Gravity Wave-Induced Instability of the Stratospheric Polar Vortex Edge

We report on a previously undocumented process capable of mixing the Northern Hemisphere (NH) winter Ertel potential vorticity (EPV)}instabilities introduced along the stratospheric polar vortex edge by breaking gravity waves (GWs). As horizontal resolution has increased, global-scale atmospheric models and data assimilation systems (DASs) are now able to capture some aspects of GW generation, propagation, and dissipation, as well as mesoscale EPV disturbances. This work examines resolved GWs, their breaking, and their interaction with the stratospheric polar vortex as seen in the NASA Global Modeling and Assimilation Office DAS during the 2021-22 NH winter. This analysis shows that tropospheric-generated GWs, breaking in the stratosphere over a substantial area, created a significant disruption of the polar vortex EPV, in turn triggering baroclinic instabilities near the edge of the polar vortex. The instabilities take the form of mesoscale vortices propagating on the edge of the stratospheric polar vortex. This work reveals two new features in the EPV analysis: high and low fluctuations at the smallest model scale created by resolved GW breaking and high values associated with mesoscale vortices along the edge of the polar vortex. SIGNIFICANCE STATEMENT: The Northern Hemisphere (NH) winter stratospheric polar vortex is typically disturbed by global-scale waves that displace, distort, and weaken the vortex; however, as the resolution of global models has increased, the role played by smaller-scale waves in disturbing the stratospheric vortex can now be evaluated. As one example, the NH winter of 2022 had unusually weak global-scale waves along with strong smaller-scale waves generated by flow over mountains, providing an ideal case for evaluating the effects brought about by the smaller-scale waves. Our examination of the 2022 NH winter reveals that the waves generated by flow over mountains, located under the stratospheric vortex, propagated up to the middle stratosphere where they broke down, interfering significantly with the vortex flow. This distortion of the vortex flow created an unstable region that led to the formation of "mesoscale vortices," relatively small eddies on the edge of the polar vortex, that then propagated coherently around the stratospheric vortex. The importance of these small-scale wave-generated, mesoscale vortices may lie in their potential to mix trace gases across the stratospheric vortex boundary.