Theme-1-3-The Bay of Bengal depression of September 1986 that caused deluge in West Bengal - A case study

Abstract

In India floods mainly occur during the monsoon season. They are caused by the heavy rainfall that occur along the track of monsoon depressions The Bay depression• of 25 September 1986 which caused devastation in the West Bengal forms the subject matter of this study. New processes which led to the development of this depression have bean identified and presented. A cyclonic circulation first formed in the lower tropospheric levels over Bangladesh and adjoining West Bengal. it interacted with the environmental cloudiness (i) a cloud cluster over the Bay of Bengal and Burma (ii) a fresh northward propagating mon oon pulse, identified in the form of an east-west trough at 500 hPa and-the associated cloudiness on cloud imagery. We observe a sequence of events ia which (i) the cyclonic circulation moves southward and organises itself into a low pressure area on the sea surface under interaction with the cloud cluster moving from east (ii) the low pressure area concentrates itself into a depression when it merges with the northward propagating monsoon pulse. The depression had a cold core at the time of formation. Subsequently it developed a warm core as a result of heavy rainfall and release of latent heat.