Prediction of the Summer Rainfall over South Africa

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Abstract
This study develops methods for the extended-range forecasting of summer rainfall in the eastern part of southern Africa. The predictand is an index (TVR) of the December–January–February precipitation in the Transvaal. The predictors are based on empirical-diagnostic analyses and include the July–August–September values of Tahiti minus Darwin pressure difference as an index (SOI) of the Southern Oscillation; the preceding January–February–March value of the 50-mb zonal wind over Singapore (U50); an index of the October–November surface westerlies along the Indian Ocean equator (UEQ); and an index of November sea surface temperature in the southwestern Indian Ocean (UKT). These predictors serve as input to stepwise multiple regression, linear discriminant analysis, and neural networking. The training period is 1954–78, and the verification period 1979–93. Regression models, using as predictors U50, UEQ, and UKT, account for more than 30% of the variance in the independent dataset. The linear discriminant analysis does not perform well. Most powerful is a neural networking model having as input information through the end of September, namely U50 and SOI, and explaining 62% of the variance in the verification period. The predictors used here could, in principle, be compiled in quasi-real time, so that the method lends itself to operational application.

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