Abstract
Abstract. Major sudden stratospheric warmings (SSWs) are extreme dynamical events where the usual strong westerly winds of the stratospheric polar vortex temporarily weaken or reverse and polar stratospheric temperatures rise by tens of kelvins over just a few days and remain so for an extended period. Via dynamical modification of the atmosphere below them, SSWs are believed to be a key contributor to extreme winter weather events at the surface over the following weeks. SSW-induced changes to the wind structure of the polar vortex have previously been studied in models and reanalyses and in localised measurements such as radiosondes and radars but have not previously been directly and systematically observed on a global scale because of the major technical challenges involved in observing winds from space. Here, we exploit novel observations from ESA's flagship Aeolus wind-profiler mission, together with temperature and geopotential height data from NASA's Microwave Limb Sounder and surface variables from the ERA5 reanalysis, to study the 2021 SSW. This allows us to directly examine wind and related dynamical changes associated with the January 2021 major SSW. Aeolus is the first satellite mission to systematically and directly acquire profiles of wind, and therefore our results represent the first direct measurements of SSW-induced wind changes at the global scale. We see a complete reversal of the zonal winds in the lower to middle stratosphere, with reversed winds in some geographic regions reaching down to the bottom 2 km of the atmosphere. These altered winds are associated with major changes to surface temperature patterns, and in particular we see a strong potential linkage from the SSW to extreme winter weather outbreaks in Greece and Texas during late January and early February. Our results (1) demonstrate the benefits of wind-profiling satellites such as Aeolus in terms of both their direct measurement capability and use in supporting reanalysis-driven interpretation of stratosphere–troposphere coupling signatures, (2) provide a detailed dynamical description of a major weather event, and (3) have implications for the development of Earth-system models capable of accurately forecasting extreme winter weather.
Highlights
Sudden stratospheric warmings (SSWs) are some of the most dramatic dynamical events in the entire atmospheric system
We investigate the potential surface impacts of the SSW, using temperature, snow cover extent and geopotential height (GPH) data from ERA5
To provide a uniform spatial average for all comparisons, throughout this study we average over the region 60–65◦ N in all analyses intended to characterise atmospheric dynamics and structure around 60◦ N, to ensure a fair comparison at all heights
Summary
Sudden stratospheric warmings (SSWs) are some of the most dramatic dynamical events in the entire atmospheric system. Over just a few days, temperatures in the winter polar stratosphere can rise by as much as 50 K, while the circumpolar wind jet which usually separates the cold polar stratosphere from midlatitudes dramatically reduces in speed and, at many heights and locations, can even reverse These dynamical changes have major effects on both weather and longer-scale atmospheric processes: in addition to direct local changes to polar ozone and other chemistry While wind speed measurements using in situ radiosondes and ground-based remotesensing techniques such as lidar and radar are routinely made at many sites, the point-based nature of these techniques inherently limits their ability to characterise the large-scale changes SSWs induce in wind patterns Due to these observational limitations, the vast majority of recent research on how SSWs affect winds above the lowest layers of the atmosphere has been carried out using models, reanalyses and winds inferred from other measured variables such as temperature.
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