Prediction of the summer monsoon rainfall over South China

The objective of this study is to derive an operational statistical prediction scheme for forecasting the summer monsoon rainfall (SMR, defined as the total rainfall between April and June) over South China (SC). The region of SC is first identified by applying the techniques of cluster analysis and factor analysis to the SMR of 43 stations over China for a period of 47 years (1951-1997). This procedure suggests 13 stations along the SC coast as having similar rainfall characteristics. Subdivisions of these into six stations in the east and seven in the west are also made. The predictands are therefore the average SMR over these stations. The potential predictors include: (a) indices that serve as proxies for the El Nino/Southern Oscillation (ENSO); (b) indices that represent the planetary-scale circulation and (c) the trend and periodicities in the rainfall time series. All predictors are monthly values from March of the previous year to February of the current year. The technique of projection pursuit regression is applied to derive prediction equations for each predictand using individual predictors. Only those equations in which the predicted (obtained from the jackknife technique) and observed SMR are significantly correlated (≥ 95%) are retained. The predictions from individual predictors are combined using a weighted average (based on the magnitude of the correlation coefficient) to produce a final forecast, which is found to be superior to those from the individual ones. An operational prediction of the SMR made in March 1998 suggested a below-normal season. However, the actual results indicate an above-normal one. Reasons for this apparent failure are discussed.