On the increasing length of the southwest monsoon season over India

Abstract

The southwest monsoon rainfall has a paramount impact on agricultural practices and water resources management across India. This study investigates long-term changes in the southwest monsoon length over India for the period from 1971 to 2020. The length of the southwest monsoon season across the country shows an increase at the rate of 1.6 days per decade during 1971-2020. The rate of increase in the southwest monsoon length coincides with the rate of delay in the monsoon withdrawal dates during this period. Although the monsoon onset date over Kerala (MOK) shows notable interannual variability, it does not show significant change during the last 50 years. Rainfall between 01 June and 30 September (JJAS) is typically considered for the southwest monsoon rainfall analyses. This study investigates the difference in rainfall between MOK and withdrawal dates (termed as MOK2WDRL), and JJAS rainfall for 1971-2020 using a rain gauge-based gridded rainfall dataset. JJAS rainfall contributes 75% to all-India annual rainfall, whereas it is 79% by MOK2WDRL rainfall. The interannual variability of all-India monsoon rainfall is also shown to be different for several years from MOK2WDRL and JJAS rainfall estimates. The correlation coefficients with Nino 3.4 index, all-India rabi crop yield, and all-India rapeseed & mustard yield are shown to be higher with MOK2WDRL rainfall than JJAS rainfall over the country.