Modelling the atmospheric response to SST dipole patterns in the South Indian Ocean with a regional climate model

Abstract

Previous observational work and experiments with global atmospheric circulation models have provided evidence that a dipole-like sea surface temperature (SST) pattern in the subtropical South Indian Ocean may influence austral summer rainfall over a southern African region stretching southeast from Zambia towards the south coast of South Africa. Increased rainfall tends to occur during positive events when the SW Indian Ocean anomaly is warm and the SE Indian Ocean anomaly is cool, and decreased rainfall during negative events when the SST anomalies are reversed. In this study, the regional climate model MM5 is used to investigate the atmospheric response to two strong events of opposite sign (1981 and 1998). In general, the MM5 response is broadly consistent with NCEP re-analyses for these years but tends to advect more low level moisture over than NCEP. The model results indicate that more moisture is advected over the subcontinent from the South West Indian Ocean during the positive event than during the negative event. This is consistent with observed rainfall over the region during these events.