Recent unprecedented weakening of Indian summer monsoon in warming environment

Abstract

All India summer monsoon rainfall (AISMR) shows a declining trend since 1953. Hence, the period 1953–2015 is considered for this study. The deviations of the summer monsoon (SM) rainfall in the recent period (1983–2015) from that of the previous period (1953–1982) have been evaluated for individual months and whole SM season using India Meteorological Department (IMD) gridded rainfall. The changes in the SM rainfall in the recent 3 decades with respect to the changes in the thermal, moisture, and circulation patterns over India and neighborhood are studied. The relationships between the winds over the Somali jet core region and the SM rainfall over India are quantified. Enhancement of rainfall activity is seen over a few subdivisions mainly Jammu & Kashmir and parts of Northwest India where the western disturbance interaction and the meridional wind component of low level jet (LLJ) have been pronounced. The decline in rainfall over major part of India in the recent 33 years is associated with weakened low level circulation, decrease in the relative humidity (RH) over the Bay of Bengal, less frequency of cyclonic disturbances formed in the Bay of Bengal, decrease in the genesis potential parameter as well as north-south sea level pressure gradient between Northern and Southern Hemispheres. The inconsistency of warming rates between Northern India and Southern India and its adjoining areas in the layer, 600–200 hPa induces reduced SM circulation. Further, the Walker and Hadley circulations specify a decline in the convective activity in SM season over the Indian mainland.