The recent decline of the long rains in East Africa

Since the late 1990s many parts of East Africa have experienced a decline in rainfall during boreal spring most notably during March-April-May (MAM), the “long rain” season in many parts of the region. A close examination of the annual cycle of rainfall across several data sets reveals that the largest decrease has occurred during the months of April and May. Interestingly, the consensus of coupled model projections from the Fourth Assessment Report of the IPCC (AR4) indicates an increase in precipitation for the region over the course of the current century. In this paper the anomalous atmospheric circulation associated with the recent decline in East Africa rainfall is first examined in different reanalysis data sets. Simulations from several atmospheric general circulation models forced with observed sea surface temperatures (SST) are then employed to examine the role of global SSTs in the recent rainfall decline. In addition, reanalysis data are employed to compute the anomalous, tropical diabatic heating field for MAM during recent years which is used to force a linear model to see which forcing regions have the greatest influence on East African rainfall. Some of these results are briefly contrasted with AR4 coupled model projections.