Extreme wet years over southern Africa: Role of Indian Ocean sea surface temperatures

Abstract

Southern Africa is a predominantly semiarid region with a high degree of interannual rainfall variability. Although much of the recent climate research has focused on the causes of drought events, the region has also experienced extremes of above average rainfall, the most recent examples being the major flooding episodes that devastated Mozambique during 2000 and 2001. This paper investigates extremely wet years over southern Africa during the twentieth century. Focusing on the two most extreme years, 1974 and 1976, we show that while ENSO serves as an important control on rainfall variability, a specific pattern of SSTs in the SW Indian Ocean, with warm anomalies in the subtropical SW Indian Ocean and cool anomalies in the northern SW Indian Ocean that is statistically independent of ENSO, plays a crucial role in generating extreme conditions. To do this, we use a series of multimodel experiments, to demonstrate first the importance of global sea surface temperatures. Through additional idealized experiments with HadAM3, we then isolate the role of SST anomalies in the Indian Ocean. The anomalies are based on the observed SSTs with the ENSO signal linearly removed. The critical influence is tied to cold SST anomalies in the Mascarene region which induce an anomalous anticyclonic circulation driving an anomalous low‐level easterly moisture flux along 10–20°S into eastern southern Africa. This results in enhanced moist convective uplift, conducive to enhanced rainfall, over a large part of southern Africa. Near surface humidity and 500‐hPa omega fields extend from eastern southern Africa into the Agulhas region in a tropical‐temperate cloud band like structure. The similarity between the reanalysis fields for the extreme years and the model experiments is striking.