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1. Surface Control of the Frequency of Stratospheric Sudden Warming Events

   https://doi.org/10.1175/JCLI-D-18-0801.1  

   Wu, Zheng ; Reichler, Thomas ( July 2019 , Journal of Climate)

   The frequency of stratospheric sudden warming events (SSWs) is an important characteristic of the coupled stratosphere–troposphere system. However, many modern climate models are unable to reproduce the observed SSW frequency. A previous study suggested that one of the reasons could be the momentum damping at the surface. The goal of the present study is to understand what determines the climatological SSW frequency and how the surface damping comes into play. To this end, we conduct a parameter sweep with an idealized model, using a wide range of values for the surface damping. It is found that the SSW frequency is a strong and nonlinear function of the surface damping. Various tropospheric and stratospheric factors are identified to link the surface damping to the SSW frequency. The factors include the magnitude of the surface winds, the meridional and vertical wind shear, the synoptic eddy activity in the troposphere, the transient wave activity flux at the lower stratosphere, and the strength of the stratospheric polar vortex. Mathematical–statistical modeling, informed by the parameter sweep, is used to quantify how the different factors relate to each other. This successfully reproduces the complex variations of the SSW frequency with the surface damping seen inmore »the parameter sweep. The results may help in explaining some of the difficulties that climate models have in simulating the observed SSW frequency.

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2. The impact of split and displacement sudden stratospheric warmings on the troposphere

   https://doi.org/https://doi. org/10.1029/2020JD033989  

   White, I. P. ( April 2021 , Journal of geophysical research)

   Although sudden stratospheric warmings (SSWs) can improve subseasonal-to-seasonal forecasts, it is unclear whether the two types of SSW - displacements and splits - have different near- surface effects. To examine the longer-term (i.e., multi-week lead) tropospheric response to displacements and splits, we utilize an intermediate-complexity model and impose wave-1 and wave-2 stratospheric heating perturbations spun-off from a control run. At longer lags, the tropospheric response is found to be insensitive to both the wavenumber and location of the imposed heating, in agreement with freely evolving displacements and splits identified in the control run. At shorter lags, however, large differences are found between displacements and splits in both the control run and the different wavenumber- forced events. In particular, in the control run, the free-running splits have an immediate barotropic response throughout the stratosphere and troposphere whereas displacements take 1–2 weeks before a near-surface response becomes evident. Interestingly, this barotropic response found during CTRL splits is not captured by the barotropically forced wave-2 events, indicating that the zonal-mean tropospheric circulation is somehow coupled with the generation of the wave-2 splits. It is also found that in the control run, displacements yield stronger Polar-Cap temperature anomalies than splits, yet both still yieldmore »similar magnitude tropospheric responses. Hence, the strength of the stratospheric warming is not the only governing factor in the surface response. Overall, SSW classification based on vortex morphology may be useful for subseasonal but not seasonal tropospheric prediction.« less
3. Limited surface impacts of the January 2021 sudden stratospheric warming

   https://doi.org/10.1038/s41467-022-28836-1  

   Davis, N. A. ; Richter, J. H. ; Glanville, A. A. ; Edwards, J. ; LaJoie, E. ( March 2022 , Nature Communications)

   Subseasonal weather prediction can reduce economic disruption and loss of life, especially during “windows of opportunity” when noteworthy events in the Earth system are followed by characteristic weather patterns. Sudden stratospheric warmings (SSWs), breakdowns of the winter stratospheric polar vortex, are one such event. They often precede warm temperatures in Northern Canada and cold, stormy weather throughout Europe and the United States - including the most recent SSW on January 5th, 2021. Here we assess the drivers of surface weather in the weeks following the SSW through initial condition “scrambling” experiments using the real-time CESM2(WACCM6) Earth system prediction framework. We find that the SSW itself had a limited impact, and that stratospheric polar vortex stretching and wave reflection had no discernible contribution to the record cold in North America in February. Instead, the tropospheric circulation and bidirectional coupling between the troposphere and stratosphere were dominant contributors to variability.
4. The Generic Nature of the Tropospheric Response to Sudden Stratospheric Warmings

   https://doi.org/10.1175/JCLI-D-19-0697.1  

   White, Ian P. ; Garfinkel, Chaim I. ; Gerber, Edwin P. ; Jucker, Martin ; Hitchcock, Peter ; Rao, Jian ( July 2020 , Journal of Climate)

   The tropospheric response to midwinter sudden stratospheric warmings (SSWs) is examined using an idealized model. SSW events are triggered by imposing high-latitude stratospheric heating perturbations of varying magnitude for only a few days, spun off from a free-running control integration (CTRL). The evolution of the thermally triggered SSWs is then compared with naturally occurring SSWs identified in CTRL. By applying a heating perturbation, with no modification to the momentum budget, it is possible to isolate the tropospheric response directly attributable to a change in the stratospheric polar vortex, independent of any planetary wave momentum torques involved in the initiation of an SSW. Zonal-wind anomalies associated with the thermally triggered SSWs first propagate downward to the high-latitude troposphere after ~2 weeks, before migrating equatorward and stalling at midlatitudes, where they straddle the near-surface jet. After ~3 weeks, the circulation and eddy fluxes associated with thermally triggered SSWs evolve very similarly to SSWs in CTRL, despite the lack of initial planetary wave driving. This suggests that at longer lags, the tropospheric response to SSWs is generic and it is found to be linearly governed by the strength of the lower-stratospheric warming, whereas at shorter lags, the initial formation of the SSW potentiallymore »plays a large role in the downward coupling. In agreement with previous studies, synoptic waves are found to play a key role in the persistent tropospheric jet shift at long lags. Synoptic waves appear to respond to the enhanced midlatitude baroclinicity associated with the tropospheric jet shift, and preferentially propagate poleward in an apparent positive feedback with changes in the high-latitude refractive index.« less
5. Sudden Stratospheric Warmings

   https://doi.org/10.1029/2020RG000708  

   Baldwin, Mark P. ; Ayarzagüena, Blanca ; Birner, Thomas ; Butchart, Neal ; Butler, Amy H. ; Charlton‐Perez, Andrew J. ; Domeisen, Daniela I. V. ; Garfinkel, Chaim I. ; Garny, Hella ; Gerber, Edwin P. ; et al ( January 2021 , Reviews of Geophysics)

   Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (∼10–50 km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are caused by the breaking of planetary‐scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid descent and warming of air in polar latitudes, mirrored by ascent and cooling above the warming. The rapid warming and descent of the polar air column affect tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode, making cold air outbreaks over North America and Eurasia more likely. SSWs affect the atmosphere above the stratosphere, producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (nonionized) particles and electron densities, and electric fields. These effects span both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal prediction. This work reviews the current knowledge on the most important aspects of SSWs, from the historical background to dynamical processes, modeling, chemistry, and impact on other atmospheric layers.