A moist‐thermal quasi‐geostrophic model for monsoon depressions

Abstract

AbstractMonsoon 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.