Role of low level flow on the summer monsoon rainfall over the Indian subcontinent during two contrasting monsoon years

The summer monsoon rainfall over the Indian subcontinent shows 1 large inter-annual variability in three important aspects, namely, the onset date, quantum of monsoon rainfall and the monsoon activity within the monsoon (June - September) period. In the present study, we look into the role of the low level flow during two contrasting monsoon years, viz., 1987 (deficit) and 1988 (excess) using the daily data from the National Centre for Environmental Prediction (NCEP) Reanalysis and European Centre for Medium range Weather Forecasting (ECMWF). We have further classified the daily data into different pentads (5 day means) to look into the role of the low level flow over the tropical Indian Ocean on the monsoon activity, during different phases such as break, weak, moderate and active rainfall conditions over the Indian subcontinent. The strength of the low level flow into the Indian subcontinent (equatorial Indian Ocean (EIO) region) is significandy higher during active and moderate (break and weak) monsoon conditions. The low level flow and moisture transport were studied using the surface pressure, surface winds, 850 mb winds and integrated columnar precipitable water during different periods ranging from break to very active conditions in the monsoon activity. The present study, thus, clearly brings out the importance of low level flow into the Indian subcontinent (Equatorial Indian Ocean) during the excess (deficit) monsoon years.