Near-Surface Zonal Flow and East African Precipitation Receipt during Austral Summer

Abstract

This paper examines the influence of moist Atlantic air masses on East African precipitation receipt during periods of westerly outbreaks across East Africa. The influence of the Indian Ocean on the interannual variability of East African rainfall is widely known, but there is also a less well known association between precipitation receipt and the inflow of moist Atlantic air. Atlantic air masses advected inland around the northern margins of the climatological mean heat low over southwestern Africa are thought to be uplifted over the high elevations encountered inland and may produce rainfall over East Africa after cooling to saturation. During austral summer [December?January?February (DJF)], Atlantic air masses converge with easterly trade wind and monsoon flows from the Indian Ocean in a complex meridionally oriented convergence zone (0°?20°S, 20°?40°E). An index of near-surface zonal flow during the austral summer period (DJF) is created for this convergence zone and is correlated with various atmospheric circulation, humidity, and precipitation datasets. Results show that although westerly flow from the Atlantic is strongly associated with rainfall receipt over East Africa, the mechanism producing rainfall is more complex than that presented above and is demonstrated to be dependent on the convergence of near-surface water vapor fluxes into the region, which allows the development of an unstable lower troposphere.