Systematic Assessment of Sudden Stratospheric Warming Thresholds

Various metrics exist for determining the occurrence of a major sudden stratospheric warming (SSW), including the rate of change in temperature and wind, the annular mode index, and geometric vortex diagnostics. The most common metric is based on the reversal of the climatological westerly zonal-mean zonal winds at 60 N and 10 hPa in the winter stratosphere. However, little systematic analysis of the precise parameters of latitude, altitude, and threshold for the wind reversal has been performed. Here, we examine how the frequency of SSWs, the changes in stratospheric temperature and zonal winds, and the surface impacts vary as a function of the stratospheric wind reversal parameters. The results provide a methodical assessment of how best to define a “standard” metric for minor, major, and final SSWs. While the continuum nature of stratospheric variability makes it difficult to identify a decisively optimal threshold, there is an envelope of thresholds that can be more rigorously justified — and the original focus on 60 N and 10 hPa does lie within this window.