Abrupt rainfall transitions over the Greater Horn of Africa: Observations and regional model simulations

We examine the yearly occurrence of a monsoon jump of approximately 20 degrees latitude during the boreal spring and summer rainy seasons over the Greater Horn of Africa (GHA). This jump is in contrast with a simple model of a smoothly varying ITCZ over the region. The rainfall jump is observed annually during April and May in three precipitation data sets and in regional climate model simulations using the PSU/NCAR Mesoscale Model (MM5). The MM5 simulations show the rainfall jump to be roughly coincident with abrupt circulation changes that occur as the Somali jet develops during April, May, and June. In particular, the cross-equatorial (meridional) branch of the Somali jet forms along the East African coast in April, bringing moisture (and rainfall) northward to the southern slopes of the Ethiopian plateau. This meridional branch forms well before the northern zonal branch of the jet, which diverts moisture eastward from southern Ethiopia and feeds the Indian monsoon. These results establish a framework for understanding the precipitation cycle over the GHA, and provide a foundation for improving subseasonal forecasts over drought-prone regions of eastern Africa.