Impacts of Atmospheric Stratification on the Behaviors of Stratospheric Weak Polar Vortex Events

Abstract Based on a highly idealized wave–mean flow interaction model, where the planetary waves are excited by the specified bottom-boundary geopotential perturbation, this study investigates the modulation of prescribed atmospheric stratification N 2 conditions on the stratospheric weak polar vortex (WPV) events’ characteristics and the underlying mechanism. In the vertically uniform N 2 experiments, strong N 2 and the consequent large refractive index squared of planetary waves preceding WPVs facilitate enhanced vertical wave propagation. The resultant WPVs are more intense because a significant convergence of wave activity flux in the upper stratosphere amplifies polar stratospheric warming and drastically collapses the polar night jet. The WPVs are also more persistent because the diabatic cooling process requires a longer period to cool the polar stratosphere and reestablish the polar vortex. In contrast, weak N 2 facilitates WPVs that are less intense and persistent. The difference in the vertical planetary wave propagation explains how N 2 shapes contrasting WPV characteristics. Considering the vertical wave propagation depends not only on N 2 but also on the mean flow, the mean flow is further fixed to isolate the effects of N 2 . The results remain the same, highlighting the crucial role of N 2 in modulating the WPV characteristics. Further experiments with vertically changing N 2 suggest that N 2 in the lower stratosphere is more efficient in modulating the WPV characteristics.