The Monsoon Low-Level Jet: Climatology and Impact on Monsoon Rainfall over the West Coast and Central Peninsular India

The monsoon low-level jet (MLLJ) originates at Mascarene high and after traveling thousands of kilometers enters India from the western boundary causing deep convection, cloudiness, and rainfall. Although its core lies at 850 hPa, it has a large vertical extent; therefore, different meteorological parameters at different levels have a large influence on the Indian summer monsoon rainfall. This study aims to examine the upper-air climatology of 9 stations on the west coast and central Peninsular India and to find out the effects of various parameters at different standard pressure levels on the Indian summer monsoon rainfall variability. We used the 34-yr (1971–2004) actual upper-air radiosonde/radio wind and standard synoptic surface observations data from these 9 stations and reported some new aspects of the MLLJ. The NCEP/NCAR and ECMWF reanalysis wind data have also been used to holistically study the features of MLLJ over sea and land areas. This study, as opposed to some recent studies, confirms the splitting of MLLJ into two branches, which can be seen on a few days during the monsoon season. Further analyses show that the change in geopotential height between 800 and 900 hPa has a strong bearing on the strength of MLLJ. The change in the upper-air pressure gradient force over the Indian landmass can cause a change in the wind speed of MLLJ during the monsoon season.