Tropical depressions forming in the monsoon trough over India have characteristics somewhat different from their counter-parts over the other tropical oceanic areas and have long been believed to be similar in structure to extratropical cyclones. The formation and structure of some monsoon depressions that have been examined, generally agree with the pattern suggested earlier by Koteswaram and George (1958, 1960). They form in the monsoon trough over the north Bay of Bengal, under the influence of divergence aloft associated with a perturbation in the upper easterlies over this area. Along with condensation heat released in the monsoon trough, thermal advection seems to take a part in the maintenance and movement of the depression as well as in the asymmetrical distribution of heavy rainfall which is confined to the south-west sector. It is concluded that the mechanism of the monsoon depression is similar to its extratropical counterpart, though no fronts or strong air mass contrasts exist. The circulation derives energy from the thermal gradient due to the differential heating of land and sea areas and is then maintained even during its long trek over land till it breaks up over the Himalayan mountains or the deserts of northwest India and West Pakistan.
Koteswaramtand et al. (Sat,) studied this question.
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