Interactions between water vapor and potential vorticity in synoptic-scale monsoonal disturbances: Moisture vortex instability

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 (q(G)) equation. The q(G) 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 because of 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.