Vortex Preconditioning Due to Planetary and Gravity Waves Prior to Sudden Stratospheric Warmings

We use reanalysis data to evaluate the evolution of polar vortex geometry, planetary wave drag, and gravity wave drag prior to split versus displacement sudden stratospheric warmings (SSWs). A composite analysis that extends upwards to the lower mesosphere reveals that split SSWs are characterized by a transition from a wide, funnel-shaped vortex that is anomalously strong, to a vortex that is constrained about the pole and has little vertical tilt. In contrast, displacement SSWs are characterized by a wide, funnel-shaped vortex that is anomalously weak throughout the prewarming period. Moreover, during split SSWs, gravity wave drag is enhanced in the polar night jet, while planetary wave drag is enhanced within the extratropical surf zone. During displacement SSWs, gravity wave drag is anomalously weak throughout the extratropical stratosphere.

Using the composite analysis as a guide, we conduct a case study of the 2009 SSW in order to evaluate the roles of planetary and gravity waves for preconditioning the polar vortex in terms of two SSW triggering scenarios: anomalous planetary wave forcing from the troposphere, and resonance due to either internal or external Rossby waves. The results support the view that split SSWs are caused by resonance rather than anomalously large wave forcing. Given these findings, we suggest that vortex preconditioning – which is traditionally defined in terms of vortex geometries that increase poleward wave focusing – may be better described by wave events that `tune' the geometry of the vortex towards its resonant excitation points.