SURFACE CYCLOGENESIS OVER SOUTH-AMERICA

The frequency of surface cyclogenesis over South America (approximately the area enclosed by 15-degrees-50-degrees-S and 30-degrees-90-degrees-W) has been calculated using 10 years (1979-1988) of data. The frequency of cyclogenesis is more in winter than in any other season. Highest frequency (139) is found in the month of May and the lowest frequency (71) is found in the month of December. In addition to seasonal variation, the frequency of cyclogenesis shows interannual variation. The occurrence of cyclogenesis is more during the years of negative Southern Oscillation index (El Nino years) and less during the years of positive Southern Oscillation index. The years of higher (lower) cyclogenesis are found to be associated with higher (lower) rainfall. This explains the negative correlation between the precipitation over southern Brazil and the Southern Oscillation index.