Barotropic growth of monsoon depressions

Abstract

Although monsoon depressions are a principal synoptic-scale element of the South Asian monsoon, producing extreme rainfall over India and surrounding regions, there exists no widely accepted mechanism explaining their occurrence. This study presents a hierarchy of numerical experiments aimed at finding such an explanation. Using a perturbation-basic state decomposition, we derive an anelastic system of equations that can represent disturbances growing in the complex, three-dimensional monsoon basic state. We find that modal solutions to these equations linearized about this basic state can explain many features of observed monsoon depressions, including their warm-over-cold core structure, westward propagation, and lower-tropospheric wind maximum. For the zonally symmetric case, these modes are barotropically unstable, drawing energy from the meridional shear of the monsoon trough. For the zonally varying basic state, modal solutions still derive energy from barotropic conversion, but fail to achieve positive net growth rates when dissipative processes are included. For the nonlinear equation set, these modes can be excited by a heating impulse, and their energy then remains roughly constant over several days as barotropic energy transfers oppose dissipative losses. Our results support the idea that the general concept of barotropic instability can explain the structure, propagation, and geographic distribution of monsoon depressions, but not their rapid growth rates. We speculate that condensational heating coupled to these waves is needed to obtain a positive growth rate.

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

“…We use the same numerical modeling framework developed in Diaz and Boos () as a starting point. In that study, we took the anelastic equations for atmospheric flow and derived a new equation set that is partitioned into a basic state and a perturbation.…”

“…The basic state (i.e., , , , , and ) used in this study is identical to that of Diaz and Boos (), who performed an extensive analysis of its instability characteristics for a dry atmosphere. It is obtained from a composite of atmospheric states preceding monsoon depression genesis derived from the ERA‐Interim reanalysis (Dee et al .…”

“…; Mishra, ). Recently, Diaz and Boos () provided additional evidence to support this idea: a normal‐mode linear instability analysis of a basic state representative of the environment preceding monsoon depression genesis over the Bay of Bengal yielded wave structures resembling monsoon depressions, the primary energy source of which was the basic‐state meridional shear. However, they also found that, although zonally symmetric basic states were unstable, a basic state with realistic zonal variations was not.…”

“…Nevertheless, they noted that these wave structures had regions of upward vertical velocity, which they speculated could couple them to convection and lead to positive net growth. Building upon the results of Diaz and Boos (), the goal of this study is to test that speculation. In other words, are monsoon depressions a variant of moist barotropic instability?…”

Mesoscale precipitation systems and their role in the rapid development of a monsoon depression over the Bay of Bengal

“…Recent studies have proposed that barotropic instability can explain the structure, propagation and geographic distribution of monsoon depressions (Mishra, 2018;Diaz and Boos, 2019). Intraseasonal oscillations such as the BSISO and QBW change the location of the strong meridional cyclonic shear zone over the BoB that strongly controls the genesis locations and tracks of the depressions (Goswami et al, 2003;Hatsuzuka and Fujinami, 2017).…”