Abstract
A paleoreconstruction of the length and intensity of the rainy season over western Africa has been recently proposed, using analysis of fossil mollusk shells from the Saloum delta region, in western Senegal. In order to evaluate the significance of local long-term reconstructions of precipitations from paleoclimate proxies, and to better characterize the spatial homogeneity of rainfall distribution in northern Africa, we analyze here the spatial representativeness of rainfall in this region, from seasonal to decadal timescales. The spatial coherence of winter episodic rainfall events is relatively low and limited to surrounding countries. On the other hand, the summer rainfall, associated with the West African Monsoon, shows extended spatial coherence. At seasonal timescales, local rainfall over the Saloum is significantly correlated with rainfall in the whole western half of the Sahel. At interannual and longer timescales, the spatial coherence extends as far as the Red Sea, covering the full Sahel region. This spatial coherence is mainly associated to the zonal extension of the Inter Tropical Convergence Zone. Coherently, summer rainfalls appear to be driven by SST anomalies mainly in the Pacific, the Indian Ocean, the Mediterranean basin, and the North Pacific. A more detailed analysis shows that consistency of the spatial rainfall coherence is reduced during the onset season of the West African Monsoon.
Highlights
The Sahel region in Western Africa is a semi-arid area located at the southern boundary of the Sahara desert and covered by grassland, shrubs, and small, thorny trees
Data Processing and Time Scale Filtering In this study, we focus on the rainfall variability over the Saloum Delta and the Sahel regions respectively (Figure 1)
Decadal rainfall variability in Sahelian West Africa seems more out of phase with east-Africa than with the Gulf of Guinea area. These results indicate that precipitation variability measured over the Saloum Delta are largely representative of the variability found over West Africa at seasonal timescales, and of the whole Sahel region at interannual and decadal scales
Summary
The Sahel region in Western Africa is a semi-arid area located at the southern boundary of the Sahara desert and covered by grassland, shrubs, and small, thorny trees. The teleconnection mechanism between the Tropical Pacific ocean and West Africa during warm ENSO events (El Niño) was explained with more details by Rowell (2001): the interaction of atmospheric equatorial Kelvin wave from the tropical Pacific with an off-equator Rossby wave (response to the zonal SST gradient between the East Indian Ocean and West Pacific) increases the large-scale subsidence over Africa This weakens convective activity and thereby reduces the monsoon flow over West Africa. The simulations of the fifth Coupled Model Intercomparison Project (CMIP5) confirm results from previous generations of models attributing a substantial role of anthropogenic emissions in driving precipitation changes in the Sahel (Biasutti, 2013) They suggest that in a warming world, the WAM season may be shifted toward the end of the year, as they report a drying spring and wetting fall. Results are interpreted in the light of present understanding of the dynamics of the WAM and heug rainfalls
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