Monsoon depression amplification by moist barotropic instability in a vertically sheared environment

Abstract

This study makes the case that monsoon depressions over South Asia can form from a variant of moist barotropic instability. Using an idealized numerical framework in which the atmosphere is partitioned into a basic state and a perturbation, we simulate vortices resembling monsoon depressions that draw energy from the meridional shear of the monsoon trough and amplify when they interact with precipitating ascent. The influence of the basic state vertical shear on the vortex induces upward velocity, which couples precipitation with a Rossby-wave-like mode arising from dry barotropic growth, allowing the vortex to intensify. Sensitivity experiments reveal that both the sheared basic state and latent heating are necessary to achieve positive growth rates and that this process requires a sufficiently large initial perturbation. Trajectory analyses suggest that the combined flow of the vortex and the large-scale monsoon transport diabatically generated potential vorticity from southwest of the vortex into the vortex center, thus enabling growth. In contrast with tropical cyclones, this mechanism does not require a feedback between surface wind speed and surface heat and moisture fluxes, though this feedback does ultimately result in a slightly stronger vortex. K E Y W O R D Sbarotropic instability, idealized numerical modeling, Indian Monsoon, monsoon depression, monsoon low-pressure system Q J R Meteorol Soc. 2019;145:2666-2684.wileyonlinelibrary.com/journal/qj

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“…(2016) found that enhanced low‐level absolute vorticity was correlated, in climatological mean data, with the genesis frequency of the global distribution of monsoon LPS. Mechanistically, amplification of LPS by barotropic instability (Diaz & Boos, 2019a, 2019b) requires the existence of a local maximum in absolute vorticity, and we generally expect vortex growth rates to increase with environmental low‐level absolute vorticity; both the absolute vorticity and the strength of its local maximum are enhanced during the genesis of depressions on active days.…”

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“…While many questions about their dynamics remain open, recent work indicates that monsoon depressions form over South Asia from moist barotropic instability due to the meridional shear of the monsoon trough, and are intensified by latent heating (Diaz & Boos, 2019a, 2019b). The background monsoonal flow hence is the source of instability for these propagating disturbances and can modulate their variability.…”

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“…being investigated. Recent work indicates that monsoon depressions form over South Asia from moist barotropic instability due to the meridional shear of the monsoon trough, and are intensified by latent heating (Diaz & Boos, 2019a, 2019b. The occurrence of depressions can be modulated by variations in the background flow.…”