Interannual variability of south-eastern African summer rainfall .2. Modelling the impact of sea-surface temperatures on rainfall and circulation

This study is concerned with the possible physical link between global sea-surface temperatures (SSTs) and south-eastern African summer rainfall. We have performed a series of general circulation model (GCM) experiments, where the model atmosphere has been forced with certain SST anomaly patterns. These have been identified by Rocha and Simmonds (in part I) to be related to drought conditions over the subcontinent. Results show that anomalously warm SSTs in the tropical Pacific and Indian Oceans, typical of ENSO events, can generate dry conditions over much of south-eastern Africa. However, those in the central Indian Ocean, which are partially independent of ENSO, dominate the rainfall response. Sea-surface temperatures in the Atlantic Ocean have little or no effect on rainfall. In the model, warming of the central Indian Ocean generates low-level cyclonic atmospheric anomalies there, which weaken the predominantly eastern flow across the eastern coast of Africa. As a result, less moisture enters the continent and reduced precipitation takes place. Cool SSTs in the south Indian Ocean further enhance this scenario. Warm surface waters in the central and eastern Pacific Ocean generate upper-level westerly wind anomalies, which extend eastwards across into the Indian Ocean. Such upper-level wind changes have been related previously to ENSO and southern African drought.