A NOTE ON THE MODULATION OF SOUTHERN OSCILLATION-SOUTHERN AFRICAN RAINFALL ASSOCIATIONS WITH THE QUASI-BIENNIAL OSCILLATION

Since the 1950's, although the association between the Southern Oscillation and South African rainfall variability appears to have been reasonably stable, there is evidence that in certain years the association has broken down completely. The Southern Oscillation-South African rainfall association is modulated with the phase of the Quasi-Biennial Oscillation in tropical stratospheric zonal winds such that high phase (low phase) wet (dry) conditions occur over the summer rainfall region in January-March only when the Quasi-Biennial Oscillation is in the westerly phase. In contrast, there is no noticeable difference in the Southern Oscillation-South African rainfall association between easterly and westerly years in October-December. The Quasi-Biennial Oscillation modulation appears to occur only when the prevalent synoptic systems over the country are of tropical origin. In the dry winter season (July-September) the Southern Oscillation-rainfall association and the Quasi-Biennial Oscillation modulation are inverse to the relationships in the peak rainfall season of January-March. Thus, a drier (wetter) than normal dry season occurs after the Southern Oscillation has been in high (low) phase and only when the Quasi-Biennial Oscillation is in easterly, and not westerly, phase.