa,
nous avons remarqué que c’est le champ de divergence du vent à 700 hPa qui s’avère être la
grandeur la mieux adaptée pour nos investigations avec la MSSA, et une oscillation à 12-13 jours a
été obtenue avec les caractéristiques d’une onde d’est.
Mots clés : Ondes d’est, mousson, oscillations, analyse spectrale, Afrique de l’ouest, Niger.
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1.31P
ON STATIC STABILITY OVER TROPICAL NORTH OF THE EQUATOR
André LENOUO
Atmospheric Sciences Lab, Dep. of Physics, University of Douala, Cameroon
1. Introduction
The atmospheric static stability parameter is defined as the difference between the dry adiabatic and
the environmental thermal lapse rate. The static stability is an important indicator in the theoretical
analysis of the thermodynamic structure of the atmosphere. In general terms, it can be regarded as
an indicator of the stability of the atmosphere in hydrostatic equilibrium with respect to vertical
motion. It is also useful in determining both the vertical and horizontal transports of many
atmospheric quantities as a consequence of its relation to temperature. As such, it plays an
important role in understanding of convective weather patterns and rainfall.
The static stability of the atmosphere has been determined in many studies covering the West Africa
region (Jegede and Balogun, 1991), the Indian peninsula (Mukerji et al. 1972), the Colorado
cyclogenic area (Hovanec and Horn, 1975), the continental United States (Cortinas and Doswell,
1997) and the West African region during the special observation period (July 14 - August 15 1979)
of the West African Monsoon Experiment (WAMEX). These studies investigated horizontal and
temporal variations of static stability. In the particular case of tropical Africa, spatial variations of
the parameter reflect the various climatic zones of the area.
2. Results
Since 1974, launching of polar orbital TIROS-NOAA satellites has enabled us to establish a quasicomplete series of twice daily measures of outgoing longvawe radiation (OLR) at the top of the
atmosphere (Diedhiou et al, 1999). We recall that the analysis of the static stability parameter is
undertaken in order to understand convective weather and rainfall and its correlation with OLR that
is proportional to nebulosity and have been used to make estimates of tropical precipitation
(Diedhiou, 1998).
To analyze the correlation between OLR and ÿ at 700 hPa (which it is the level where the
convection is more important), the figures 1 gives the time variation of static stability in units of 103
K.Pa-1 (in left) and OLR in units of W.m-2 (in right) over southern zone Eq-10°N (Fig 1a), middle
zone 10°-20°N (Fig 1b), northern zone 20°-30°N (Fig 1c) and entire zone Eq-30°N (Fig 1d). There
are strong correlations between OLR and σ in the northern zone with high values in August, which
corresponds to the period of maximum rainfall. The correlation coefficient in the northern area is
around 0.92. In southern area this coefficient drops to 0.68. But as it is shown in figure 1b, there is
little correlation (0.21) between these parameters in the middle zone. The absence of a definite
relationship between σ and OLR in the middle area of study can be explained by the fact that the
Inter-tropical convergence zone (ITCZ) also in influences rainfall here. The ITCZ is a seasonal
migrating line that arrives in this latitude in June-August and is characterized by the effects of
easterly waves. In this period, the mid-tropospheric waves are observed from the Ethiopian
221
Highlands through the Sahel to the coast of West Africa (Burpee 1972). One can note that in the
entire zone, monthly variations of σ and OLR have acceptable correlation (0.78). In general, OLR is
larger when the atmospheric conditions are more stable. This indicates that the maximum
convective activity does not occur over the warmest. It is important to note that OLR data represent
a mean temperature averaged over the time period analyzed, hence, when the environment is stable,
less one has cloud.
3. Conclusion
By using the proportionality relationship between the temperature fields and the static stability at a
given level, the thermodynamic structure and climatic patterns of the tropical region of Africa were
examined. We showed that a fairly strong correlation exists between σ and OLR in the Northern
zone with high value in August. An acceptable correlation existed in the Southern zone and no
definite relationship existed in the middle zone. The annual cycles of σ and OLR anomalies show
identical patterns of variations. Therefore, the static stability parameter can be considered as a good
indicator to understand the sahalian rainfall variability.
References
Burpee, R.W., 1972, The origin and structure of easterly waves in the lower troposphere of North Africa. JAS,
29,77-90.
Cortinas, John V. Jr. And Charles A. Doswell III, 1997, Climatology of tropospheric static stability across the
contiguous United States, Pre-prints, 16th Conference on Weather Analysis and Forecasting Phoenix, AZ, Amer.
Meteor. Soc.
Diedhiou, A, 1998, Easterly wave regimes over West Africa and the tropical Atlantic. Thesis, University Paris 12,
230pp (In French).
Diedhiou, A. , S. Janicot, A. Vitard , P. de Felice and H. Laurent, 1999, Easterly wave regimes and associated
convection over West Africa and tropical Atlantic : results from the NCEP/NCAR and ECMWF reanalyses, Climate
Dynamics, 15, 795-822.
Hovanec, R.D. and L.H. Horn, 1975, Static stability and the 300mb isotaches field in the Colorado cyclogenic area.
Mon. Wea. Rev., 103, 628-638.
Jegede, O.O and E.E. Balogun, 1991, A contribution of the thermodynamic structure of the atmosphere over
continental West Africa : Static stability measure. Atmos. Res., 26, 75-90.
Contact
Email: [email protected] or [email protected]
222
Figure 1 : Correlations between OLR and static stability (σ) at the AEJ level during the 12-year
(1982-1994) study from NCEP/NCAR data over southern (a), middle (b), northern (c) and entire (d)
zone.
223
1.32P
ANALYSIS OF CONVECTION AND ITS ASSOCIATION WITH
AFRICAN EASTERLY WAVES
Ademe MEKONNEN, Chris THORNCROFT and Anantha AIYYER
Dept. of Earth and Atmospheric Science, University at Albany, USA
Several studies based on composite analysis show the presence of a coherent relationship between
African easterly waves (AEWs) and convection. Our understanding of the association between
convection and AEWs is mainly based on the analysis in West Africa, a region where the AEWs are
at their peak amplitudes and have stronger coherence with convection. However, we still lack a
detailed quantitative description of the relationship between the synoptic scale convection and
incipient AEWs, including how this varies in space and time.
We know very little about the nature of AEWs in central and eastern Africa (east of 10E). There is
still no consensus regarding the source region of AEWs. We suggest that convective outbreaks in
central and east Africa trigger AEWs that propagate in West Africa. We will show that convection
triggered in the vicinity of Darfur (west Sudan) plays a key role in initiating AEWs. In some years,
however, convection over Ethiopian highlands has also an important role.
The contribution of synoptic timescale convection to the total variability will be highlighted. The
presentation will focus on the year-to-year variability of the synoptic scale convective systems
propagating from the source regions in eastern Africa. We will also assess the extent to which these
regions are important for initiating AEW development downstream. Westward/eastward
propagating structures will be identified based on space-time filtering methods. Brightness
temperature and ECMWF 40-years reanalyses data sets are used in this research.
Corresponding author
Ademe Mekonnen - Department of Earth and Atmospheric Science, State University of New York
at Albany, 1400 Washington Ave., Albany, NY 12222-0001, USA
[email protected] - web page: www.atmos.albany.edu/student/ademe/AEW.html
224
ANALYSE DE LA CONVECTION ET DE SON ASSOCIATION
AVEC DES ONDES EST AFRICAINES
Plusieurs études basées sur des composites montrent l'existence d'une relation cohérente entre la
convection et les ondes est africaines (AEW). Notre compréhension de cette relation est
principalement basée sur les analyses en Afrique de l'ouest, où les AEWs ont leurs amplitudes
maximales et sont les plus liées à la convection. Pourtant, il nous manque toujours une description
détaillée et quantitative de la relation entre la convection à l'échelle synoptique et des AEWs
naissantes, y compris leur variabilité spatio-temporelle.
Nous avons peu de connaissance des AEWs en Afrique centrale et de l'est (est de 10E). Il n’y a
toujours pas d'accord sur la source des AEWs. Nous proposons que les évènements convectifs dans
ces régions puissent initier des AEWs, qui ensuite se propagent vers l'ouest. Nous allons montrer
que la convection dans la région de Darfur a un rôle clé pour l'initiation des AEWs. Il y a certaines
années où la convection sur l'Ethiopie a un rôle aussi important.
La contribution de la convection aux échelles temporelles synoptiques à la variabilité totale sera
mise en évidence. Cette présentation focalisera sur la variabilité d'une année à l'autre de ces
systèmes et leur propagation. Nous allons également déterminé à quel point ces régions ont de
l'importance pour l'initialisation du développement en aval. Des structures qui se propagent vers
l’ouest où vers l’est seront identifiées avec les méthodes de filtrage spatio-temporel. La température
de brillance et les réanalyses ERA40 sont utilisées.
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1.33P
226
227
INFLUENCE DE L’ACTIVITE DES ONDES 6 – 9-JOURS SUR LA
DISTRIBUTION SPATIALE DE LA PLUIE DE MOUSSON SUR LE NORD DE
L’AFRIQUE : VARIABILITE INTRA-SAISONIERE ET INTER-ANNUELLE
Une méthode composite est utilisée pour déterminer la structure spatiale de l’onde d’est de 6 – 9jours et la variation intra-saisonnière et inter-annuelle de la perturbation des précipitations
associées.
Les ondes de 6–9-jours augmentent les précipitations dans la bande de latitude 7.5°N – 17.5°N
dans la zone d ‘étude avec un maximum vers le Fouta Djallon en Guinée et le Lac Tchad, et font
décroître les précipitations au nord de 15°N et au sud de 7.5°N. Les zones d’augmentation
(diminution) des précipitations sont associées aux circulations cycloniques (anticycloniques) et aux
valeurs négatives (positives) de la perturbation de l’altitude géopotentielle. La modulation des
précipitations est faible en juin et en septembre où les ondes de 6 – 9-jours sont peu actives. Ces
ondes sont par contre très actives en juillet et août qui sont les deux principaux mois de fortes pluies
dans la zone d’étude.
Concernant la variabilité inter-annuelle, les ondes de 6 – 9-jours sont intermittentes et
l’augmentation ou la diminution des précipitations dépend de l’activité de l’onde. Quand elles sont
très actives, elles sont associées à d'importants tourbillons cycloniques et de forts accroissements
des précipitations.
Comparées aux ondes Africaines, les phénomènes de 3 – 5-jours se caractérisent par une extension
méridienne des valeurs positives de la perturbation des précipitations dans la bande 5°N – 15°N,
principalement à l’est du méridien de Greenwich tandis que les ondes de 6 – 9-jours se distinguent
par une extension zonale des valeurs positives de la perturbation des précipitations entre 7.5°N et
15°N, de l'est à l'ouest de la zone d'étude.
Contact
David Momkam : [email protected]
228
1.34P
WEST AFRICAN MONSOON DYNAMICS / EVIDENCE OF CONVECTIVELY
COUPLED KELVIN WAVES
FLORE MOUNIER(1), GEORGE N. KILADIS(2) AND SERGE JANICOT(3)
(1) Laboratoire de Métérologie Dynamique/Institut Pierre Simon Laplace, Palaiseau, France
(2) Aeronomy Laboratory, NOAA,Boulder, Colorado
(3) LOCEAN) /IPSL, Institut de Recherche pour le Développement, Paris, France
1. Introduction
Climatologies of propagating anomalies of convection in the equatorial region (WK99, WKW00,
Roundy and Frank, 2004) show that equatorial Rossby and Kelvin waves, along with the MJO,
contribute to a substantial amount of the tropical convection variance. These results suggest that,
during the northern spring, up to 20% of the local variance near the equator and the Atlantic InterTropical Convergence Zone (ITCZ) can be ascribed to Kelvin wave activity. Straub and Kiladis
(2003, hereafter SK03) show that Kelvin waves are primary modulators of convective activity
within the Pacific ITCZ, and that their structures resemble that of theoretical Kelvin waves. Here,
we demonstrate evidence of Kelvin wave modulating convection over both West and Central Africa
at a level at least as large as easterly waves do, and being present also out of the northern summer.
2. Detection of convectively coupled Kelvin waves
A spatial EOF (SEOF) analysis has been performed on Kelvin filtered OLR values over the domain
(10°S–30°N/30°W–30°E) for June-September 1979-2000. To determine the number of Principal
Components (PC) to be retained, the Scree test and the North rule of thumb were used after taking
into account the effect of the autocorrelation on the number of independent data. Following these
constraints, PC1/PC2 form an “effectively degenerate multiplet” well separated from PC3. PC1 and
PC2 represent respectively 16.3% and 15.3% of the filtered total variance and as a pair represent
eastward propagating Kelvin waves. Here, focus is given to the combination of EOF1 and EOF2
(“EOF12”) of the dominant mode that explains more than 30% of the filtered total variance.
In the following, we will call wet (dry) Kelvin phase when a Kelvin wave occurrence over West and
Central Africa is associated with negative (positive) OLR anomalies, that is higher (weaker)
convective activity in the ITCZ. Figure 1 shows the wet minus dry composite sequence associated
to EOF12 reconstructed ITCZ index (10°W-10°E/7.5°N-12.5°N), for the non-filtered OLR
(shaded), the non-filtered 925hPa wind (vectors) and 925hPa geopotential fields. The OLR data are
from the NOAA (National Oceanic and Atmospheric Administration) (see Liebmann and Smith,
1996) and are used as a proxy for deep convection; and wind and geopotential comes from the
updated version of the NCEP/NCAR reanalysis the NCEP-DOE AMIP-II Reanalysis (R-2) (see
Kalnay et al., 1996 and Kanamitsu et al., 2002). High significant non-filtered OLR anomalies
confirms that the Kelvin wave signal has a large impact on convection on Africa and the tropical
Atlantic. It yields a typical amplitude of the convection modulation of up to 35 W/m2 (that is about
plus or minus 15 W/m2 around the mean) associated with this wave activity. The OLR anomaly
pattern evolves from a zonal orientation over the Atlantic to a northwest-southeast one over West
229
and Central Africa and a meridional oriented one over East Africa before disagregating. As in the
upper-troposphere, the near-surface structure is in rough agreement with theoretical equatorially
trapped Kelvin wave solution on an equatorial β-plane (Matsuno 1966). As in Kelvin waves over
the Pacific (SK03), passage of the wave is preceded by easterly and followed by westerly wind
anomalies, in phase with negative and positive geopotential perturbations respectively, the whole
structure propagating at the same speed. More precisely, westerly anomaly winds correspond with
the negative OLR anomalies while easterly anomalies precede the negative OLR anomalies by
about two days. Most of the flow is in the zonal direction as predicted by theory, although the
strong monsoonal heating over West Africa does favor meridional southerly inflow. This wind field
anomaly pattern leads to an enhancement of moisture inland zonal flow component along the
Guinean coast and off the Fouta-Djalon orography at the location of the “westerly low-levels jet”
pointed out by Grodsky and Carton (2001). The convergence of westerly and easterly flows
contributes then to the enhanced convection over West and Central Africa. Also, as in the Pacific
(SK03), there is a tendency for the dynamical
fields to be much more symmetric about the
equator, with large Southern Hemisphere
signals, even though the convection is
concentrated well north of the equator, at the
latitude of the ITCZ (about 10°N at the full
development of the African monsoon). The
effect of strong heating over the African
continent results however in a perturbation of
the classical Kelvin wave structure when
compared to the open ocean over the Atlantic,
although the OLR signal continues to
propagate eastward.
Figure 1 : Composite time sequences based on
the OLR Kelvin-filtered ITCZ index (red box)
reconstructed by SEOF12. From the
standardized ITCZ index time series, we
retained the dates (called t0) where this index
is maximum (minimum) and greater (lower)
than 1.5 to define a dry (wet) Kelvin phase.
The respective wet minus dry Kelvin phases
composite sequences are shown for nonfiltered OLR (shaded), non-filtered 925hPa
wind field (vectors) and non-filtered 925hPa
geopotential field (contours); shown OLR and
wind values are significant at the 95% level.
These sequences go (top left to bottom right)
from t0-2 days to t0+2 days with a one day lag
(day marked on the right corner). OLR
anomalies are expressed in W.m-2, geopotential
in mgp, and vector scale (m.s-1) is displayed
below. The ITCZ box is displayed on each
map.
230
3. Conclusion
Kelvin wave appears to be a synoptic scale weather system factor modulating convection in the
ITCZ over tropical Atlantic and West and Central African monsoon at least as strong as African
easterly waves.
4. References
Grodsky S. A. and Carton J. A., 2001. Coupled land/atmosphere interactions in the West African
Monsoon. Geophys. Res. Lett., 28, 1503-1506.
Kalnay, E. et al. 1996: The NCEP/NCAR 40-year reanalysis project. Bull. Amer. Meteorol. Soc.,
77, 437-471.
Kanamitsu M., Ebisuzaki W., Woollen J., Yarg S-K., Hnilo J. J., Fiorino M. & Potter G. L.,
2002. NCEP/DOE AMIP-II reanalysis (R-2). Bull. of Amer. Met. Soc ., 83, 1631-1643.
Liebmann B. and Smith C. A., 1996. Description of a complete (interpolated) outgoing longwave
radiation dataset. Bull. of Amer. Met. Soc., 77, 1275-1277.
Matsuno T., 1966. Quasi-geostrophic motion in the equatorial area. J. Met. Soc. Japan, 44, 25-42.
Roundy P. E. & Frank W. M., 2004. A climatology of Waves in the Equatorial Region. J.Atmos.
Sci., 61, 17, 2105-2132.
Straub K. H. & Kiladis G. N., 2003. The observed structure of convectively coupled Kelvin
waves: comparison with simple models of coupled waves instability. J. Atmos. Sci., 60, 1655-1668.
Wheeler M. & Kiladis G. N., 1999. Convectively coupled equatorial waves: analysis of Clouds
and temperature in the wavenumber-frequency domain. J. of Atmos. Sci., 56, 374-399.
Wheeler, M., G.N. Kiladis and P.J. Webster, 2000. Large scale dynamical fields associated with
convectively coupled equatorial waves. J. Atmos. Sci., 57, 613-640.
Contact
Corresponding author address e-mail: [email protected]
231
1.35P
DOMINANT CONVECTIVELY COUPLED 15 DAYS PERIODICITY KELVIN
WAVES MODE AND ITS INFLUENCE DURING THE WEST AFRICAN
MONSOON (WAM) ONSET
Flore MOUNIER (1), Serge JANICOT (2) and George N. KILADIS (3)
(1) Laboratoire de Météorologie Dynamique/Institut Pierre Simon Laplace, Palaiseau, France
(2) LOCEAN / IPSL, Institut de Recherche pour le Développement, Paris, France.
(3) Aeronomy Laboratory, NOAA, Boulder, CO, USA
1. Introduction
The intra-seasonal variability of the West and Central African convection during the monsoon
season has been address only by few studies. Indeed, much of the researches have been on the
decenal or interannual variability due to the important drought of the Sahel the second half of the
twentieth century and on synoptic time scale of easterly waves. In between these two levels of
variability, Kiladis & Weickmann (1997) showed at the 6-30-days time scale connections between
convection in the region 5°-15°N/10°-20°E and the moisture advection over West Africa during the
northern summer. More recently, Sultan et al. (2003) highlighted, based on a regional rainfall index
computed on the area 12.5°-15°N / 10°W-10°E and filtered between 10 and 60 days, a westward
propagating signal of convection along the Sahelian latitudes, with a dominant periodicity around
15 days. Matthews (2004) focused on the 20-200 period range and identified by an empirical
Orthogonal Function (EOF) analysis a dominant mode over the whole African monsoon region
which might arise as a remote response to the intra-seasonal Madden-Julian Oscillation (MJO) over
the warm pool region. Grodsky & Carton (2001) showed that intra-seasonal modulation of
convection may also occur during northern spring in the ITCZ over the Tropical Atlantic with a
dominant periodicity between 10 and 15 days.
Moreover, Mounier & Janicot (2004) highlighted evidences of two independent modes of
convection at intra-seasonal time scale in the west African monsoon; the first one being coherent
with the dominant "Guinean" 15-days mode that interests this study. One of the questions they raise
was whether the fluctuations associated with this mode are an inherent feature of the WAM or a
larger scale one independent dynamically to the WAM. And finally, the impact and the dynamics of
convectively coupled Kelvin waves for the particular case of the West African monsoon was
presented in (Mounier et al. submitted). Here these points are discussed.
2. Dominant convectively coupled 15 days periodicity Kelvin waves mode
The dominant mode of convection at intra-seasonal timescale of the West African summer monsoon
is the “Guinean” mode of about 15-days periodicity. It depicts a stationary modulation of the ITCZ
convection without any significant change in its latitudinal location. However, over the tropical belt,
it is associated to an eastward propagating modulation of convection strength, even visible on scale
as small as cloud cluster (not shown here) and also to a modulation of the zonal wind component
over the eastern equatorial Atlantic. The propagative nature of this "Guinean" mode established in
Mounier & Janicot (2004) make an extended S-EOF (ES-EOF) analysis interesting. Therefore, an
ES-EOF analysis was performed on temporally refine 10-25-day bandpass filtered OLR values,
232
over the domain (120°W-40°E / 10°S-30°N) for the monsoon season (June to September) of years
1979 to 2000. The OLR data are from the NOAA (National Oceanic and Atmospheric
Administration) (see Liebman and Smith, 1996). Figure 1 portrays components of the “Guinean”
mode resulting from an ES-EOF analysis. ES-EOF3 and ES-EOF4 are in temporal quadrature, they
depicts the "Guinean" mode and its eastward propagation nature. It also highlights the scale of the
signal. This propagation happens along the position of the ITCZ during the WAM period (7.5°N).
ES-EOF1 and ES-EOF2 are not presented as they are out of the scope of our mode.
The analysis of the associated circulation has highligted interaction between convectively coupled
Kelvin waves of lower periodicity and the dominant 15-days "Guinean" mode. Indeed, study of the
dominant 15-days "Guinean" show that the appearing stationary convection modulation mode is
linked, on a larger scale, to an eastward propagation along the ITCZ position. Convection anomalies
growing on the west side of the Atlantic and propagating then eastward up to the east side of the
African continent. This propagation is coherent horizontally and vertically to convectively coupled
Kelvin waves. However, part of it could also be coherent with the Grodsky & Carton (2001)
hypothesis on the monsoon flux modulation by ocean-continent interactions.
Figure 1 : Representation of ES-EOF3 and ES-EOF4 resulting from an ES-EOF analysis performed
on 10-25-days filtered OLR data over the domain 120°W-40°E / 10°S-30°N. Negative values
(dotted) are for convection enhancement and positive value are for convection weakening. Variance
percentages are respectively for EOF3 and EOF4, 2.91% and 2.71%. The sequence goes from t0
(top) to t0+8days (bottom) with a 2 days gap in between.
233
3. Influence of 15-days mode and Kelvin waves on the WAM onset
To go further into the understanding of the interaction between convectively coupled Kelvin waves
and the dominant 15-days periodicity mode a real time analysis was performed on a critical stage of
the WAM, the onset of year 1984. The year 1984 onset was dated on the 3d of July following
Sultan & Janicot (2003) technique. The year 1984 being particularly dry, rainfall over the Sahel is
depleted compared to the averaged ones. Indeed, the Sahel has encountered a persisting drought
since the seventies, year 1984 being the driest one. It is characterised by a reinforcement of northeast and south-east trade winds and a reduction of the sea surface temperature over the tropical
Atlantic and by a decrease in the averaged velocity of the Tropical Easterly Jet (TEJ). However our
concerned is on interactions between convectively coupled Kelvin wave and the ITCZ shift, and
year 1984 is on this point a suitable example to characterise it. Indeed, in 1984, the ITCZ did stay
south of its averaged position during the full monsoon season. Then, links with Kelvin waves may
be facilitated.
This 1984 onset was characterised (not shown) by the arrival of a Kelvin wave train that modulated
the “Guinean” mode. A study of other onset cases will be needed to understand how important this
modulation of the “Guinean” mode by Kelvin waves is with regards to the onset.
4. Conclusion
The Some questions arise from this analysis: which mechanism is responsible of the birth of these
Kelvin waves in the Western side of the Atlantic. Is this mechanism modulated by the Central and
North American monsoon? How much of the Indian monsoon convection anomaly could be
imputable to African “Guinean” mode? The answer to these questions might help to better
understand connection between the three dominant monsoon regimes.
Reference
Grodsy S. A. & Carton J. A., 2001. Coupled land/atmosphere interactions in the West African
Monsoon. Geophysical Research Letter, 28, 1503-1506.
Kiladis, G.N. and K.M. Weickmann, 1997. Horizontal structure and seasonality of large-scale
circulations associated with submonthly tropical convection. Mon. Wea. Rev., 125, 1997-2013.
Kiladis, G.N., C.D. Thorncroft and N.M.J. Hall, 2005. A new perspective on African easterly
waves. Part I: Observations. Submitted to J. Atmos. Sci.
Liebmann B. & Smith C. A., 1996. Description of a complete (interpolated) outgoing longwave
radiation dataset. Bulletin of American Meteorological Society, 77, 1275-1277.
Matthews, 2004. Intraseasonnal variability over Africa during northern summer. Journal of
Climate, 17, 2427-2440.
Mounier, F., S. Janicot and G. Kiladis, 2005. The West African monsoon dynamics. Part IV: The
15-day “Guinean” mode and its interaction with Kelvin waves. Submitted to J. Climate.
Sultan B. & Janicot S., 2003. The West African monsoon dynamics. Part II: the “preonset” and
“onset” of the summer monsoon. Journal of Climate, 16, 3407-3427.
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INTERACTIVE ASPECTS OF THE INDIAN AND AFRICAN
SUMMER MONSOON SYSTEMS
P. Sanjeeva RAO (1) and D.R. SIKKA (2)
(1) Department of Science and Technology, New Delhi-110 016, India
(2) 40, Mausam Vihar, New Delhi-110 051, India
The Indian and African summer Monsoon systems interact with each other on sub-seasonal scale.
This study addresses to understand the mutual interactions of such sub-seasonal variability of the
two neighboring regional monsoon systems through data analysis.
The study uses the re-analysis and OLR (outgoing long wave radiation) data for the last five years
to reveal the large scale organization of convective episodes on synoptic (~5 days) and low
frequency (15-50 day) scales. It is found that synoptic scale organization over the Indian sector
influenced by the eastward migration of large scale convective episodes on the low frequency
scales.
The convective organization on the synoptic scale over the African Monsoon System is influenced
by the pulsatery character of lower mid-troposphere and upper troposphere wind regimes moving
westward over the African sector. The study points out for the need to organize simultaneous field
campaigns over the Indian and African Monsoon Systems so as to bring out observational features
of possible interactions between the two neighboring Monsoon Systems which could be validated
through modeling studies.
Further, major results of the two regional process studies carried out in India on the Bay of Bengal
Monsoon and the Arabian Sea Monsoon experiments carried out during 1999, 2002 and 2003
summer seasons are presented. The paper would also share the planning of a major field study on
the coupled land-atmosphere-ocean-biosphere system to be carried out during the monsoon seasons
during 2007-2010 as `Continental Tropical Convergence Zone (CTCZ)’ under the Indian Climate
Research Programme.
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UTILISATION DES DONNEES MSG DANS DES ETUDES DE CAS DE
PERTURBATIONS (POUSSIERE, ORAGE ET LIGNE DE GRAINS)
OBSERVEES A NIAMEY EN 2004
Garba ADAMOU (1), Issa Salifou El MAHAMAN (1), A.E. Nazaire ITOU (1),
Arona DIEDHIOU (2), Amadou Thierno GAYE (3) et Greg JENKINS (4)
(1) Ecole Africaine de la Météorologie et de l’Aviation Civile, Niamey, Niger
(2) IRD, Niamey, Niger (3) LPASF, ENSUT, Dakar Fann, Sénégal
(4) Howard University, Washingtown, USA
La mise en orbite du satellite MSG-1 le 28 Août 2003, rebaptisé METEOSAT-8 suivant l’ordre de
succession amorcé par MPG marque ainsi le début d’une ère nouvelle en météorologie. Sa mise en
orbite terminée, MSG-1 fournit actuellement neuf nouveaux produits et trois autres améliorés à des
intervalles de temps plus courts. Ce premier modèle d'une toute nouvelle série de satellites
météorologiques est destiné à fournir aux services météorologiques européens et des tiers (pays
africains et autres), des données de meilleure qualité à une fréquence plus élevée.
L’année 2004 qui coïncide avec la mise en place de la phase opérationnelle de MSG a été marquée
par l’occurrence de phénomènes météorologiques d’échelle planétaire. Plus localement on a observé
à Niamey des perturbations qui ont retenu l’attention par leurs particularités saisonnières et
structurelles et ont fait l’objet d’études de cas que nous présentons dans cet article.
Il s’agit d’une situation de brume de poussière observée du 2 au 6 Mars, de la ligne de grains du 29
Avril singularisée par son caractère très précoce de saison de pluie et la quantité de pluie générée.
Le troisième cas est un orage évoluant en complexe convectif de méso échelle.
La diversité des canaux de MSG nous a permis de bien cerné l’évolution spatio-temporelle de ces
perturbations.
Contact
ADAMOU Garba - EAMAC, BP 746, Niamey, Niger
Tel : 00227 93 54 33 - Mail : [email protected] [email protected]
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STATISTIQUES CONCERNANT LES TYPES DE PRECIPITATION ET DES
CONDITIONS DE CISAILLEMENT ET DE STABILITE DANS LA ZONE
HUMIDE SOUDANIENNE (HAUTE VALLEE DE L´OUEME, BENIN)
PENDANT LA CAMPAGNE DE RADIO-SONDAGE D´IMPETUS EN 2002
Les observations de surface et d´altitude récoltées durant le projet IMPETUS sont utilisées afin de
subdiviser les cumuls de pluie dans la zone soudanienne de l´Afrique de l´ouest pendant la période
du mousson de l´été en différents types et sous-types de systèmes de précipitation.
Le sous- type de précipitation le plus marquant etait composé d’amas nuageux extensif, qui se
déplaçaient vite et vivaient longtemps. Ils se formaient loin en amont au-dessus du plateau du
Nigeria central pendant l´après-midi et arrivaient sur la Haute-Vallée de l´Ouémé (HVO) après
minuit. Ces systèmes convectifs organisés (OCSs advectifs, sous-type Ia) représentaient 50 % des
cumuls totals de pluie sur l’HVO au Bénin en 2002.
On a trouvé que les sous-types Ia et IIa (OCSs se formant sur place) passent ou se forment quand un
environnement fortement cisaillé en présence de couches séches moyennes et profondes sur l’HVO.
Ces systèmes se formaient le plus fréquemment hors du pic de la saison de mousson. Le deuxième
type majeur de convection organisée, ici nommé système convectif de méso-échelle (sous-type Ib,
IIb et IIIb), contribuait pour 26% des précipitations annuelles sur l´HVO. Ils apparaissent dans un
environnement troposphérique moins cisaillé et plus humide, principalement au moment du pic de
la saison des pluies.
Un deuxième groupe de présence de pluies prononcées apparait pendant des situations synoptiques
exceptionnelles, pendant lesquelles un tourbillon cyclonique au nord de l’HVO, aboutissait à un
écoulement profond d´Ouest. Durant cette période, le jet africain d´est était absent. Ces types de
récipitation du type tourbillon (sous-types IIIa, IIIb, et IIIc) contribuent pour à peu près 9% des
précipitations annuelles.
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DECLENCHEMENT ET DUREE DE VIE D’UN EVENEMENT
CHASSE SABLE AU SAHEL
Ibrahima HAMZA
EAMAC
Situé au cœur des préoccupations lithométéores sur les horizons ouest africains, le phénomène
chasse sable demeure encore un problème d’actualité au niveau des spécialistes météo et autres
hommes de science de la région. Outre la question de son origine et ses impacts sur les troubles de
visibilité et la qualité de l’air, on s’interroge également sur la portée érosion du sol et dégradation de
l’environnement.
Les méthodes d’approche du phénomène varient d’un domaine d’activité à un autre, en fonction de
la spécialité de chacun. Il en est de même pour les objectifs visés. Les résultats attendus sont alors
subordonnés à la nature des données utilisées et aux moyens de traitement mis en œuvre.
Pour de nombreuses applications, les besoins exprimés en la matière sont limités à la connaissance
de la climatologie du phénomène sur la région. Mais, pour le cas particulier de la météorologie
opérationnelle l’impératif premier est de prévoir l’occurrence du phénomène à temps quasi réel.
La présente étude est consacrée pour l’essentiel au volet climatologique avec une classification des
occurrences de ces phénomènes basée principalement sur la dynamique de la composante
atmosphérique au cours des épisodes chasse sable (intensité, la durée en zone source, la forme et
l’étendu du nuage de brume associé).
Nous désignons ici par chasse sable, tout lithométéore résultat d’un soulèvement de particules de
sable ou de poussière du sol par le vent et leur mise en suspension dans l’atmosphère avant d’être
évacuées par les courants aériens. Les investigations ont été menées principalement sur les cartes
synoptiques de surface du centre météorologique principal de Niamey (Niger) de la période 2000 à
2004, et sur les carnets d’observation des stations d’Agadez et Bilma (nord Niger) de la période
1981 à 1988. Les résultats obtenus font ressortir les points suivants:
De la dimension spatiale du phénomène
Deux catégories de chasse sable se dessinent sur les cartes synoptiques de surface: les évènements
chasse sable isolés (très localisés) et des systèmes brumeux qui intéressent plusieurs sites
simultanément et sur des centaines de kilomètres. Ces derniers donnent naissance à des nuages de
brume qui couvrent les horizons ouest africains.
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De la durée du phénomène
Les évènements isolés ont une durée de vie très courte, généralement inférieure à 3 heures et
s’observent seulement en matinée. Les systèmes brumeux se rencontrent en toute heure de la
journée et peuvent durer plusieurs heures (voire plusieurs jours).
Des mécanismes du déclenchement
Les évènements isolés sont de 2 classes : les chasses sable d’évolution diurne du vent et ceux de
l’accélération dans les gorges de reliefs. Tous les deux se produisent au moment de la hausse de
pression matinale. On note qu’ils sont associés à une perturbation de l’onde barométrique diurne
locale. Quant aux systèmes brumeux, on les rencontre dans les perturbations synoptiques ou de
méso-échelle du champ de pression. Ce sont: les grains orageux associés à la convection profonde
en période de mousson, et les chasse sable et tempête de poussière associées aux évolutions rapides
des centres d’action de la région. Dans l’un ou l’autre cas, l’onde barométrique locale est fortement
perturbée.
Caractéristiques de l’environnement atmosphérique associé
En saison sèche, sur le plan synoptique, l’analyse des cartes de surface ne révèle aucun indice
d’identification d’événement chasse sable isolés. Par contre, le vent chasse sable associé aux
systèmes brumeux est un vent de gradient de pression: c’est le cas dans les renforcements des
dorsales maghrebo-libyennes, le passage des thalwegs extra tropicaux ou le creusement de
dépression sur le Sahara et le Sahel.
En saison humide, le domaine d’occurrence de grains orageux se situe sur le front de la cellule de
hausse du champ d’isallobares. Le vent chasse sable est associé à la dynamique de la cellule
orageuse appartenant au système convectif.
C’est là un aspect de la mousson africaine qui doit être pris en compte dans toute analyse
multidimensionnelle du phénomène.
A ce stade d’investigation, deux grandeurs atmosphériques semblent décrire l’occurrence d’un
événement chasse sable sur une localité: le gradient de pression et la perturbation de l’onde
barométrique diurne locale.
Contact
HAMZA Ibrahim : [email protected]
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VARIABILITY OF DRY AIR IN THE MID-TROPOSPHERE OVER THE LAST 20
YEARS: SATELLITE AND TRAJECTORY CLIMATOLOGIES
Rémy ROCA, Hélène BROGNIEZ and Laurence PICON
Laboratoire de Météorologie Dynamique, Paris, France
Over the northern Saharan and Mediterranean regions, the water vapour distribution in the midtroposphere has a profound impact on the radiative budget of the West African monsoon, while over
Sahel, humidity has been shown to play an important role in modulating the squall lines occurrences
through its influence on the convective inhibition and convection thermodynamics. This key
variable is documented using the METEOSAT observations as well as an ensemble of backtrajectories over the last 20 years.
First over the dry northern regions, the METEOSAT archive analysis reveals significant interannual
variability with free tropospheric humidity ranging from 5% to 10%. A detailed investigation of this
interannual signal using backtrajectories technique highlights the important role of the extra-tropical
dry air that mix with tropical air over this region. The relative fraction of extra-tropical air explains
the interannual variability. Implications for our understanding of the radiative sinks of the monsoon
will be discussed.
Second over Sahel, the long term climatology of dry air intrusions will be presented. Based on
backtrajectories, the analysis shows a strong variability of the origin of mid-tropospheric dry air.
For instance, the 1992 season appears as the most commonly associated with these extratropical
intrusions. Some other years hardly any dry air intrusions are observed. The relationship between
the frequency of occurrence of these events and their impact on the precipitation will be discussed.
The variability of the intrusions will also be related to the large scale tropical variability index of El
Nino.
Finally a brief discussion oriented towards the forecast of the water vapour distribution will be
given.
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RECHERCHES SUR LES FACTEURS INFLUENÇANT LA CYCLOGENESE
ATLANTIQUE A PROXIMITE DES COTES AFRICAINES
Saïdou Moustapha SALL (1) et Henri SAUVAGEOT (2)
(1) Laboratoire de Physique de l'Atmosphère Siméon Fongang, UCAD, Dakar, Sénégal
(2) Laboratoire d'Aérologie, OMP, Toulouse, France
Les systèmes convectifs qui naissent sur les flancs du Fouta Djallon, les plateaux du Niger et du
Nigeria ou encore plus à l’Est sur le Soudan évoluent souvent jusqu’au Sénégal. Au cours de leur
déplacement d’est en ouest, ces systèmes subissent d’importantes modifications et ont souvent
évolués sous forme de lignes de grains. A la fin de leur parcours continental, certaines perturbations
s’affaiblissent et se dissipent sur l’océan non loin de la côte, alors que d’autres se renforcent. Ces
dernières semblent jouer un rôle déterminant dans la genèse des cyclones tropicaux. Sur les
transformations possibles de ces systèmes convectifs au niveau de l’interface terre– mer nous nous
sommes posé deux questions fondamentales : quels sont les processus qui influencent sur le
renforcement ou le non renforcement sur le bassin Atlantique des systèmes convectifs d’origine
africaine ? Peut-on à partir d’un indice d’instabilité, prévoir à l’échelle journalière le phénomène de
cyclogénèse qui peut être une conséquence du renforcement d’un système convectif sur l’océan ?
C’est la recherche de réponses à ces questions qui a fait l’objet de notre travail.
Pour répondre aux questions précédentes, nous avons exploité les données du radar de Dakar, les
réanalyses du NCEP, le rayonnement ondes - longues sortant au sommet de l’atmosphère, les
images du satellite Météosat et les ressources de la base de données du National Hurrican Center.
L’étude du comportement moyen de paramètres météorologiques significatifs lors de la traversée
des côtes africaines des systèmes convectifs montre l’importance du flux de mousson dans le
renforcement des systèmes convectifs et l’affaiblissement des flux de moyennes et hautes couches
(AEJ et TEJ) dans le cas de renforcement conduisant à un cisaillement vertical du vent zonal plus
faible.
Nous avons observé l’évolution des systèmes convectifs en phase de renforcement en dehors du
champ radar dans le but d’analyser dans quelle mesure ils sont liés à la cyclogénèse sur
l’Atlantique. Les résultats ont montré que 62.5 % des systèmes étudiés se sont transformés en
cyclones tropicaux. On a noté également la présence de perturbations cycloniques dont la trace de
nébulosité n’est pas reliée à une convection d’origine africaine. L’arrivée des perturbations telles
que les ondes d’est pourrait expliquer le déclenchement de ces cyclones. Cependant nous avons
observé que d’autres dépressions tropicales peuvent naître à partir de l’éclatement sur l’océan de
certains systèmes en phase de renforcement sur la côte.
La coïncidence entre une onde d’est et un système convectif sur l’Atlantique non loin des côtes
africaines, où une importante quantité d’eau précipitable est observée, dans l’atmosphère, une forte
convergence dans les basses et moyennes couches et un faible cisaillement vertical du vent zonal
ont été à l’origine de la bonne organisation de la perturbation qui est devenu plus tard le cyclone
tropical dénommé « Cindy », d’une violence jamais observée dans cette zone (Sall and Sauvageot,
2005).
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Nous avons construit un indice pour mieux comprendre les variations à l’échelle journalière de ces
perturbations d’origine africaine et pour prévoir le phénomène de cyclogénèse sur l’Océan
Atlantique (Sall et al., 2005). Les composantes de cet indice ont été calculées à partir de l’humidité
dans les couches moyennes, le cisaillement vertical du vent, la température potentielle équivalente
et le réchauffement dans les couches supérieures, le tourbillon vertical et le tourbillon potentiel.
Nous avons observé une assez bonne correspondance entre le maximum de l’indice de cyclogénèse
et le début de quelques dépressions tropicales à proximité des côtes africaines. Nous avons montré
la capacité de l’indice à déterminer la présence simultanée de deux perturbations cycloniques sur le
bassin Atlantique. Enfin on a trouvé une bonne concordance entre la variation spatio-temporelle de
cet indice et celle de la nébulosité durant des périodes caractérisées par la présence de perturbations
cycloniques. Cela a permit de mettre en évidence la trace de ces systèmes avec comme seule
variable l’indice de cyclogénèse.
Bibliographie
SALL, S. M., and H. SAUVAGEOT, H., 2005: Cyclogenesis of the African Coast: the case of Cindy in August
1999. Mon. Wea. Rev., 133, 2803-2813.
SALL, S. M., SAUVAGEOT, H., GAYE, A., VILTARD, A. and de FELICE, P., 2005: A Cyclogenesis Index for
Tropical Atlantic off the African Coasts, accepted in Atmos. Research.
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THREE MCS CASES OCCURRING IN DIFFERENT SYNOPTIC
ENVIRONMENTS IN THE SUDANESE WET ZONE (UPPER OUÉMÉ VALLEY,
BENIN) DURING THE 2002 IMPETUS
UPPER-AIR CAMPAIGN
J. M. SCHRAGE (1), Andreas H. FINK (2) , Volker ERMERT (2)
and Epiphane D. AHLONSOU (3)
(1) Dept. of Environmental and Atmospheric Sciences, Creighton Univ., Omaha, NE, USA
(2) Institute of Geophysics and Meteorology, University of Cologne, Cologne, Germany
(3) National Meteorological Service of Benin, Cotonou, Benin
1. Introduction
Tropical West Africa experiences several different precipitation regimes during the monsoon
season. While a wide variety of mesoscale convective systems (MCSs) are important in the subSahelian wet zone, highly organized squall line systems are perhaps the most extensively studied.
The more southerly moist region is clearly characterized by a broader variety of MCSs with varying
degrees of apparent organization. Using ECMWF operational analyses, METEOSAT infrared
imagery, and experimental data gathered during the IMPETUS field experiment, the synoptic
structure and evolution of three examples of MCSs in the sub-Sahelian latitudes of Benin are
examined in this study (Fig. 1). Only a few characteristics of each system are presented here.
2. Case I (14/15 September 2002)
Case I was a West African squall line that developed over the central Nigerian highlands. High
resolution surface observations showed that after the passage of the gust front, a period of heavy
convective rain was followed by a brief period of light rainfall and a prolonged period of moderate
rain—all characteristic of squall lines. In the stratiform region, temperatures at the surface dropped
due to subsaturated conditions and the evaporation of precipitation.
At 700mb, a wake vortex developed behind the squall line in the ECMWF analyses. The upper-air
observations collected at Parakou, Benin during the IMPETUS project were supplied in real-time to
ECMWF and were assimilated into the operational analyses. Detailed examination of the records of
the ECMWF showed that the special observations clearly improved the representation of such
structures in the analyses.
3. Case II (18 September 2002)
Case II was also a West African squall line that was initiated over the highlands of central Nigeria.
While Case I experienced an atmosphere that was, at best, only moderately unstable, with
insufficiently dry conditions in the lower and middle troposphere. In contrast, Case II propagated
across the UOV in an environment that was considerably less stable and more conducive to
production of strong downdrafts and robust outflow due to dry conditions at 700 hPa. Consistent
with the local environmental conditions, Case II was faster and exhibited stronger wind gusts and
mesohighs.
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4. Case III (29/30 July 2002)
Precipitation was considerably less widespread during Case III, with only about 75% of the rain
gage stations reporting precipitation. However, Case III had the highest observed rainfall totals of
any of the three cases, with an average rainfall of 33.8 mm in the Oueme Valley of Benin.
Case III was clearly not a squall line in the classic sense and occurred in a different kinematic and
thermodynamic environment. Rather, Case III was found to be associated with a low-level vortex
to the north of the observation region.
5. Conclusions
An analysis of three MCSs that occurred in the summer of 2002 has shed light on the range of
synoptic environments in which these systems can occur. The ECMWF analyses, especially when
augmented by special observations, yielded important clues to the structure and evolution of these
systems. The upcoming AMMA field campaign will further enhance the undestanding of these
features.
Figure 1: METEOSAT infrared imagery every three hours from 0°N to 15°N for (a) Case I, (b)
Case II and (c) Case III. At times when the 00 UTC image was not available, it has been replaced
by the 23 UTC image. Cases I, II and III are highlighted, as are the longitudes of the UOV domain.
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TROIS CAS DE MCS APPARAISSANT DANS DES SITUATIONS
SYNOPTIQUES DIFFERENTES DANS LA ZONE HUMIDE SOUDANAISE
(HAUTE VALLEE DE L´OUEME) PENDANT LA CAMPAGNE DE
RADIOSONDAGE 2002 D´ IMPETUS
Trois systèmes convectifs de méso-échelle, apparaissant dans la zone humide sous-sahéliene de
l´Afrique de l´Ouest sont examinés en utilisant des observations obtenues pendant la phase terrain
d’IMPETUS en 2002 (Approche Intégrée pour la Gestion Efficiente des Ressources Hydriques en
Afrique de l´Ouest) ainsi que des analyses opérationnelles ECMWF et des images infrarouges
METEOSAT.
Cet ensemble de données permet l´analyse de l´environnement à l´échelle synoptique où les MCS
sont inclus, combinée avec des observations à haute résolution des paramètres de surface pendant le
passage de ces systèmes.
Deux cas s´avèrent être des lignes de grains s´étendant vite vers l´ouest à travers la Haute Vallée de
l´Ouémé au Bénin central (HVO). Le cas I s´étendait vers une troposphère basse et moyenne plutôt
humide et faiblement cisaillée au dessus du HVO. L’intensité de l´instabilité convective était
modeste. Au contraire, le cas II était lié à une basse troposphère assez cisaillée avec instabilité forte
et humide. Ce cas était également favorable à la production de
fort courants descendants du fait de la présence d’air sec. Des changements plus forts dans les
paramètres de surface sont observés. Dans les deux cas, un maximum d´un tourbillon cyclonique
bougeait vers l´ouest à la suite d’un amas convectif. Le premier cas semblait interagir à l´aval avec
la formation d´un thalweg d´une onde est africaine.
Le troisième cas s´avère d´être une configuration de convection plus stationnaire combiné avec un
tourbillon dans l´écoulement de la mousson.
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IMPACT RELATIF DES CONDITIONS DE SURFACE OCEANIQUE ET DU
SIGNAL ANTHROPIQUE SUR LA MOUSSON EN AFRIQUE DE L’OUEST
EME
SIECLE
PENDANT LE 20
C. CAMINADE et L. TERRAY
CERFACS, Toulouse, France
Certains travaux ont montré l’existence de relations robustes entre des modes de température de
surface océanique (TSO) et les précipitations en Afrique de l’Ouest. En particulier, de nombreuses
études ont apporté la preuve que les modèles atmosphériques étaient capables de restituer la
tendance en précipitation sur le Sahel (contrastant une période humide de 1950 à 1970 avec une
sécheresse forte pendant les années 70 et 80) lorsque le seul forçage appliqué est celui des TSO
observées. A l’inverse, la question de l’impact des émissions en gaz à effet de serre (GES) et
aérosols sulfatés sur le système de mousson ouest africain reste encore ouverte. Dans cette étude,
nous nous focaliserons sur les impacts relatifs des modes de TSO d’une part, et des émissions
anthropiques (GES et sulfates) d’autre part, sur la mousson ouest africaine sur la période 19501999. Des simulations numériques réalisées avec le modèle de climat ARPEGE de Météo France
sont utilisées pour caractériser la réponse atmosphérique aux différents types de forçages externes
considérés (TSO / GES+sulfates). Deux ensembles de simulations sont réalisés. Le premier
ensemble (référencé par GOGA) est forcé par les TSO observées sur la période 1950-1999. Le
second ensemble (GOGA+GES) inclut l’effet additionnel des GES et des sulfates. En premier lieu,
l’influence des différents bassins océaniques (et les mécanismes physiques associés) sur la
variabilité des précipitations ouest africaine est étudiée à différentes échelles de temps
(interannuelle, décennale à multi décennale)
Une comparaison entre la variabilité des précipitations sahéliennes (10°N-20°N, 15°W-45°E )
observées (données CRU2) et simulées (GOGA) est faite sur la figure 1. Le modèle reproduit la
tendance négative des précipitations observées sur le sahel en dépit d’une nette sous-estimation. Les
corrélations spatiales entre les TSO observées et l’index de pluie sur le sahel aux échelles de temps
interannuelle (HF) et décennale (BF) sont montrées sur la figure 2. L’influence forte du phénomène
El Niño sur la variabilité interannuelle des précipitations sur le Sahel est reproduite par la moyenne
d’ensemble du modèle (Fig. 2, colonne de gauche). L’impact de la composante océanique basse
fréquence sur la tendance simulée des pluies sahéliennes (caractérisé par des corrélations fortes sur
le bassin Sud Atlantique et l’Indien) est très similaire à celle observée.
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Fig. 1: Anomalies de précipitations normalisées (pointillés) et brutes (trait plein) sur le Sahel pour
les observations (CRU2, rouge) et la moyenne d’ensemble du modèle (bleu) pendant Juillet, Août,
Septembre. L’enveloppe bleue montre la déviation maximum-minimum dans les 10 simulations
considérées.
Fig. 2: Gauche : Corrélations entre les TSO observées (jeux de données Reynolds 2) et l’index de
précipitations sahélien observé (haut) et simulé (bas) filtrés haute fréquence (filtre passe haut avec
une coupure à 8ans). Droite : idem mais les données sont filtrées basse fréquence (filtre basse bas
avec une coupure à 8 ans). Les régions colorées sont significatives au seuil de 95% suivant un t-test
de Student.
L’effet additionnel d’un forçage anthropique (GOGA+GHG) ne modifie pas de manière
significative les liens existants entre les différents bassins océaniques et la variabilité des
précipitations sur le Sahel (quelle que soit l’échelle de temps considérée).
D’autre part, l’impact relatif des différents forçages externes (TSO / GES) par rapport à la
variabilité interne de l’atmosphère sur le système de mousson ouest africaine est quantifié au moyen
d’une analyse en variance appliquée aux deux ensembles. Les résultats confirment le rôle
prépondérant des TSO et un impact relativement faible du signal anthropique (forçage direct) sur la
variabilité des précipitations ouest africaine.
Contact
Email: [email protected] - [email protected] - Tel : 05.61.19.30.40
251
RELATIVE IMPACT OF OBSERVED OCEANIC VERSUS ANTHROPOGENIC
EXTERNAL FORCING UPON THE WEST AFRICAN MONSOON DURING THE
20TH CENTURY
At different time scales, previous studies highlight robust links between natural modes of variability
of global sea surface temperatures (SST) and precipitation over West Africa. In particular,
numerous works provide evidence that the atmospheric models can reproduce the observed sahelian
rainfall trend (contrasting wet conditions during the 1950s and early 60s followed by a marked
drought over the 1970-1990 period) when forced only with observed SST. By contrast, the
influence of anthropogenic greenhouse gases (GHG) and sulfate aerosols emissions upon the West
African Monsoon (WAM) still remains an open question. In this study, we focus on the relative
impact of observed SST modes versus anthropogenic (GHG and sulfate aerosols) external forcing
on the WAM system over the 1950-1999 period. Numerical experiments using the Météo-France
ARPEGE Global Climate Model are used to characterize the atmospheric response to the different
external forcing signals. Two ensemble simulations are performed. The first ensemble hereafter
referred as GOGA is forced by observed SST over the second part of the 20th century. A second
ensemble named GOGA+GHG includes the additional effect of observed GHG and sulfate aerosols
time evolution. As a first step, the influence of different oceanic basins upon West African rainfall
and the associated physical mechanisms are investigated at different time-scales. Figure 1 compares
rainfall long-term variability in the sahelian region (10°N-20°N, 15°W-45°E) in observations
(CRU2 data set) and in the GOGA configuration. The model is able to reproduce the rainfall
negative trend despite a clear underestimation compared to observations. The associated SST
correlation patterns at interannual (HF) and decadal to multi-decadal (LF) time scales are shown in
Fig 2. The influence of the El Niño Southern Oscillation on sahelian precipitation variability at
interannual time scale is well reproduced in the model ensemble mean (Fig 2, left panel). The
impact of the slower (LF) oceanic components variability (with strong loadings in the South
Atlantic and Indian basin) in forcing the sahelian rainfall trend is present in the model (Fig 2 right
panel). This latter result is consistent with the earlier finding on the main role played by the SST
forcing upon the sahelian rainfall trend.
Fig 1: Standardized (dashed line) and raw (solid) recipitation anomalies over the Sahel during JulyAugust September (JAS) for CRU2 observation (red) and the model ensemble mean (blue). The
blue colored area depicts raw aximum to minimum deviation in the 10 simulations
252
Fig 2: Left column: correlations between high pass filtered (with an 8-year cut-off) observed SST
(Reynolds 2 data set) and sahelian rainfall index (upper view: CRU observation, lower view: model
ensemble mean). Right column: idem for low-frequency components (low pass filtered with an
eight year cut-off). Shading depicts areas significant at the 95% level as estimated by a Student ttest.
The additional effect of anthropogenic external forcing does not significantly modify the linear link
between sahelian precipitation and the oceanic basins variability at different time scales (not shown
here).
An attempt is done to quantify the relative impact of the different external forcing versus the
internal variability of the atmospheric signal (noise) using analysis of variance (1 and 2 way)
methods (applied to the two ensembles). We suggest the predominant role of SST forcing upon the
sahelian rainfall variability, and a weak non-significant impact of anthropogenic direct forcing in
the late period.
Contact
Email: [email protected] - [email protected] - Tel : 05.61.19.30.40
253
1.45P
IMPACT OF GLOBAL WARMING ON THE WEST AFRICAN MONSOON :
PRELIMINARY ASSESSMENT OF THE IPCC4 CLIMATE SCENARIOS
HERVE DOUVILLE, AURORE VOLDOIRE AND MATHIEU JOLY
Météo-France/CNRM, 42 av. Coriolis, 31057 Toulouse Cedex, France
Recent climate scenarios produced for the fourth assessment report of the Intergovernmental Panel
on Climate Change (IPCC4) have been analysed to evaluate both their ability to simulate the 20th
century West African Monsoon climate and their uncertainties in the projected monsoon response to
global warming. Using the results of eleven coupled ocean-atmosphere models, it is shown that the
main uncertainties in the response of the global hydrological cycle originate from the tropics, where
even the sign of precipitation change remains uncertain. West Africa appears as a critical area
where the model climatologies show serious deficiencies and where the anticipated climate change
is particularly uncertain (Figure 1).
Given the large interannual fluctuations of tropical precipitation, Douville et al. (2005) suggested
that this variability can be used as a surrogate of climate change to better constrain the hydrological
reponse in the IPCC4 climate projections. While the simulated sensitivity of global land
precipitation to global mean surface temperature indeed shows a remarkable similarity between the
interannual and climate change timescales respectively, the model ability to capture the ENSOprecipitation teleconnections remains a limited constraint on the IPCC4 climate projections. Only
the models that exhibit the highest precipitation sensitivity clearly appear as outliers. Besides
deficiencies in the simulation of the ENSO-tropical rainfall teleconnections, uncertainties in the 21st
century evolution of these teleconnections also represent an important contribution to the model
spread, thus emphasizing the need for improving the simulation of the tropical Pacific variability to
provide more reliable scenarios of the global hydrological cycle. This last remark is reinforced by a
CCA analysis applied on summer precipitation over West Africa and simultaneous or lagged
tropical sea surface temperatures (Joly et al. in this proceedings report).
Moreover, Douville (2005) argued that the strong natural variability of West African precipitation is
a major obstacle for the regional detection of anthropogenic climate change based on surface
thermal indicators. This issue is here illustrated over Sudan and Sahel using the IPCC4 20th century
simulations of the coupled models shown in Figure 1. Like in situ observations, models show a
strong relationship between detrended seasonal anomalies of surface air temperature and
precipitation during the summer monsoon season. As a result, linear regressions can be used to
remove the precipitation influence on observed and simulated linear trends in surface air
temperature. This strategy is very efficient to reduce the spread in the simulated surface warming
and make it more consistent with the instrumental record (Figure 2).
References
- Douville H., D. Salas-Mélia, S. Tyteca (2005) On the tropical origin of uncertainties in the global land precipitation
response to global warming. Climate Dyn. (in press)
- Douville H. (2005) Detection-attribution of global warming at the regional scale: How to deal with precipitation
variability. Geophys. Res. Letters (submitted)
- Joly et al. (2005) African monsoon in IPCC simulations : modes of variability and climate change. Proceedings of
the First AMMA International Conference.
254
.
Figure 1: Summer (June to September) precipitation change (2071-2100 minus 19712000 in mm/day) simulated in the IPCC4 SRES-A2 scenarios from eleven coupled
ocean-atmosphere models. Note that the multi-decadal variability (1976-2000 minus
1951-1975) of precipitation observations is also shown, but should not be necessarily
interpreted as a climate change signal. Also shown are the present-day (1971-2000)
summer precipitation distributions for both models and observations (black contours
for the 2, 4, 6 and 8 mm/day isolines respectively). Note the model difficulty for
simulating the late 20th century monsoon precipitation, as well as the multi-model
spread in the late 21st century monsoon projections.
255
Figure 2: a) Raw and corrected linear trends in surface air temperature (in °C/century)
averaged over Sudan and Sahel for increasing time intervals with a minimum of 30
years (1971-2000) and a maximum of 100 years (1900-2000). Black lines denote the
observed trends, while the color lines correspond to three replications of the NCAR
20th century IPCC4 simulations. b) Same as a), but for a multi-model ensemble of
IPCC4 simulations. Besides the ensemble mean trends (thick red line), the spread
among the models is illustrated though the minimum and maximum values (thin solid
red line) as well as plus/minus one standard deviation (thin dashed red line).
256
1.46P
SAHEL RAINFALL VARIABILITY AND RESPONSE
TO GREENHOUSE WARMING
Reindert J. HAARSMA, Frank M. SELTEN, Suzanne L. WEBER
and Michael KLIPHUIS
Royal Netherlands Meterological Institute, De Bilt, The Netherlands
Historical Sahel rainfall variability
To investigate the historical Sahel rainfall variability we used the NCAR atmospheric GCM CCM3
in T31L18 resolution. Using observed SSTs from 1945 in an ensemble of 10 members we were able
to simulate the historical decadal variations in the Sahel including the drought during the 70-90s,
although the modeled decline in rainfall is less than observed.
For these SST-forced experiments an analysis of the covariability of rainfall and mean sea level
pressure (MSLP) over northern Africa during JAS, by means of a singular value decomposition
(SVD) analysis, reveals that anomalous large Sahel rainfall is related to an anomalous low centered
over the northeast Sahara (referred to as Sahara Low). The southern meridional pressure gradient of
the Sahara Low induces, in case of a negative phase, enhanced moisture advection from the
southwest thereby increasing the Sahel rainfall. We checked these results for the NCEP/NCAR
reanalysis and the CMAP rainfall data. A similar SVD analysis confirms the relation between the
MSLP distribution over the northeastern part of the Sahara and the Sahel rainfall.
To answer the question what determines the variability of the Sahara Low we made a regression
analysis of the amplitude of the Sahara Low with global surface air temperatures (SAT). This
regression analysis indicates that on interannual time scales a deeper Sahara Low is related to
warmer temperatures over the Sahara and the Eurasian continent. On decadal time scales the
regression analysis shows a significant contribution from the Indian and South Atlantic ocean SSTs.
The Northern Hemisphere continental heating still remains significantly correlated with the Sahara
Low
Anthropogenic changes
For studying the anthropogenic changes we used the NCAR CSM 1.4 model, where CCM3 is
coupled to an ocean model, which has 25 vertical levels and 3.6o longitudinal resolution. The
latitudinal resolution ranges from 0.9o in the tropics to 1.8o at higher latitudes. No artificial
corrections in the heat exchange between atmosphere and ocean are applied.
The simulation period is 1940-2080. Until 2000 the forcing includes temporally evolving estimates
of solar radiation, volcanic aerosols and major greenhouse gases (GHGs). From 2000 onwards all
these forcings are kept constant, except for the GHGs, which increase according to a "business-asusual" (BAU) scenario, similar to the SRES-A1 scenario. An ensemble of 62 simulations was
produced, each differing in a small random perturbation to the initial atmospheric temperature field.
The SVD analysis of Sahel rainfall and MSLP of the coupled integrations for the control period
from 1940-2000 yielded very similar patterns as obtained in the SST forced runs. Over the tropics
257
and subtropics the ensemble mean anthropogenic warming at the end of the integration period (Fig.
1a) is less over the oceans compared to the continents.
Due to the heating over the Sahara we expect a decrease in MSLP over the Sahara and a subsequent
increase in Sahel rainfall, similar as for the historical Sahel rainfall variations. Figs. 1b show that
indeed there is a decrease in MSLP of about 0.5 hPa over the Sahara and an increase in the order of
1-2 mm day-1 (25-50%) in Sahel rainfall during JAS. The pattern of anthropogenic change in Sahel
rainfall is very similar to the dominant pattern of interannual Sahel rainfall variability.
To test the hypothesis that an enhanced heating of the Sahara deepens the Sahara Low, thereby
increasing the Sahel rainfall due to the increased moisture transport into the Sahel region, we
performed an additional experiment. We compared a 30-year control integration of the atmospheric
model with prescribed SSTs computed from the coupled integration for the period 1960-1960, with
a similar integration in which we applied an enhanced heating of 4 Wm-2 over the Sahara. Due to
this heating the temperature over the Sahara increases about 1oC and Fig.1c shows that for JAS this
indeed causes an anomalous low over the Sahara and an anomalous rainfall pattern that is very
similar to the anthropogenic pattern in the coupled model (Fig. 1b).
Conclusion
Due to the enhanced heating of the Sahara in case of greenhouse warming we expect an increase in
Sahel rainfall between 1980 and 2080 of about 1-2 mm/day (25-50%) due to a deepening of the
Sahara Low.
Figure 1. Coupled integrations: a) Difference
in SAT [K] during JAS between 2050-2080
and 1950-1980. For SAT the global average
mean between 50oS and 50oN is subtracted
from it. b) Difference in rainfall [mm day-1]
(shaded) and MSLP [hPa] (contours) during
JAS between 2050-2080 and 1950-1980. c) As
b but now for the difference between the run
with the Sahara heating and the control run
(see text). The rainfall and MSLP have been
reduced by a factor 2 to facilitate comparison
with b.
This has been confirmed using an ensemble of
62 coupled model runs forced with a business
as usual scenario. The ensemble mean increase
-1
in Sahel rainfall between 1980 and 2080 is about 1-2 mm day (25-50%) during July-September,
thereby strongly reducing the probability of prolonged droughts.
References
Haarsma, R.J., F.M. Selten, S.L. Weber and M. Kliphuis, 2005: Sahel rainfall variability and response to greenhouse
warming. Geophys. Res. Lett., 32, L17702, doi:10.1029/2005GL023232.
258
1.47P
AFRICAN MONSOON IN IPCC SIMULATIONS : MODES OF VARIABILITY
AND CLIMATE CHANGE
Mathieu JOLY, Aurore VOLDOIRE and Hervé DOUVILLE
Météo-France, CNRM/GMGEC/UDC, Toulouse, France
Number of studies have stressed the influence of Sea Surface Temperatures (SSTs) on the summer
West African monsoon (WAM) variability. In particular, atmospheric processes linking sahelian
rainfall with ocean basins have been well documented. However, as coupled ocean-atmosphere
models are powerful tools for climate modeling, we can wonder if the observed teleconnections can
be realistically simulated by coupled models. Here teleconnections are assessed with a Maximum
Covariance Analysis (MCA) over the period 1901-2000. Often called Singular Value
Decomposition (SVD), MCA is generally used in climate research in order to identify pairs of
coupled spatial patterns which explain as much as possible the covariance between two variables.
In this study MCA is applied to Sahel rainfall and inter-tropical SST anomalies, with prior
subtraction of a 3rd degree polynomial in order to isolate inter-annual variability. Significance of the
results has been tested on the basis of a moving blocks bootstrap approach (Wilks 1997), and
heterogeneous correlations have been systematically compared to mean and standard-deviation
maps, as well as separate Principal Component Analysis (PCA) of each field, and a VARIMAX
rotation of the 10 first singular vectors (Cheng and Dunkerton 1995). Applied to observed data, the
MCA reveals two important modes of co-variability between monsoon precipitation, and SSTs:
259
Mode 1: SCF=60%
Var=17%
Var=30%
Mode 2: SCF=21%
Var=14%
Var=6%
Figure: MCA heterogeneous vectors (isolignes) coloured where significance level 95% is
reached. SCF=Squared Covariance Fraction & Var=explained Variance
•
The first coupled mode statistically links the whole WAM region variability with the eastern
Pacific SSTs, and in a weaker way with the Indian ocean (cf. fig.). This confirms the strong
relationship between El Niño events and the sahelian droughts. The first precipitation Expansion
Coefficient (EC1) shows a strong decadal signal (confirmed by a spectral analysis), that
corresponds to the well-known trend of sahelian rainfall during the second half of the century. Such
a strong decadal variability seams absent of SST ECs. However if we proceed to a MCA after prior
filtering of high-frequencies in the observed datasets, it yields a strong first coupled mode with two
SST patterns: the first one located in the Indian ocean is anti-correlated to the precipitation EC1,
and the other is situated in the north Pacific basin with stronger positive correlations. A possible
impact of Indian Ocean at multi-decadal time scales has been evoked in a number of papers, but a
possible influence of the north Pacific ocean would need further examination, that is out of the
scope of this study.
•
The second mode deals with a likely influence of the Guinea Gulf on the monsoon (cf. fig.).
The precipitation pattern reveals that warmer than average SSTs in the Guinea Gulf is strongly
correlated with higher than average precipitation along the coast and lower precipitation over the
Sahel. Running correlations between principal components confirmed that whereas ENSO
teleconnection got stronger at the end of the century, Guinea Gulf linkage dominated during the wet
period (Janicot et al. 2001).
260
But the most concerning issue of this study is that - applied to IPCC4 simulations of the 20th
century for 12 coupled models - the MCA gives very heterogeneous results.
•
Two models do not have any consistent teleconnexion in their 20th century simulation. In
one case, the model shows an obvious lack in SST variability, especially in key areas such as the
ENSO region. In the other case, SST variability is weak but more realistic, however there is no
significant heterogeneous correlations, and the SCF accounted by the first coupled mode is not
significant at the level 95% (moving block bootstrap test).
•
Five models exhibit an overwhelming first coupled mode, that explains more than 80% of
the SCF. This feature is always associated with stronger than usual SST explained variance for this
first mode, and in two cases precipitation explained variance is quite high too. Although
precipitation modes are completely different in those four simulations, SST modes look quite alike,
with a strong El Niño pattern in the Pacific. There is an exception: one model shows a spurious
ENSO pattern displaced on the west of the Pacific basin. Note that in four cases, the strong signals
in the equatorial Pacific are realistically correlated with the Indian basin, but this corresponds also
to a synchronous signal in the Atlantic, that is not present with the observed datasets.
•
In the five remaining simulations, two coupled modes are likely to coexist, as with the
observations. The first mode is here always related to the Pacific basin, but only one model shows a
realistic ENSO pattern. The others present an extremum in the west of the basin, which seams a
common feature of five of the models we studied. Looking at the second mode, it appears that four
simulations exhibit a Guinea Gulf pattern, often with a dipole-like counterpart above the equator.
Yet the associated precipitation pattern is almost always unreadable.
We also noted that within this subset of simulations, and in the context of this study, the resolution
of the models had no obvious impact on the realism of the simulated teleconnexions. Besides,
despite generally consistent SST signals, precipitation patterns revealed by the MCA seamed
sometimes quite spurious. Finally, spectral analysis of calculated ECs showed that none of the
models simulated multi-decadal variations comparable to that of the observed sahelian precipitation
over the last century. Next step should be to analyse the evolution of those teleconnections under
greenhouse gases scenarios, but the present validation study indicates that this should be assessed
with great care.
References :
Cheng, X. and T. J. Dunkerton (1995). Orthogonal rotation of spatial patterns derived from Singular Value
Decomposition analysis. Journal of Climate, 8, 2631-2634.
Janicot, S., S. Trzaska and I. Poccard (2001). Summer Sahel-ENSO teleconnection and decadal time scale SST
variations. Climate Dynamics,18, 303-320.
Wilks, D. S. (1997). Resampling hypothesis tests for autocorrelated fields. Journal of Climate,10, 65-82.
Contact :
Mathieu Joly : [email protected]
261
1.48P
ETUDE DES OCCURRENCES DES EXTREMES DES PRECIPITATIONS
JOURNALIERES DANS LA REGION DE L’AFRIQUE CENTRALE, DE LEUR
TENDANCE ET DE LEUR VARIABILITE SOUS LE CLIMAT PRESENT OU
PERTURBE, A L’AIDE DES DONNEES D’OBSERVATIONS ET DES
SIMULATIONS D’UN MODELE GLOBAL A MAILLES VARIABLES
(ARPEGE-CLIMAT)
Guy Merlin NGUENANG et François Mkankam KAMGA
LAMEPA, Université de Yaoundé, Cameroun
En plus de la grande variabilité climatique observée depuis quelques décennies en Afrique et ayant
entraîné notamment de grandes sécheresses, il est de plus en plus admis que les concentrations de
plus en plus élevées des gaz à effet de serre dans l’atmosphère va entraîner des changements
climatiques à l’échéance de quelques décennies. La détection et l’attribution des changements dit
anthropiques nécessitent une bonne connaissance du climat présent et la comparaison au climat
perturbé.
Or de nombreux auteurs pensent que les changements climatiques seront davantage perçus à travers
l’intensification des phénomènes extrêmes (sécheresse, inondation, orages). Nous présentons une
étude des occurrences des extrêmes annuels des précipitations journalières au Cameroun et les
e
comparons aux simulations de Arpège-climat. D’autres indices tels que les 90 percentiles sont aussi
analyses. Il en ressort que comme beaucoup de modèles climatiques, dans cette expérience, Arpège
a sous-estimé largement les pluies extrêmes.
262
1.49P
OVERVIEW ON THE MODELLING STUDIES IN THE GERMAN GLOWAIMPETUS PROJECT, WHICH DEALS WITH FRESHWATER MANAGEMENT
IN DIFFERENT REGIONS OF WEST AFRICA
Heiko PAETH
Meteorologisches Institut der Universitaet Bonn Auf dem Huegel, Germany
This presentation gives an overview on the modelling studies in the German GLOWA-IMPETUS
project, which deals with freshwater management in different regions of West Africa. A large
quantity of global and regional climate model simulations is taken into account in order to evaluate
the relative importance of natural and anthropogenic factors in the climate system. The regional
climate model performs well over the African continent. The overall aim is to provide a fairly
realistic prediction of future African climate conditions under enhanced radiative forcing and
ongoing land degradation. Different global climate models do not draw a homogeneous picture of
the response of African precipitation to an enhanced greenhouse effect but most models exhibit an
intensification of the West African monsoon. A variety of sensitivity studies with the regional
climate model highlights the outstanding role of land degradation in slowing down the hydrological
cycle over tropical Africa. This is partly compensated partly amplified by increasing greenhousegas concentrations, which mainly act through oceanic heating in the low latitudes.
Atmospheric aerosols over Africa, primarily from biomass burning, may cause a further reduction
in rainfall in the equatorial region. At the same time, tropical Africa may warm up by partly more
than 2K within less than 25 years due to changes in the energy fluxes from land surface. There is
evidence that land use changes may even be a more relevant player in near-future African climate
change than enhanced radiative forcing and, hence, need to be taken into account in climate change
scenarios for Africa.
Submitted by :
Heiko Paeth - Meteorologisches Institut der Universitaet Bonn Auf dem Huegel 20 D-53121 Bonn,
Germany - Tel. : (49) (0)228 / 73 51 86 Fax : (49) (0)228 / 73 51 88
Mail : [email protected] - MIUB : http://www.meteo.uni-bonn.de
Home : http://www.meteo.uni-bonn.de/Deutsch/Mitarbeiter/HPaeth
Group: http://www.meteo.uni-bonn.de/forschung/gruppen/klimadynamik
263
1.50P
STUDY OF LATE 20TH AND 21ST CENTURIES HIGH RESOLUTION
GCM SIMULATION OVER WESTERN AFRICA
A. SARR (1) and A. NODA (2)
(1) Direction de la Météorologie Nationale du Sénégal, Dakar-Yoff, Dakar-Sénégal
(2) Meterological Research Institute, MRI/JMA, Japan
The MRI GCM, in the framework of the IPCC fourth Assessment AR4, is run at very high
horizonal resolution of 20 km. The last decade of the 20th century is simulated as a control run of
10 years and the last dekade of the 21st centuries is simulated using A1B scenario.
In this study, with the known weekness of GCMs due to their coarse resoluton, we investigate the
performance of the model at high resolution compared to CRU and CMAP data for the present time.
Future projections are then investigated with a main focus on precipitation and temperature over
West Africa and mainly the Sahel area. This vulnerable region has experienced the most devatating
drougth in the 1970s and 1980s, and the initiation of a recovery in the late 1990s.
It is therefore important to investigate the future behavior of the climate system in this region in the
context of climate change. Agriculture and water ressources being critical in various sectors, it’s of
high importance to have indications on the expected behavior of rainfall regime even with large
uncertainties.
264
1.51P
EXAMINING INTERANNUAL RAINFALL VARIABILITY OVER THE SUDANSAHEL : A NEW PERSPECTIVE
Zakariya’u D. ABDULRASHEED
Nigerian Meteorological Agency, Nigeria
Since 1970’s abnormal rainfall fluctuations especially in the Sudan-Saharan African and
particularly the Sahel region of Nigeria has had a lot of impact on human activities. This is
particularly true for agriculture which is the main stay of the region’s economy. It has also
necessitated drought with the attendant conditions depending on their distribution and
characteristics of the extreme climate event.
Daily rainfall data for 31 years (1969-1999) for twelve stations in the Nigerian Sahel were collected
and analysed to show Interannual Variability over the region. A new Index proposed as Monsoon
Quality Index MQI (Usman, 2000) was applied to the data. Results show clearly the extreme
shortfall in rainfall over the Sahel, in 1973, 1983 and the good years of mid and late seventies and
eighties, MQI is able to depict location specific features as well as spatial variability.
This shows that the index is capable of providing the signals in the Interannual Variability over the
region.
Objectives of the Paper
Specifically, the objectives of the study were to:
(i)
(ii)
(iii)
(iv)
Review various operational indices in use for determine rainfall variability.
On the basis of (i), make conclusion as to whether they are enough to analyse particularly
agricultural drought, frequency, severity and duration within the Sudan-Sahel.
On the basis of (ii) assess the nature of drought occurrences in Sudan-Sahelian Nigeria
between 1969-1999.
To compare paterns depicted by MQI under (iii) with results of two earlier studies (Iwegbu
1993 and Oladipo, 1993) as a means of assessing MQI’s viability as a drought index.
Method of Data Analysis
Data:
The data used in this study covered daily rainfall data collected from the Meteorological Agency,
Oshodi-Lagos. Over a period of 31 years (1969-1999). This was collected for twelve (12) stations in
the Sudano-Sahelian region of Nigeria namely:1.
2.
3.
4.
Yola
Minna
Jos
Bauchi
265
5.
6.
7.
8.
9.
10.
11.
12.
Yelwa
Zaria
Maiduguri
Kano
Gusau
Nguru
Sokoto
Katsina
Method of Analysis
Derived Rainfall Parameters:
Pentade Rainfall Totals:
Pentade rainfall totals was calculated for five (5) days periods within each year staring from 1st
January i.e. Jan……………………………… Feb.
(1-5) (6-11) (31-4)
Feb…………………… Dec……………………
(5-11)
(1-5)
Jan………………………………….. etc.
(6-11)
This amount to 73 Pentads in each year.
Number of Breaks
A break for this work was taken as any pentads period with less than 5mm of rain. The number of
breaks is used to amplify any problem in the distribution.
Monthly Total Rainfall
Monthly totals were calculated from the daily amounts in a year. The month in each year that has
the highest amount has a special place in the assessment of variability.
Annual Rainfall Total
The annual rainfall total was calculated as the total of all the monthly totals in a year i.e. from
January to December.
(Ri)2
=
i = 12
∑ (rm)
i= 1
where i
=
rm =
Months
Monthly rainfall
MQI
MQI
=
Monsoon Quality Index
MQI
=
rmMi x Nbi
1
(Ri)2
where
i
=
rmmi =
Ri
=
Nbi
Year Identifier
Highest Monthly rainfall total
Annual rainfall total (the squaring is to remove the possibility of any bias)
=
Number of breaks in a year (as defined in ……..)
266
Table : MONSOON QUALITY INDEX CLASSIFICATION
MQI
CLASS
1
2
3
4
MQI VALUES
<0.005
>0.005 <0.01
≥0.01 <0.015
≥0.015 <0.02
RAINFALL
PERFORMANCE
Good
Fair
Poor
Very Poor
5
>0.02
Extremely Poor
DROUGHT
CATEGORY
Normal
Mid Drought
Severe Drought
Very
severe
Drought
Extremely
severe
Drought
0.015< MQI ≥0.01
0.01 ≥ MQI <0.015
The new perspective
The MQI measures the quality of the rains in terms of both annual amount and seasonal spread.
Time Space Cross Section:
Fig. 4.32 describes the severity of drought occurrences at different times and stations. Towards the
extreme northern part of the country, drought incidences of all types are more common while
locations in the Sudan Savannah such as Yola, Minna, Jos, Bauchi, Yelwa and Zaria experience
more normal years with only some patches of mild droughts in 1973 and 1993. These can be
explained by the influence of relief features on the flow of the southwest Monsoon. Nguru (on the
whole) the worst affected station with an apparent drought cycle of 5 years; followed by Maiduguri,
Katsina and Kano all with an apparent cycle of 10 years.
Station Specific temporal Variation:
Fig. 4.33-4.44. Rainfall decreases interland and towards the northeastern Nigeria. Relief features
play a very significant influence on the arising from the Plateau. On the windward side more normal
years are experienced but as you move further places like Kano and Maiduguri experienced some
form of drought with Nguru on the extreme end of the northeast having the worst drought years.
Frequency of Drought Types
Interdecadal
From the table 5, it can be observed that mild drought occurred more commonly in the 1970’s and
shows a decreasing trend into the 1990’s with 80’s more prone to drought than any decade. Another
decreasing trend is to be seen in the frequency of very severe drought. The severe drought and
extremely severe drought forms depicts an increasing trend.
Latitudinal
Fig. 4.45: Latitudinal location play a significant role in drought frequency and distribution as one
move into the hinterland drought years increases, Oguntoyinbo (1982) explained that the latitudinal
falling of rainfall is interrupted by the effect of the relief, but the longitudinal location has no effect
on drought frequency.
Comparison between MQI Patterns and Earlier Research
From results obtained using the MQI for the twelve stations over 31 years period (1969-1999), Fig.
4.1 – 4.31, it is observed that works by Iwegbu (1993) and Oladipo (1993); Fig. 4.47 – 4.48
267
respectively are restricted to only meteorological drought unlike the MQI which looks at
agricultural drought.
Some of the difference between MQI and these earlier works are:
(i)
While MQI looks at agricultural drought, earlier works by Iwegbu (1993) looked at
meteorological drought only;
(ii)
From the MQI Pattern, Fig. 4.1 – 4.31, it is observed that MQI has a definite pattern but
earlier works do not have, you observe some pockets of drought forms even in areas that did not
experience any form of drought Fig. 4.47 – 4.48.
(iii) Earlier works only looks at rainfall totals, but MQI looked at spatial distribution patterns of
rains.
(iv)
Earlier works examined rainfall variability on both sides of normal while MQI has an
emphasis on agricultural drought intensity level.
(v)
MQI has both longitudinal and latitudinal shift with the latitudinal shift given a better
picture. Earlier works look at meridroual spread. Because of these variations, even periods where
we experience extremely severe drought like Nguru and Maiduguri, earlier works reported normal
years or just mild drought, especially where it is more glaring that drought occurrence was very
serious at that period.
Conclusion
The recent (mainly 1970-73) Sahelian drought has evoked considerable research into important
atmospheric mechanisms that might have led to the catastrophic failure of the region’s precipitation,
and the resulting loss of human and animal life associated with it (Adedokun 1978). The approach
to the problem posed by this atmospheric catastrophe is at least necessarily two folds; (a) through a
quests for mechanism that are behind the shortage or failure of the rains and (b) through seeking a
forecast of the region’s precipitation using it’s historical records. In order to mitigation planning
purpose, the MQI has been compared with other indices and the other indices have been observed to
have similar or common short comes. In the use of long term mean rainfall and for standard
deviation values, which will require seldom available large data to compute? MQI on the other
hand, can be used even if a new station with only a year of data is involved (Usman 2000).
Recommendation
The serious problem caused by drought in the Sudano-Sahelian zone of Nigeria from its onset to
date has shown proper measures must be taken in order to utilize our environment properly for
today’s and tomorrow’s generation as Usman (2000) puts it. “Drought is a natural hazard that is
said to differ from all other hazards and especially the agricultural drought which has a lot of socioeconomic impact on the people”. Indeed, it is recommended that more research into the SudanoSahelian region of Nigeria should be done for early warning purposes to reduce its effect on
agriculture and as earlier proved the MQI was tested and found to be a good measure for the
analysis of drought by depicting its pattern of occurrence and distribution. It should in a matter of
urgency be accepted as better measures of rainfall variability especially in the Sudano-Sahelian
region of Nigeria.
Contact
Nigerian Meteorological Agency, Central Forecast Office, P.M.B. 1215, Oshodi-Lagos, Nigeria - Email: [email protected]
268
1.52P
DYNAMIQUE ET TENDANCES CLIMATIQUES FUTURES DANS LE GOLFE DU
BENIN A TRAVERS L’EXEMPLE DE
QUELQUES STATIONS COTIERES
Pessiézoum ADJOUSSI et Adoté BLIVI
Centre de Gestion Intégrée du Littoral et de l’Environnement, Département de géographie,
Université Lomé, TOGO
Ce travail sur la crise climatique dans les stations côtières du golfe du Bénin présente les résultats
des analyses menées sur la base de séries temporelles fournies par les DMN du Bénin, Ghana et du
Togo suite au dérèglement avéré du climat actuellement observé sur le littoral. Ces résultats
indiquent un important réchauffement et une tendance à la disparition progressive de la petite saison
de pluies qui est tantôt inexistante tantôt centrée uniquement sur le mois d’octobre. L’analyse des
hauteurs d’eau mensuelles décennales, basée sur ces données dégage une baisse évidente des pluies
au cours de la petite saison pluvieuse avec un assèchement du mois de septembre prolongeant la
durée de la petite saison sèche. La station d’Accra présente des valeurs de 61,5 mm au mois de
septembre au cours de la décennie 1971-1980, cette hauteur est passée à 69,9 durant la période
1981-1990, puis à 20 mm en 1991-2000. Les cinq (5) autres stations disposant de séries assez
longues et homogènes, Lomé-Ville, Lomé-Aéroport, Baguida, Aného, Akatsi présentent la même
tendance. Ceci atténue le rythme bimodal connu par le passé.
Du point de vue physique, la diminution du nombre de jours de pluies accompagnée de la
diminution des précipitations, peut s’expliquer par le comportement actuel des facteurs aérologiques
et la variabilité des SST dans le golfe du Guinée. En réalité, dans l’Est Atlantique le fait que
l’épaisseur de la couche de mélange océanique soit faible, entraîne une forte variation des SST aux
échelles saisonnières et inter-annuelles créant des anomalies convectives.
Mots-clés : Variabilité climatique, crise climatique, saison, facteurs aérologiques, SST
Soumis par :
Pessiézoum ADJOUSSI - Marine biodiversity data manager - Département de Géographie/CGILE Université de Lomé - BP 1515 - Lomé - Togo - Tél : (228) 221 68 17/ 222 48 65 - Cel : (228) 914
21 76 - Email : [email protected] / [email protected]
269
1.53P
IMPACTS DE L’OSCILLATION NORD ATLANTIQUE (NAO)
SUR LA VARIABILITE CLIMATIQUE EN AFRIQUE DE L’OUEST :
CAS PARTICULIER DE LA GUINEE
Alpha Boubacar BARRY
D.N. Météorologie, Conakry, Guinée
En Afrique de l'Ouest et plus particulièrement en Guinée, les principales sources de revenu de la
population sont l'agriculture et l'élevage qui dépendent de la variabilité climatique et en particulier
de la pluviométrie. Mais celle-ci est très variable dans l'espace et le temps.
Les données sur la pluviométrie quotidienne des 70 dernières années pour au moins une quarantaine
de stations au niveau de la Guinée ont servi de base pour le calcul du début, de la fin et de la durée
de la saison pluvieuse. Les cartes établies à partir de ces résultats montrent des isolignes parallèles
entre elles et ayant une configuration plus ou moins zonales. Les pluies commencent au début du
mois de Mars et se terminent en décembre dans la zone subéquatoriale (sud de 8°N) de la Guinée.
Dans la zone septentrionale, subsahélienne (nord 12°N) de la Guinée, les pluies débutent
tardivement (début Juin) et se terminent tôt (début Octobre). Ainsi la longueur de la saison passe de
10 mois dans la zone subéquatoriale à 4 mois dans la zone subsahélienne.
Une corrélation significative est trouvée entre les anomalies standards normalisées du début et de
fin des pluies et l'index normalisé de l'Oscillation Nord Atlantique. Par contre, si une bonne
corrélation est trouvée entre la quantité des pluies et l’anomalie des SST dans l’Océan Atlantique
Sud , une très faible liaison semble s’établir avec le début et la fin de la saison pluvieuse.
Contact :
AL. B. Barry - D.N. Météorologie - BP 566 Conakry, Guinée - Tel : (224) 545492
270
1.54P
INTEGRATION OF WEATHER SYSTEM VARIABILITY TO
MULTIDECADAL REGIONAL CLIMATE CHANGE: WEST AFRICAN
SOUDANO-SAHEL ZONE, 1951-1998
Michael A. BELL (1) and Peter J. LAMB (2)
(1) Research Institute for Climate Prediction, The Earth Institute at Columbia Univ.,
Palisades, NY, USA (2) Cooperative Institute for Mesoscale Meteorological Studies and School
of Meteorology , The University of Oklahoma , Norman, Oklahoma USA
This study examines the characteristics of westward propagating mesoscale convective systems
(Disturbance lines, DLs) to regional climate variability on intraseasonal-to-multidecadal timescales
o
o
over West African Soudano-Sahel zone (10 -18 N) during 1951-98. The analysis is based on daily
raingauge-based indices of DL frequency, size, and intensity for four 440 km square “catchments”
o
o
across the region (18 W-4 E) and time series of 10-day and seasonal DL frequency/magnitude.
This approach is validated using TAMSAT satellite IR cold cloud duration statistics for the same
1995-98 DLs. Results obtained for all catchments are remarkably similar on each timescale. Longterm (1951-98) average DL size/organization increases monotonically from early June through late
August, and then decreases strongly during September. In contrast, average DL intensity maximizes
10-30 days earlier than DL size/organization and is more symmetrically distributed within the rainy
season for all catchments except the westernmost, where DL intensity tracks size/organization very
closely.
The predominant mode of rainfall extremes involves near season-long suppression or enhancement of
the seasonal cycles of DL size/organization and intensity, especially during the late July-late August
rainy season peak. Some other extreme seasons result only from peak season anomalies. On the
multidecadal scale, the dramatic decline in seasonal rainfall totals from the early 1950s through the
mid-1980s is shown to result from pronounced downtrends in DL size/organization and intensity.
Surprisingly, this DL shrinking/fragmentation/weakening is not accompanied by increases in
catchment rainless days (i.e., total DL absence). Like the seasonal rainfall totals, DL size/organization
and intensity increase slightly after the mid-1980s.
271
INTEGRATION DE LA VARIABILITE DU TEMPS AUX CHANGEMENTS
CLIMATIQUES REGIONALES DECENNALES EN AFRIQUE DE L’OUEST:
ZONE SOUDANO-SAHELIENNE DE 1951 A 1998
Cette étude examine les caractéristiques des systèmes convectifs de Méso-échelle de propagation
d’ouest (lignes de grains, LDGs) à la variabilité climatique régionale à l’échelle intra-saisonnière à
multi décennale en zone Soudano-Sahélienne d’Afrique de l’Ouest (10o-18oN) sur la période 195198. Notre analyse est basée sur les indices des fréquences des LDGs, leurs tailles, et leurs intensités,
calculés dans quatre zones de 440 km carrés chacune et situées entre 18oW et 4oE. Les séries
décadaires et saisonnières de la fréquence/intensité des LDGs ont été également utilisées. Cette
approche a été validée par l’utilisation des données infrarouges des statistiques de la durée des
nuages froids obtenues par satellite TAMSAT sur la période 1995-98.
Les résultats obtenus pour chacune des zones sont curieusement très similaires pour différente
échelle temporelle. La moyenne à long terme (1951-98) de la taille/organisation des LDGs accroît
d’une façon monotone à partir du début du mois de Juin jusqu’à la fin du mois d’Août, puis décroît
drastiquement en Septembre. Par contre, l’intensité moyenne des LDGs maximisée 10-30 jours plut
tôt que la taille/organisation, est systématiquement distribuée au cours de la saison des pluies pour
tous les degrés carrés à l’exception de celui plus à l’ouest, où l’intensité et la taille/organisation des
LDGs varient étroitement.
Le mode prédominant des extrêmes pluviométriques (très excédentaire ou déficitaire) implique la
suppression ou le renforcement du cycle saisonnier de la taille/organisation et de l’intensité des
LDGs spécialement durant le pic de la saison des pluies (fin juin – fin Août). D’autres extrêmes
sont seulement le résultat des anomalies de la saison. A l’échelle multi décennale, l’impressionnant
déclin du total saisonnier des pluies observé à partir du début des années 1950 à mi-1980 serait le
résultat de la réduction de la taille/organisation et de l’intensité des LDGs. Surprenant encore est
que cette réduction/fragmentation/affaiblissement des LDGs n’est pas accompagnée par une
augmentation du nombre de jours sans pluie au niveau des zones étudiées (ex. total de jours sans
LDGs). Il est a noté cependant, que la taille/organisation et l’intensité des LDGs ont accru
légèrement à partir de la mi-1980 comme d’ailleurs le total saisonnier de pluies.
272
1.55P
INTERACTION OF AFRICAN MONSOON AND THE LARGE-SCALE
ATMOSPHERIC CIRCULATION AND OCEANS ON INTRASEASONAL TO
MULTIDECADAL TIMESCALES
Dan C. COLLINS, Carlos D. HOYOS and Peter J. WEBSTER
Earth and Atmospheric Sciences, Georgia Institute of Technology
Recent studies have identified intraseasonal variability in the African monsoon system (Sultan and
Janicot 2003; and Mathews 2004), and a relationship between African variability, the large-scale
system and the South Asian monsoon (Hoyos and Webster 2004; and Collins and Webster 2005).
We explore the African intraseasonal and interannual variability and its physical forcing by the
large-scale atmosphere and ocean-atmosphere interactions using the operational ECMWF 1°x1°
reanalysis atmospheric circulation data and Reynolds sea surface temperature data. A key aspect of
this study is the separation of African monsoon variability into timescales from intraseasonal to
multidecadal climate change, using multivariate singular spectral analysis and wavelet analysis.
Signals at different timescales are extracted from large-scale variables using correlation analysis or
composites relative to African climate or SST variations. To determine the forcing of the African
th
monsoon system by anthropogenic climate change in the 20 century, we make use of a set of
coupled global GCM experiments made under the auspices of the IPCC.
Because ocean-atmosphere interactions and large-scale atmospheric circulation are forced by
atmospheric composition, the response of the African monsoon to climate change is assessed. As a
first step, the ability of the models to simulate the large-scale changes associated with African
climate variability is evaluated. Using the conditional entropy of model-generated probability
distributions, we calculate the information content of GCM simulations of the regional variability.
273
1.56P
ANALYSE DE L’EVOLUTION CLIMATIQUE ACTUELLE AU SENEGAL
B. DIAGNE (1), P. SAGNA (2) et A. SENE (1)
(1) Direction de la Météorologie du Sénégal, Dakar, Sénégal
(2) Département de Géographie, UCAD Dakar, Sénégal
Différentes études menées sur l’Afrique de l’ouest de manière globale et sur le Sénégal de manière
plus particulière ont montré une variation parfois inquiétante des relevés météorologiques qui
s’intègre dans une dynamique planétaire de changement climatique. Parmi les aspects de ce
changement figurent le glissement des régions climatiques et surtout la recrudescence des
phénomènes météorologiques extrêmes (pluies exceptionnelles, orages violents, sécheresse, vagues
de chaleur et de froid) observés au cours des dix dernières années. A travers notre analyse de
l’évolution climatique récente au Sénégal, basée sur les précipitations, les températures, l’indice de
sécheresse et l’indice bioclimatique plusieurs résultats ont été obtenus.
L’analyse de la pluviométrie a été faite à partir des données de la Direction de la Météorologie du
Sénégal, sur la base des trois normales OMM, 1901-1930 ; 1931-1960 et 1961-1990 et sur celle de
la période sèche et plus récente 1971-2000. La période 1901-1930 se caractérise par une
augmentation des précipitations selon un gradient Sud-Nord et des isohyètes sensiblement parallèles
de valeurs comprises entre 1500 mm et 450 mm (figure 1). La forte pluviométrie des régions
méridionales s’explique en grande partie par l’influence des mouvements de la Zone Intertropicale
de Convergence et des apports importants des lignes de grains. Les parties septentrionales doivent
en partie leur faible pluviométrie à l’influence des alizés maritimes issus de l’anticyclone des
Acores.
Avec la période 1931-1960, on note un début de glissement des valeurs vers le sud et surtout
l’apparition de l’isohyète 400mm sur la partie nord-ouest, axe Saint-Louis – Podor (figure 2). Une
stabilité pluviométrique est constatée au niveau des régions centrales, entre les latitudes 12° 30 et
15° 30 avec les isohyètes 600 mm, 800 mm et 1000 mm. Plus au sud, l’isohyète 1400 mm observé
précédemment disparaît de la zone de Kédougou, ce qui dénote une péjoration importante qu’on
retrouve aussi jusqu’à la latitude Kolda.
Pour la période 1961-1990 la baisse est plus notable avec l’isohyète 400 mm qui se positionne sur
l’axe Linguère-Matam aux environs de 14° 45 de latitude, laissant la partie nord du territoire dans
une situation de risque climatique structurel (figure 3). L’isohyète 600 mm subi aussi une baisse
très sensible pour se stabiliser sur les régions du centre-sud, aux environs de 14° N.
La période 1971-2000 reflète mieux la baisse constatée de la pluviométrie depuis le début des
années 1970 (figure 4). Le Sénégal se retrouve partagé en deux parties :
•
274
une vaste superficie en dessous 500 mm qui est une zone à risque pour l’agriculture pluviale
et qui est caractérisée par un court séjour et un début tardif de l’installation de la mousson
vers la deuxième quinzaine de juillet, par des pluies sous forme d’averses pluvio-orageuses
et par des périodes sèches assez fréquentes ;
•
une autre au-dessus de l’isohyète 500 mm, pour laquelle l’agriculture pluviale présente une
meilleure garantie de réussite de la production avec un séjour plus long de la mousson et une
longueur de la saison des pluies relativement plus importante et des pauses pluviométriques
plus courtes.
Il faut également noter la disparition de l’isohyète 1200 mm et la cassure en deux parties de
l’isohyète 1000 mm sur les zones de Ziguinchor et Kédougou.
17
Normale pluviométrique: Période 1931-1960 en mm
Normale Pluviométrique : Période 1901 -1930 en mm
17
PODOR
PODOR
SAINT-LOUIS
16
LINGUERE
LINGUERE
RANEROU
RANEROU
15
15
BAKEL
14
KOUNGHEUL
TAMBA
NIORO
VELINGARA
SIMENTI
13
BAKEL
DAKAR
DIOURBEL
DIOURBEL
FATICK
KAOLACK
14
MATAM
LOUGA
MATAM
LOUGA
DAKAR
SAINT-LOUIS
16
FATICK
KAOLACK
KOUNGHEUL
NIORO
VELINGARA
13
SIMENTI
KOLDA
KOLDA
KEDOUGOU
ZIGUINCHOR
KEDOUGOU
ZIGUINCHOR
TAMBA
12
12
-18
Figure 1 : Normale 1901-1930
Figure 2 : Normale 1931-1960
-17
-16
-15
-14
-13
-12
-11
Normale pluviométrique : période 1961-1990 en mm
17
PODOR
SAINT-LOUIS
16
LOUGA
MATAM
LINGUERERANEROU
15
DAKAR
BAKEL
DIOURBEL
FATICK
KAOLACK
KOUNGHEUL
NIORO
14
TAMBA
VELINGARA
SIMENTI
KOLDA
13
KEDOUGOU
ZIGUINCHOR
12
-18
-17
-16
-15
-14
-13
Figure 3 : Normale 1961-1990
-12
-11
Figure 4 : Moyenne 1971-2000
Les températures, aussi bien minimales que maximales, ont connu des hausses différentes ces
dernières décennies dans les principales stations du pays. Ziguinchor, localité du sud du Sénégal,
dans un climat sud soudanien laisse apparaître une hausse d’environ 0,5°C entre 1941 et 2000. Les
hausses les plus significatives se situent après 1970.
L’indice de sécheresse et l’indice bioclimatique ont permis de dégager des modifications
climatiques à l’intérieur des différents domaines que sont : le domaine sahélien, le domaine nord
soudanien et le domaine sud soudanien ainsi que de leur nuance littorale. On a pu ainsi observé au
cours des 50 dernières années, à partir d’une analyse décennale, le passage de certaines stations de
la semi-aridité à l’aridité et pour d’autres du caractère sec à subhumide à celui de semi-aride. Ainsi,
malgré une tendance générale à la dégradation des conditions climatiques au Sénégal on observe
des nuances régionales très significatives dans cette évolution.
275
Ainsi l’analyse du climat actuel au Sénégal permet de retenir que :
-
la variation de la pluviométrie annuelle au Sénégal est fortement liée à l’évolution
ascendante de la ZCIT et des phénomènes associés ;
la période 1901 à 1960 très humide et qu’une baisse importante de la pluviométrie
annuelle constatée sur la période 1961-1990 est plus marquée sur la période 1971-2000 ;
la perte de ressource pluviale de l’ordre de 200mm sur environ une distance de 300 km
est enregistrée sur le Sénégal ;
la recrudescence des phénomènes météorologiques extrêmes constatée au cours des dix
dernières années est liée certainement au réchauffement de la planète ;
les tendances générales dans l’évolution des températures sont à la hausse ;
les différents indices utilisés pour déterminer l’évolution climatique confirment la
dégradation du climat actuel au niveau du Sénégal.
ANALYSIS OF THE CURRENT CLIMATIC EVOLUTION IN SENEGAL
Different studies carried out on West Africa in general and in particular on Senegal have sometimes
shown a worryiing variation on meteorological statements. Throughout our analysis of the recent
climatic evolution in Senegal based on rainfalls, temperatures, drought and bioclimatic indexes,
many results have been obtained. First, there is throughout the pluviométric déficit which has
started since 1968, a very important migration of isohyets towards the south. The pluviometric
deficit by 2020 will be about 6% in Saint-louis, 7% in Matam; 10% in Dakar; 23% in Kedougou
and 24% in Ziguinchor. Then, minimum as well maximum temperatures have increased differently
these last decades in the main stations of the country. Finally, the drought and bioclimatic indexes
allowed to underline climatic modifications inside different zones such as : the sahelian zone, the
north and south soudanian zones just as their coastal difference. So, we have noticed during the last
50 years, from an analysis per decade, a change from semi dryness to aridity in some stations and
for others from a dried to subhumid nature to that of semi-dryness. Thus, despite of general
tendency of the deterioration of climatic conditions in Sénégal, we notice very significant regionals
differences in this evolution.
276
1.57P
INFLUENCES OF CLIMATIC VARIABILITY ON COASTAL
OCEANOGRAPHY OF BENIN
Roger DJIMAN and Zacharie SOHOU
Fisheries and Oceans Research Centre of Benin, Cotonou, Benin
Monthly average of air temperature at Cotonou Cadjehoun station (06° N X 002°23 E) over 10
years (from 1975 to 1984), as well as fifteen and monthly averages of Ocean surface temperatures
in Cotonou for the same period have been studied and analyzed.
The rains (precipitations) heights were analyzed and correlated with air and sea surface
temperatures. On the basis of data collected by our Centre, variations of sea water temperatures and
salinities according to the depths have also been studied.
It's a question of identifying themocline zones to see variations in depth, of the upwelling
phenomenon during a year. An attempt at correlation of the monthly production of pelagic species
(Sardinella spp.), indicator of the phenomenon has enabled to see the composite correlation with the
air temperature.
Contact
R. DJIMAN : Head, Fisheries and Oceans Research Centre of BENIN (CRHOB), P.O. Box 031665, Cotonou, Bénin - Phone : (229) 32 62 14 - [email protected] / [email protected]
[email protected]
Z. SOHOU : Researcher Fisheries and Oceans Research Centre of BENIN (CRHOB), P.O. Box 031665, Cotonou, Bénin - Phone (229) 32 62 14 - [email protected]
277
1.58P
THE MEAN CONDITIONS AND VARIABILITY OF THE CLIMATE OVER
SENEGAL DURING THE RECENT DECADES
Souleymane FALL (1), Dev NIYOGI (1) and Fredrick SEMAZZI (2)
(1) Purdue University, Department of Agronomy, West Lafayette, USA
(2) North Carolina State University Department of Mathematics, Raleigh, NC, USA
This paper describes the mean climate conditions over Senegal in the recent decades and analyzes
the rainfall variability and its relationship with global climate.
Observed rainfall and temperature records for 20 stations over Senegal have been compiled,
digitized, and their quality ascertained. Monthly, seasonal and annual averages are computed to
generate temperature and rainfall totals and climate indices averaged over the study period. Trends
in precipitation and temperature are examined using a linear regression analysis and interpolation
maps. The spatial distribution patterns are mapped and analyzed using ArcGIS Spatial Analyst.
To investigate the climate variability over Senegal an Empirical Orthogonal Function (EOF)
analysis is performed for the 1979-1998 period, using rain-gauge and CMAP rainfall data over
Senegal, and CMAP data only over West Africa. Two additional datasets are used to examine the
relationship with global climate: the 2° latitude x 2° longitude NOAA Extended Reconstructed SST
data (ERSST) provided by the NOAA-CIRES Climate Diagnostics Center in Boulder (Smith and
Reynolds, 2003), and the NCEP/NCAR 51-year (1948-1998) reanalysis wind data (Kalnay et al,
1996) at the 850 mb level.
With regard to the mean conditions, the 1971-1998 period agrees well with traditional settings in
terms of temperature and rainfall distribution: a north – south gradient in rainfall and an east - west
gradient in temperature variations are observed. The trend analysis tells a different story: (i) air
temperature showed a positive and significant warming trend throughout the country, except in the
Southeast; a significant correlation is found between the temperature index for Senegal and the
Pacific sea surface temperatures during the January-April period, especially in the El Niño zone; (ii)
in contrast to earlier regional-scale studies, precipitation does not show a negative trend and has
remained largely unchanged, with few locations showing a positive (but not significant) trend
particularly in the Northeast and Southwest regions.
The EOF-based analysis of rainfall variability reveals that the time series of the first West African
mode agrees strongly with Lamb’s rainfall index. This agreement confirms that EOF1 for West
Africa is the representative mode of rainfall variability over the region. One of our major findings is
that EOF2 for West Africa is well correlated with EOF1 for Senegal rainfall. This mode, which is
traditionally neglected in the studies of variability over the region, is the one that is important in
explaining the variability of rainfall over Senegal. This relationship is supported by the projection
of NCEP winds on EOF2 mode. EOF2 for West Africa, which is well correlated with Senegal
rainfall, is associated with a strong coupling between the wind circulation over the tropical Atlantic
and eastern tropical Pacific. This circulation pattern is associated with sitive anomalies over
Senegal. A further confirmation is shown by the grid-point correlation between the time series of
EOF2 over West Africa and the Atlantic SST. There is a positive correlation between south Atlantic
278
SSTs and rainfall anomalies over Senegal. The highest coefficients are observed during the wet
season (up to 0.70 in August). This relationship is strong contrast from what is usually observed in
many studies. The typical circulation associated with positive anomalies over the country is a
moisture convergence which takes place over the Guinea Gulf, in conjunction with warm waters in
this area.
1.59P
VARIABILITE ET TENDANCE CLIMATIQUES EN COTE D’IVOIRE :
CAS DE LA STATION DE LAMTO
Mamadou FOFANA, Abé Vincent ATTI et Marie AKADJE
Station géophysique de LAMTO, Côte d’Ivoire
La station climatologique de LAMTO est située dans une zone de contact forêt savane en Côte
d’Ivoire. Les données recueillies permettent d’étudier les variations interannuelles et mensuelles
dans sa zone sur quatre décennies, de 1964 à 2004. Elle prend en compte les paramètres climatiques
importants tels que la pluviométrie, les températures, l’insolation.
De 1964 à 2004, malgré la stabilité de ces régimes, une hausse de ces différents paramètres est
observée, ce qui entraînerait un réchauffement climatique. Mais, la pluviométrie surtout, a des
décalages dans ces saisons des mois les plus pluvieux qui va souvent de juin à mai ou à juillet et sa
petite saison sèche en août qui est quelquefois un mois pluvieux.
L’analyse des données in-situ sera appuyée par d’autres technologies telles que les images
satellitaires comme NOAA, METEOSAT.
Ces études serviront donc à former une base de données pour une meilleure connaissance de
phénomènes de la mousson africaine et aussi un indicateur pour l’agronomie.
279
1.60P
ATLANTIC OCEAN HEATING AND RAINFALL EXTREME EVENTS
RELATIONSHIPS IN WEST AFRICA DURING JAS
K. Yves KOUADIO, Delfin A. OCHOU and Paul A. ASSAMOI
Laboratoire de physique de l’atmosphère, Université de Cocody, Abidjan, Côte d’Ivoire
Abstract
This study focused on the statistical relation between the tropical Atlantic and rainfall in Côte
d'Ivoire and in Ghana. The Principal Components Analysis of rainfall anomalies of 43 stations
made possible to highlight periods of strong rainfall from July to September (JAS) at the Guinean
coast. A comparison between the SST of the southern basin and the annual chronicle of the second
principal component shows that the oceanic heating is overall related to a rise of rainfall in JAS,
except for a few particular years.
Introduction
Côte d’Ivoire and Ghana are two countries of west Africa bordering the Gulf of Guinea (4-11°N /
9°W-1.2°E). They are subjected to both monsoon flux influence coming from Atlantic ocean and
the continental dry air mass (Harmattan) coming from the Saharan zone. The seasonal and
interannual variations of the rainfall influence considerably the dynamic ecological, the agicultural
outputs and the economy of this area. The drought of 1983 and 1994 for example had a significant
socio-economic impact, with the increase in fires of vegetation, the loss of certain cultures and the
fall of the outputs of the food crops. Several studies suggested the existence of the Sea Surface
Temperature (SST) of the tropical Atlantic and rainfall relationships in West Africa (Lough, 1986).
This study focuses on a main goal : rainfall predictability during July to September (great rainy
season in Sahel, small dry season on the Guinean coast and oceanic cold season in the Gulf of
Guinea). It is based on the statistical relation between rainfall and the meteo-oceanic parameters
such as pseudo-wind stress (PWS) and (SST).
Factorial study of the seasonal rainfall variation
This study is based on the Principal Components Analysis (PCA) of the standardized rainfall
anomalies on 43 stations in Côte d’Ivoire and in Ghana. It highlights the different modes of climate
variability (figure 1). We are interested particularly to the second eigenvector which correspond to
the July-September (JAS) season.
Figure 1 : The first three eigenvectors of a Principal Components Analysis of standardized rainfall
anomalies in Côte d’Ivoire and in Ghana.
280
SST-rainfall chronicles, SST/rainfall and PWS/rainfall correlations
The interannual chronical (figure 2, left) identified as the second eigenvector, shows an opposite
relation between the rainfall and the SST of tropical Atlantic south basin. This relation is confirmed
by the SST/rainfall correlation and the PWS (JAS)/rainfall significant complex correlation vectors
(located in 10°N) (figure 2, right). The area delimited by Côte d’Ivoire and Ghana is generally
characterized by a decrease of rainfall in the south (Guinea region) and an increase of rainfall in the
north (Sahelian region) during the July-September season (kouadio et al., 2003). However, an
opposite situation can occurred during particular years in which the rainfall rises in the south and
decreases in the north.
2 x facteur2
2,5
SSTSAT_JAS_as
2
1,5
1
0,5
0
-0,5
-1
-1,5
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
1970
1968
1966
1964
-2
Figure 2 : (left) Annual chronicle of the second eigenvector of the standardized rainfall anomalies
in Côte d’Ivoire and in Ghana and the annual standardized anomalies of SST in the south basin of
the tropical Atlantic. (right) correlation of SST/rainfall (colour) and PWS/rainfall (arrow).
Some particular years during JAS in the Guinean region
A first group, for example JAS 1966, is obtained during the cold SST years in the Gulf of Guinea.
This result is different from those generally obtained (Fontaine and Janicot, 1996). It may be linked
to local factors influencing the increase of rainfall during JAS. However these phenomena exist,
their weak frequency of appearance is drowned in the second group. This one, for example 1984
and 1987, concerned the abnormal heating of the upwelling zones and generally, the global heating
of the Atlantic south basin. However, we noted that SST anomalies observed in the upwelling zones
are highest to those observed during the global heating of the south basin.
Figure 3 : (above) Monthly anomalies of the standardized rainfall anomalies in Côte d’Ivoire and
in Ghana of some particular years and (bottom) the corresponding monthly anomalies of SST.
Conclusion et perspectives
In order to better determine the continental atmospheric and oceanic factors responsible of this
rainfall characteristics, the study of these groups must be deepened by the use of meteo-oceanic
parameters. It will be able to quantify by a statistical and a modelling studies, the influence of each
281
parameter in the rainfall variability. A study on the agricultural economy impacts will be considered
thereafter.
Bibliography
Fontaine B. et Janicot S. : Sea surface temperature fields associated with West African rainfall anomaly types. Journal
of Climate, 9 (1996), 2935-2940.
Kouadio K.Y., Ochou D.A. et Servain J.: Tropical Atlantic and rainfall variability in Côte d’Ivoire. Geophysical
Research Letters, 30 (2003), 5, 15,1-15,4.
Lough J.M.: Tropical Atlantic sea surface temperature and rainfall variations in Subsaharan Africa. Mon. Wea. Rev.,
114 (1986), 561-570.
Contact
K.Y. Kaoudio - Laboratoire de physique de l’atmosphère, Université de Cocody, 22 BP 582,
Abidjan 22, Côte d ’Ivoire) - [email protected]
RELATION ENTRE LE RECHAUFFEMENT OCEANIQUE ET LES
EVENEMENTS PLUVIEUX EXTREMES EN AFRIQUE DE L’OUEST DURANT
LA PERIODE DE JUILLET A SEPTEMBRE
Cette étude se focalise sur la relation statistique entre l’Atlantique tropical et la pluviométrie en
Côte d’Ivoire et au Ghana.
L’Analyse en Composantes Principales des anomalies pluviométriques de 43 stations a permis de
mettre en évidence des périodes de forte pluviométrie de juillet à septembre (JAS) au sud. Une
comparaison entre la SST du bassin sud et la chronique annuelle de la première composante
principale montre que le réchauffement océanique est globalement lié à une hausse de la
pluviométrie en JAS, à l’exception de quelques années particulières.
Soumis par
OCHOU Abé Delfin - Université de Cocody Laboratoire de Physique de l'Atmosphère et de
Mécaniques des Fluides (LAPA-MF), 22 BP 582, Abidjan 22, Côte d’Ivoire
Tél :(225) 07 94 53 84 - email : [email protected] / [email protected]
282
1.61P
INFLUENCE OF INTERTROPICAL FRONT ON RAINFALL VARIABILITY
IN WEST AFRICA SAHELIAN COUNTRIES
Mouhamadou Issa LELE and Peter J. LAMB
Cooperative Institute for Meteorological Studies and School of Meteorology,
The University of Oklahoma, Norman, OK 73019, USA
This study examines the intraseasonal and interannual variability of the Intertropical Front (ITF),
also called Intertropical Discontinuity (ITD), and total rainfall over West Africa Sahelian countries
during 1974-2003.
The analysis is based on three data sets -- daily dew point temperatures, daily rainfall, and
NCEP/NCAR reanalyses. Dew point temperatures were computed from observed daily minimum
o
o
o
temperature and maximum relative humidity for synoptic stations in the Sahel (10 -25 N, 12 Wo
24 E). Observed daily raingauge data were used to study the performance of the monsoon in
association with the ITF positions. Global circulation features associated with the advance and
retreat of the ITF and the occurrence of extreme conditions in the area were explored using
NCEP/NCAR reanalyses data. Preliminary results suggest that delayed northward excursion of the
ITF does not necessary imply drought conditions in the Sahel.
Composite analysis of wet and dry episodes, in fact, suggests that a northward displacement of the
ITF may account for wetter conditions, but that droughts years are likely to be caused by changes in
convergence strength during the rainy season regardless of the ITF position. The depth of the
monsoon westerlies seems to be the primary factor controlling the strength of the convection that
produces rainfall in the region.
283
INFLUENCE DU FRONT INTERTROPICAL (FIT) SUR LA VARIABILITE DES
PRECIPITATIONS EN AFRIQUE DE L’OUEST :
CAS DES PAYS DU SAHEL
Cette étude examine la relation entre la variabilité intra-saisonnière et interannuelle du Front
Intertropical (FIT) également appelé Discontinuité Intertropicale et la pluviométrie dans les pays du
Sahel, sur la période 1974-2003. L’objectif est de mieux comprendre les relations qui existent entre
la montée ou le retrait du FIT avec les précipitations au Sahel. Trois types de données ont été
utilisés à cet effet : la température journalière du point de rosée, la pluviométrie journalière et les
données ré-analysées de NCEP/NCAR. Nous avons utilisé la formule de Clausius-Clapeyron pour
calculer les températures du point de rosée à partir des données d’observation des températures
minimales et des humidités maximales pour les stations comprises entre 10o-25oN et 12oW-24oE.
Les relevées pluviométriques journalières ont été utilisées pour mettre en évidence le lien entre les
précipitations et la position moyenne du FIT. En outre, les conditions atmosphériques relatives a la
circulation générale pouvant influencer la montée ou le retrait du FIT ont été étudiées.
Les résultats préliminaires obtenus suggèrent qu’une montée tardive du FIT n’est pas
nécessairement la cause des situations de sècheresse dans le Sahel. L’analyse comparative des
années sèches et pluvieuses montre qu’une montée précoce du FIT peux être associée a des
conditions humides dans le Sahel, mais que les années sèches sont probablement liées à un
affaiblissement de la convergence et cela peu importe la position du FIT. L’épaisseur de la mousson
semble être le facteur primordial contrôlant le degré de convergence pouvant occasionner des
précipitations dans la région.
284
1.62P
NEED FOR LONG-TERM BROADBAND SURFACE IRRADIANCE
MEASUREMENTS IN WEST-AFRICA
Beate G. LIEPERT
Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA
Recent studies suggest that the observed steady increase in the so-called dimming (decline of
surface solar radiation observed at sites worldwide), which occurred mainly during the second half
of the last century has been halted in industrialized countries due to major efforts in reducing
industrial air pollution.
On the other hand, satellite observations and sparse ground measurements indicate that the
reductions in solar radiation are still ongoing in East Asia and South and Central Africa. The impact
of the dimming of sunlight on a global scale revealed the possibility of slowing down the water
cycle even with global warming because of the importance of surface solar heating on evaporation.
It can be assumed that in contrast to heavily industrialized countries most of the dimming in West
Africa is continuing to rise due to the different sources. Most of pollution is from biomass burning
from small fires and households.
Satellites observe only large fires and have problems in identifying local sources. Here we present
satellite (ISCCP) based estimates of changes in surface solar radiation for the last 15 years in Africa
and discuss possible influences of these changes on the cycling of water. We argue that more
broadband solar radiation recordings at the surface are needed to correctly monitor the long-term
impact of biomass burning on the energy budget and hence water cycle.
285
1.63P
CHARACTERIZATION AND VARIABILITY OF KIREMT
RAINY SEASON OVER ETHIOPIA
Zewdu Tessema SEGELE and Peter J. LAMB
Cooperative Institute for Mesoscale Meteorological Studies and School of Meteorology
The University of Oklahoma - Norman, OK 73019 – USA
Ethiopia has been ravaged by severe drought for many of the last 35 years, primarily due to the
failure of its main (Kiremt) rainy season in boreal summer. Kiremt quality results from the timing
of its onset and cessation and the frequency and duration of intervening dry-spells. To address these
key aspects of Kiremt variability, we analyzed specially constructed sets of research quality, daily
rainfall and rawinsonde data for the longest available periods (25–57 years).
The analyses produced wide-ranging results of considerable value to Ethiopia. The long-term
average spatial progression of the southwest-to-northeast Kiremt onset and its reverse cessation are
documented, along with measures of their interannual variability. Treated on a similar geographical
basis is the local vulnerability to Kiremt interruption by dry-spells. Rawinsonde data for central
Ethiopia are analyzed to place these long-term mean surface Kiremt characteristics in the context of
the annual cycles of tropospheric wind, temperature, and pressure.
Investigation of the rich interannual Kiremt variation focuses on onset, cessation, and growing
length (which excludes dry-spells) anomalies. The analyses begin with the compositing of
indicative tropospheric profiles for sets of extremely dry and wet Kiremt seasons. This is followed
by examination of 1961–99 time series of the above Kiremt parameters, which prompts case study
investigations of the highly contrasting 1984 (very dry) and 1996 (much wetter) Kiremts in terms of
both Ethiopian rainfall and the tropospheric circulation of the surrounding region. Finally,
correlation analyses are used to investigate relations between the above key Kiremt parameters for
the most drought-prone (northeastern) part of the Kiremt region and global tropical-subtropical sea
surface temperature patterns, including the ENSO phenomenon.
286
CARACTERISTIQUES ET VARIABILITES DE LA SAISON
DES PLUIES KIREMT EN ETHIOPIE
L’Ethiopie a été au cours de ces 35 dernières années ravagée par une série de sècheresse à cause du
déficit enregistrée par sa plus importante saison des pluies (Kiremt) durant l’été boréal. La qualité
de la saison Kiremt est déterminée par l’effet combiné de son démarrage, de sa fin, de sa fréquence,
ainsi que par la longueur des séquences sèches. Afin d’investiguer ces paramètres essentiels de la
variabilité de Kiremt, nous avons procédé à une analyse méthodique d’une série de précipitations
journalières de haute qualité et des données de radiosondage sur la plus longue période disponible
(25-57 années).
Les résultats obtenus sont variables et sont d’une importance capitale pour l’Ethiopie. La
progression spatiale sud-ouest à nord-est de la date (moyenne à long terme) du début de Kiremt
ainsi que son effet inverse caractérisant sa fin, sont documentés. La vulnérabilité aux séquences
sèches au cours de la saison Kiremt a été étudiée. Les données de radiosondage pour le centre de
l’Ethiopie ont été analysées afin de placer les moyennes à long terme des caractéristiques de surface
de la saison Kiremt dans le contexte du cycle annuel troposphérique du vent, de la température et de
la pression.
Dans l’étude de la variabilité interannuelle de Kiremt, nous avons mis l’accent sur les anomalies de
la date de démarrage, de la fin, et de la longueur de la saison (exclusion faites des épisodes secs).
Nous avons procédé à une étude comparative des paramètres troposphériques indiquant les années
sèches et humides de la saison Kiremt. Cette étude comparative est suivie de l’analyse des séries
chronologiques des paramètres de Kiremt sur la période 1961-99 et qui par ailleurs, nous a incités à
l’analyse de cas fortement opposés des années 1984 (très sèche) et 1996 (beaucoup plus humide) en
terme de précipitation en Ethiopie et de la circulation troposphérique de la région environnante. En
fin, l’analyse des corrélations est utilisée pour documenter la relation existante entre les paramètres
de la saison Kiremt de la région la plus sèche (nord-est) et les températures de surface de l’Océan
global.
287
1.64P
TOWARDS HIGH-RESOLUTION TREE-RING PROXY CLIMATE RECORDS
FOR THE WEST AFRICAN SAHEL SAVANNA ZONE
Tarhule AONDOVER (1) and Ibrahim BOUZOU MOUSSA (2)
(1) Department of Geography, University of Oklahoma , Norman, Oklahoma, USA
(2) Univ. Abdou Moumouni, Dépt de Géographie, Niamey, Niger
Sharply diminished seasonal rainfall totals in the Soudano Sahel zone of West Africa since 1968
“provide the most dramatic example worldwide of climate variability that has been directly and
quantitatively measured” (Hulme 2001, p.20) and “represents one of the strongest inter-decadal
th
signals on the planet in the 20 century” (Lebel et al. 2003, p.52). Despite this global significance, it
is unclear - primarily because of the short length of instrumental records - whether mega droughts
are a recurring feature of the Sahelian climate or whether the current drought represents a new
phenomenon in the climate dynamics of the region (Street-Perrot et al. 2000; Hulme 2001). There is
a need, therefore, for high-resolution (annually resolved) paleoclimatic proxies that provide a longer
temporal perspective for contemporary climate in the Sahel. Such information could facilitate the
incorporation of low frequency mega-droughts in long-run climate models of the West African
monsoon.
This paper reports preliminary findings from a study exploring the feasibility of using tree rings to
develop proxy records of annual seasonal rainfall, effectively extending the length of instrumental
records. The study has identified 6 indigenous West African tree species that satisfy all
requirements for chronology development. These are Cassia sieberiana, Cordyla pinnata, Daniella
oliveri, Isoberlinia doka, Tamarindus indica (Caesalpiniaceae), and Acacia seyal (Mimosaceae).
Biweekly tree growth time series based upon band dendrometers installed on 45 members of
Daniella oliveri and Isoberlinia doka trees in Burkina Faso and Niger suggests that microenvironmental conditions may play a greater role in tree growth than previously acknowledged. To
further investigate this hypothesis, an automated environmental monitoring station was installed at
Torodi (Niger) in December, 2004 to collect information on climatic variables that impact tree-ring
formation. Twenty two additional band dendrometers were also installed on a third tree specie
(Tamarindus indica) in the vicinity of the environmental station. The result so far indicates that
asynchronous growth (driven by microenvironmental conditions) complicates conventional tree ring
analysis using ring width indices. Consequently, we analyzed the inter-ring variations in stablecarbon isotope composition of cellulose in Tamarindus indica and compared the results with other
13
climatically sensitive tree-ring δ C series. Our ultimate goal is to explore the links between SSTs
tree ring chronologies developed using either stable carbon isotopes or ring width indices.
288
1.65P
ANALYSIS OF THE SEASONAL CYCLE OF THE AFRICAN MONSOON :
REGIONAL APPLICATIONS ON CAMEROON
M. BELLA-MEDJO, S. JANICOT and B. SULTAN
Laboratoire d’Océanographie et du Climat: Expérimentation et Approche Numérique (LOCEAN ex
LODYC), Université Pierre et Marie Curie, Paris, France
The African monsoon is characterized by the seasonal migration of the Inter Tropical Convergence
Zone (ITCZ). Most of the studies of the African monsoon focused on the northern summer period
and on the West Africa zone. Therefore, only few studies deal with the analysis of the African
monsoon in Central Africa and outside the northern summer period. That’s why we have focus our
study in Cameroon whose latitudinal extension from Central Africa to the Sahelian area allows us to
follow the whole migration of the ITCZ.
We will show first a statistical analysis of the seasonal cycle of rainfall over West Africa over the
1925-1992 period by using the monthly rainfall dataset of CRU with the 3.75° x 2.75° and 0.5° x
0.5° resolution.
Then, we will use the 0.5° monthly rainfall dataset to describe regional rainfall indexes over the
Cameroon region by using the rotated extended Empirical Orthogonal Functions (EOFs). This
method allows to point out the relationships between the seasonal cycle and the annual cycle. These
relationships are given by the spatial patterns of the weight of each month on the interannual
variability.
After regionalizing the seasonal cycle over Cameroon, we will study the anomalies of cotton
production in the northern Cameroon. Thus, we will compute the correlation between cotton
production and rainfall over this region of Cameroon. This study will be done by using in one hand
the cotton production dataset provided by the SODECOTON (SOciété de DEveloppement du
COTON), and on the other hand the CRU 0.5° monthly rainfall dataset.
Submitted by
Marthe Bellla-Medjo - [email protected]
289
1.66P
THE HYDROLOGICAL ONSET AND WITHDRAWAL INDEX FOR
THE WEST AFRICA MONSOON
G.A. DALU, M. GAETANI, V. CAPECCHI, M. BALDI and F. GUARNIERI
IBIMET, CNR, Roma, Italy
From March to November the rainfall in the West Africa up to 20°N is related to the seasonal
migration of the ITCZ. In the coast of Guinea there are two rainy seasons, since, in its meridional
excursion, the ITCZ rainbelt crosses this region twice, in spring and in fall, while in summer
establishes itself in the Sudan-Sahel region. The monsoonal air penetrates inland driven by large
meridional gradients of the moist static energy (MSE) in the PBL, the moist monsoonal air in the
PBL behaves as a density current about 2 km deep.
Using the NCEP reanalysis2 daily data and the GPCP pentad rainfall data, we compute the moisture
transport integrated from the surface to 850 hPa (VIMT) in the West Africa region, to elaborate the
hydrological onset and withdrawal index (HOWI). We use the HOWI to diagnose the onset and
withdrawal of the West Africa monsoon (WAM).
The onset of the WAM occurs in late June, when the ITCZ experiments an abrupt latitudinal shift
from the coast of Guinea (5°N) to 10°N. The climatological date is June 24. We compute the HOWI
in the Sahel (10°W – 10°E, 13°N – 18°N), where the VIMT has pronounced variability around the
onset date.
Analyzing the HOWI from 1979 to 2004, we find that the mean date for the onset occurrence is
June 28 with 11 days standard deviation and that the mean date for the withdrawal is September 27
with 18 days standard deviation.
Submitted by
Dr.MarcoGaetani, CNR, Roma, Italy - e-mail:[email protected]
290
1.67P
LE PHENOMENE DE MOUSSON DANS LE GOLFE DE GUINEE
ENTRE LE TOGO ET LE BENIN
Kodjovi Sidéra EDJAME
Laboratoire de Climatologie Physique, Département de Géographie, Univ. de Lomé, Togo
Comme la plupart des pays du golfe de Guinée, le climat du Togo et du Bénin est largement
influencé par deux grandes masses d’air : la mousson (vent à secteur dominant Sud-Ouest) et
l’harmattan (vent à secteur dominant Nord-Est).
La mousson représente pour toute l’Afrique de l’Ouest une masse d’air chargé d’humidité
susceptible de favoriser la pluviométrie. En réalité, son arrivée dans le golfe de Guinée coïncide
avec le début du développement des phénomènes convectifs de grande ampleur. Contrairement à ce
que l’on pourrait penser, la mousson commence sa pénétration sur les côtes ouest-africaines dès la
mi-février. Elle s’étale tout doucement sur l’arrière-pays au fil du temps, apportant les premières
pluies véritables déjà au cours du mois de mars. Sa progression en latitude fait déplacer la zone de
séparation des deux masses d’air de direction opposée communément appelée Front Inter Tropical
(FIT) du Sud vers le Nord. La position du FIT permet finalement de suivre la répartition territoriale
des différentes saisons aussi bien au Bénin qu’au Togo.
L’épaisseur de la couche de mousson augmente régulièrement sur tout l’ouest- africain pour
atteindre son maximum entre Juillet et Août.
Lorsque la trace au sol du FIT sera au-delà du 13 ème parallèle, les pluies s’amenuisent dans le sud
du Bénin et du Togo et s’intensifient au contraire dans la partie septentrionale de ces deux pays.
Au cours des dernières années la répartition des pluies en nette corrélation avec le déplacement du
FIT connaît un certain bouleversement. Serait-il dû au phénomène de réchauffement planétaire en
cours, à l’origine des évènements climatiques extrêmes ? Cette communication a pour but
d’apporter des éléments de réponse à cette interrogation.
Mots clés : Mousson, harmattan, golfe de Guinée, FIT, réchauffement planétaire, évènements
climatiques extrêmes
291
1.68P
ETUDE DE LA VARIABILITE SPATIO-TEMPORELLE DES PLUIES
MOYENNES MENSUELLES DE LA REGION SUD-OUEST
DE LA COTE D’IVOIRE
Bi Tié Albert GOULA, Issiaka SAVANE, Vamoryba FADIKA,
Yves N’GUETTIA et Koffi KOUADIO
Laboratoire Géosciences et Environnement, Abidjan, Côte d’Ivoire
La variabilité spatio-temporelle des précipitations à différentes échelles en Afrique de l’Ouest est
bien connue. Cette étude a pour objectif de montrer cette variabilité des pluies mensuelles à une
2
échelle plus réduite, sur un domaine de la côte Atlantique de 317 km au sud-ouest de la Côte
d’Ivoire sous l’influence de la mousson. Ce sont les isohyètes des moyennes mensuelles de 18
postes pluviométriques sur la période 2000-2002 qui ont été analysées et complétées par une étude
géostatistique. Pendant la grande saison des pluies (avril à juillet) où les alizés humides en
provenance de l’océan Atlantique souffle sur le sud, le gradient de pluviométrie s’établit du nord-est
au sud-ouest alors qu’en saisons sèches (décembre à mars et août à septembre), les précipitations
mensuelles sont plus irrégulièrement reparties, avec très souvent plusieurs maxima localisés en
différents endroits du domaine à part le Sud. Les écarts entre hauteurs pluviométriques mensuelles
moyennes peuvent atteindre 185 mm, pendant les fortes précipitations (mai et juin), pour des
stations qui ne sont distantes que de 30 km au plus. L’étude géostatistique montre différents types
de variogramme mensuels avec des portées ajustées au modèle sphérique variant 5,94 km à 11,51
km pour les mois de janvier, février, avril, août, septembre et octobre. Les portées comme les écarts
entre pluie des stations augmentent avec l’importance des précipitations. Les variogrammes des
autres mois ont pu être ajustés au modèle de puissance avec des pentes qui varient de 0,5 à 0,33. Le
paramètre λ varie de 0,86 à 0,98.
Mots clés : Variabilité, pluie mensuelle, géostatistique, mousson africaine, sud-ouest, Côte d’Ivoire.
Contact :
Laboratoire Géosciences et Environnement, UFR des Sciences et Gestion de l’Environnement,
Université d’Abobo-Adjamé, Abidjan, Côte d’Ivoire - [email protected]
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VARIABILITE SPATIO-TEMPORELLE DES PLUIES MENSUELLES DU
SUD-OUEST DE LA COTE D’IVOIRE ET EVOLUTION
DU FRONT INTER TROPICAL (FIT)
Bi Tié Albert GOULA (1), Théophile LASME (2), Issiaka SAVANE (1),
Yves N’GUETTIA (1) Vamoryba FADIKA(1), Koffi KOUADIO (1)
et Bernard SRHOUROU(3)
(1) Laboratoire Géosciences et Environnement, Univ. Abobo-Adjamé, Abidjan, Côte d’Ivoire
(2) Laboratoire des Sciences et Techniques de l’Eau, Univ de Cocody, Abidjan, Côte-d’Ivoire
(3) Sodexam, Direction de la Météorologie Nationale, Abidjan, Côte d’Ivoire
La présente étude permet de montrer la variabilité spatio-temporelle des pluies mensuelles à une
2
échelle spatiale plus réduite, sur un domaine de la côte Atlantique de 317 km au sud-ouest de la
Côte d’Ivoire. Cette zone subit l’influence directe de la mousson ouest africaine. Les séries
pluviométriques mensuelles de 18 postes d’observation de la période 2000-2002, ont été analysées
et complétées par une étude géostatistique.
Ainsi, durant la grande saison des pluies (avril à juillet) où les alizés humides en provenance de
l’océan Atlantique soufflent sur le sud, le gradient pluviométrique s’établit du nord-est au sud-ouest
alors qu’en saisons sèches (décembre - mars et août - septembre), les précipitations mensuelles sont
plus irrégulièrement reparties, avec très souvent plusieurs maxima localisés en différents endroits
du domaine hormis le Sud. Les écarts entre hauteurs pluviométriques mensuelles moyennes peuvent
atteindre 185 mm, pendant les fortes précipitations (mai et juin), pour des stations qui ne sont
distantes que de 30 km au plus.
L’étude géostatistique montre différents types de variogramme mensuel ajustées au modèle
sphérique avec des portées variant de 5,94 km à 11,51 km pour les mois de janvier, février, avril,
août, septembre et octobre. Les portées tout comme les écarts pluviométriques des stations
augmentent avec l’importance des précipitations. Les variogrammes des autres mois ont pu être
ajustés au modèle de puissance avec des pentes qui varient entre 0,5 et 0,33. Le paramètre λ varie de
0,86 à 0,98.
L’analyse a été complétée par l’étude de l’évolution du FIT avec les divers paramètres d’analyse.
Mots clés : Variabilité, pluie mensuelle ; géostatistique ; mousson africaine ; front inter-tropical ;
Côte d’Ivoire.
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1.70P
VARIATIONS SAISONNIERES ET INTERANNUELLES DES ONDES
DE GRAVITE EN AFRIQUE EQUATORIALE
Pétronille KAFANDO (1), Fabrice CHANE-MING (2) et Monique PETITDIDIER (3)
(1) LPCE, Université de Ouagadougou, Burkina Fasso
(2) LPA, Université de la Réunion, France
(3) CETP, Vélizy, France
1. Introduction
L’atmosphère est caractérisée par la présence importante de structures ondulatoires de propriétés et
d’échelles très variées. Si on fait une analyse spectrale du vent ou d’autres traceurs atmosphériques
on obtient des pics dans une gamme de période qui peut aller de l’ordre de 6mn, période de Brunt
Väisälä correspondant à la « respiration de l’atmosphère », jusqu’à des périodes de plusieurs jours.
Au voisinage de relief, on observe des ondes stationnaires tant que le vent incident est stationnaire.
Les ondes de gravité correspondent à la réponse de l’atmosphère à des perturbations locales ; ces
ondes n’existeraient pas sans la force de gravité. Ces ondes assurent le couplage entre les différentes
régions de l’atmosphère ; la forme de ce couplage dépendant de la période des ondes et de leur
extension horizontale et verticale. Ces ondes jouent un rôle important dans la dynamique à petite et
grande échelle (météorologie locale et circulation globale) par l’intermédiaire du transport vertical
d’énergie et de quantité de mouvement. A petite échelle, ces ondes entretiennent des interactions
complexes avec la turbulence. Depuis 1960, date de la mise en évidence des ondes de gravité dans
l’atmosphère de nombreuses études théoriques et à partir de données expérimentales ont été
effectuées. Les études climatologiques des ondes de gravité font encore l’objet d’une étude active
notamment sur les variations géographiques et temporelles de l’activité des ondes de gravité et de
leurs caractéristiques spectrales dans un objectif commun de les représenter au mieux dans les
modèles numériques compte tenu de leur importance, leurs petites échelles et les limites en matière
de résolution numérique(Wang and Geller, 2003). De nombreuses zones géographiques ont été
étudiées cependant la climatologie de la région tropicale et équatoriale africaine, objet de cette
étude, n’existe pas.
Cette étude est juste à son début. Nous allons présenter les données utilisées pour cette étude et
l’étude préliminaire effectuée à partir de ces données.
2. Les données
Les données concernées par cette étude sont des séries temporelles de paramètres météorologiques
soit obtenues à une altitude fixe, au sol ou sur un mat, soit des profils en fonction de l’altitude. Les
données déjà sélectionnées pour cette étude sont les radiosondages de paramètres météorologiques
(Pression, Température, Humidité, Vent) et d’ozone, et les données de radar profileur de vent.
Les radiosondages météorologiques de sites africains sont disponibles auprès de l’ASECNA et du
site web de l’Université du Wyoming. Les radiosondages d’Ozone font partie des bases de données,
SHADOZ et NDSC (Network for the Detection of Stratospheric Change).
294
Deux radars UHF-profileur de vent, appartenant à l’ASECNA, sont installés sur l’aéroport de
Bamako (Mali), et très prochainement sur l’aéroport de Ouaga Dougou. Un radar VHF-profileur de
vent de la communauté scientifique française sera installé à Djougou au Bénin en 2006 dans le
cadre de l’expérience AMMA.
3. Etude préliminaire
Dans l’étude préliminaire nous avons utilisé les données d’une seule station de radio-sondages. La
première étape a consisté à valider les données, c'est-à-dire d’éliminer les points et profils aberrants.
Ensuite nous avons tracé les pour chaque année la variation de chacun des paramètres et déterminer
leur variation annuelle et interannuelle. Ensuite nous avons mis en évidence la présence d’ondes
dans les données.
L’étape suivante consiste à utiliser les techniques originales d’analyse des données développées par
F. Chane-Ming(2002) afin d’extraire les caractéristiques des ondes à partir des radiosondages.
4. Conclusion
Cette étude qui vient juste de démarrer va permettre de déterminer les variations saisonnières et
annuelles des caractéristiques des ondes et de les corréler aux événements météorologiques de cette
région afin de déterminer les interactions possibles. Citons les jets et la convection, sources connues
des ondes, l’oscillation quasi-biennale, l’ENSO, la mousson…. De ces études une paramétrisation
des caractéristiques des ondes sera tentée car c’est le seul moyen de traduire leur impact dans les
modèles météorologiques actuels.
Des questions sont toujours à l’étude. Dans des études de cas qui seront faites à partir des données
obtenues lors de l’expérience AMMA et d’une modélisation, le spectre des ondes générées par les
sources de chaleur liées aux nuages et les jets sera déterminé et comparé aux données
expérimentales.
Ces travaux apporteront une contribution originale à la climatologie des ondes de gravité dans la
zone tropicale, où leur rôle est très important, et leur interaction avec la météorologie, et aussi à une
meilleure connaissance de leur sources et du spectre ainsi généré.
Références
Chane-Ming, F., G. Roff, L. Robert and J. Leveau, Gravity wave characteristics over Tromelin Island during the
passage of cyclone Hudah, Geophys. Res. Lett., 29, 6, 10.1029/2001GL013286, 2002.
Thompson, A.M., J.C. Witte, R.D. McPeters, S.J. Oltmans, F.J. Schmidlin, J.A. Logan, M.Fujiwara, V.W.J.H.
Kirchhoff, F. Posny, G.J.R. Coetzee, B. Hoegger, S. Kawakami, T. Ogawa, B.J. Johnson, H. Vömel and G. Labow,
Southern Hemisphere Additional Ozonesondes (SHADOZ) 1998-2000 tropical ozone climatology 1. Comparison with
Total Ozone Mapping Spectrometer (TOMS) and ground-based measurements, J. Geophys. Res., Vol. 108 No. D2,
8238, doi: 10.1029/2001JD000967, 30 January 2003.
Thompson, A.M., J.C. Witte, S.J. Oltmans, F.J. Schmidlin, J.A. Logan, M. Fujiwara, V.W.J.H. Kirchhoff, F. Posny,
G.J.R. Coetzee, B. Hoegger, S. Kawakami, T. Ogawa, J.P.F. Fortuin, and H.M. Kelder, Southern Hemisphere
Additional Ozonesondes (SHADOZ) 1998-2000 tropical ozone climatology 2. Tropospheric variability and the zonal
wave-one, J. Geophys. Res., Vol. 108 No. D2,8241, doi: 10.1029/2002JD002241, 31 January 2003.
Wang L. and M.A. Geller, Morphology of gravity-wave energy as observed from 4 years(1998-2001) of high vertical
resolution U.S. radiosonde data, J. Geophys. Res., 108, D16, 4489, doi : 10.1029/2002JD002786, 2003.
295
1.71P
AN EXTREME SAHARAN DUST OUTBREAK IN SPRING 2004 AND ITS
IMPACT ON THE ONSET OF THE WEST AFRICAN MONSOON
Peter KNIPPERTZ (1) and Andreas H. FINK (2)
(1) Institute of Atmospheric Physics, Mainz, Germany
(2) Institute of Geophysics and Meteorology, Köln, Germany
Between 4 and 6 March 2004 a large-scale, strong, and persistent outbreak of Saharan dust onto the
adjacent tropical and subtropical Atlantic Ocean was observed in satellite imagery (Fig. 1). The dust
front is initially related to a density current caused by strong evaporational cooling along a
precipitating cloud band that penetrates into the northern Sahara ahead of an upper-level trough. At
later stages massive upper-level convergence, sinking, low-level divergence, and an explosive
anticyclogenesis over northwest Africa cause strong northerly flow and a quick spreading of the
dust front to the south and west (Fig. 1). The strong pressure gradients over North Africa are further
enhanced by the formation of a cyclone ahead of the upper-trough. Surface observations show that
the event was accompanied by unusual sensible weather conditions across large parts of North
Africa including cold temperatures and high wind speeds in the Sahara and extreme precipitation in
Libya.
Fig. 1: Meteosat visible satellite images for 12 UTC on (a) 4 and (b) 6 March 2004.
High-resolution surface observations at various stations in southern West Africa from this period
show that the day before the arrival of the dust front was characterized by a heat wave at the Guinea
coast. The maximum temperature at Cotonou (Benin) of 37ºC exceeded the highest observed value
for 1931–90 (Fig. 2a). Observations reveal anomalous subsidence, below-average cloud cover, low
relative humidity (Fig. 2a) and a delayed onset of the cooling monsoonal southwesterlies on that
day (Fig. 2b). The latter is consistent with a weakening of the continental heat low as indicated by
large positive pressure tendencies at Cotonou (Fig. 2b). A pronounced drop in temperature, relative
humidity and visibility, as well as strong northerly winds, characterizes the arrival of the dust front
early on 5 March 2004 (Fig. 2).
296
Fig. 2: (a) Hourly METAR temperature (solid) and 3-hourly SYNOP relative humidity (dashed)
observations at Cotonou (Benin) between 00 UTC 4 March and 23 UTC 6 March 2004. (b) As in
(a) but for METAR 24-hour pressure tendency (dashed), and hourly visibility (solid) and wind
(barbs).
The dust outbreak had a distinct impact on the onset of the African monsoon in the spring of 2004.
The Intertropical Convergence Zone (ITCZ) was pushed to an unseasonally southerly position in
the aftermath of the dust outbreak (see Fig. 1b). Over the following two weeks the cold air over the
North African continent and the large dust loadings hindered the establishment of the African heat
low, resulting in an unusually late and prolonged Harmattan episode. Radiation observations from
Cotonou show that it took in fact about two weeks until the total down-welling shortwave radiation
and the 24-hour net radiation reached the positive values observed before the dust outbreak. This
shift in the heat low and ITCZ position was associated with a delayed northward progression of the
African monsoon and widespread dry anomalies along the Guinea Coast.
Fig. 3: 10-minute total downwelling shortwave radiation
(line) and 24-hour net fullspectrum radiation (grey bars)
at Cotonou for 1–21 March
2004.
Reference
Knippertz, P. and Fink, A. H., 2005: Synoptic and dynamic aspects of an extreme springtime Saharan dust outbreak. Q.
J. R. Meteor. Soc., in press.
Contact
Peter Knippertz - Institute of Atmospheric Physics, University of Mainz, Becherweg 21, D-55099
Mainz, Germany - Email: [email protected]
Andreas H. Fink - Institute of Geophysics and Meteorology, University of Cologne, Kerpener Str.
13, D-50937 Köln, Germany; Email: [email protected]
297
UNE IRRUPTION EXTREME DE POUSSIERE SAHARIENNE AU PRINTEMPS
2004 ET SA REPERCUSSION SUR LE DEBUT
DE LA MOUSSON AFRICAINE OCCIDENTALE
En mars 2004 on a observé dans les images satellite une irruption à grande échelle, forte et
persistante de poussière saharienne sur l'Océan Atlantique tropical et sous-tropical voisin.
Initialement le front de poussière est relié à un courant de densité provoqué par le refroidissement
fort causé par l’évaporation de précipitation le long d'une bande de nuages qui pénètre dans le
Sahara du Nord devant un thalweg d´altitude. Pendant des phases postérieures, de la convergence
d´altitude massive, de la décrue, de la divergence à basse altitude et une anticyclogenèse explosive
en Afrique du Nord-ouest causent un écoulement du nord fort et une propagation rapide du front de
poussière vers le sud et l'ouest. Les gradients de pression forts en Afrique du Nord sont encore
renforcés par la formation d'un cyclone devant le thalweg d´altitude.
Les observations de surface prouvent que l'événement a été accompagné des conditions de temps
inhabituelles à travers de grandes parties de l'Afrique du Nord comprenant des températures froides
et des vitesses de vent fortes au Sahara, de la précipitation extrême en Libye et une vague de
chaleur à la Côte de Guinée. L’irruption de poussière a déplacé la zone de convergence
intertropicale dans une position plus vers le sud que normal. L'air froid sur le continent africain du
nord et les grands chargements de la poussière gênent l'établissement de la dépression de chaleur
africaine, ayant pour résultat un épisode de Harmattan exceptionnellement tard et prolongé. Cet
événement est associé à une progression retardée de la mousson africaine vers le nord et des
sécheresse répandues le long de la Côte de Guinée pendant mars 2004. Les processus décrits sont
documentés dans des observations de surface à haute résolution de vent, de température, de point de
rosée et de rayonnement à de diverses stations en Afrique occidentale tropicale.
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DEFICIT PLUVIOMETRIQUE RECORD AU SENEGAL EN 2002
J. B. NDONG et P. SAGNA
Laboratoire de Climatologie et d’Environnement. Département de Géographie
Université Cheikh Anta DIOP de Dakar, Dakar, Sénégal
Le Sénégal a connu en 2002 un déficit pluviométrique record qui nous a rappelé, par certains
aspects, les années 1968, 1983 et 1991. Ce déficit, par rapport aux résultats de notre analyse de la
situation météorologique et du déroulement de la saison pluvieuse est dû à plusieurs facteurs. Il est
lié tout d’abord à une migration cahoteuse de la trace au sol de l’Equateur Météorologique (FIT),
puis à un faible nombre de jours de présence de la mousson notamment dans les stations
septentrionales et centrales du pays et à une faible épaisseur de ce flux ce qui a un rapport avec le
potentiel précipitable advecté. Il est enfin lié à un nombre réduit de passages de lignes de grains
dont l’efficacité pluviométrique a été moindre et à des remontées peu importantes sur le Sénégal de
la Zone Intertropicale de Convergence. Les totaux pluviométriques ont été faibles surtout dans la
zone septentrionale du pays où les quantités recueillies n’ont pas atteint 250 mm (quantité inférieure
à 300 mm qui est la limite des cultures sous pluie au Sahel). La situation n’a été guère meilleure
dans les domaines nord soudanien et sud soudanien. Dans ce dernier, il n’y a que la station de
Kédougou qui a enregistré une quantité supérieure à 1 000 mm. Les importants déficits se sont
traduits par la non satisfaction des besoins en eau des plantes dans plusieurs localités. Or, dans les
zones sahélienne et soudanienne où l’agriculture reste prédominante et fragile, ces importants
déficits ont eu des répercussions sérieuses sur l’ensemble de l’économie et ont constitué un facteur
essentiel de modification de l’équilibre du milieu naturel et de l’organisation spatiale. Ce travail
propose d’utiliser les données des précipitations mensuelles de 23 stations réparties de façon
homogène sur l’ensemble du territoire pour cerner les fluctuations majeures qui ont caractérisé le
déroulement de la saison des pluies 2002 et montrer les impacts sur la campagne agricole 2002 –
2003.
Mots-clés : Déficit pluviométrique, Sénégal, flux de mousson, zones climatiques, agriculture.
Soumis par :
Pascal SAGNA - Département de géographie, Université Cheikh Anta DIOP de Dakar, Dakar,
Sénégal – email : [email protected]
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300
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CHARACTERIZATION OF THE MONSOON CIRCULATION DIURNAL CYCLE
AROUND THE MONSOON ONSET
B. SULTAN (1), P. DROBINSKI (2) and S. JANICOT (1)
(1) LOCEAN / IPSL, France (2) SA / IPSL, France
Introduction
Previous studies have depicted a coherent diurnal cycle in the West African monsoon winds (Parker
et al. 2005). The monsoon winds are the weakest in the afternoon when the turbulent boundary layer
is deep and the strongest overnight when the boundary layer turbulence is much weaker. The
present study addressed the question of this diurnal cycle of the monsoon circulation around the
monsoon onset. It is based on a Principal Component Analysis (PCA) and wavelet analyses applied
to composite surface wind and temperature fields around the onset date (Sultan et al. 2003).
The two spatio-temporal modes of the diurnal cycle
A composite analysis around the onset date, detected by the ITCZ shift from 5N to 10N (see Sultan
and Janicot 2003), is applied to the 1979-2000 NCEP/NCAR reanalyses at the main synoptic hours
0000, 0600, 1200 and 1800 UTC. It reveals an enhancement of the heat low circulation and
monsoon surface winds from 1800 to 0600 UTC at the time of the monsoon onset in coherence with
the previous work from Parker et al. (2005).
Figure 1 : PC1 and PC2 loadings (top) and time series (bottom)
302
To highlight the temporal and spatial patterns of the diurnal cycle of NCEP/NCAR fields around the
onset of the monsoon, we apply a PCA on 1800-0600 differences of the 1979-2000 composite mean
of winds and surface temperatures. The meridional and zonal components of the surface wind and
the surface temperatures have been considered as the variables of the input matrix of the PCA. The
two first modes explain around 75% of the variance of the diurnal cycle of surface wind and
temperatures. The first mode shows a northward extension of the wind anomalies in the heat low
and a decrease of the temperature diurnal cycle in the Inter-Tropical Convergence Zone around
10°N. The temporal pattern of this mode is gradual with a seasonal peak reached 40 days after the
onset of the monsoon (t0). The second mode highlights the evolution of the diurnal cycle of the
surface wind between 10°N and 15°N. This diurnal cycle is characterized by a nocturnal jet (Parker
et al. 2005) whose intensity increases rapidly 50 days before t0 and reaches its maximum at the time
of the monsoon onset. Thereafter the wind diurnal cycle decreases rapidly. The PCA shows a
similar temporal pattern for surface temperatures but with spatial anomalies located northward.
Indeed, the second mode characterizes an increase of the temperatures diurnal cycle maximum
around the onset date north of 10°N near the Atlas Mountains.
A wavelet analysis applied to wind speed
In order to document the temporal pattern of the wind diurnal cycle already pointed out by the
second PC, we applied a wavelet analysis to a wind speed index (with 4 values per day) near the
area of PC2 maximum loadings in wind fields, e.g. 10°W-10°E ; 10°N-15°N. The wavelet modulus
shows highest values for the diurnal cycle between t0 - 40 days and t0. Thereafter, the diurnal cycle
amplitude decreases in coherence with the PC2 time series.
Conclusion
This study has pointed out two independent modes describing the spatio-temporal variability of the
diurnal cycle in surface wind and temperatures. While the first mode appears to belong to a gradual
and seasonal pattern linked with the northward migration of the whole monsoon system, the second
mode is characterized by more rapid time variations with a peak of both temperature and wind
anomalies around the monsoon onset date. The latter mode is connected with the time pattern of the
nocturnal jet reaching its highest values around the onset date as it is suggested by a wavelet
analysis applied to surface wind near the location of the nocturnal jet. However, further studies have
to be done to explain the temperatures anomalies associated with the second mode, showing an
increase of the diurnal cycle in Northern Africa, and to connect this variability to the wind pattern.
References
Parker DJ et al. (2005) The diurnal cycle of the west African monsoon circulation, Quat. J. of the Royal Meteorol. Soc.,
in press.
Sultan B. et S. Janicot (2003), The West African monsoon dynamics, Part II : The “pre-onset” and the “onset” of the
summer monsoon, Journal of Climate, 16, 3407-3427.
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L’EVOLUTION SAISONNIERE DU CYCLE DIURNE DE LA MOUSSON
Des études récentes ont mis en évidence un cycle saisonnier dans les vents de mousson en Afrique
de l’Ouest. Ces vents sont les plus faibles dans l’après-midi quand la couche limite turbulente est
faible et les plus forts la nuit quand cette couche limite est plus faible.
L’objectif de notre étude est de caractériser l’évolution saisonnière de ce cycle diurne de la
mousson en Afrique de l’Ouest avec un accent mis sur le moment de la mise en place de la
mousson. Une analyse composite autour de cette date de mise en place de la mousson est appliquée
aux champs de réanalyses NCEP/NCAR pour chacune des heures 0000, 0600, 1200 et 1800 UTC.
Elle montre un renforcement de la circulation associée à la dépression thermique saharienne entre
1800 et 0600 UTC au moment de la mise en place de la mousson.
Pour mettre en évidence l’évolution spatio-temporelle du cycle diurne, on réalise une Analyse en
Composante Principale (ACP) sur les champs composites de températures et de vent de surface en
différence entre 1800 et 0600. Les résultats de cette ACP permettent de discuter des interactions
entre le cycle saisonnier et particulièrement le démarrage de la mousson et le cycle diurne des
champs de surface.
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LOW-LEVEL CIRCULATIONS ASSOCIATED WITH
THE WEST AFRICAN MONSOON
CHIDONG ZHANG AND PHOEBE WOODWORTH
Rosenstiel School of Marine and Atmospheric Science
University of Miami, Miami, FL, USA
In situ sounding observations from selected sites in West and Central Africa are used to document
the seasonal cycle in the lower troposphere associated with the seasonal migration in precipitation
of the West African Monsoon (WAM). The main purpose of this exercise is to provide a description
of the seasonal cycle free of errors and biases of numerical models, which can be substantial in this
region. Many features in the circulation typically associated with the WAM exist through almost the
entire seasonal cycle in the region. The low-level easterly jet, known as the African Easterly Jet
during the monsoon season, remains prominent and closely related to the rainband in other phases
of the seasonal cycle, even when the rainband is located near the equator. The surface southerly
monsoon onshore flow penetrates through the rainband almost in the entire season cycle. The depth
of the southerly surface monsoon flow undergoes a seasonal variation, which is greatest during the
peak season of the monsoon. The most unique feature is a low-level (~700 hPa) northerly flow,
which penetrate through the rainband in various phases of the seasonal cycle. These observations
are synthesized into a schematic describing the salient features in the low-level circulation
associated with the rainband in different phases of the seasonal cycle. This schematic circulation
pattern in West Africa is in contrast to the traditional conceptual circulation pattern associated with
the marine ITCZ and other monsoon systems (see figure). The results from this study call for more
research attention to the low-level meridional circulation for a full understanding of the dynamics of
the West African Monsoon. This study also advocates the study of the West African Monsoon to be
conducted in a broad context of seasonal cycle in the region and in comparison with the marine
ITCZ.
305
(a) Phase I
700 hPa
Equ
Africa
(b) Phase II
500 hPa
Eq
u
Africa
(c) Phase III
500 hPa
X
Africa
Equ
(d) Marine ITCZ
700 hPa
Equ
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SIGNATURE OF LARGE-SCALE MODELS ALONG A MERIDIAN TRANSECT
OVER WEST AFRICA : AMMA-CROSS
F. GUICHARD (1), F. FAVOT (1), F. HOURDIN (2), I. MUSAT (2),
J.-F. GUEREMY (1), J.-P. LAFORE (1), P. MARQUET (1) and P. RUTI (3)
(1) CNRM-GAME (CNRS & Météo-France), Toulouse, France
(2) Laboratoire de Météorologie Dynamique, CNRS, IPSL, Paris, France
(3) ENEA, Roma, Italy
I. Context
West Africa is characterized by well defined strong meridian surface gradients (illustrated in Fig.
1), coupled to specific atmospheric circulations, such
as the African easterly jet (AEJ) which is present
.
during the monsoon season. The location of the AEJ
itself is strongly constrained by meridian surface
.
temperature and moisture gradients (Thorncroft &
Blackburn QJRMS 1999). In particular, inter-annual
.
variability of the West African monsoon (WAM) is
accompanied by changes in these basic structures.
.
Synoptic variability in turn is dominated by African
easterly waves which are dynamically linked to the
no
dat
AEJ. The structure and variability of these basic large10°
10°
scale features involve complex interactions with soil, Figure 1: Shortwave albedo 1st decade of June 1997
(POLDER-1, POSTEL, AMMASAT).
surface, turbulent and convective processes occurring
on different scales. Finally, the WAM exhibits specific
seasonal variations, with an abrupt monsoon onset to be compared to a more progressive latitudinal
retreat (Sultan & Janicot J. Climate 2003).
While current numerical weather prediction (NWP) analyses seem able to reasonably capture these
large-scale atmospheric features, the extent to which large-scale models are able to properly
reproduce these observations remains unclear, and likely sensitive to changes in the physical
parametrizations.
The objective of this starting study is therefore to get a more precise view concerning the ability of
the large-scale models involved in the AMMA project to simulate these fundamental features of the
WAM. Beyond, we also want to explore and compare the mechanisms and feedbacks involved in
the WAMs as depicted by these models.
To do so, we follow an approach proposed by Siebesma et al. (QJRMS 2004), and apply it to the
Western African region: we define a North-South cross-section, over which we compare model
behaviours with analyses and observational products, with the help of dedicated diagnostics.
307
II. Case-study and specifications for models and simulation diagnostics
-1
(mm.day )
The meridian cross section
2000
2003
2000
2003
TAMS
DRY
DRY
spans 10W to 10E (Fig. 1),
YEA
WET
YEA
WET
and extends from 20S to
YEA
YEA
2
20°
20°
40N. Two recent contrasted
1
years: 2000 (year of the
1
JET2000 experiment) and
1
2003
(MOZAIC
data
15°
15°
1
available for chemistry) have
1
been chosen. They present
8
distinct northern migration of
6
the
monsoon,
with
10°
10°
4
significantly more (less)
2
rainfall North (South) of 8N
0
in 2003 (Fig. 2). This is
accompanied by typical M J J A S O M J J A S O
M J J A S O M J J A S O
changes in the dynamical
Figure 2: May to October time-latitude series of rainfall as given by TAMSAT &
GPCP products for the two selected years, [10W-10E] average decadal values.
structures such as seen on the
August average zonal wind
(Fig. 3): for instance, in 2003, the monsoon flow is thicker, the AEJ weaker and locater further
north, while the TEJ is stronger, when compared to the same monthly mean for 2000. Note however
that in 2003 the Eastern Atlantic sea surface temperatures (SST) anomaly was positive towards the
Equator and South of it, i.e. the 2003 WAM does not fall in the category of wet years previously
identified, that are characterized by similar rainfall patterns, but by an opposite correlation with SST
anomalies. Figure 2 also underlines the significant differences existing between available rainfall
datasets (here TAMSAT and GPCP products, GPCP overestimates rainfall rates compared to
TAMSAT).
(m.s-1)
NCEP/NCAR reanalysis
5
200
TEJ
0
(hPa)
-5
400
-10
-15
600
AEJ
800
1000
-20
2000
DRY
YEAR
10°S
-25
-30
-35
0°
10°N
20°N
30°N
10
(hPa)
200
5
0
400
600
800
-5
2003
WET
YEAR
-10
-15
-20
-25
1000
Figure 3: 2000 & 2003 August [10W,10E] average zonal
wind, from NCEP/NCAR reanalysis.
308
Time-varying SST fields been provided to modellers (files & information available from
http://amma-mip.lmd.jussieu.fr/), and a list of model outputs has been defined: it consists in timelatitude-height and time-latitude series of daily-mean fields averaged between 10W and 10E. These
include atmospheric fields (winds, temperature, humidity, convective heating...), surface and top of
the atmosphere variables and quantities such as heat, water and radiative fluxes. Reanalyses and
data products over [10W, 10E] are available as figures at http://www.cnrm.meteo.fr/ammamoana/transect/index.html (contact [email protected] for pw).
III. Models, simulations, discussion of preliminary results
So far, preliminary simulations from three general circulation models (GCMs) ARPEGE-Climat,
ECHAM4 & LMDZ4 have been realized. They differ in many respects. For instance, even if the
radiation scheme is the same among two models, their treatment of aerosols is distinct (which may
affect the heat low properties and AEJ, Tompkins et al. GRL 2005). Soil initialization methods and
the associated spin-up duration also vary according to the land-surface scheme that is used. Beyond
these first simulations, we are planning to perform additional simulations, with increased resolution
and/or updated parametrizations.
All the simulations reasonably depict the broad seasonal migration of rainfall. The differences
among them concern in particular: the range of the northern monsoonal extension of rainfall, the
rainfall retreat phase, the magnitude of intraseasonal variability. Differences appear between rainfall
observational products and the reanalyses and among reanalyses as well. At the same time, while all
GCMs are able to produce an AEJ, simulated AEJs differ in their latitudinal position and strength.
These distinct behaviours of models are quite robust along the monsoon season. Differences among
simulations are not restricted to the monsoon season though.
In addition, these results are found to be sensitive to the convection scheme (LMDZ4) and to the
vertical resolution (ECHAM4). For LMDZ4, differences among runs performed with different
convection schemes are larger than among ensemble runs all performed with the same convection
scheme.
Finally, the contrasted rainfall regimes between 2000 and 2003 seem to be captured by ARPEGEClimat.
IV. Perspectives
Further work will involve in particular the analysis of surface and TOA variables, heat, water and
radiative fluxes, and how they are related to convective activity and cloud cover. (Fig. 4 shows the
seasonal variation of some of them as depicted by the NCEP/NCAR reanalysis). The surface flux
climatology database developed within AMMA should be particularly helpful for this study as well
as AMMA-SAT and ISCCP products. Links with the chemistry-transport modelling studies should
also be strengthened through our choice of years (simulations available).
A complementary and coupled study AMMA-MAPS will allow to assess how representative this
cross-section analysis is and to relate it to the synoptic and intra-seasonal variability simulated by
models versus observed over West Africa. (see http://amma-mip.lmd.jussieu.fr for further
information.)
309
310
θe (K)
340
20°N
300
10°N
320
290
0°
300
θ (Κ)
-2
LE (W.m )
20°N
200
100
10°N
50
100
0°
0
-2
H (W.m )
J F M A M J
0
J A S O N D
J F M A M J J
A S O N D
Figure 4 : NCEP/NCAR reanalysis, year 2000 seasonal latitudinal
variation of 10W, 10E daily mean potential temperature θ, equivalent
temperature θe close to the surface, and surface sensible and latent heat
fluxes H & LE.
SIGNATURE DE MODELES DE GRANDE ECHELLE LE LONG D’UN
MERIDIEN EN AFRIQUE DE L’OUEST : AMMA-CROSS
L’Afrique de l’ouest présente de forts gradients méridiens en surface, qui se combinent avec des
circulations atmosphériques spécifiques, telles le jet d’est africain (JEA) qui se développe pendant
la mousson. La position du JEA est elle-même fortement contrainte par les gradients méridiens de
température et d’humidité à la surface. En particulier, la variabilité inter-annuelle de la mousson en
Afrique de l’ouest (MAO) s’accompagne de modifications de ces structures de base. La variabilité
synoptique est à son tour dominée par les ondes d’est africaines, qui sont elles-mêmes reliées au
JEA. La structure et la variabilité de ces phénomènes de grande échelle résultent d’interactions
complexes avec les processus intervenant au niveau du sol, de la surface, et les processus
convectifs, interactions qui se manifestent à différentes échelles. Finalement, la MAO présente des
variations saisonnières particulières, comme un saut de mousson rapide en début de saison, tandis
que le retrait de la mousson est plus progressif.
Ces structures de grande échelle sont relativement bien vues par les analyses issues des systèmes
numériques de prévision du temps. Cependant, il n’est pas évident que les modèles de grande
échelle soient capables de les reproduire correctement, étant donné que ces structures sont
probablement sensibles aux paramétrisations physiques utilisées.
L’objectif de cette étude est de fournir des éléments plus précis concernant la capacité des modèles
de grande échelle à reproduire ces structures fondamentales de la MAO. Pour ce faire, nous avons
défini un transect nord-sud en Afrique de l’ouest, couvrant la bande de longitude [10°O,10°E], avec
lequel nous comparons, pour les années 2000 et 2003, le comportement des modèles avec les
analyses et des produits d’observation, via des diagnostics adaptés. Le cas d’étude et des résultats
préliminaires seront présentés à la conférence.
310
1.77P
VALIDATION DES SORTIES PLUVIOMETRIQUES DES MODELES DE
CIRCULATION GENERALE (GCMS) AU SAHEL, AUX ECHELLES
MENSUELLE ET JOURNALIERE
Mohamed HAMATAN
LTHE, Grenoble, France et AGRHYMET, Niamey, Niger
La meilleure connaissance des impacts futurs des changements climatiques sur les divers secteurs
socioéconomiques et les stratégies d’adaptation, reste la principale préoccupation des pays du Sahel.
En effet ces pays sont considérés comme les plus vulnérables aux effets néfastes des changements
climatiques.
Les modèles climatiques constituant les principaux outils d’évaluation des impacts du climat sur
l’écosystème, il convient donc d’en mesurer leurs performances au Sahel.
Cette étude vise, à partir de quelques indicateurs, à valider les sorties des modèles climatiques au
Sahel sur la période 1961-90, aux échelles mensuelle (signal pluviométrique annuel, cumul
pluviométrique interannuel, gradient Nord-Sud et biais) et journalière (pluie journalière maximale,
pluie journalière supérieure à un seuil, séquence sèche, séquence pluvieuse, …),
L’étude permettra d’identifier parmi les six GCMs proposés par le GIEC, ceux reproduisant au
mieux la variable climatique pluie au Sahel selon les deux échelles de temps. Aussi, une évaluation
zonale est faite en décomposant le Sahel en quatre (4) zones identifiées à partir d’une étude
préalable menée sur la base de données mensuelles.
Ainsi les meilleurs modèles, donnant les meilleures valeurs aux critères d’évaluation pour une zone
donnée, peuvent être exploités pour des études d’impacts du changement climatique.
Mots clés :, GCM, critère de performance, pluie, changement climatique, Sahel.
Contact
Mohamed Hamatan - LTHE/Grenoble et AGRHYMET/Niamey (BP 11011 Niamey, Niger)
Tel. : +227 56 18 96 / 73 31 16 - Fax : +227 73 24 35 - [email protected]
311
1.78P
SENSITIVITY OF THE ATMOSPHERIC TRANSPORT AND MONSOON
RAINFALL TO THE PARAMETERIZATION OF MOIST CONVECTION IN THE
LMDZ4 CLIMATE MODEL
Frédéric HOURDIN (2), Ionéla MUSAT (2), Marie-Angèle FILIBERTI (1),
Jean-Yves GRANDPEIX (2) and Mai PHAM (3)
(1) Institut Pierre Simon Laplace, (2) Laboratoire de Météorologie Dynamique
(3) Service d’Aéronomie
(1, 2, 3) Université Pierre et Marie Curie, Paris, France
The LMDZ general circulation model is the atmospheric component of the IPSL coupled model
IPSL-CM4 which has been used to perform climate change simulations for the 4th IPCC assessment
report. During the development phase of the coupled model, the atmospheric model has undergone
a series of evolutions. As for the simulation of the tropical climate, the replacement of the Tiedtke
convection scheme by Emanuel's scheme has a major and generally positive impact. The vertical
distribution of convective heating is affected with a contrasted behavior over land and ocean. The
convective heating is relatively higher over continents with Emanuel. As a consequence, large scale
low level convergence over continents is reinforced with Emanuel's scheme. Over West Africa, the
monsoon rainfall extends farther north with the new parametrization in better agreement with
observations. The modification of the convection has also a strong impact on the transport of
atmospheric constituents. Tests performed with idealized tracers are analysed to understand how the
modification of the parametrized convective transport and the associated modification of the large
scale atmospheric circulation contribute to the changes in tracer transport. This first attempt could
serve as a basis for an intercomparison of tracer transport in the chemistry transport models used in
the frame of AMMA
LMDZ4 is the atmospheric component of the Institut Pierre Simon Laplace Coupled Model (IPSLCM4). It is also used for the simulation of the direct and inverse transport of trace species (Hourdin
et al., 2005) and is coupled to a module for INteractive Chemistry and Aerosols (INCA,
Hauglustaine et al., 2004). The simulations below are done with the IPCC configuration (horizontal
resolution of 3.75°x2.5° and 19 levels, Marti et al, 2005), except that the model is forced by the
mean seasonal cycle of the sea surface temperature and uses a simple bucket model for surface
hydrology.
312
Figure 1 : July cross-section over West-Africa (averaged 10W-10E) for the convective heating
(grey scale, W/m2), zonal wind (thick contours, m/s) and meridional stream function (with arrows)
for two simulations performed with the LMDZ4 model with either Tiedtke or Emanuel scheme for
the parametrization of convection.
Two versions of LMDZ4 are used here, which differ only by the representation of convective
processes for which we use either Tiedtke's or Emanuel's scheme. Both schemes attempt to
parametrize explicitely the convective mass fluxes in the updraughts and downdraughts as well as
the induced motion in the environment. In the Tiedtke scheme, the convective cloud is idealized as
one saturated updraught and one saturated downdraught compensated by a slow subsidence in the
environment. The version used here is close to the original version of Tiedtke (1989) and relies on a
“closure” in moisture convergence. In the Emanuel (1993) scheme (see also Grandpeix et al., 2005),
the convective cloud is represented by multiple buoyancy sorted saturated draughts (both ascending
and subsiding). The backbones of the convective clouds are regions of adiabatic ascent originating
from some low-level layer and ending at their level of neutral buoyancy. The closure and triggering
take into account both tropospheric instability and convective inhibition. Emanuel's scheme is one
of the few schemes simulating precipitating downdraughts with an intensity comparable to CRM
simulations (Guichard et al., 2004). The radiative effect of the convective clouds is computed with a
probability distribution function of the total subgrid scale water coupled to the convective scheme
(Bony et Emanuel, 2001).
The sensitivity of the large scale circulation to convection (a large scale convergence globally
stronger and higher over continents with Emanuel's, see Hourdin et al., 2005) has a clear signature
over West Africa. This is illustrated adopting the cross-section framework (AMMA-CROSS) of the
AMMA-MIP (Model Intercomparison Program, see poster by F. Guichard and others) in which the
fields are averaged in longitude over the region 10W-10E. In July, the convective heating is larger
over continents with Emanuel's (north of 5N) and peaks higher in altitude (grey scale, Fig. 1). The
separation between the Inter Tropical Front (ITF) at 20N in the lower troposphere and Inter
Tropical Convergence Zone (ITCZ) is also somewhat clearer with Emanuel's and the return branch
of the North Hadley cell is located higher. The African Easterly Jet is better represented with
Emanuel's scheme. The convective parametrization has also a strong impact on both the large scale
(indirectly) and parametrized transport of atmospheric tracers.
313
Figure 2 : Latitude-pressure cross-section (July, averaged 10W-10E) of the CO concentration
(contours, ppmv) and of the concentration of an idelized tracer emited in the lower troposphere over
Africa, south of 10N (grey scale, arbitrary units).
The CO concentration simulated with the INCA module are shown in Fig. 2 (contours). When
compared with the Tiedtke's scheme (left) the simulation with Emanuel's (right) shows a stronger
and isolated maximum at the top of the convective region (5N, 200hPa). The contrasts are also
generally stronger with Emanuel's, with a region almost free of CO north of 20N. Those difference
are analysed further by using idealized tracers emited in the boundary layer (we force the tracer
concentration to be larger than an hyperbolic tangent function decreasing from 1 at the surface to 0
above, with a sharp transition at 850 hPa) over different regions of the globe. The tracer emited over
the African continent south of 10N (grey scale in Fig.2), behaves very similarly to the CO obtained
in INCA. Idealized tracers can be used further to isolate the relative importance of large scale and
parametrized convection. For the results shown here, the role of the parametrized transport is
responsible for most of the differences observed in Fig. 2. (not shown).
Main conclusions
1. Changing the convection scheme (from Tiedtke's to Emanuel's) has a major impact not only on
the climate (here a better representation of the AEJ and mean meridional circulation when using
Emanuel) but also on the transport of chemical compounds.
2. The transport of tracers contains important information on the large scale and convective
transport.
3. The AMMA-CROSS approach and the use of idealized tracers may serve as a framework for
intercomparison of chemistry transport models for AMMA.
References
Bony and Emanuel, 2001, J. Atmosph. Sci., 58, 3158-3183.
Emanuel, 1993, AMS Meteorol. Monographs, 24 (46), 185-192.
Guichard et al., 2004, QJRMS, 130 (604C), 3139-3172
Grandpeix et al., 2005, QJRMS, in press.
Hauglustaine et al., 2004, J. Geophys. Res., D18, 4314-+.
Hourdin et al., 2005, QJRMS, in press.
Marti et al., 2005, http://dods.ipsl.jussieu.fr/omamce/IPSLCM4/DocIPSLCM4/
Tiedtke, 1989, Mon. Wea. Rev., 117, 1179-1800
314
1.79P
MODEL CHARACTERIZATION OF WAM MEAN SEASONAL CYCLE
T. LOSADA (1), J. GARCIA (1), B. RODRIGUEZ-FONSECA (1), H.-Y. MA
and C. R. MECHOSO (2)
(1) Departamento de Geofisica y Meteorologia, UCM, Spain
(2) Department of Atmospheric Sciences, UCLA, USA
In the framework of the AMMA-EU project, an ensemble of WAM simulations were performed
with the UCLA AGCM . The work was carried out with the high resolution version of the model
(2.5º longitude by 2º latitude, and 29 vertical sigma layers). The simulations were 21-year-long and
used the Reynolds climatological SST as boundary conditions. The results are compared with the
NCEP-NCAR reanalysis data. The transition between spring (MAM) and the rainy season (JAS) is
studied using the precipitation, surface and 850 hPa temperature, 850 hPa humidity and divergence
fields.
The results show that the model overestimates precipitation in general, but particularly so in eastern
equatorial Africa, However, the observed spatial patterns of relative humidity are well reproduced
by the simulation, in particular the spring to summer northern migration of the humidity maximum
from the Guinean coast. Similar overestimations are noticeable in the Asian-Monsoon region. The
air temperature at the surface is underestimated over the continent and overestimated above the
oceans, but the late is correct at 850 hPa level. The spatial distribution of temperature and its
seasonal evolution are well simulated at both, the surface and 850 hPa.
Looking at divergence field it can be seen that during the rainy season (JAS) exists a clear
latitudinal shift in the dry convection. During spring (MAM) the model dry convection is in good
agreement with the observations. The wet convection during summer (JAS) is well simulated. The
model output shows two principal areas of divergence, one centred around 650hPa and the second
around 200hPa. There are similar features in the observation.
Results and discussion
Figure 1 shows the averaged seasonal precipitation during march-may (MAM), may-july (MJJ) and
july-september (JAS). Precipitation is overstimated over the entire region during the period of
study. The maximum of precipitation located on the Guinean coast in MAM reaches values of 12
mm/day, whilst the observed record (not shown) is around 9 mm/day. The model reproduces the
seasonal migration of the ITCZ up to the north, located between 5N-10N during MJJ and 10N-12N
during JAS (figure 1 center, right) [1][2]. However, although the location and magnitude of de
maximum in both sides of the Guinean coast are well represented, the simulated longitudinal
rainbelt over Sahel is overstimated, compared with the observations. Maximum precipitation in
eastern equatorial Africa appears specially overstimated in the three seasonal periods plotted.
In order to study in depth the behaviour of the WAM precipitation pattern, the monthly
precipitation, 850 hPa humidty and 850 hPa air temperature evolution is represented (figure 2 left,
center). These diagrams have been performed for the 10W-10E longitudinal window, and from 0 to
315
25N. As it was observed in figure 1, the precipitation (fig 2 left, center, contours) is greater than the
observational records. Although the slope changes in the rainbelt [3]can be identified, in the daily
evolution (figure 2, right) these changes are more clear. In this way, the results show the pre-onset
(at the middle of may) connected to the monsoon winds arrival and the begining of the rainy season,
and the onset (at the end of june), associated with the northward shift of the ITCZ. It can be seen a
clear relation between precipitation and humidity at low tropospheric levels. Both Huvmuller
diagrams are very similar, with the maximum humidity values located within the higher values of
precipitation (figure 2, left). In fact, the simulated humidity is greater than the observed one, which
is consistent with the overstimation of precipitation found in the model simulation. The 850hPa
temperature and precipitation diagrams (figure 2, center) show both fields changing together, with
minimum values of air temperature located below the maximum of precipitation. It is important to
emphasize on the presence of anomalously high values of simulated precipitation northern 15N
(over Sahara region) during June and July, with respect to the observations, accompanied by the
corresponding positive humidity anomalies and negative air temperature anomalies.
To complete the simulated mean seasonal cycle desciption, fields of divergence and vertical
velocity have been represented in latitude-heigh cross-sections, for 10W-10E and for MAM, MJJ
and JAS (figure 3). During spring (figure 3, left), the location of wet (0-5N) and dry (10N-15N)
convection are in good agreement with the observations [2][4], although wet convection is
overstimated. The seasonal latitudinal shift of both vertical motions is well simulated. However,
during MJJ (figure 3, center), the width of the wet convection is extended beyond 12N, further
north than the 10N limit reached in the observations. This could be the cause of the anomalous
maximum of precipitaion found in June and July. Wet and dry convection simulated during JAS
(figure 3, right) is in acordance with observations, although simulated wet convection (5N-10N) is
somehow weaker than the observed one. Two principal areas of divergence, associated with both
upward motions, appear in the model results: one (dry) is centred on 650hPa and the second (wet)
around 200hPa.
Figure 1 : simulated seasonal mean precipitation. Left: march-may. Center: may-july.
Right: july-september
Figure 2 : Simulated time-latitude diagrams. Left: monthly mean precipitation (contours) and
850hPa humidity (shaded). Center: monthly mean precipitation (contours) and 850hPa air
temperature
316
Figure 3 : Simulated vertical sections of Omega (contours) and divergence (shaded) seasonal
means, as function of latitudes, averaged over 10W-10E. Left: march-may. Center: may-july.
Right: july-september.
References
[1] Sultan, B. and S. Janicot, 2000 : Abrupt shift of the ITCZ over West Africa and intra-seasonal variability
Geophysical Research Letters, 27, 3353-3356.
[2] Sultan, B S. Janicot and A. Diedhiou. 2003: The West African Monsoon Dynamics. Part I: Documentation of
Intraseasonal Variability. Journal of Climate: Vol. 16, No. 21, pp. 3389-3406.
[3] Sultan, B. and S. Janicot, 2003 : The West African monsoon dynamics. Part II: The pre-onset and the onset of the
summer monsoon. J. Climate, 16, 3407-3427.
[4]Fontaine B, Philippon N, Trzaska S, Roucou P,2002: Spring to summer changes in the West African monsoon
through NCEP/NCAR reanalyses (1968-1998). JGR 107 (D14): Art. No. 4186
317
1.80P
SIMULATIONS OF THE 2000 AND 2003 AFRICAN MONSOON SEASONS
WITH THE UCLA AGCM
Belen RODRIGUEZ-FONSECA (1), T. LOSADA (1), J. GARCIA (1), I. POLO
and C. R. MECHOSO (2)
(1) Departamento de Geofisica y Meteorologia, UCM, Spain
(2) Department of Atmospheric Sciences, UCLA, USA
In the framework of the WP4.1 of the AMMA-EU Project, a first intercomparison exercise will
focus on testing the performance of general circulation models (GCM) and regional models (RCM)
over the west African Monsoon (WAM) region for the years 2000 and 2003. These years are
characterized by dry and wet WAM, respectively. In this work, the results from two 10-member
ensemble simulations for the 2002 and 2003 WAMs using the high resolution UCLA Atmospheric
GCM forced with global observed SST anomalies are presented.
(a.1)
(a.2)
(b.1)
(b.2)
(c.1)
(c.2)
Figure 1: 2003 – 2000 UCLA model precipitation results. Ensemble time evolution of the 5-day
mean zonal precipitation for the 10E-10W longitudinal band and between 0 to 25N for (a) year
2000; (b) year 2003; (c) 2003 minus 2000. Column 1 corresponds to model results, whilst column
2 corresponds to CMAP observed one
318
Figure 1 (a, b) shows the main results obtained for the precipitation. In particular, the ensemble
time evolution of the 5-day mean zonal precipitation for the 10E-10W longitudinal band and
between 0 to 25N shows that the main changes in the rainbelt slope from March to December are
well represented by the model. Also, this ensemble precipitation evolution for each of the simulated
years shows that the August maximum is larger in 2003 than in 2000.
In order to know the significant differences between both simulated WAM years, a T-test has been
performed between the 2 10-member samples of each year, representing only those regions in
which the equal mean null hypothesis is refused. As has been pointed out by other authors [1], the
model represents how the 2003 precipitation maximum is located northern and later that the 2000
one; although for the year 2003, the model represents the northern shift of the ITCZ northern
boundary up to the north from the beginning of spring. Also, it can be seen how the preonset of
2003 is, in general, drier [3].
Preliminary results show how, in general, for both simulations, there is a good reliability of the
model mainly during the preonset and over the ocean.
Figure 2: Annual evolution, as a function of months and latitudes (10ºW-10ºE) of ensemble 5-day
means simulated moist static energy (MSE) for 2000 (left), 2003 (right).
The model also shows how, at the beginning of spring, the moist static energy (MSE) meridional
gradients are stronger over the continents (north 5ºN). Also, it can be seen how, after this, they relax
and the monsoon air mass is transported northward into the continent. The model slightly shows
how, before the wettest rainy season(2003), the moist static energy (MSE) meridional gradients
tend to be stronger. Nevertheless, the model clearly shows how after the wettest rainy season
(2003), the moist static energy (MSE) meridional gradients tend to relax later [3] (figure 2).
The large difference in the quality of the simulation to the east and west of about 10°E is also
apparent in the geographical distribution of the 600 mb geopotential height and zonal wind, as
shown in Fig. 3.
The speed of the African easterly jet (Fig 3), located above 5N-10 N in JAS at 600 hPa, decreases
during the 2003 monsoon more than during 2000 [2].
319
Figure 3: JAS ensemble model (left) versus NCEP observed (right) 600 hPa geopotential height
(contours) and zonal wind (shaded) for 2003 (top) and 2000 (bottom).
The latitudinal shift of the dry and wet convection from MAM to JAS is well reproduced by the
model for 2000 and 2003 (fig 4). Fig 4 also shows how, around 10N, the model reproduces
subsidence in low levels for the 2003 that, although deeper and wider, agrees with observations but
not with other observational studies [1,3]. During the preonset, convection is higher over 5N in
2000 that in 2003 and, during the monsoon season, the dry and wet convections over 5N and 20N
respectively are deeper during 2003 than 2000.
Figure 4: ensemble simulated 2003 (right), 2000 (left), latitude-height cross-section (averaged over
10ºW-10ºE), of the zonal wind (u,contours), omega (w,shaded) and (u,w,arrows) for MAM(top),
and JAS (bottom).
References :
[1] Janicot, S., B. Sultan, Intra-seasonal modulation of convection in the West African monsoon, Geophys.
Res.
Lett., 28(3), 523-526, 10.1029/2000GL012424, 200
[2] Sultan, B Serge Janicot and Arona Diedhiou. 2003: The West African Monsoon Dynamics. Part I: Documentation of
Intraseasonal Variability. Journal of Climate: Vol. 16, No. 21, pp. 3389–3406
[3] Fontaine B, Philippon N, Trzaska S, Roucou P,2002: Spring to summer changes in the West African monsoon
through NCEP/NCAR reanalyses (1968-1998). JGR 107 (D14): Art. No. 4186
320
1.81P
CONVERGENCE DU FLUX D’HUMIDITE SUR L’ATLANTIQUE TROPICAL ET
PREVISION SAISONNIERE DES PRECIPITATIONS
SUR L’AFRIQUE DE L’OUEST
V. BENOIT (1), P. ROGEL (2), Y. M. TOURRE (3) et L. JARLAN (4)
(1) CERFACS/Météo France, Toulouse, France
(2) CERFACS, Toulouse, France
(3) MEDIAS-France and Lamont Doherty Observatory of Columbia University, USA
(4)Météo France/CNRM, Toulouse, France and ECMWF, Reading, UK
Un modèle dynamico-statistique de prévision des pluies en Afrique de l’Ouest est mis en place à
partir de prédicteurs issus des prévisions saisonnières multi-modèles rétrospectives du projet
DEMETER. Une analyse des précipitations en ACP (rotation suivant le critère Varimax), permet de
mettre en évidence trois modes expliquant un maximum de variance sur les régions Sénégal, Sahel
et Guinéenne. On peut montrer que ces trois modes sont associés à la variabilité thermique de la
surface de l’Atlantique tropical.
Lorsque l’on analyse en ACP la divergence du flux d’humidité calculé à partir des paramètres
DEMETER, trois modes liés à la position de l’ITCZ et les conditions thermiques de l’océan sont
également mis en évidence. Ces trois modes sont aussi fortement corrélés avec les régimes de
précipitations.
Le modèle probabiliste développé utilise donc ce lien statistique, via la construction de relations
conditionnelles entre les probabilités d’occurrence des quantités de pluies et de la
divergence/convergence du flux d’humidité sur l’Atlantique tropical. Les scores de prévision
probabilistes sur la saison des pluies utilisant ces liens statistiques sont significativement supérieurs
aux scores de prévision utilisant directement les précipitations issues des modèles.
321
1.82P
INTERANNUAL VARIABILITY AND PREDICTABILITY OF THE WEST
AFRICAN MONSOON CIRCULATION
S. CORTI (1), F. MOLTENI (2) and F. KUCHARSKI (2)
(1) ISAC-CNR Institute of Atmospheric Sciences and Climate, Bologna, Italy
(2) ICTP International Centre for Theoretical Physics, Trieste, Italy
Atmospheric variability and predictability of the West African Monsoon is investigated using
observational data and a large ensemble of AMIP-like experiments with an atmospheric general
circulation model of intermediate complexity. The variability of the atmospheric flow can be
thought as a superposition of an internal part, due to intrinsic dynamical variability, and an external
part, due to the variations of the boundary forcing (i.e. sea surface temperatures (SSTs) and landatmosphere interaction). The two components are identified by performing a 75-member ensemble
of atmospheric simulations with prescribed, observed SSTs in the period 1949 - 2002. The large
number of realizations allows the estimation of statistics of the atmospheric variability with a high
confidence level. The analysis performed focuses on interdecadal and interannual variability. The
model reproduces well the structure of the observed trend. The model results are compared with the
observed dataset and the degree of predictability of the large scale monsoon circulation is evaluated.
Submitted by
Susanna Corti – Istituto di Scienze dell'Atmosfera e del Clima - Consiglio Nazionale delle Ricerche
[Institute of Atmospheric Sciences and Climate - National Research Council], Via Gobetti, 101 I40129 - Bologna - Italy – Phone : +39 051 6399603 – Fax : +39 051 6399658 email:
[email protected]
322
1.83P
BURKINA FASO SEASONAL RAINFALL PREDICTABILITY :
SAHELAN OR GUINEAN REGIMES
Guillaume NAKLOUMA (1) and Ousmane NDIAYE (2)
(1) Direction de la Météorologie Nationale, Burkina Faso
(2) IRI, Columbia University, Palisades, New York, USA
An EOF analysis of 59 stations in Burkina Faso delineates sahelian (BFS) and guinean (BFG)
climate modes of variability, respectively Northern and Southern part of Burkina Faso. These two
regions are investigated separately for seasonal predictability.
The Sahelina part of Burkina is closely related to Pacific SST near the ENSO region whereas the
guinean part of Burkina is related to the Atlantic near the gulf of Guinea. Both seem to have a
common signal coming from the Indian Ocean. These results are further confirmed with GCM
simulations forced with SST.
The sahelian part can be predicted by using Pacific SST directly and GCM output. However the
Guinean region can be predicted only by Atlantic SST and not accurately by GCM output. Results
of the study will be presented.
1.84P
ANALYSIS AND DOWNSCALING OF SUB-SEASONAL VARIABILITY OF
SENEGAL RAINFALL WITH WEATHER CLASSIFICATION
Vincent MORON, Andy W. ROBERTSON, M. Neil WARD and Ousmane NDIAYE
IRI, The earth institute at Columbia University Palisades, New York, USA
A k-means clustering scheme is used to summarize daily atmospheric variability at regional-scale
around Senegal during the boreal summer season 1961-1998 from the European Centre for Medium
Weather Forecast Reanalyses (ERA-40). The weather regimes characterize typical phases of subseasonal features while other states describe particular phases of the seasonal cycle. The temporal
behavior is characterized by a systematic evolution, together with considerable variability at subseasonal and inter-annual time scales. Daily rainfall occurrence at 13 gauge stations in Senegal is
found to be moderately conditioned on regional atmospheric conditions identified by the clusters.
The clustering is then used to downscale the 24-run ensemble of the ECHAM4.5 general circulation
model forced by prescribed sea surface temperatures. The simulations are able to capture quite well
the inter-annual changes in daily rainfall occurrence and also mean length of dry and wet spells at
some individual stations.
Submitted by : Vincent MORON [email protected]
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1.85P
GETTING AROUND POOR GCM RAINFALL SIMULATION FOR
SENEGALESE SEASONAL FORECASTING: CIRCULATION
ALTERNATIVE
Ibrahima NDIAYE (2), Ousmane NDIAYE (1) and Amadou GAYE (1)
(1) LPAO-SF, Ecole Supérieure Polytechnique, U,C A Dakar, Senegal
(2) IRI, Columbia University, Palisades, New York, USA
Using 21 rain gauges over Senegal with good spatial coverage, we investigate the potential of
seasonal forecasting for the JAS season over the period 1968-2003. We compare predictability from
Sea Surface Temperature and Model Output Statistic of ECHAM forced with observed and
persisted SST. Senegalese stations rainfall are closely related to the Equatorial South Atlantic SST
near the gulf of Guinea with an out of phase. The first Principal Component of the lower level wind
(UEOF1GCM) over the tropical Atlantic capturing 40% of the variance offers a good predictability
of Senegalese rainfall. Using GCM wind gives higher skill than precipitation simulated by the
model. UEOF1GCM is as good as the SST signal and very similar to that of the reanalysis.
Dans ce travail, nous avons étudié les relations entre la pluie et différents indices dont elle dépend
en vue de la prévision saisonnière. En effet, prévoir la pluie revient à déterminer les réponses
atmosphériques aux fluctuations des différentes variables qui influent de façon directe ou indirecte
et de déterminer l’importance de leurs influences.
Pour se faire, la méthodologie utilisée est le Downscaling. C’est une méthode de réduction d’échelle
qui permet de faire le lien entre les modèles de circulation général et les modèles d’échelle fine. Il
permet d’augmenter la résolution spatiale des modèles suivant trois procédés particuliers :
dynamique, statistique, hybride. Dans cette étude nous avons utilisé la méthode statistique.
Les données utilisées sont les données de vent et de pression réduite à la surface de la mer
provenant des réanalyses du NCEP1 qui sont des données d’observation du vent zonal à 925Hpa en
JAS2 et sur la période 1950 2003. Elles couvrent notre période d’étude avec une résolution de
2.5°X2.5°. Nous avons utilisé aussi les données de pluie de 34 stations de la base de données de la
Direction de la Météorologie Nationale du Sénégal sur la période d’étude et avec une couverture
spatiale du pays suivant la disponibilité des données. Comme prédicteurs nous avons utilisé le vent
zonal issu du modèle ECHAM4.5forcé par les SST sur la période 1950 à 2003 et les données des
SST provenant du service de météorologie des Etats- Unis (NWS), obtenues par mesure in situ et
par satellite avec une résolution de 2°X2° sur presque toute l’étendue du globe et couvrant notre
période d’étude.
Par une analyse en composantes principales des données de pluie mensuelles des 34 stations du
Sénégal, nous avons isolé deux modes EOFs3 ou composantes principales qui représentent environ
60% de la variance totale. La cohérence de ce choix se justifie par le fait qu’on a une bonne
corrélation entre ces deux EOFs et les stations initiales. On a donc utilisé ces EOFs puisqu’elles
sont anticorrélées entre elles et captent la majeure partie de la variance. Pour recherchez les
prédicteurs qui permettent de déminer la variabilité du système, nous avons corrélé ces EOFs
d’abord avec le vent zonal observé du NCEP puis on a effectué la comparaison avec les cartes
obtenues en utilisant le vent du modèle ECHAM4.5 forcé par les SST et les SST du signal de base
sous les tropiques. Ces résultats montrent que le modèle ECHAM4.5 donne des informations plus
324
proches de l’observation. Pour confirmer ce résultat nous avons choisi de faire la prévision avec ces
deux prédicteurs en utilisant le fichier des EOFs constitué de 21 stations. L’analyse canonique de
corrélation donne les previsions suivantes:
Performance de prévisions avec NCEP
Performance des prévisions avec ECHAM
Performance de prévision avec SST du signal de base
Les corrélations obtenues avec les sorties du modèle ECHAM 4.5 sont positives et presque
identiques à celles de l’observation, alors que celles des SST du signal de base sont négatives qui
est donc anticorrélé avec la pluie. On a aussi effectué la validation croisée entre l’observation et la
prévision. Cette validation croisée nous a permis d’une part de confirmer les résultats précédents et
d’autre part de prouver la validité du modèle. En effet, le modèle permet d’identifier les années
humides (situées au dessus de la normale) et les années sèches (en dessous de la normale) et ceci
n’est valable que si le prédicteur utilisé est les sorties du modèle ECHAM.
Ainsi, l’utilisation du modèle dynamique permet de capter les variations locales du système
atmosphériques qui sont presque invisibles dans le cas d’une téléconnéction de grande échelle.
1 - National Center for Environemental Prediction
2 - Juillet Août Septembre
3 – Empirical Orthogonal Fonction
Mot-clé : Downscaling, modèle, corrélation, prévision, prédicteur, validation croisée
325
1.86P
USING TROPICAL ATLANTIC CIRCULATION TO PREDICT
SAHEL RAINFALL
Ousmane NDIAYE (1) and M. Neil WARD (2)
(1) DEES, Columbia University, USA
(2) International Research Inst., Columbia University, USA
In this study we examine the SST signal associated with the loss of skill of JAS rainfall in the Model
Output Statistic (MOS) corrected forecasts over Sahel. The ECHAM4.5-GCM does not simulate
well the interannual rainfall variability in time and space over Sahel region but it captures the
decadal trend during 1950-2004 period. We show also that the GCM is capable of reproducing the
wind circulation at 925hPa fairly well in the interannual and decadal time scale. An EOF
transformation of the wind field is used to forecast the rainfall in Sahel. The first EOF of the zonal
wind capturing 34% of the variance, is capable of recovering the skill of the GCM from 0.10 to 0.57
during the period 1968-2002. When we use forecasted SST by persisting SST anomaly of June over
JAS climatology a skill of 0.54 is obtained using the MOS. When May SST anomaly is persisted
through JAS climatology the skill drops to 0.23.
1. Introduction
The Sahel region of West Africa has a well known short rainy season from June to September
followed by a long dry season. The rainy season itself is marked by a strong interannual variability
embedded in low frequency multidecadal trend variability. A forecast of the rainy season is crucial
for the population in Sahel (Ndiaye et al. 2001) helping them for a better preparedness with their
limited economic resources. Many studies have related Sahel rainfall variability to different
components of ocean boundary layer through its coupling with the atmosphere. The key Oceanic
regions identified include the tropical Atlantic (Lamb 1978), the Pacific Ocean El Niño/Southern
Oscillation (ENSO) region (Ward 1998) and an interhemispheric gradient of near global tropical
SST (Folland et al. 1986, Rowell et al. 1995, Ward 1994).
The objective of this study is to further investigate the forecast lead time using persisted SST to
force GCM for seasonal prediction. We will investigate the SST evolution critical for the GCM skill
sensitivity. The period of interest is July-August-September (JAS) season.
2. Data and experiments
A Sahelian rainfall index is constructed using 156 stations within the domain 18W-30E and 12N20N over JAS 1950-2004 period. The atmospheric General Circulation Model used in this study is
ECHAM4.5 with an approximate horizontal resolution of 2.8 degrees. Two sets of ensemble run are
used in this study as in Goddard and Mason 2002. “Simulation” runs were generated by forcing the
ECHAM4.5 with observed simultaneous monthly-mean SSTs. “Retrospective” runs refers here to
forecasts using SST anomaly persistence. The SST persistence is designed as follow : for each
month the SST anomaly (SSTA) is added to each of the following 5 months SST climatology. Those
five months are then used to force the atmospheric GCM.
326
3. Results
The GCM simulates quite well the mean circulation at the lower level. The year to year variation of
the wind is also well captured over the Atlantic basin. We compare also our Sahel index with a
similar index computed with GCM simulated rainfall. The correlation between those two indices is
0.54 over the whole period including the trend but drops to 0.1 over the period 1970-2004 without
the trend. ECHAM4.5 has little skill in simulating the rainfall over Sahel region during the recent
periods 1970-2004. We will use EOF1 of 925hPa zonal GCM wind as predictors of Sahel rainfall
(referred here as Model Output Statistics -MOS). EOF1 captures 33.8% of the total variance of the
raw zonal wind field. Table I summarizes the skill of Sahel index over different periods using raw
GCM rainfall and MOS corrected. The skill of Sahel rainfall with the trend is still high with MOS
correction 0.69 versus 0.54 with the raw GCM rainfall. Both MOS correction and raw GCM capture
the trend. Before the trend 1950-1969 the skill is similar between GCM and MOS correction (Table
I). Over the period 1970-2004 the MOS correction is capable of recovering the rainfall skill of the
GCM simulation from 0.06 to 0.54. This is an indication that the GCM is capable to correctly
simulating the low level winds associated with the rainfall but fails to get to directly simulate the
rainfall.
Table I : Skill of GCM simulation and MOS correction
Period
GCM
rainfall
MOS
Uwind
1950-04
0.54
0.69
Sahel index
1950-69
1970-04
0.36
0.06
0.33
0.54
We also look at the GCM skill using different SST boundary conditions at zero up to three months
before the rainy season : JAS, June, May and, April (table II). Persisted June SSTa runs have the
same skill than observed SST. The skill deteriorates form 0.54 with June persisted SST to 0.23 and
0.22 with respectively May and April persisted SST. From June to May we lost 27% of the rainfall
predictable variance compare to only 3% between JAS and June SST. This loss of skill from June to
May is tremendous and needs further investigation.
Table II : MOS and GCM skill.
Raw GCM rainfall (obs. SST)
MOS of GCM Observed SST
MOS of GCM Persisting June SSTA
MOS of GCM Persisting May SSTA
MOS of GCM Persisting April SSTA
Sahel index JAS 1968-2002
0.10
0.57
0.54
0.23
0.22
4. Conclusion
In this study we have investigated the lead-time associated with global SST to forecast rainfall over
Sahel. ECHAM4.5-GCM does not simulate correctly the interannual Sahel rainfall due to at least its
coarse resolution and maybe the parameterization of convective rainfall and a bad representation of
spatial vegetation changes. We have shown also that the wind is a better predictor than the raw
327
rainfall from the GCM. Using the wind at 925hPa can correct the GCM rainfall from a skill of 0.10
to 0.57 over the period 1968-2002. Furthermore there is possibility of using SST boundary condition
during June to obtain a similar skill of 0.54. However the skill drops from 0.54 with June SST to
0.23 with May SST. We will study the source of such a change by looking at SSTA Boundary
conditions between May and June.
Acknowledgments
I want to acknowledge Dr Lisa Goddard for her useful comments and advice.
References
Folland, C. K., T. N. Palmer, and D. E. Parker, 1986: Sahel rainfall and world wide sea temperatures, 1901-85. Nature, 320, 602607.
Goddard L., and S. J. Mason, 2002: Sensitivity of seasonal climate forecasts to persisted SST anomalies. Clim. Dyn., 19, 619-631.
Lamb P. J., 1978: Case study of Tropical Atlantic surface circulation patterns during recent sub-saharan weather anomalies : 1967
and 1968. Mon. Wea. Rev., 106, 482-491.
Ndiaye O., J-Y. Le Hesran, J-F. Etard, A. Diallo, F. Simondon, M. N. Ward, and V. Robert, 2001: Climate variability and the
number of deaths attributable to malaria in the Niakhar area, Senegal, from 1984 to 1996, Cahier Sante, Vol. 11, 25-33.
Palmer T. N., 1986: Influence of the Atlantic, Pacific and Indian Oceans on Sahel rainfall. Nature, 322,251-253.
Rowell D. P., C. K. Folland, K. Maskell, and M. N. Ward, 1995: Variability of summer rainfall over tropical North Africa (190692): Observations and modeling. Quart. J. Roy. Meteor. Soc., 121, 669-704.
Ward M. N., 1998: Diagnosis and short lead time prediction of summer rainfall in tropical North Africa at interannual and multidecadal timescale.
J. Climate, 11, 3167-3191.
PREVISION DES PLUIES AU SAHEL BASEE SUR LA CIRCULATION
DE L’ATLANTIQUE TROPICAL
Résumé
Dans cette étude, nous examinons le signal SST associé a la perte de performance sur les prévisions
de pluies de JAS en utilisant une correction de sortie de modèle au Sahel. Le modèle ECHAM4.5 ne
parvient pas à simuler la variabilité spatio-temporelle des pluies au Sahel mais il parvient à capter la
tendance décennale sur la période 1950-2004. Nous montrons ici que le MCG est capable de bien
reproduire la circulation du vent au niveau 925hPa. Une transformation du vent par ACP est utilisée
pour prévoir la pluie au Sahel. La première CP du vent zonal captant 34% de la variance permet de
retrouver la performance sur les pluies du modèle de 0.1 à 0.57 sur la période 1968-2002. Quand la
TSM de Juin est persistée sur la climatologie de JAS un score de 0.54 est obtenu par ACP du vent.
Quand la TSM de Mai est utilisée le score devient 0.23.
1. Introduction
La région sahélienne de l’Afrique de l’Ouest est bien connue par sa courte saison pluvieuse de juin à
septembre suivie d’une longue saison sèche. La saison pluvieuse se caractérise par une forte
variabilité interannuelle, le tout dans une variabilité à tendance multi-décennale. Une bonne
prévision des pluies au Sahel est cruciale pour les populations (Ndiaye et al. 2001) car elle leur aide
à une meilleure préparation avec leurs ressources économiques limitées. Beaucoup d’études ont lié
la variabilité des pluies au Sahel à différentes composantes océaniques de par son couplage avec
l’atmosphère. Les régions clés identifiées sont la région tropicale (Lamb 1978), l’Océan Pacifique
dans la zone ENSO (Ward 1998) et le gradient de température quasi-global sur les tropiques
328
(Folland et al. 1986, Rowell et al. 1995). L’objectif de cette étude est d’approfondir la question
relative aux échéances de la prévision saisonnière des pluies en forçant le MCG par les SST
persistées. Nous regarderons de près l’évolution de TSM cruciale à la sensibilité sur la performance
du MCG. La période d’intérêt est la saison de juillet-août-septembre (JAS)
2. Données et expériences
Un indice de pluie sur le Sahel est établi à partir de 156 stations d’observations dans le domaine
18W-30E et 12N-20N sur la période JAS 1950-2004. Le Modèle de Circulation Générale (MCG)
atmosphérique utilisé dans cette étude est ECHAM4.5 avec une résolution spatiale de 2.8 degrés
environ. Deux sortes d’expériences ont été conduites comme l’ont fait Goddard et Mason (2002) :
une dite “Simulation” qui utilise les TSM observées pour forcer le MCG à chaque pas de temps et
une autre dite “Rétrospective” où les TSM sont d’abord prévues par persistance et ensuite utilisées
pour faire tourner le MCG. La persistance de la TSM est faite de la manière suivante : pour chaque
mois l’anomalie de la TSM (TSMA) est ajoutée à la climatologie des TSM des 5 mois qui le suit
pour ensuite forcer le MCG atmosphérique.
3. Résultats
Le MCG simule bien la circulation du vent dans les basses couches. La variation interannuelle du
vent est aussi bien simulée sur l’Atlantique tropicale. Nous avons comparé notre indice de pluie
avec un indice similaire obtenu à partir des pluies du MCG. La corrélation entre ces deux indices est
de 0.54 sur toute la période comprenant la tendance mais diminue à 0.1 sur la période 1970-2004
sans la tendance. Nous allons utiliser la CP1 du vent zonal du MCG à 925hPa comme prédicteurs
pour les pluies au Sahel (cette méthodologie sera appelée MOS pour Model Output Statistics). La
CP1 capte 38% de la variabilité du vent zonal. Le tableau I donne un résumé du score sur les pluies
au Sahel pour différentes périodes en utilisant le MOS et les pluies direct du MCG. Le score sur la
période avec la tendance reste très forte 0.69 avec le MOS comparée à 0.54 avec le MCG. Tous
MOS et les pluies MCG capte bien la tendance. Avant la tendance 1950-1969 le score est
comparable entre les deux (Tableau I). Sur la période 1970-2004 le MOS est capable de recouvrir un
skill de 0.54 qui n’était que de 0.06 avec les pluies du MCG. Ceci est une indication que le MCG est
capable de simuler correctement le vent des basses couches qui sont associés aux pluies mais reste
incapable de produire directement les pluies.
Table I : Score du MCG par simulation directe et par correction MOS
Period
Pluie du MCG
MOS du vent
1950-04
0.54
0.69
Sahel index
1950-69
1970-04
0.06
0.36
0.33
0.54
Nous avons aussi regardé le score du GCM en utilisant différentes conditions (TSM) aux limites
(CL) à des échéances de 0 à 3 mois avant la saison pluvieuse JAS : CL en JAS, juin, mai et avril
(tableau II). Les CL de juin donnent presque le même résultat que celles de JAS. Mais le score se
détériore des CL de juin de 0.54 à respectivement 0.23 et 0.22 pour ceux de Mai et d’Avril. De juin
à mai, une perte de 27% sur la prévision est donc notée comparée à 3% entre les CL de JAS et de
juin. Cette perte entre juin et mai est importante et mérite d’être étudiée dans le futur.
329
Table II : Skill par le MOS et le MCG.
Pluie du MCG (TSM observée)
MOS avec TSM observée
MOS persistant les TSM de juin
MOS persistant les TSM de mai
MOS persistant les TSM d’avril
Indice du Sahel JAS 1968-2002
0.10
0.57
0.54
0.23
0.22
4. Conclusion
Dans cette étude, nous avons regardé le délai temporel associé au Signal global des TSM sur les
pluies au Sahel. Le MCG ECHAM4.5 ne parvient pas à capter la variabilité interannuelle des pluies
qui peut être expliquée au moins par une résolution très faible du modèle, une paramètrisation des
pluies et une mauvaise représentation dans le modèle de la végétation. Nous avons montré aussi que
le vent est un meilleur prédicteur que la pluie du MCG. En utilisant le vent à 925hPa, on peut
corriger la pluie du MCG pour passer d’un score de 0.10 à 0.57 sur la période 1968-2002. Il existe
aussi une possibilité d’utiliser les CL de juin pour obtenir un score de 0.54 mais cependant le score
descend de 0.54 avec les TSM de juin à 0.23 avec les TSM de mai. Nous allons étudier les raisons
d’un tel changement en regardant l’évolution des CL sur la période de mai à juin.
Remerciements
Les auteurs voudraient remercier Lisa Goddard pour ses commentaires et conseils utiles.
Bibliographie
Folland, C. K., T. N. Palmer, and D. E. Parker, 1986: Sahel rainfall and world wide sea temperatures, 1901-85.
Nature, 320, 602-607.
Goddard L., and S. J. Mason, 2002: Sensitivity of seasonal climate forecasts to persisted SST anomalies. Clim. Dyn.,
19, 619-631.
Lamb P. J., 1978: Case study of Tropical Atlantic surface circulation patterns during recent sub-saharan weather
anomalies : 1967 and 1968.
Mon. Wea. Rev., 106, 482-491.
Ndiaye O., J-Y. Le Hesran, J-F. Etard, A. Diallo, F. Simondon, M. N. Ward, and V. Robert, 2001: Climate variability
and the number of deaths attributable to malaria in the Niakhar area, Senegal, from 1984 to 1996, Cahier Sante, Vol. 11,
25-33.
Palmer T. N., 1986: Influence of the Atlantic, Pacific and Indian Oceans on Sahel rainfall. Nature, 322,251-253.
Rowell D. P., C. K. Folland, K. Maskell, and M. N. Ward, 1995: Variability of summer rainfall over tropical North
Africa (1906-92): Observations and modeling. Quart. J. Roy. Meteor. Soc., 121, 669-704.
Ward M. N., 1998: Diagnosis and short lead time prediction of summer rainfall in tropical North Africa at interannual
and multidecadal timescale. J. Climate, 11, 3167-3191.
330
1.87P
PRECIPITATION AND SST PATTERNS RELATED TO WAM
I. POLO, B. RODRIGUEZ-FONSECA and E. SERRANO-MENDOZA
Departamendo de Geofisica y Meteorologia, UCM, Madrid, Spain
SST anomalies can modulate the meridional energy gradients and, consequently, the intensity of the
WAM. Therefore, it is important to determine the SST-WAM relationship for seasonal to
interdecadal predictability. Since the statistical association between observed Sahel rainfall
variability and tropical Atlantic SST [1, 2] some studies have shown possible implications of the
tropical Pacific and Indian Oceans on WAM [3, 4, 5, 6, 7, 8].
The Guinea rainfall pattern related to the equatorial SST mode occurs at interannual timescales and
owes its existence to ITCZ shifts, whilst the rainfall variability over the semi-arid Sahel (between
10ºN to 20ºN) represents an internal mode of the African summer monsoon variability [8]. At
interannual to decadal scales, positive SST anomalies in the eastern equatorial Pacific, mainly
associated with east-west divergent circulation over the tropical Atlantic, are found to coincide with
negative rainfall anomalies over West Africa (WA) [5, 6].
We investigate the SST-precipitation patterns related to WAM performing Multiple Maximum
Covariance Analysis [MCA, 9] to the monthly anomalous precipitation over WA in summer and the
monthly tropical Atlantic, Indian and Pacific, as well as Mediterranean SST anomalies from
February to September. Also, the WAM impact on the Mediterranean climate has been studied
through the MCA between the summer SST and the North Atlantic precipitation during the next
months. All these analyses have been done with monthly anomalies, using CMAP data set for the
precipitation field, ERSST data set for the SST variable and ERA 40 for the wind and SLP fields
Correlations
for the JJAS
season
1# pt-sst Atl
3# pt-sst Atl
1# pt-sst Pac
4# pt-sst Pac
1# pt-sst Ind
WAM
continental
domain
-0.63
-0.45
-0.57
WAM
oceanic
domain
-0.60
-0.29
-0.41
Western
Sahel
Central
Sahel
Eastern
Sahel
Central
Sudan
-0.5
-0.38
-0.46
-0.57
-0.43
-0.58
-0.41
-0.55
-0.40
-0.52
Central
Guinea
Gulf of
Guinea
-0.67
-0.35
-0.64
-0.73
-0.66
-0.54
-0.41
during the period 1979-2001.
Table 1. Correlation coefficients between the rainfall WAM indexes and the precipitation expansion coefficients, from
the MMCA between the WA precipitation and the SST over the different ocean basins
Table 1 shows the correlation coefficients between the rainfall WAM indexes and the precipitation
expansion coefficients for several modes, and from the MMCA performed between the WA
precipitation and the SST over the different ocean basins. All the SST structures related to WAM
persist from four months in advance. The first mode of the different analyses (Atlantic, Pacific and
Indian ocean) is related to positive rainfall anomalies over the Guinea coast (figure 1.a top panel).
The Atlantic SST patterns related to this precipitation mode show positive SST anomalies over the
Guinea Gulf [8] and negative SST anomalies over the Mediterranean Sea (figure 1.b top panel)
during the whole time sequence. The associated Hadley circulations anomalies show that the ITCZ
is displaced southward (figure 1 c top panel) when compared to its climatological behavior.
331
Figure 1. Modes of the MMCA between the summer precipitation over WA and the SST over the Atlantic and the
Mediterranean sea. a) Precipitation heterogeneous map in mm day-1 b) SST homogeneous map in ºC c) zonal (contours),
meridional (vectors) and vertical (shaded areas) wind regression map averaged from 10ºW to 10ºE in Pa s-1. For the
leading mode (top panel fvar=16% and ruv=0.7) and the third mode (bottom panel, fvar=7% and ruv=0.66). d)
Precipitation regression map from the MMCA between the summer SST and the precipitation over the Atlantic months
after. Precipitation in autumn for the mode related to the Guinea Gulf and Mediterranean SST anomalies (top panel) and
precipitation in winter for the Atlantic SST meridional mode (bottom panel).
The third mode (figure 1. bottom panel) describes positive summer rainfall anomalies over he Sahel
and positive SST anomalies over the Subtropical North Atlantic and Mediterranean Sea from
February to June. This structure is associated with positive anomalous ascents over Sahel and
strengthening of the Tropical Easterly Jet (figure 1. c bottom panel).
The leading mode of the Pacific SST and WAM shows La Niña anomalies related to positive
rainfall anomalies over the Guinea coast and continental region [6]. The Indian Ocean SST patterns
related to WAM have shown structures significantly associated to the Pacific El Niño events (not
shown). All these patterns are analyze in the light of the recent literature. Also we study how the
SST patterns that are related to WAM (i.e figure 1.b), are affecting to autumn (figure 1.d top panel)
and winter (figure 1.d bottom panel) extratropical North Atlantic precipitation.
References
[1] Hastenrath S., (1978), On modes of tropical circulation and climate anomalies, J. Atmos. Sci., 35, 2222-2231.
[2] Lamb P. J., (1978), Large-Scale tropical Atlantic surface circulation patterns associated with sub-Saharan weather
anomalies, Tellus, 30, 240-251
[3] Folland C. K., T. N. Palmer and D. E. Parker, (1986), Sahel rainfall and worldwide sea temperatures, Nature 320,
602-607
[4] Shinoda M. and R.Kawamura, (1998), Tropical rainbelt, circultation, and sea surface temperatures associated with
the Sahelian rainfall trend, J. Meteorol. Soc. Jpn., 72, 341-357
[5]Ward M. N., (1998), Diagnosis and short-lead time prediction of summer rainfall in tropical North Africa at
interannual and multidecadal timescales, J. Clim., 11, 3167-3191
[6] Janicot S., S. Trzaska and I. Poccard, (2001), Summer Sahel-ENSO teleconnection and decadal time scale SST
variations, Clim. Dyn, 18, 303-320
[7] Rowell, D. P.,(2001), Teleconnections between the tropical Pacific and the Sahel, Q. J. Roy. Meteorol. Soc., 127,
1683-1706
[8] Gianini A., R. Saravannan and P. Chang, (2005), Dynamics of the boreal summer African monsoon in the NSIPP1
atmospheric model, Clim. Dynam., DOI 10.1007/s00382-005-0056-x
[9] Bretherton, S. B., C. Smith and J. H. Wallace (1992), An Intercomparation of methods for finding coupled patterns
in climate data, J. Clim., 5, 541-560.
Contact : I. POLO (email : [email protected])
332
1.88P
SIMULATION OF SUBSEASONAL CHARACTERISTICS OF RAINFALL IN
CENTRAL WEST AFRICA USING A HIDDEN MARKOV MODEL
Sylwia TRZASKA, Andrew ROBERTSON and Pauline DIBI-KANGAH
International Research Institute for Climate and Society
The Earth Institute at Columbia University, Palisades, NY, USA
Introduction
One of the major challenges in tailoring seasonal climate forecasts to meet societal needs is that the
potential users of climate information are often concerned with the characteristics of high-frequency
weather at a particular location. Very often however studies addressing synoptic/subseasonal and
interannual scales are done separately. Here we present results obtained using a Hidden Markov
Model as a diagnostic tool.
Data and Methods
An original dataset of 68 years of daily rainfall in 41 stations spreading from the coastal area to arid
regions in Central West Africa has been compiled from the data obtained from National
Meteorological Services of Côte d’Ivoire, Burkina Faso and Mali. Daily rainfall occurrence for the
period 1951-98 is analyzed using the Hidden Markov Model (HMM). HHM has been recently used
in daily rainfall analysis in different tropical regions and as a method for downscaling General
Circulation Model outputs (Robertson et al. 2004, Robertson et al. 2005).
HMM associates T daily rainfall probablities on a spatial network of M stations R1..M t=1..T with a
small number of discrete (hidden) states S t=1..n. Most probable sequence of states can be then fit to
the sequence of daily rainfall observed at the stations and atmospheric conditions prevailing in each
hidden state inferred.
R1
R2
S1
S2
…
Rt
…
RT
St
…
Sn
Figure 1: Graphical representation of an HMM
The basic assumption here is that the multivariate precipitation observation Rt at time t depends
only on the state St and is independent from prior rainfall observations. The second assumption is
that the hidden state process St is first-order Markov i.e. depends only on St-1 (cf. figure 1). In
addition, rainfall observation at each station at time t is supposed independent from observations at
other stations at time t. This however does not imply spatial independence of the rainfall process.
333
Results
HMM has been applied to 48 May-to-October sequences of daily rainfall occurrence
(rainfall>1mm/day) in 41 stations to derive spatial patterns of rainfall occurrence probabilities
associated with n=1..6 states and the most probable sequence of states associated with given
observations. Probability distribution patterns and the mean occurrence of each state in each month
for a four-state decomposition are shown in figure 2.
This particular decomposition isolates two states (#3 and #1), mostly prevalent in May-June and
September-October, associated with reduced probability of rainfall in the Sahel (state #3) and
moderate probability of rainfall with southward gradient (state #1). The two other states peak in the
core of the rainy season in the Sahel and are associated with enhance/reduced probability of
precipitation occurrence in the northern/southern part of the domain (state #2) and with moderate
rainfall probability over the whole domain (state #4).
State #1 has a tendency to occur later in the season than the state #3 and could be interpreted as a
transition state. Similarly the ‘enhanced sahelian/reduced coastal precipitation’ state #2 occurs later
than the state #4. Note that the two ‘early’ states can still occur during the peak rainy season in the
Sahel, reflecting the dry spells during the monsoon. Thus, the HMM decomposition into 4 states
captures the typical seasonal evolution of rainfall in
a)
the region with reduced (enhanced) probability of
100%
100%
rainfall in the Sahel earlier (later) in the season but
also higher frequency variability within each of the
seasons.
The decomposition into 2 states (not shown) captures
the basic seasonality of the rainfall over the region
while decomposition into 6 states (not shown) gives
more detailed view of the subseasonal characteristics.
100%
100%
b)
Figure 2: a) Four-state HMM rainfall probabilities
(proportional to circle surface, scale in upper right
corner) for 1951-98 MJJASO daily rainfall
occurrence. b) mean monthly occurrence of each
state in the most probable sequence of states
obtained with Viterbi algorithm.
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Based on the inferred most probable sequence of
states, atmospheric conditions associated with each
state have been computed by compositing daily
atmospheric conditions from NCEP Reanalysis
(Kalnay et al. 1996) and analysed as departures from
the 6-months average for the MJJASO season (not
shown).
As expected the dry state #3 is associated with deep
convection in SACZ and equatorial Africa, with
shallow convection in the coastal West Africa and
with strong south- and westward winds, consistent
with atmospheric circulation in boreal spring. In the
transition state #1 the anomalies are similar but with
strongly reduced amplitude over West Africa while
the deep convection centers vanish, pointing towards
an evolution in the seasonal cycle, more favorable for
precipitation over West Africa.
The two states peaking in the core of the Sahelian
rainy season share very similar atmospheric patterns
associated with the West African monsoon
circulation and differ mainly by a cyclonic circulation
localized over Central West Africa. The main
difference is thus of much smaller time and spatial
scale and the decomposition seems to capture the occurrence of synoptic disturbances.
The frequency of occurrence of each state displays substantial interannual variability. In the core of
the monsoon, enhanced occurrences of states #3 and #1 with low probability of rainfall in the Sahel
are related to warm SST anomalies in the eastern equatorial Pacific and Gulf of Guinea respectively
(not shown). The states #2 and #4, favoring rainfall over the Sahel, are more frequent during cold
anomalies in the Pacific and Gulf of Guinea, respectively. These results are consistent with previous
analyses carried on larger spatial and temporal scales (e.g. Janicot et al., 2002).
Conclusion
By applying the Hidden Markov Model decomposition to 48 years of May-to-October daily rainfall
occurrence in a network of 41 stations in Central West Africa we were able to identify rainfall
patterns related to the mean seasonal cycle of rainfall but also modifications of these on subseasonal
and local scales. Interannual variability of the occurrence of the related hidden states pointed to the
traditional association between rainfall anomalies and global/regional SST.
References
Janicot S., Trzaska S. and I. Poccard, 2002: Summer Sahel-ENSO teleconnection and decadal time scale SST
variations. Clim. Dyn., 18, 303-320.
Kalnay E. and co-authors, 1996: The NCEP/NCAR 40-year reanalysis project, BAMS, 77, 437-571.
Reynolds, R.W. and T.M. Smith, 1994: Improved global Sea Surface Temperature analyses using optimum
interpolation. J. Climate, 7, 929-948
Robertson A.W., Kirshner S. and P. Smyth, 2004: Downscaling of daily rainfall occurrence over Northest Brazil
using Hidden Markov Model. J. Climate, 17, 4407-4424.
Robertson A.W., Kirshner S., Smyth P., Charles S.P. and B.C. Bytes, 2005: Subseasonal-to-interdecadal variability
of the Australian monsoon over north Queensland. Q.J.R. Meteorol. Soc., in press.
335
SIMULATION DES CARACTERISTIQUES SUBSAISONNIERES DES
PRECIPITATIONS EN AFRIQUE DE L’OUEST CENTRALE
A L’AIDE DU MODELE DE MARKOV CACHE
Une des principales difficultés dans l’application des prévisions saisonnières dans différents
secteurs économiques réside dans le besoin des utilisateurs potentiels à disposer d’une information
sur des événements localisés dans le temps et l’espace. Alors que la limite théorique de la prévision
déterministe du temps est de l’ordre de 10 jours, à échéance plus longue une prévisibilité des
caractéristiques statistiques de la variabilité sub-saisonnière peut exister en relation avec les
forçages aux limites, à évolution plus lente. Malheureusement ce type d’information à l’échelle
locale est mal représenté dans les MCGA utilisés dans la prévision saisonnière et dont la maille
reste encore très lâche. Diverses méthodes ont été développées afin de traduire les résultats des
prévisions dynamiques en information localisée, allant de générateurs aléatoires de temps aux
modèles dynamique régionaux.
Dans ce travail le Modèle de Markov Caché (HMM) est utilisé comme outil de diagnostic et de
désagrégation. L’hypothèse de base sous-tendant cette méthode est que les précipitations
journalières observées dans un réseau de stations peuvent être décrites avec un nombre (réduit) de
types de temps ayant un comportement markovien dans le temps. Elle peut donc être utilisée, tout
d’abord, à diagnostiquer les types de temps associés avec des structures spatiales des précipitations
observées dans un réseau de stations et simuler des scénarios de séquences de types de temps ;
ensuite à désagréger des sorties de MCG vers les échelles journalières et stationnelles. Pour leur
utilisation comme outil de désagrégation les HMM sont étendus vers le modèle non-homogène
(non-homogeneus hidden Markov Model – NHMM) où les probabilités de transition entre les états
ne sont pas fixes dans le temps. De tels modèles ont été utilisés dans la désagrégation des
précipitations au Nordeste brésilien, en Afrique de l’Est et au Sénégal ainsi qu’en Australie.
Cette étude se concentre sur l’Afrique de l’Ouest centrale (Côte d’ivoire, Burkina Faso et Mali) et
se base sur les observations journalières de précipitations dans une quarantaine de stations. Les
données de réanalyse NCEP et des sorties de MCGA forcés avec des TSO observés seront
également utilisés. Des tests de sensibilité à la taille de l’échantillon seront également conduits.
Mots clés : Variabilité et prévisibilité des précipitations journalières et stationnelles, caractéristiques
sub-saisonnières, désagrégation, prévision saisonnière, Afrique de l’Ouest centrale.
336