Abstract
The conditions in the upper troposphere leading to the formation of severe tropical cyclones in the Bay of Bengal have been examined. It is noticed that the westward movement of anticyclonic vortices over pre-existing sea level troughs is favourable for the formation of these cyclones. The formation, structure and dissipation of two cyclones in the Bay are discussed. The initial intensification of the sea level trough took place with the by-passing of the upper anticyclonic vortex, while the rapid ‘winding up’ of the cyclone occurred with an in-phase superposition of troughs in the easterlies and westerlies as suggested by Riehl. While the trough in the broad westerlies interlocked with the cyclonic trough in the easterlies south of the sub-tropical ridgeline in the case of the post-monsoon cyclone, a trough in the broad easterlies got interlocked with a cyclonic trough in the westerlies north of the subtropical ridgeline in the case of the pre-monsoon cyclone.
The disturbances start with cold air in the centre and develop a warm core during the stage of their intensification. Their dissipation occurs with the inflow of cold air from neighbouring jet streams or wave troughs in the high troposphere.
In all these respects the Indian cyclones bear a close similarity with the Typhoons of the Pacific and Hurricanes of the Atlantic oceans.
