Climate Change at the Margin in East Africa

There remain several gaps in our understanding of the underlying physical processes that drive the climate of equatorial eastern Africa. Eastern Africa is located at the margin of the major convection centers over the central African continent and the Indian Ocean/South Asia, making the seasonality of its rainfall rather complex. The geographical focus in this paper will be on regions exhibiting a bimodal rainfall annual cycle in eastern Africa, generically referred to as the "long rains," typically from March-May, and the "short rains" from October-December. The thrust of the paper will be an assessment of key terms in water budget of the region within the framework of CMIP5 coupled climate model simulations and projections. The water budget is separted into "dynamical" and "thermodynamical" components, emphasizing changes in atmospheric circulation and water vapor content, respectively. Dominant processes in the current climate are first examined in simulations before consider changes in these terms under increasing anthropogenic greenhouse gas forcing (RCP8.5). As the CMIP5 models generally show the region becoming wetter in the future, a key question we explore is what processes contribute most to the increasing rainfall trend. Inherent model biases, particularly relating to unrealistic characteristics in sea surface temperatures when compared with observations, will be briefly described in the context of circulation changes in model projections for eastern Africa.