Existence of ‘mini-break’ like condition during early Indian monsoon onset years

Abstract

In the context of increasing probability of advancement in onset date of Indian Summer Monsoon during recent years, precipitation and circulation features of early monsoon onset events are investigated in detail using reanalysis data and model simulations. During the onset pentad (± 2 days from the onset day), an anomalous increase in precipitation is observed over the west coast of peninsular India and Bay of Bengal during early onset years compared to normal onset years. The monsoon low level jet (LLJ) also becomes stronger during the onset pentad of early onset years particularly over the southern peninsular India. However, after a week from the onset day of early years, precipitation is considerably decreased and the LLJ become weaker. Moreover, convection over the eastern Arabian Sea and western peninsular India suppresses during that period which in turn inhibits the systematic northward propagation of monsoon rainbands. This induces a ‘mini break’ like condition immediately after a week from the onset day during early onset years. Analysis of the sea surface temperature (SST) shows that, over the north eastern Arabian Sea, a lower value of SST exists during early onset years compared to normal onset years which may have a triggering role in the formation of ‘mini break’ like condition. This aspect is examined using sensitivity simulations with the Advanced Research Weather Research and Forecasting model. Simulations with reduced SST over the north eastern Arabian Sea during normal onset years exhibits considerable reduction in precipitation over the west coast of India and weakens the LLJ over the peninsular India. Whereas simulations with enhanced SST over the north eastern Arabian Sea during early onset years show increase in the convective activities over the eastern Arabian Sea and west coast of peninsular India, and the existence of ‘mini break’ like situation disappears from those years. Thus numerical simulations confirm the relation between SST over the north eastern Arabian Sea and the formation of ‘mini break’ during early onset years.