Recent changes in El Nino Southern Oscillation events and their implications for Southern African climate

Since the late 1970s, El Nino episodes have been unusually recurrent, while the frequency of strong La Nina events has been low. With the long El Nino sequence of 1991-1995, concern has been expressed about the possibility of climatic change in the equatorial Pacific. However, changes in the frequency of El Nino-Southern Oscillation events and earlier persistent El Nino and La Nina sequences can be detected in the historical and palaeoclimatic records. The recurrent warm event conditions of the first half of the 1990s are the result of the persistence of an anomalously warm pool near the date line which shifted the main centre of convection over Indonesia toward the centre of the equatorial Pacific Ocean. The eastward shift of the convection centre has allowed the penetration of westerly wind anomalies, associated with Madden-Julian wave activity, further into the western and central Pacific, thus initiating sequences of downwelling Kelvin waves. It has been suggested that the warm pool near the date-line may be a result of an abrupt warming trend in sea-surface temperatures throughout the tropical Indian and Pacific oceans. The abrupt warming has been attributed to the enhanced-greenhouse effect, but may equally be indicative of inter-decadal variability. The recent changes in El Nino events are therefore not necessarily an indication of climatic change. Although the generally dry conditions over parts of southern Africa over the last 15-20 years may be attributed in part to the relatively high ratio of warm to cold events, no long term change in the mean annual rainfall of the subcontinent can be implied at this stage.