Role of subtropical Rossby waves in governing the track of cyclones in
the Bay of Bengal
Abstract
The cyclones during November in the Bay of Bengal follow two distinct
tracks. Some cyclones move west-northwestward and make landfall at
Odisha, Andhra Pradesh, Tamil Nadu or Sri Lanka coast while others move
north-northeastwards and make landfall at West Bengal, Bangladesh or
Myanmar coast. We found that there is a 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 a part of a sub-tropical wave train.
This wave train is triggered by an anomalous upper-level convergence
over the Mediterranean region near the subtropical westerly jet
entrance. The jet acts as a Rossby waveguide and excites an eastward
propagating wave train which propagates from the East
Atlantic/Mediterranean region and reaches the Indian subcontinent in
four days. This wave train induces an anomalous cyclonic circulation
over 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-northward
moving cyclones, there is no Rossby wave intrusion over the Indian
subcontinent, hence the cyclones move in west-northwestward direction
assisted by the climatological winds which are from east to west over
the south and central Bay of Bengal. This shows that the track of the
cyclone in the north Indian Ocean can be modulated by the atmospheric
changes in the extratropics and can act as a precursor for the
prediction of the track of the cyclone in this region.