Recent Changes in Onset and Withdrawal Characteristics of Monsoon over India in Relation to Variability of SST and Convection over the Indian Ocean

The  onset  of  the  southwest  monsoon  over  Kerala  (Southern tip of India) is  very  crucial  as  it  marks  the  beginning  of  the rainy  season  for  Indian  land  mass.  The  onset  of  the  broad  scale  Asian  monsoon  occur  in  many  stages  associated with the significant  transitions  in  the  large-scale  atmospheric  and  ocean  circulations  over  the  region.  Along with this, the changes in sea surface temperature (SST) and convective activity over the north Indian Ocean also play crucial roles during the onset and advance of monsoon over India. Recent analysis (based on data from 1971 to 2019) by India Meteorological Department (IMD) on the onset & withdrawal of southwest monsoon over India compared to the earlier onset and withdrawal dates (based on the rainfall data from 1901 to 1940) has found variations in onset and withdrawal dates over different parts of India.

 

        The variability of SST and convective activity over the north Indian Ocean (Bay of Bengal and Arabian Sea) on inter-annual time scales and their association with the onset and withdrawal of southwest monsoon over India has been analysed by using 42-year (1980-2021) monthly mean outgoing longwave radiation (OLR) data. The 42-year period is categorised into two groups of 21 years each (Former: 1980-2000 & Later: 2021-2021). The inter-annual variability of SST shows significant increasing trends 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 increasing during later period compared to the former period with a magnitude of difference is higher over the Arabian Sea compared to the Bay of Bengal.

         The analysis also indicated that there is a rapid progress of monsoon to the north after its onset over the southern tip of India leading to early onset over parts of northern India.  Similarly, there is a delayed withdrawal of monsoon from northwest India in later period compared to the former period, which is basically, due to the increasing convective activity over the north Arabian Sea and neighbourhood during the onset and withdrawal phase of monsoon.