The Formation and Growth of Monsoon Depressions within the South Asian Monsoon

Monsoon depressions are an important component of intraseasonal variability within the South Asian monsoon. However, there is no widely accepted theory to explain their formation and growth. 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 non-linear 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. When coupled with a microphysics parameterization to represent moist convection, these wave modes amplify into disturbances closely resembling monsoon depressions. In this case, they derive their energy from both condensational heating and the meridional shear of the monsoon trough as a type of moist barotropic instability.