Adiabatic westward drift of Indian monsoon depressions

Abstract

A large fraction of the rain received by continental India is produced by cyclonic vortices with outer radii of about 1000 km that are contained within the larger scale South Asian monsoon flow. The more intense occurrences of these vortices are called monsoon depressions; these consist of bottom-heavy columns of relative vorticity that propagate to the northwest in time-mean low-level eastward flow. Previous studies have argued that this apparent upstream propagation is caused by dynamical lifting west of the vortex centre, with the resulting ascent producing vortex stretching that shifts the vortex to the west. Here, analysis of over 100 Indian monsoon depressions is used to show that low-level vortex stretching has a spatial structure inconsistent with the observed propagation and is balanced by other terms in the low-level vorticity budget. Instead, monsoon depressions are shown to consist of potential vorticity maxima that have peak amplitude in the middle troposphere and propagate westward by nonlinear, horizontal adiabatic advection (i.e. beta drift). The precipitating ascent in monsoon depressions makes a more minor contribution to the total storm motion and primarily acts to maintain the upright structure of the vortex. These results suggest a new view of Indian monsoon depressions as potential vorticity columns that propagate primarily by adiabatic dynamics.

Modeling the Interaction between Quasigeostrophic Vertical Motion and Convection in a Single Column

“…Observations show a dipole heating profile in the southwest quadrant of the monsoon depression vortex, which may contribute to the axial asymmetry of the vortex and its westward advection (Boos et al , 2016. One may imagine using the CQG modeling framework to examine what determines this structure by simulating a convective column in the southwest quarter forced by the advection associated with the monsoon depression vortex.…”

The movement of Indian monsoon depressions by interaction with image vortices near the Himalayan wall

“…Boos et al () showed how IMD movement can be inferred through the advection of the 450 hPa PV maximum in the IMD by the local flow at that level, which is enhanced by beta drift, in which the PV anomalies induced by the meridional flow around the vortex generate a meridional northwestward advection of the vortex. Our arguments relating to the advection of the 850 hPa vortex maximum are probably complementary to the results of Boos et al (), in that we invoke the advection of the IMD structure, rather than wave‐propagation mechanisms presented by earlier authors. Boos et al () also show how the alignment of the 500 hPa PV structure with that at 700 hPa can explain the upwind advection of the MD PV at the lower level.…”

“…Hoskins and James, , p. 339). However, vorticity tendencies due to stretching remain significant (Boos et al ). Previous authors (Krishnamurti, ) have shown that baroclinic effects in monsoon depressions are in fact strong, and undoubtedly the effects of deep convection in the monsoon system introduce significant vorticity tendencies in the environment of a monsoon depression.…”

“…22To clarify, this is different from the mechanism proposed by Boos et al (), which is horizontal nonlinear advection of the mid‐level (500 hPa) potential vorticity maximum.…”

Structure, characteristics, and simulation of monsoon low‐pressure systems in CFSv2 coupled model

“…Boos et al . [] have shown that these depressions are associated with potential vorticity columns that have a peak amplitude in the midtroposphere. They argue that horizontal, nonlinear adiabatic advection is responsible for their westward propagation.…”