Formation and structure of a monsoon revival depression over the Bay of Bengal during a 'break' monsoon period

Abstract

Formation and structure of a monsoon revival depression (MRD) over the Bay of Bengal during the monsoon 'break' of August 1977 are studied with the aid of six hourly rawinsonde ascents made by a polygon of four oceanographic research vessels of the USSR participating in the joint Indo-USSR Monsoon '77 expedition. It is found that a mid-tropospheric cyclone (MTC) initially developed over the East Central Bay (ECB) under the influence of an upper easterly wave (UEW) moving westwards across the Arakan coast of Burma and descended to the surface as MRD after the MTC had moved northwestwards for more than 24 hour. The MTC formed at the west-east interface (WEF) between the monsoon westerlies and the upper easterlies and developed upwards and downwards under the influence of the upper vorticity advection in the UEW. It was oscillatory, reaching its maximum vertical extent upto 200 mb (as a narrow core) just before it descended to the sea level. The MTC in the mid-troposphere was located to the southwest of the MRD in the lower troposphere and was warm cored while the MRD was cold cored. It is postulated that all monsoon depressions have similar origins and structures.

The structure of a break monsoon cell in the vertical and the mechanism of the formation of the MRD by superposition of the MTC and UEW are postulated schematically. It is pointed out that monsoon revival after a break takes place not by northward movement of a fresh oceanic intertropical zone of convergence (OITC) from the south Bay but by the return of the continental OITC from Burma to the north Bay under the influence of a UEW.