GCM studies of the African summer monsoon

A 37-year simulation of global climate by a 9-level GCM on an 8°×10° grid showed realistic interannual variation of the computed precipitation over the African Sahel. The model includes an interactive ocean so that interannual variations of sea-surface temperature (SST) also occur. Comparison of an ensemble of five summers that were rainy over the Sahel with five summers of simulated drought showed that insufficient ambient moisture was the immediate cause of the lack of moist convection. The drier conditions are shown to result from weaker moisture advection over the southeast Atlantic Ocean. Weaker southerly winds there and lower sea-level pressure gradients seemed to result from anomalously warm SST. Such SST anomalies have been linked to Sahelian drought in previous observational studies. These regional circulations that were conducive to lower rainfall rates during the north African summer monsoon were not manifestations of the more generalized zonal mean circulation.