Modelling the atmospheric response over southern Africa to SST forcing in the southeast tropical Atlantic and southwest subtropical Indian Oceans

The sensitivity of the atmospheric circulation and rainfall over southern Africa to various regional sea surface temperature (SST) patterns observed during Benguela Nino events is investigated using an atmospheric general circulation model (AGCM). The model used is the United Kingdom Meteorological Office (UKMO) HadAM3 and experiments using idealizations of observed regional SST anomalies during various Benguela Nino events are performed. It is found that SST forcing in the tropical southeast Atlantic induces a regional baroclinic response and that a Benguela Nino is capable of forcing anomalous rainfall along the Angolan and northern Namibian coastal regions on its own, via changes in low level moisture convergence, uplift and evaporation over the SST forcing. An experiment with the tropical southeast Atlantic SST anomaly shifted slightly further north produced a larger circulation and rainfall response in the model. Additional experiments with various SST anomalies in the southwest Indian Ocean combined with those in the southeast Atlantic were performed. These experiments are motivated by the fact that South Indian Ocean SST anomalies of varying signs often occur at the same time as Benguela Nino events. The results of these experiments suggest that, depending on its sign, magnitude and location, SST forcing from the southwest Indian Ocean may augment or oppose the southern African rainfall anomalies occurring during a Benguela Nino event to varying degrees. Copyright (C) 2009 Royal Meteorological Society