The influence of the decadal-scale variability of summer rainfall on the impact of El Nino and La Nina events in South Africa

Regions of correlation between South African late summer rainfall and sea-surface temperature (SST) in the NINO3 region were identified. Regions identified for late summer showed a much better spatial coherence than was the case for early summer. These regions were then compared with eight homogeneous rainfall regions in South Africa, identified by Mason (Mason, S.J. 1998. Int. J. Climatol., 18, 147-164). For the period 1955-1991, near-decadal epochs of above- and below-normal late summer rainfall were identified, with Cramer's t-test and cumulative deviations, for five of the eight homogeneous regions which possessed areas with rainfall correlated with the NINO3 SST time series. According to these tests, the 1960s and 1980s are characterized by below-normal rainfall, while the 1950s and 1970s are characterized by above-normal rainfall. Turning points between these epochs are ca. 1959/1960, 1970/1971, 1980/1981 and 1991/1992. This corresponds to cycles in summer rainfall in South Africa identified by other authors. To estimate rainfall anomalies without the effect of El Nino/La Nina, rainfall anomalies due to NINO3 were estimated by a linear regression of rainfall against NINO3 SSTs. These were subtracted from the standardized rainfall anomalies to obtain the residual anomalies that give an indication of the rainfall without the effects of El Nino/La Nina. From these time series similar epochs and turning points were evident. El Nino seasons during epochs of below- and above-normal rainfall were compared. The same was done with La Nina seasons. The impact of El Nino and La Nina is influenced in such a way that during an epoch of above-normal rainfall a moderating effect is evident on the severity of El Nino events so that even rainfall above the normal rainfall is experienced on average during such events. The opposite is also true in that an epoch of below-normal rainfall will have a negative effect on the enhancement of rainfall usually associated with a La Nina event. Extreme events of rainfall (droughts and floods), usually occur when an Fl Nino event occurs during an epoch of below-normal rainfall and when a La Nina event occurs during an epoch of above-normal rainfall. Copyright (C) 1999 Royal Meteorological Society.