Change in the association between Indian Ocean sea-surface temperatures and summer rainfall over South Africa and Namibia

The EI Nino-Southern Oscillation (ENSO) signal in Indian Ocean sea-surface temperatures has weakened since the late-1970s. resulting in important changes in the association between tropical western Indian Ocean sea-surface temperatures and December-February rainfall over South Africa and Namibia. Prior to the late-1970s, warm events in the tropical Indian Ocean occurred frequently in association with ENSO warm events, and both were typically associated with anomalously dry conditions over much of South Africa and Namibia. Over the most recent two decades, however, sea-surface temperature variability in the tropical western Indian Ocean has become significantly less dependent upon ENSO. At the same time, warm (cold) events in the tropical western Indian Ocean have become associated with wet (dry) conditions over the north-eastern half of South Africa and northern Namibia since the late-1970s. These changes in rainfall-sea-surface temperature associations can be successfully simulated using a general circulation model. Such changes in the climate system have important implications for the predictability of southern African rainfall, and highlight the need for an operational forecast system to predict Indian Ocean sea-surface temperatures. Copyright (C) 1999 Royal Meteorological Society.