Precursors of northeast monsoon rainfall variability during extreme epochs of the global warming era

Abstract

The smoothed time series of northeast monsoon rainfall over south peninsular (SP) India has revealed two extreme active epochs (1877-1887 and 2005-2015) and two extreme weak epochs (1899-1909 and 1980-1990) during 1871-2016. Only the recent two epochs 1980-1990 and 2005-2015 were chosen due to data constraints. For these two epochs, the distribution of zonal wind pattern at the 850 and 200 hPa levels, latent heat flux, vertical shear in the zonal wind between 850 and 200 hPa, sea surface temperature and outgoing long wave radiation (OLR) were studied. The vertical profile of zonal wind over the mean position of the subtropical westerly jet and the maximum wind reversal were also analysed. Further, the frequencies of dry days, little rainfall days (0-20 mm), moderate rainfall days (20-60 mm), heavy rainfall days (60-100 mm) and very heavy rainfall days (>100 mm) were evaluated. SP India has experienced active/weak monsoon conditions every one to two decades alternately. An active monsoon epoch shows a higher north-south temperature gradient over the Bay of Bengal than a weak monsoon epoch. An upper level subtropical westerly jet over north India, strong tropical easterlies in the lower troposphere and low vertical shear in the zonal wind over the Bay of Bengal were observed during the active epoch, which facilitates the transportation of good amounts of moisture to SP India. The low OLR values over SP India in the active monsoon epoch indicate more convective activity over that region. Enhanced rainfall activity over SP India in the active monsoon epoch is due to the enhanced frequency of very heavy, heavy, moderate and little rainfall events and the reduced frequency of dry days. K E Y W O R D S active epoch, latent heat flux, northeast monsoon, weak epoch, zonal wind