An observational study of the relationship between East African rainfall and Indian Ocean SST

Emily Black

In 1997-1998, the sea surface temperature of the Indian Ocean was unusually high off the African coast and low near Sumatra. This resulted in reversal of the normal climatological gradient in Indian Ocean SST - with the SST between October 1997 and June 1998 increasing from east to west rather than from west to east. Analysis of 128 years of SST data puts the 1997-8 event in the context of long-term variability in Indian Ocean SST. It is shown that there have been several previous incidences of similar SST anomaly patterns. These events are used to investigate the climatic impact of Indian Ocean SST on the surrounding continents. In all the years examined, reversed SST gradients are associated with anomalous zonal winds and OLR. In October and November, when the SST pattern is most pronounced, the normally weak westerly winds over the Central Indian Ocean are replaced by moderately strong easterlies. In Africa the OLR is unusually low, indicating high rainfall while in Indonesia it is high which is indicative of drought. The relationship between SST and East African rainfall is further elucidated by collating precipitation data from seven East African stations. It is demonstrated that extreme rainfall at each of the stations is associated with a localised SST low off the North Australian coast and a weaker SST high east of Madagascar. These observations imply that high East African rainfall is associated with strong remote SST anomalies rather than with small, local fluctuations. Thus, greater understanding of large-scale Indian Ocean dynamics could lead to improved predictability of East African rainfall.