Multi-model Analysis of the West African Monsoon: Seasonal Evolution and the Monsoon Onset

The economy of West Africa and particularly in the Sahel region depends primarily on agriculture. Most of its agricultural productivity is achieved during the monsoon season (July- September) when the greatest amount of rain is recorded. The goal of this paper is to study the seasonal evolution of West African rainfall with a special focus on the onset of monsoon using five (5) regional climate models (RCMs) involved in the CORDEX (Coordinated Regional climate Downscaling Experiment) program. These RCMs have a horizontal resolution of 0.44° × 0.44° (about 50 km) and are initialised and forced to their lateral boundaries by the ERA-Interim reanalysis. The results show that the intra-seasonal variability of the West African monsoon (WAM) is well reproduced by RCMs despite the presence of some biases. The analysis of the dynamics of the West African monsoon shows that almost all models simulate a strong increase of the heat flux over the Sahara desert especially on its western part which promotes a substantial rise in the monsoon flow over the Sahel. This increase of low-level monsoon flow may induce an enhancement of the low-level specific humidity and the convective activity over the Sahel just after the onset. Moreover, the RCMs show a northward position and an intensification of the AEJ just before the monsoon onset. The upper levels divergence seasonal evolution is similar with the rainfall band with collocation of the maxima suggesting that it may be used as a tool to better forecast the WAM onset.