Comment on wcd-2022-13

Abstract

Natural variations in the strength of the northern stratospheric polar vortex, so-called polar vortex events, help to improve sub-seasonal to seasonal (S2S) predictions of winter climate. Past research on polar vortex events has been largely focused on stratospheric sudden warming events (SSWs), a class of relatively strong weakenings of the polar vortex. SSWs are defined when the polar vortex reverses its normal westerly wind direction. In this study, however, we use an alternative definition, based on the weighted time-integrated upward wave activity flux at the lower stratosphere prior to the onset of the events. We use a long control simulation with a stratosphere-resolving model and the ERA5 reanalysis to compare various aspects of the wave activity definition with traditional SSWs over the Arctic. About half of the wave events are identical to SSWs, while the other half create on average more robust surface signals than the SSWs that do not concur with the wave events. There exist several other advantages over traditional SSWs: the wave activity flux definition creates more robust surface signals, captures with one simple criterium a variety of different event types, lengthens the prediction horizon of the surface response, and can be more meaningfully applied over the southern hemisphere. We therefore conclude that that the wave driving represents a useful early indicator for stratospheric polar vortex events, which exploits the stratospheric potential for creating predictable surface signals better than SSWs.