El Nino, climate change, and Southern African climate

The El Nino phenomenon involves a large-scale warming of the equatorial eastern and central Pacific Ocean. Recent developments in the El Nino-Southern Oscillation (ENSO) phenomenon have raised concerns about climate change, In this review paper, these recent developments are critically assessed and forecasts of possible future changes are reviewed. Since the late-1970s, El Nino episodes have been unusually recurrent, while the frequency of strong La Nina events has been low. Prolonged/recurrent warm event conditions of the first half of the 1990s were the result of the persistence of an anomalously warm pool near the date line, which, in turn, may be part of an abrupt warming trend in tropical sea-surface temperatures that occurred in the late-1970s. The abrupt warming of tropical sea-surface temperatures has been attributed to the enhanced-greenhouse effect, but may be indicative of inter-decadal variability: earlier changes in the frequency of ENSO events and earlier persistent El Nino and La Nina sequences have occurred. Most forecasts of ENSO variability in a doubled-CO2 climate suggest that the recent changes: in the tropical Pacific are anomalous. Of potential concern. however, is a possible reduction in the predictability of ENSO events given a warmer background climate. El Nino events usually are associated with below-normal rainfall over much of southern Africa. Mechanisms for this influence on southern African climate are discussed. and the implications of possible changes in ENSO variability on the climate of the region are assessed. Recent observed changes in southern African climate and their possible relationships with trends in ENSO variability are investigated. The El Nino influence on rainfall over southern Africa occurs largely because of a weakening of tropical convection over the subcontinent. A warming of the Indian Ocean during El Nino events appears to be important in providing a teleconnection from the equatorial Pacific Ocean. The abrupt warming of the tropical Pacific and Indian oceans in the late-1970s is probably partly responsible for increasing air temperatures over southern Africa, and may have contributed to a prolongation of predominantly dry conditions. A return to a wet phase appears to have occurred, despite the persistence of anomalously high sea-surface temperatures associated with the late-1970s warming, and a recordbreaking El Nino in 1997/98. Copyright (C) 2001 John Wiley & Sons, Ltd.