Bimodal Character of Cyclone Climatology in the Bay of Bengal Modulated by Monsoon Seasonal Cycle

The annual cycle of tropical cyclone (TC) frequency over the Bay of Bengal (BoB) exhibits a notable bimodal character, different from a single peak in other basins. The causes of this peculiar feature were investigated through the diagnosis of a genesis potential index (GPI) with the use of the NCEP Reanalysis I dataset during the period 1981-2009. A methodology was developed to quantitatively assess the relative contributions of four environmental parameters. Different from a conventional view that the seasonal change of vertical shear causes the bimodal feature, it was found that the strengthened vertical shear alone from boreal spring to summer cannot overcome the relative humidity effect. It is the combined effect of vertical shear, vorticity, and SST that leads to the GPI minimum in boreal summer. It is noted that TC frequency in October-November is higher than that in April-May, which is primarily attributed to the difference of mean relative humidity between the two periods. In contrast, more supercyclones (category 4 or above) occur in April-May than in October-November. It is argued that greater ocean heat content, the first branch of northward-propagating intraseasonal oscillations (ISOs) associated with the monsoon onset over the BoB, and stronger ISO intensity in April-May are favorable environmental conditions for cyclone intensification.