Idealized Simulations of Monsoon Depressions

Monsoon depressions (MDs) are cyclonic vortices embedded in the baroclinic zones of monsoon circulations. They play a significant role in the meteorology of the tropics by producing a large fraction of monsoon precipitation. Since they form and intensify in regions of strong easterly shear, their growth has previously been attributed to moist baroclinic instability. However, recent studies show that monsoon depressions fail to satisfy necessary criteria for baroclinic growth and display characteristics similar to those of the more general class of tropical depressions.

Previous studies of tropical depression spinup, however, use shear magnitudes that are considerably weaker than those in monsoon regions. Additionally, studies of tropical depression spinup primarily focus only on disturbances that intensify into strong tropical cyclones (TCs), with little emphasis on weaker vortices. Here, we examine the dynamics of monsoon depressions in idealized cloud resolving simulations, with an emphasis on the intensification of cold core vortices in environments of strong vertical shear.

We perform numerical simulations using the System for Atmospheric Modeling (SAM), with a horizontal resolution of 2 km. An axisymmetric vortex is introduced into a region of strong wind shear and its intensification is analyzed. Moist static energy (MSE) sources and sinks are examined to determine the role of feedbacks responsible for the intensification of the vortex. The sensitivity of growth rates to mean wind shear, sea surface temperature, background surface wind and moisture profiles is also measured.