Monsoon Depression Amplification by Moist barotropic Instability in a Vertically Sheared Environment

TitleMonsoon Depression Amplification by Moist barotropic Instability in a Vertically Sheared Environment
Publication TypeJournal Article
Year of Publication2019
AuthorsDiaz, Michael, and Boos William R.
JournalQuarterly Journal of the Royal Meteorological Society
Number145
Date Published06/2019
Abstract / Summary

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 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 their presence does ultimately result in a slightly stronger vortex.

URLhttps://rmets.onlinelibrary.wiley.com/doi/10.1002/qj.3467
DOI10.1002/qj.3585
Journal: Quarterly Journal of the Royal Meteorological Society
Year of Publication: 2019
Number: 145
Date Published: 06/2019

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 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 their presence does ultimately result in a slightly stronger vortex.

DOI: 10.1002/qj.3585
Citation:
Diaz, M, and WR Boos.  2019.  "Monsoon Depression Amplification by Moist barotropic Instability in a Vertically Sheared Environment."  Quarterly Journal of the Royal Meteorological Society.  https://doi.org/10.1002/qj.3585.