Mechanisms of growth and propagation of monsoon synoptic vortices

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 propagate toward the northwest, are roughly 2,000 km in diameter, and have precipitating ascent southwest of the vortex center. 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.

Here we present observationally based composites of the dynamical structure of synoptic vortices in the Indian and Australian monsoons, and use these structures to discriminate between previously proposed hypotheses for mechanisms of disturbance growth and propagation. Vortices are identified in reanalysis data using an automated tracking algorithm and composite life cycles are produced for both strong and weak storms. Synoptic vortices in the Indian monsoon consist of horizontally localized potential vorticity (PV) maxima that grow in amplitude over several days and have a bimodal vertical structure, with the dominant PV maximum at 500 hPa and a weaker, secondary maximum at 750 hPa. The PV anomalies tilt westward with height in a region of westward vertical wind shear and the estimated eddy heat transport is poleward in a region of poleward temperature gradient; both characteristics are inconsistent with the previously suggested hypothesis that monsoon depressions draw energy from the baroclinicity of the mean state. We present diagnostics to show how the PV structure propagates westward and how the energy of the storm grows over time. Because observational estimates of PV and total energy tendencies contain large biases and uncertainties, we also present initial analyses of the PV and energy budgets from an ensemble of idealized numerical simulations of monsoon depressions. Observational composites of synoptic vortices in the Australian monsoon are compared with the Indian monsoon composites and with the simulated vortices.