Sea surface temperature-rainfall relationships and associated ocean-atmosphere coupling mechanisms in the southern African region
The relationships between interannual sea surface temperature variability (SST) of the oceans surrounding southern Africa and summer rainfall variations over South Africa are investigated using statistical, observational and mechanistic approaches. Positive correlations are identified between summer rainfall and SSTs of the Mozambique/ Agulhas Current region, the Agulhas Retroflection region and the northern Benguela Current system. These relationships are stronger when rainfall anomalies associated with the Southern Oscillation are not considered. The observation of significant lag relationships involving temperatures of the Agulhas Current system suggests that future prediction efforts for summer rainfall will benefit from a consideration of SST anomaly patterns east and south of Africa, in combination with other atmospheric indices. Surface winds, heat fluxes and atmospheric boundary layer characteristics are investigated using compositing analyses to assess pertinent ocean-atmosphere coupling mechanisms. Easterly wind anomalies across the southwest Indian Ocean and over source regions of the Agulhas Current accompany and precede the local oceanic "warm events" which correspond with higher rainfall. Thus a class of event is identified in which warmer waters along the east coast and stronger easterly wind forcing accompany wetter seasons over South Africa. The atmospheric boundary layer is considerably warmer and moister in association with positive SST anomalies along the east coast and increased tropical airflow. Consequently, moisture convergence and tropical convection are increased over the eastern interior near 20° to 25°S. South of Africa, positive SST anomalies generate surface heat flux anomalies, increasing instability and moisture levels within the boundary layer. Horizontal heat flux gradients are strengthened across the Agulhas/Subtropical Convergence SST front and the conditions necessary for cyclogenesis and westerly wave amplification are optimized. Tropical-temperate troughs account for most of the abnormal rainfall during local Agulhas "warm events". The presence of positive SST anomalies east and south of southern Africa increases the likelihood of their formation by intensifying tropical and temperate components. The contribution provided by each component is influenced by the position and magnitude of the SST anomaly as well as the season of occurrence. Conceptual models are presented which summarize the most important ocean-atmosphere coupling mechanisms associated with rainfall variations of southern Africa.
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