“Interactions between water vapor and potential vorticity in synoptic-scale monsoonal disturbances”
South Asian monsoon low pressure systems, referred to as synoptic-scale monsoonal disturbances (SMDs), play an important role in the monsoon system, responsible for up to half of India’s rainfall. It was thought that these systems grow due to a variant of baroclinic instability that includes the effects of 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 that may explain the growth of SMDs and may also be applicable to other modes of tropical variability. Moisture is prognostic and is coupled to precipitation through a simplified Betts-Miller scheme. Interactions between moisture and potential vorticity (PV ) in the presence of a moist static energy gradient can be understood in terms of a “gross” PV equation. Gross PV summarizes the dynamics of SMDs and reveals the relative role that moist and dry dynamics play in these disturbances, which is largely determined by the gross moist stability. Linear solutions to the coupled PV and moisture equations reveal Rossby-like modes that grow due to a moisture-vortex instability. Meridional temperature and moisture advection to the west of the PV maximum moisten and destabilize the column, which results in enhanced convection and SMD intensification through vortex stretching. This instability occurs only if the moistening is in the direction of propagation of the SMD and is strongest at the synoptic scale.