Weakening trend of the southwest monsoon current through peninsular India from 1950 to the present

Abstract

The southwest monsoon current in the form of a crossequatorial low level jetstream (LLJ) with its core at an altitude of about 1.5 km and core wind speeds of 80 – 100 km/h is a conduit carrying moisture generated over south Indian Ocean and the Arabian Sea that supports the June to September monsoon rainfall in India. During spells of active (strong) monsoon, the core of the LLJ passes through peninsular India between latitudes 12.5 and 17.5 N. In break (weak) monsoon spells, LLJ bypasses India and flows to its south through the latitude belt 2.5 to 7.5 N. The mean June to September monsoon flow through India from surface to 1.5 km altitude between latitudes 12.5 and 17.5 N, had a si gnificant decreasing trend and that between 2.5 and 7.5 N a significant increasing trend during the recent half century, i.e. 1950 to the present. The dur ation of break (weak) monsoon spells in a monsoon season has increased by about 30% during this period. These changes have resulted in the number of days during the monsoon season 1 June to 30 September with daily average rainfall less than 8 mm/day increasing and days with rainfall more than 12 mm/day decreasing through 45.4% and 78.1% respectively, during the last 53 years. These are alarming findings for a cou ntry whose food production and economy depend heavily on monsoon rainfall.