Did Similar Sea Surface Temperature Patterns Force the Recent Dry Long Rains and Wet Short Rains Over East Africa?

East Africa, defined here as the countries of Somalia, Kenya and southern Ethiopia, relies on precipitation during its two rainy seasons, the ‘short rains' of October-December and the ‘long rains' of March-May, to support the region's rain-fed agriculture and water resources. During March-May 1979-2013, the linear trend in Pacific Ocean sea surface temperatures (SSTs) forced a strong decline in East Africa precipitation, which resulted in water shortages and acute food insecurity. However, during October-December 1979-2013, the linear trend in Pacific Ocean SSTs forced above average precipitation over East Africa although the linear SST trend patterns during March-May and October-December were very similar. Here, we examine how similar patterns of SSTs, highlighted by a cool eastern Pacific and warm Indo-west Pacific Ocean, dynamically forced dry conditions during March-May and pluvial conditions during October-December over East Africa using model simulations forced by the observed linear trend in monthly 1979-2013 SST over the globe and over the Pacific.

During March-May, the global and Pacific SST-forced simulations drive similar reductions in East Africa precipitation through an increase in the regional climatological subsidence via an enhancement the Walker Circulation, corroborating previous work that Pacific SST were the primary driver of precipitation. During October-December, the global and Pacific SST-forced simulations drive increases in East Africa precipitation as a result of increases in the regional moisture flux from the Indian Ocean. However, the global and Pacific SST-forced simulations drive different regional circulations and moisture flux despite the similar outcome of precipitation over East Africa, which indicate important contributions from both the Indian and Pacific Ocean SST.