Dynamical and Surface Impacts of the January 2021 Sudden Stratospheric Warming in Novel Aeolus Wind Observations, MLS and ERA5

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 Kelvin over just a few days. 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. Due to the major technical challenges involved in measuring wind from space, SSW-induced changes to the structure of the polar vortex have never previously been directly observed at the global scale. Here, we exploit novel observations from ESA's flagship Aeolus wind-profiler mission, supported by additional temperature and geopotential height data from NASA's MLS limb sounder and the ERA5 reanalysis. This allows us to directly examine wind and related dynamical changes associated with the January 2021 major SSW, the first such event in the Aeolus data record. 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-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.

Mid-Latitude Mesospheric Zonal Wave 1 and Wave 2 in Winter 2020–2021

“…The Z1 and Z2 amplitude variations are even smaller in the lower stratosphere at about 20 km (<300 m, Figure 2, see color scales), which is evidence of relatively low planetary wave activity. This is consistent with absence of a zonal wind reversal at these altitudes at 55-65° N during the SSW onset [26]. The weak SSW manifestations in the wave 1-2 amplitudes in the lower-middle stratosphere may explain why the SSW 2021 cannot be clearly classified as having a splitting or displacement of the stratospheric polar vortex [26].…”

“…This paper is focused on the coupling between zonal planetary wave 1 and wave 2 in the mid-latitude stratosphere and mesosphere during SSW 2021. The SSW 2021 was accompanied by an increase in temperature in the stratosphere (a decrease in the mesosphere), accelerating sharply from January 1 and reaching a maximum (minimum) on January 5 [25][26][27]. Unlike many others, this SSW was a mixed event not easily classifiable as having either a splitting or displacement of the stratospheric polar vortex [26], suggesting the combined effects of zonal wave 1 and wave 2.…”

“…The MERRA-2 reanalysis shows a sharp deceleration of zonal wind at 10 hPa, 60° N, with maximum weakening from 1 to 5 January 2021 (vertical lines in Figure 1a) and the appearance of weak easterlies in early January. Relative to the MLS climatology of 2004-2020, the stratospheric temperature poleward of 60°N increased by 20 K, the zonal wind reversed in early January at 10 hPa, 55-65° N, and the SSW event became defined as major [26,27]. In the ERA5 reanalysis, a major SSW began on 5 January 2021, and the average temperature of the Arctic stratosphere at 10 hPa rose by close to 30 K in under a week before the onset of the SSW [25].…”

“…At mid-latitude 50° N, the SSW effect consisted of a warming by about 10 K at 10 hPa in late December-early January (Figure 1b). A simultaneous cooling of about 25 K occurred in the polar mesosphere, suggesting distinct height-separated regimes in the stratosphere and mesosphere [26].…”

“…Unlike temperature and zonal wind (Figure 1a,b), both zonal wave components in the stratosphere showed no noticeable anomalies during the SSW onset (1-5 January, vertical lines in Figure 1c). In the mesosphere at 0.01 hPa (80 km), Z2, exhibited a sharp amplitude peak of about 1000 m on 5 January (dashed curve in Figure 1d), just on the date of the strongest easterly in the mesosphere [26]. The wave-1 amplitude, Z1, maximized after the SSW onset (solid curve in Figure 1d), which is consistent with the enhancement of mesospheric wave 1 after the zonal wind reversal described in earlier studies [16,18].…”