Being restricted in their vertical development by the Tibetan high, monsoon depressions propagate westward against monsoon westerlies embedded in the Indian monsoon trough. The cause of this peculiar propagation has not been well explained. Special characteristics of individual depressions were revealed from observations of previous studies; particularly, the major rainfall of a depression occurs over its west–south-west sector. The latent heat released by this rainfall forms east–west differential heating across the depression in developing an east–west asymmetric circulation. Because this east–west circulation is a part of the depression's divergent circulation, a spatial quadrature relationship exists between this divergent circulation and the depression. Based on these characteristics, a westward propagation mechanism of the depression is introduced. The depression's rainfall is supported by the convergence of water vapor transported by the low-level divergent circulation. In turn, the divergent circulation is maintained through a feedback of the latent heat released by the rainfall. The upward branch of the east–west circulation coupled with the convergent center of the low-level divergent circulation generates a negative streamfunction tendency. The depression is propagated westward by a dynamic interaction between rainfall/convection and this monsoon disturbance through the negative streamfunction tendency. The spatial quadrature relationship between a depression and its east–west (divergent) circulation rejuvenates the water vapor supply maintaining diabatic heating and the divergent circulation, and perpetuating the generation of negative streamfunction tendency ahead of the depression. The entire process from the maintenance of east–west differential heating to the generation of negative streamfunction tendency west of a depression will not diminish until the cessation of water vapor supply.
Budget analyses of heat, water vapor, velocity potential, and streamfunction for 143 depressions identified over 24 summers (1979–2002) were performed with ERA-40 reanalyses and three daily rainfall data sources. The westward propagation mechanism of monsoon depressions was illustrated/confirmed in terms of these budget analyses.