The Baroclinic Processes of Monsoon Depressions

Abstract

Two cases of summer monsoon depressions in the vicinity of the Bay of Bengal are analyzed to study the importance of the baroclinic mechanism. Both cases show a baroclinic structure with well-defined warm and cold sectors, the latter being situated to the east of the former in a region where the thermal wind is easterly throughout the troposphere. In a developing depression, the geopotential and the temperature fields differ in phase such that warm advection from the north occurs to the west of the depression center and cold advection from the south to the east. There is also strong convergence to the west and divergence to the east of the depression center in the lower troposphere, and vice versa in the upper troposphere. Thus a divergent secondary circulation exists in the zonal-vertical plane with warm air rising to the west and cold air sinking to the east. Marked increases in upward motion and relative divergence (divergence at 200 mb minus divergence at 850 mb) in the southwest sector occur during the period of development. Intensity changes of the depressions are found to be strongly influenced by thermal advection from mid-latitude disturbances to the north, giving further evidence of the importance of baroclinic processes.