The movement of Indian Monsoon Depressions by interaction with image vortices near the Himalayan Wall

It is argued that a simple explanation for the westward propagation of Indian Monsoon Depressions (IMDs) is the interaction of the depression vortex with the Himalayan “wall”. This interaction can be modelled by simulating an IMD as a point vortex in a horizontal plane (at 850 hPa) and invoking image vortices behind the barrier. Solenoidal flows associated with the image vortices allow the boundary conditions at the Himalayas to be met, and cause the IMD vortex to propagate parallel to the barrier, toward the west. This simple model is tested against propagation speeds for observed IMDs. The histogram of observed propagation speed, normalised by the point-vortex model prediction, has a mean of 1.08 and standard deviation of 0.68. The model also explains the observed intensification of flow on the Himalayan side of the IMD which is a key process in enhancing rainfall to the Indo-Gangetic Plain in the monsoon season.