Is Pre-monsoon Rainfall Activity Over India Increasing in the Recent Era of Global Warming?

Abstract

The 21 year moving averages of all-India pre-monsoon rainfall (PMR) shows epochal changes. The interesting fact is the presence of an increasing trend during 1970–2015. This change is closely associated with global warming. The tendencies of rainfall during this period are evaluated over India using gridded rainfall data. The differences in the mean rainfall between the global warming period (1970–2015) and the pre-global warming period (1901–1969) are evaluated. It seems that the major area of India is having positive differences and positive tendencies. In this study, the active and weak pre-monsoons are discriminated during 1979–2015. The composite mean differences in the rainfall of active and weak pre-monsoons have been evaluated using the daily gridded rainfall data. And also, the composite mean frequencies and their differences of different rainfall events i.e., wet, dry, very light rain, light rain, moderate rain, rather heavy, heavy rain, very heavy rain and extremely heavy rain days are analyzed. The factors responsible for the spatial and temporal evolution of the rainfall composite patterns are elucidated with the help of changes in the thermal, moisture and circulation patterns over India and neighborhood. The scrutiny reveals that the rainfall of the active/weak pre-monsoons in the recent 37 years is primarily associated with (1) the strengthened/weakened low level circulation, (2) increase/decrease in the convective available potential energy (CAPE), sea surface temperature (SST), vertical integrated moisture flux convergence (VIMFC) and relative humidity (RH) and (3) decrease/increase in the outgoing long-wave radiation (OLR). Furthermore, an attempt is made to understand the concurrent/forecast relationships between PMR and the Atlantic Multi-decadal Oscillation Index (AMO), Effective Strength Index (ESI), Southern Oscillation Index (SOI) and Sunspot number of pre-monsoon/winter season.