The structure of the monsoon depression and the observed flow features prior to and at the time of monsoon depression formation (compositeof 15depressions) are examined. The composite monsoon depression (transient eddy) has a scale of 25 0 longitude and extends up to 300 mb and has the greatest intensity at 700 mb. It shows north-northeast to south-southwest tilt in the lower levels indicating that it may draw upon zonal kinetic energy for its growth. The disturbance has lower temperatures to its west and tilts westwards with height indicating that eddy available potential energy is not converted from zonal available potential energy by large scale advection. There appears to be a reduction of vertical shear at the time of formation of monsoon depressions and this possibly aids cumulus convection. The profiles of potential vorticity indicate extremes (i) in the upper troposphere and (ii) at several midtropospheric levels in the region of the monsoon trough indicating the possibility of combined barotropic-baroclinic instability. Using multi-level quasi-geostrophic model and employing the eigen-valuetechnique it is shown that the monsoon zonal current is not baroclinically unstable. A barotropic stability analysis is also done for monsoon zonal current in the lower and middle tropospheres. It yields very slowly growing unstable modes at lower tropospheric levels with wave lengths of 2500 km and 5000 km.
In order to study the evolution of flow patterns associated with 'strong' and 'break' monsoon we have studied six cases each of such epochs during the period 1965 to 1973. As we are interested in the departure of the flow patterns from the normal, we have subtracted long term climatological pentad means from the daily data and constructed anomaly wind, height and thickness charts at standard levels. The several contrasting features during the epochs as well as prior to the epochs. A few days prior to the beginning of the strong monsoon epoch (usually in association with a depression), an eastwest anomaly trough is seen extending from SE Asia to peninsular India in the lower and, middle tropospheres. The trough over peninsular India shifts northwards with the advance of the epoch. Westward movement of a cyclonic anomaly from across Southeast Asia IS also seen. In the upper troposphere a warm anomaly ridge isseen to the northwest of India. Prior to the beginning of breaks, however, an anomaly ridge is seen extending from peninsular India to Malaysia in the lower and middle tropospheres. This feature also shows some northward shift with the advance of the epoch. In the upper troposphere, an anomaly trough is seen to the northwest of India.
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