Wind shear in a monsoon depression

Abstract

DEPRESSIONS of the Bay of Bengal are an important component of the south-west monsoon circulation. Moving WNW, they account for much of the rainfall in India. Synoptic conditions favouring formation of monsoon depressions, so far described, are: (1) trailing of the axis of the monsoon trough into the head Bay of Bengal; (2) movement of a low pressure system from the east across Burma; and (3) the building downwards of a midtropospheric cyclonic circulation¹. Studying the structure of monsoon depressions in their later stages of evolution, using principally upper air data of land stations, earlier workers² have invoked upper divergence as a crucial factor for development. Gray³ questions the role of upper divergence in tropical cyclogenesis and emphasises that small vertical wind shear is a primary factor in storm development. The mechanism of initial formation over the Bay, however, has remained unexplored for want of adequate data. To gain further insight, a close network of ships and coastal and island stations was organised in the joint Indo–USSR Monsoon-77 Experiment (Fig. 1). Four Soviet research ships in polygon formation recorded for the first time, upperwind observations over the Bay of Bengal. The formation of a depression in the north Bay just to the east of the ships on 19 August and its passage across the polygon the next day was monitored. Sharp decrease in tropospheric vertical wind shear was identified along the coast to start with and subsequently over the north-east of the polygon, before formation of the depression. Marked anticyclonic vorticity first appeared in the upper troposphere and was then totally replaced by cyclonic vorticity of comparable magnitude. This cyclonic vorticity gradually descended to the lower troposphere. The above features, hitherto undocumented, are described here.