Regional dynamical downscaling over West Africa: model evaluation and comparison of wet and dry years

In this study, a 25-year regional climate model run over West Africa is evaluated and examined with respect to causes of interannual rainfall variability related to the West African Monsoon. West African rainfall has been subject to strong interannual and decadal variability throughout the past 50 years. Known driving forces for this variability are large-scale changes in Atlantic sea surface temperatures (SSTs), variability due to global atmospheric circulation changes, like for instance variability related to El Nino-Southern Oscillation, but also regional and local-scale changes in land use and vegetation cover. The interaction of these impact factors with West African synoptic and subsynoptic processes is still not completely understood. One reason for this lack of knowledge is that basic features of West African climate, including the African Easterly Jet (AEJ), African Easterly Waves (AEWs) as well as monsoon dynamics, are very complex multiscale phenomena. Climate modeling in West Africa requires the ability to simulate these effects, which can only be achieved by mesoscale atmospheric models. Using the regional climate model REMO from the Max-Planck Institute for Meteorology in Hamburg, a 25-year dynamical downscaling study was undertaken in order to evaluate a tool, which will then be used for the examination of causes of rainfall variability in West Africa. The model was used on a 0.5 degrees grid over North Africa northward of 15 degrees S. The model evaluation leads to some confidence in the reliability of the modeled climate. A detailed examination of composites of selected wet and dry years in the Guinean coast region elucidates the role of SST forcing and external atmospheric forcing for interannual rainfall variability. In general, abundant monsoonal rainfall comes along with warm tropical Atlantic SSTs, enhanced latent heat fluxes from the ocean to the atmosphere and stronger surface wind convergence near the Guinean Coast. This is accompanied by large-scale dynamical changes in strength and direction of both the Tropical Easterly Jet (TEJ) over the Indian Ocean and the Subtropical Jet (STJ) over the Near East and the Caucasian region.