Some dynamical characteristics and thermal structure of monsoon depressions over the Bay of Bengal

In this paper, some of the dynamical characteristics and the thermal structure of the monsoon depressions formed during July-August 1979 in the Bay of Bengal are presented. The vorticity budget terms are examined for the eastern and western halves of three depressions in a Lagrangian reference frame. Two of the depressions moved westward and one northward. The vorticity divergence term and the horizontal advection term (in the upper troposphere) apparently caused the westward movement, whereas the vertical advection term and tilting term had little influence on the movement. The residual term is predominantly negative in the western half of the monsoon depression where convective activity and rainfall are greater, compared to the eastern half. This corroborates the inference of Sui and Yanai that the appearance of large vorticity budget residuals is associated with intense cumulus convection. The computational study of energetics shows that the zonal available potential energy was the source for eddy available potential energy. This, in turn, was converted into eddy kinetic energy on most of the days during the life cycle of monsoon depressions. Eddy kinetic energy was converted into zonal kinetic energy on many days, thus maintaining the monsoon current. Computations of the momentum transport show that the transport was southward over the region 18°N to 25° N during the strengthening of the depressions and northward during the decaying stage. From analysis of thermal structure, the August depression (6–13 August 1979) had a warm core until two days after it reached peak intensity; subsequently it had a cold core.