Propagation and Growth of Monsoon Depressions

Like Earth's extratropical atmosphere, monsoon circulations contain baroclinic zones with strong vertical shear in which synoptic scale cyclones grow. Intense occurrences of these cyclones are called monsoon depressions; these storms have high rain rates, outer diameters of about 2,000 km, and frequently transit land surfaces without great loss of intensity. Despite the importance of these synoptic vortices, which have been estimated to produce half the summer monsoon rainfall of continental India and Australia, they have been studied relatively little and their governing mechanisms are poorly understood. The few studies that exist have argued that monsoon depressions grow by baroclinic instability of the mean monsoon flow and propagate by interaction with the vertical shear of the mean state. The observed propagation is remarkable because it consists of westward movement of a lower tropospheric vortex in a region where the time mean low-level flow is eastward.

Here we show that these previous theories for storm growth and propagation are inconsistent with observed structures of South Asian and Australian monsoon depressions. Monsoon depressions consist of localized potential vorticity (PV) maxima that grow in amplitude over several days and are centered in the middle troposphere. We show that the observed PV structure precludes a dry baroclinic instability mechanism, is inconsistent with existing models of moist baroclinic instability, and seems to have more in common with the general class of oceanic tropical depressions. We also show that the observed westward propagation is caused by adiabatic advection of the mid-tropospheric PV maximum. These findings constitute a new view of the dynamics of an important but little-studied class of storms.