Moisture-Convection Feedbacks and the Growth of Indian Monsoon Low-Pressure Systems

Monsoon low pressure systems, referred to as synoptic-scale monsoonal disturbances (SMDs), are modes of monsoonal variability that have a large influence in rainfall over the South Asian monsoon region. It has long been thought that these systems grow due to moist baroclinic instability, a variant of baroclinic instability that includes the effects of deep convection. Recent work, however, has shown that this framework is inconsistent with the observed structure and dynamics of SMDs.

Here we present an alternative framework where moisture is prognostic and is coupled to precipitation through a simplified Betts-Miller parameterization. Unstable Rossby-like wave solutions are obtained that arise from interactions between moisture, convection and the anomalous circulation in the presence of a background temperature gradient. Warm air that is advected south by the anomalous flow is lifted along the sloping isentropes of the monsoon region, which moistens and destabilizes the column to the west of the low-pressure center. The moistened lower troposphere enhances convection which, in turn, causes the low pressure system to intensify through vortex stretching. It is shown that during the active months of the South Asian monsoon unstable growth occurs only if the moist wave can propagate westward against the low-level westerly flow. For parameter values that resemble the observed monsoonal background state, growth of these waves is largest at the synoptic scale. Expanding the framework to include meridional moisture gradients leads to a more general framework that suggests this ``moisture-vortex" instability may operate on other synoptic-scale low pressure systems such as easterly waves.