Bay of Bengal: Coupling of pre-monsoon tropical cyclones with the monsoon onset in Myanmar in a Changing Climate

The pre-monsoon tropical cyclone (TC) activity and the monsoon evolution in the Bay of Bengal (BoB) are both influenced by the Madden Julian Oscillation (MJO), but the two do not always occur in unison. This study examines the conditions that allow the MJO to modulate the monsoon onset in Myanmar and TC activity concurrently. Using the APHRODITE gridded precipitation and the ERA-Interim reanalysis datasets, composite evolutions of monsoon rainfall and TC genesis are constructed for the period of 1979-2010. It is found that the MJO exhibits a strong interannual variability in terms of phase and intensity, which in some years modulate the conditions for BoB TCs to shortly precede or form concurrently with the monsoon onset in Myanmar. Such a modulation is absent in years of weaker MJO events. Further understanding of the interannual variability of MJO activity could facilitate the prediction of the monsoon onset and TC formation in the BoB. To understand how BoB tropical cyclones themselves have changed in the recent epoch, 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.