Premonsoon Ridge Location over India and Its Relation to Monsoon Rainfall

Abstract

The location of the 500-hPa ridge axis during April over India is one of the most important long-range predictors for the summer monsoon rainfall. This paper presents a comprehensive analysis on its space-time variability during the premonsoon season and its relation with the monsoon rainfall. Data on the daily latitudinal locations of the 500-hPa ridge axis along three longitudes during March, April, and May, as well as all-India rainfall and subdivisional monsoon rainfall for the period 1967?90, have been used. The analysis involves correlations between the running means of the premonsoon ridge locations over windows of 15, 21, and 31 days, and the subsequent monsoon rainfall. The ridge location in March shows negative correlation with the all-India summer monsoon rainfall, while that in April shows positive correlation. The anticorrelation of the March ridge was more dominant with the monsoon rainfall of the peninsular India, while the positive correlation of the April ridge was more dominant with the monsoon rainfall of northern India. Regression equations for the prediction of the monsoon rainfall have also been developed.