South African rainfall, the Southern Oscillation and a Southern Hemisphere semi-annual cycle



Significant differences in rainfall over central South Africa are known to occur between opposite extremes in the phase of the Southern Oscillation, but details of both temporal and spatial aspects of the modulation of South African rainfall with the phase changes of the Oscillation remain to be described. Zero-lag correlations between the Tahiti—Darwin Southern Oscillation Index and monthly and 3-month seasonal rainfall over South Africa suggest that the rainfall—Southern Oscillation Index association is best defined in the late summer season January—March and in a north-west to south-east aligned zone across the central summer rainfall region of South Africa. Rainfall in this zone is directly related to the Southern Oscillation Index, increasing during high phase summers. An apparent semi-annual cycle in the rainfall—Southern Oscillation Index correlations over central South Africa is in phase with the November and February turning points of a semi-annual cycle in atmospheric circulation parameters over southern Africa. A plausible circulation mechanism is suggested to account for some of the spatial and temporal characteristics of the association between the Southern Oscillation and South African rainfall.