Instability of the Distorted Polar Night Vortex: A Theory of Stratospheric Warmings

Abstract

The properties and stability of forced planetary waves in a continuous baroclinc shear are investigated gated using a weakly nonlinear analysis. A steady solution exists which under certain conditions may be unstable via interaction with a free traveling wave of the same wavenumber. This wave then grows at the expense of the zonal mean available potential energy. The parameter range in which instability occurs is extended by finite-amplitude perturbations. Theoretical predictions are compared with results from a highly truncated model of the winter atmosphere. Agreement is good, even after assumptions of the theory have broken down. The theory does become inapplicable, however, following a mean wind reversal (stratosphere warming); in an cases studied, such warming were found to occur subsequent to the instability. The theory appears to explain many features of observed and model warmings. Results of the study suggest that warmings arise from the intrinsic instability of the winter atmosphere. They can occur in the presence of steady forcing?no precursor tropospheric pulse of planetary wave energy is necessary. In these experiments, interaction with critical lines (of zero wind speed) plays no part in the generation of warmings.