Relationships between South African rainfall and SST anomalies in the southwest Indian Ocean

Previous work has indicated the importance of sea surface temperature (SST) anomalies in the Southwest Indian Ocean for rainfall variability over South Africa. Here, these links are further explored via bandpass filtering of the rainfall and SST data in several bands on the interannual and interdecadal scale that are known to be significant for global climate variability. This procedure suggests that warmer SST in the Southwest Indian Ocean tends to be associated with wetter conditions over eastern and central South Africa and vice versa. An ensemble of experiments with an atmospheric general circulation model forced by an idealization of the warming in the Southwest Indian Ocean leads to statistically significant rainfall increases over large areas of eastern South Africa and neighbouring regions. The mechanism appears to involve changes in the convergence of moist air streams originating from the Indian Ocean and from tropical southern Africa. The magnitude of the rainfall anomalies accumulated over a 90 day season was of the order of 90-300 mm and, therefore, represent a significant fraction of the annual total. These model results reinforce the observational work suggesting that SST anomalies in the Southwest Indian Ocean are linked with significant rainfall anomalies over eastern South Africa.