Changes in Convective Activity Over the Indian Ocean and Neighbourhood in Relation to Onset, Withdrawal and Break Features of Monsoon

The variability of convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on inter-annual and longer time scales and their association with the onset, withdrawal, break and seasonal rainfall over India during the monsoon season from June to September (JJAS) has been investigated by using 40-year (1980–2019) monthly mean outgoing longwave radiation (OLR) data. The 40-year period has been divided into two groups of 20 years with the first 20 years from 1980 to 1999 (hereafter called F8099) and second 20 years from 2000 to 2019 (hereafter called L2019). It is seen that the mean sea surface temperature (SST) over the north Indian Ocean shows higher values during period L2019 compared to that in period F8099. The inter-annual variability of SST shows a significant increasing trend both over the Arabian Sea and the Bay of Bengal with a comparatively higher rate of increase of SST over the Arabian Sea. Associated with this increasing SST, the degree of moist static convective instability and associated convective rainfall is also increased during period L2019 compared to period F8099 with a magnitude of difference that is higher over the Arabian Sea compared to the Bay of Bengal. On an inter-annual scale, a significant decreasing trend of OLR anomaly (thus, an increase in convective activity) is also noticed mainly over the Arabian Sea, which is correlated with the monsoon rainfall over most of India. The analysis also indicated that there is rapid progress of monsoon to the north after its onset over the southern tip of India leading to early onset over northern India along with delayed withdrawal phase of monsoon from northwest India in period L2019 compared to period F8099 is basically due to the increasing convective activity over the north Arabian Sea and neighbourhood during the onset and withdrawal phase of monsoon, respectively. During the peak monsoon phase of July–August, convective activity over the North Bay of Bengal (NBAY) is correlated positively with the break frequency, whereas the convective activity over the North Arabian Sea (NARA), South Arabian Sea (SARA) and South Bay of Bengal (SBAY) regions are related inversely with the frequency of monsoon break.