A moist-thermal quasigeostrophic model for monsoon depressions

Abstract

Monsoon depressions (MDs) are synoptic-scale storms that occur during the summer phase of the global monsoon cycle and whose dynamical mechanisms remain incompletely understood. To gain insight into the dynamics governing the large-scale structure of MDs, we formulate an idealised moist-thermal quasi-geostrophic model that includes distinct thermal and moisture fields in simple forms. A linear-stability analysis of the model, with basic states corresponding to typical monsoon conditions, shows three distinct mode classifications: thermal-Rossby modes, heavy precipitating modes, and a moist-thermal mode. In the linearised model, the presence of a background precipitation gradient strengthens thermal-Rossby modes by coupling the dynamics to latent heating. The separation of heavy precipitating modes from fast-propagating thermal-Rossby modes is further examined with numerical experiments of large-amplitude MDs. Wind-induced evaporation is found to amplify large-amplitude MDs in conditions analogous to those over the northern Bay of Bengal. An energetic analysis shows the pathways by which the MDs derive energy from the background state. A further series of experiments through a continuum of meridional temperature gradients demonstrates the sensitivity of large-scale MD dynamics to the background state and suggests a possible mechanism to explain variations in the propagation direction of MDs.