Environmental conditions modulating tropical cyclone formation over the Bay of Bengal during the pre-monsoon transition period

Abstract

Globally, the highest formation rate of super tropical cyclones (TCs) occurs over the Bay of Bengal (BoB) during the premonsoon transition period (PMT), but TC genesis has a low frequency here. TCs have occurred over the BoB in only 20 of the past 36 years of PMTs (1981-2016). This study investigates which environmental conditions modulate TC formation during the PMT over the BoB by conducting a quantitative analysis based on the genesis potential parameter, vorticity tendency equation, and specific humidity budget equation. The results show that there is a cyclonic anomaly in the TC genesis group compared to the non-TC genesis group, which is mainly due to the divergence term. A significant difference in vorticity contributes to TC formation over the BoB during the PMT. Furthermore, anomalous cyclonic flow enhances ascending motion, transporting moisture to the midlevel atmosphere. A change in specific humidity (SH) causes an increase in relative humidity, which contributes positively to TC formation. The vertical wind shear also makes a small positive contribution. In contrast to the previous three terms, the contribution from the instability term associated with 500-and 850-hPa air temperatures is negative and almost negligible. In addition, the synoptic-scale disturbance energy is more powerful in the TC genesis group than in the non-TC genesis group, which is favorable for TC breeding. Together, these conditions determine whether TCs are generated over the BoB during the PMT.