A new class of vacillations of the stratospheric polar vortex

Abstract

A new class of persistent vacillations of the winter polar vortex, under the action of topographic wave forcing and radiative cooling, is identified in numerical integrations of the rotating shallow water equations. The vacillations are obtained provided only that care is taken to prevent the unconstrained growth of tropical easterlies that otherwise develop as the result of persistent angular momentum deposition at low latitudes. The vacillation cycle involves purely barotropic dynamics and is characterized by a dynamically controlled rapid splitting and rapid reformation of the vortex followed by a more gradual period of vortex intensification under the influence of radiative relaxation. The onset of the splitting occurs when the frequency of the free mode of the vortex approaches that of the forcing and resembles a resonant excitation. Experiments with an alternative basic state suggest that the vacillations are a robust feature of the topographically forced and radiatively relaxed vortex. In contrast to the behavior found in models with vertical structure, the period of the vacillation cycles here increases with increasing forcing amplitude. A wide range of forcing amplitude exists over which the vortex exhibits distinct regime transitions between a strong, vacillating state and a state in which the vortex is weak and the zonal mean polar flow nearly zero. Comparison with observational reanalysis suggest that the vacillation cycles obtained here may be relevant to the dynamics of some sudden warming events and that the onset of a radiatively dominated regime may be usefully linked to the loss of vortex area following such an event.