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 Oscill ation and South African rainfall variability appears to have been reas onably stable, there is evidence that in certain years the association has broken down completely. The Southern Oscillation-South African ra infall association is modulated with the phase of the Quasi-Biennial O scillation 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 w esterly phase. In contrast, there is no noticeable difference in the S outhern Oscillation-South African rainfall association between easterl y and westerly years in October-December. The Quasi-Biennial Oscillati on modulation appears to occur only when the prevalent synoptic system s over the country are of tropical origin. In the dry winter season (J uly-September) the Southern Oscillation-rainfall association and the Q uasi-Biennial Oscillation modulation are inverse to the relationships in the peak rainfall season of January-March. Thus, a drier (wetter) t han normal dry season occurs after the Southern Oscillation has been i n high (low) phase and only when the Quasi-Biennial Oscillation is in easterly, and not westerly, phase.