Empirical Prediction of the Summer Monsoon Rainfall over India

We have examined 46 years (1939–84) of observed data to study synoptic and statistical relationships between the summer monsoon rainfall over India, the Southern Oscillation, and the midtropospheric circulation over India. The change in Darwin pressure from January to April and the latitudinal position of the April 500-mb ridge along 75°E are taken as two quasi-independent predictor parameters to develop a regression equation to predict the summer monsoon rainfall. Verification of predictions on independent data shows that the root-mean-square error for predicted rainfall is 36 mm, which is less than half of the standard deviation (82 mm) and only about 4% of the mean rainfall (857 mm). Since the observations needed to define the predictor parameters would be available well before the monsoon season, and since the performance of the empirical prediction formula is reasonably good, this method can be of some possible use for long-range forecasting of seasonal mean rainfall over India.