Perspectives on moist baroclinic instability: implications for the growth of monsoon depressions

Little is known about the genesis and growth mechanisms of monsoon depressions, despite the great importance of these storms for the hydrological cycle of the Asian-Australian monsoon region. Of the few theoretical studies that have examined this issue, most have attributed the amplification of monsoon depressions to some form of baroclinic instability or stable baroclinic growth, highly modified by the diabatic effects of moist convection. Here a simple criterion, namely the upshear tilt of potential vorticity anomalies, is argued be necessary for dry or moist baroclinic growth. Reanalysis data is then used to assess whether a large ensemble of South Asian monsoon depressions has vertical structures consistent with this criterion. The evolution of these monsoon depressions is compared with that of ensembles of hurricanes and diabatic Rossby waves, the latter being prototypical examples of moist baroclinic instability. During their amplification phase, monsoon depressions do not exhibit an upshear tilt of potential vorticity anomalies. Many similarities are found between developing monsoon depressions and hurricanes but few with diabatic Rossby waves. Thus, the mechanism responsible for the intensification of monsoon depressions remains unknown, but these results indicate greater similarity with the general process of tropical depression spin-up than with moist convectively coupled baroclinic instability.