Role of subtropical Rossby waves in governing the track of cyclones in the Bay of Bengal

The cyclones during November in the Bay of Bengal follow two distinct tracks. Analysis for the period 1982-2019 shows that some cyclones move west-northwestward and make landfall at the Odisha, Andhra Pradesh or Tamil Nadu coast of India, or the Sri Lanka coast, while others move north-northeastwards and make landfall at the West Bengal, Bangladesh or Myanmar coast. Our analysis shows there is a significant difference in the steering winds governing these two different cyclone tracks. The north-northeastward moving cyclones are associated with an anomalous upper-level cyclonic circulation over India which is part of a subtropical Rossby wave train triggered by an anomalous upper-level convergence over the Mediterranean region. This wave train propagates along the subtropical westerly jet from the east Atlantic/Mediterranean region and reaches the Indian subcontinent in 4 days. It induces an anomalous cyclonic circulation over the Indian landmass and provides south-to-north and west-to-east steering over the Bay of Bengal, causing the cyclones to move in a north-northeastward direction. On the other hand, for west-northwestward moving cyclones, there is no Rossby wave intrusion over the Indian subcontinent, hence the cyclones move in a west-northwestward direction assisted by the beta effect and climatological winds which are from east to west over the south and central Bay of Bengal. This shows that the track of cyclones in the north Indian Ocean can be modulated by atmospheric changes in the extratropics and can act as a precursor for the prediction of the track of cyclones in this region.