Role of Upper-Level Rossby Wave Propagation in Governing the Track of the Tropical Cyclones in the Bay of Bengal

Tropical cyclones in the Bay of Bengal during the post-monsoon season follow two different tracks. Some cyclones move west-northwestward and make landfall at the Odisha, Andhra Pradesh, and Tamil Nadu coast while others move north-northeastwards and make landfall at West Bengal, and Bangladesh coast. Analysis for the period 1982-2019 shows that there is a significant difference in the upper-level winds that prevail during these two distinct cyclone tracks. These upper-level winds that govern the track of the cyclone in the Bay of Bengal are modulated by Rossby wave propagation along the sub-tropical westerly jet. These Rossby waves originate over the East Atlantic/Mediterranean region, and gets trapped in the sub-tropical westerly jet. The cold air intrusion and anomalous convergence over the Mediterranean act as a trigger for the start of the Rossby waves at the jet entrance region. The westerly jet acts as a waveguide and excites cyclonic/anticyclonic wind anomalies as a wave train along the jet axis which reaches the Indian region and Bay of Bengal in about four days. The propagation of these Rossby waves modulates the winds over the Bay of Bengal and leads to the establishment of an anomalous cyclonic circulation over India's landmass. This cyclonic circulation provides steering in the south to north direction over the Bay of Bengal and steers the cyclone northward towards the West Bengal/Bangladesh coast. On the other hand the cyclones which forms in the Bay of Bengal in the absence of the upper level Rossby waves, move west-northwestwards due to the beta drift and absence of any anomalous steering in the south to north direction. This shows that the track of the cyclone in the Bay of Bengal can be modulated by the large-scale atmospheric changes in the subtropics. An improved understanding of this teleconnection can improve the cyclone track predictability over the Bay of Bengal.