Using time series motifs to examine preconditioning of the stratospheric polar vortex

Sudden stratospheric warmings (SSWs) are dramatic events that strongly disrupt the stratospheric polar vortex (hereinafter, the polar vortex) during winter. While the triggering mechanisms behind SSWs are still not fully understood, it is thought that preconditioning of the vortex may be a common puzzle piece across mechanisms. Herein, work is presented that uses a time series motif discovery technique to search for specific features of vortex preconditioning in reanalysis data. The motifs found through this analysis agree well with prior theoretical and modeling studies that have suggested specific vortex preconditioning characteristics. The motifs specifically show that prior to most major SSWs, potential vorticity gradients sharpen in the vortex edge region, and the vortex shrinks in the middle and upper stratosphere. These “canonical” preconditioning signals are often apparent by more than 10 to 20 days before the central dates of major SSWs. However, these same signals also appear in a majority of years without major SSWs, as they tend to be associated with minor warming disturbances confined mostly to the middle-to-upper stratosphere. This result seems to indicate that preconditioning of the vortex may be a necessary but insufficient condition for the occurrence of major SSWs.