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A new understanding 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 intense rain rates, exhibit outer radii of about 1,000 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 rainfall of the Indian and Australian monsoons, they have been studied relatively little and their governing mechanisms are poorly understood.  The relatively 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 existing 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, and is inconsistent with existing models of moist baroclinic instability.  We also show that the observed westward propagation is caused by simple advection of the mid-tropospheric PV maximum by the time mean wind.  These findings constitute a new view of the dynamics of an important but little-studied class of storms.