Mixing Processes Following the Final Stratospheric Warming

Abstract

The dynamics responsible for the mixing and dissolution of the polar vortex during final stratospheric warmings is investigated. A GCM and an associated offline N2O transport model are used to simulate the dynamics and transport during a Northern Hemisphere final stratospheric warming. The results are compared with those obtained from LIMS data for the final warming of 1979. In particular we examine the potential vorticity evolution in the two datasets, the modeled N2O evolution and the observed O3 evolution. With the cessation of Rossby wave activity following the final warming the large horizontal wind shears and vigorous horizontal mixing associated with periods of enhanced wave activity during the winter months are not observed. In the simulated warming the flow field rapidly becomes zonally symmetric and tracer anomalies are trapped in the associated easterly flow regime. In the LIMS dataset potential vorticity anomalies are observed over two months following the breakup of the polar vortex. Ozone anomalies associated with those long-lived vortices are protected from mixing and are also long lived. Following each warming the remnants of the originally intact vortex (defined in terms of the potential vorticity, N2O, O3 fields) gradually homogenize with the atmosphere at large. Two processes leading to this homogenization have been identified following the final warmings: 1) the potential vorticity field becomes decorrelated from that of the chemical tracer; 2) the vortex remnants begin to tilt dramatically in the vertical direction. As the vortex remnants tilt in the vertical, their vertical depth scale is greatly reduced facilitating vertical mixing; horizontal mixing is enhanced as the decorrelation between the potential vorticity and chemical anomalies leads to a rapid stirring of the chemical anomaly on the horizontal plane.