Role of the Bay of Bengal warming in the Indian summer monsoon rainfall trend

The Bay of Bengal (BoB) sea surface temperatures (SSTs) are found to be negatively correlated with the Indian summer monsoon (ISM) rainfall at seasonal, monthly and daily time scales. It is intriguing that the ISM seasonal mean rainfall is reported to be decreasing in the recent decades while no prominent warming has been observed over the BoB. We posit that the reason why BoB warming is not apparent is because BoB surface waters stay largely above the convective threshold of ~ 28 °C and further warming may be perceivable only in terms of an atmospheric response. This is consistent with prominent increase in the latent heat flux (LHF) over BoB during the pre-monsoon season. However such an increasing trend in LHF is not seen for the monsoon season. The increased surface evaporation leading to an increase in LHF over BoB enhances convection aloft which drives a stronger low level pre-monsoon circulation. We hypothesize that the dynamic response of the atmosphere associated with the pre-monsoon circulation changes driven by the BoB warming can explain, at least in part, the observed ISM rainfall trend. This hypothesis is tested by diagnosing SST-forced simulations with the Community Atmosphere Model version 6. The results suggest that the BoB warming does generate dynamic and thermodynamic responses which enhance moisture convergence (divergence) and convection over BoB (central India) during the pre-monsoon season. As a consequence, evapotranspiration is enhanced over the Indian subcontinent leaving the ground drier and colder. The net result is a weaker land–ocean thermal contrast and a weaker monsoon circulation and hence reduced seasonal rainfall totals.