10-Role of Southern Hemispheric Equatorial Trough in Long Range Forecasting.

Abstract

Satellite observed weekly mean cloud cover data for the premonsoon months of April and May over the Indian Ocean between 20S and 20N latitudes and 40 to 100E longitudes have been studied for the years 1972 to 1991 (except for the year 1978). Features of the southern hemispheric equatorial trough and their relationship with the performance of the southwest monsoon have been identified. An overall negative relationship between the activity of SHET during pre-monsoon months and the subsequent southwest monsoon activity has been observed. The. features of the activity of SHET have been quantified by assigning an activity index ranging from 1 to 20. The activity index so assigned is highly correlated (CC ranging from -0.51 to -0.89) for those meteorological subdivisions of northwest and peninsular India where the rainfall variability is more. This relationship between the southern hemisperic equatorial trough activity index and the seasonal rainfall in each meteorological subdivision has been used to develop regression equations for seasonal forecasting of SW monsoon rainfall. The technique reported here appears promising in producing useful long range rainfall forecast for the SW monsoon season. In addition to this the results of this study are helpful in understanding the physical processes taking place over the Indian Ocean during the pre-monsoon months which also continue to appear during the period June to September leading to the development of Asian summer monsoon and its different phases.