Links between South African summer rainfall and temperature variability of the Agulhas and Benguela Current systems


  • N. D. Walker


Close associations are identified between interannual sea surface temperature (SST) variability within the Agulhas Current system, the Agulhas Retroflection region, and the Benguela Current system and summer rainfall behavior of South Africa. The relationships between local ocean temperatures and summer rainfall are strongest when extreme global-scale atmospheric events, i.e., Southern Oscillation high- and low-index phases, are excluded from the time series. The results indicate that rainfall prediction efforts may benefit from a consideration of SST anomalies within the western portion of the south Indian Ocean, in combination with other meteorological indices. Surface winds, heat fluxes, and atmospheric boundary layer characteristics are investigated as ocean-atmosphere coupling mechanisms. Easterly wind anomalies across the southwest Indian Ocean and over source regions of the Agulhas Current accompany and precede local oceanic “warm events” which co-occur with increased rainfall. Whereas previous climatic research has revealed that intensification of the South Atlantic Anticyclone co-occurs with wet periods over the summer rainfall region, this research has revealed an additional mode of climate variability involving enhanced atmospheric circulation within the southwest Indian Ocean. The simultaneous occurrence of strong easterly winds and a moister atmospheric boundary layer produces increased moisture flux convergence over tropical and subtropical regions of the eastern subcontinent. South of Africa, altered surface heat flux distributions enhance low-level baroclinicity and instability within the marine boundary layer, optimizing conditions for the intensification of mid-latitude frontal systems. These processes provide mechanisms which increase the likelihood of tropical-temperate trough formation across southern Africa, the major rain-producing system in summer.