Intensification of pre-monsoon tropical cyclones in the Bay of Bengal and impacts from aerosols

We analyze multiple global reanalysis and precipitation datasets in order to explain the dynamic mechanisms that lead to an observed intensification of the monsoon trough and associated tropical cyclone (TC) activity over the Bay of Bengal (BOB) during the premonsoon month of May. We find that post-1979 increases in both premonsoon precipitation and TC intensity are a result of enhanced large-scale monsoon circulation, characterized by lower-level cyclonic and upper-level anticyclonic anomalies. Such circulation anomalies are manifest of the tropospheric expansion that is caused by regional warming. The deepened monsoon trough in the BOB not only affects TC frequency and timing, but also acts to direct more cyclones towards Myanmar. We propose that increasing sea surface temperature in the BOB has contributed to an increase in cyclone intensity. Our analyses of the Community Earth System Model single-forcing experiments suggest that tropospheric warming and a deepening of the monsoon trough can be explained by two discreet anthropogenic causes—an increase in absorption due to aerosol loading and an increase in the land-ocean thermal contrast that results from increased greenhouse gases. The ensuing circulation changes provide favorable conditions for TCs to grow and to track eastward towards Myanmar.