Variability of the Indian Summer Monsoon and Tropical Circulation Features

Abstract

The Indian economy is very closely linked with the variable performance of the summer monsoon. The incidence of dry and wet conditions over the country has been examined for the period 1871?1978 by examining the normal monsoon rainfall and its variability over the region by means of the Index of Dryness over India (IDI) and the Index of Wetness over India (IWI). These are respectively defined as the country's percentage area characterized by dry and wet conditions. These series are found to be homogeneous, random, highly variable and positively skewed. The year in which IDI (IWI) exceeds the mean by more than two times the mean deviation from the mean is taken as a year of large-scale drought (flood). The occurrence of large-scale droughts or floods is found to be random in time continuum. The IDI and IWI show consistently significant correlation with the Southern Oscillation Index (SOI) and with sea surface temperature (SST) anomalies of the equatorial eastern Pacific for the concurrent and succeeding seasons. The relationships of the indices of dryness and wetness over the country with SOI and SST anomalies are expected to be useful in understanding the implications of the large-scale anomalies in the performance of the Indian summer monsoon.