African rainfall and its relation to the upper air circulation

Abstract

Abstract The rainfall distribution over Africa analysed for the period 1951–1975 shows a downward trend at 15°N not present at other latitudes. A widespread reduction in rainfall is observed during the drought years 1972 and 1973. Upper air analyses have been used to construct mean monthly vertical motion patterns and water vapour budgets but the results are not sufficiently precise to examine year‐to‐year differences. Changes in the August flow patterns show that low rainfall in the Sahel is associated with the virtual disappearance of the 850‐mb trough near 8°N and weakening of the easterly jet above it. an empirical orthogonal function analysis of the 850‐, 500‐ and 200‐mb temperatures from 150 stations shows that these changes are global in extent. the downward trend in the Sahel rainfall has evidently been paralleled by a weakening of the northern hemisphere circulation. These results suggest that local modification of surface conditions is not the principal cause of the decline in Sahel rainfall over the last two decades.