Sudden Stratospheric Warming

A Sudden Stratospheric Warming (abbreviated SSW) is an abrupt, large-scale meteorological breakdown of the wintertime stratospheric polar vortex in which the polar stratosphere (between ~10 and 50 km altitude, or 100 to 1 hPa) warms by 30 to 50 K (30 to 50 °C) over the span of just 3 to 7 days, accompanied by a deceleration and reversal of the circumpolar westerly polar night jet into an easterly flow. First discovered in February 1952 by German meteorologist Richard Scherhag using high-altitude radiosonde balloons launched over West Berlin (initially termed the "Berlin Phenomenon"), and explained dynamically in 1971 by Japanese meteorologist Taroh Matsuno, major Sudden Stratospheric Warmings occur roughly six times per decade in the Northern Hemisphere (and only once in the observational record of the Southern Hemisphere, in September 2002). Because the circulation anomalies generated during an SSW descend from the stratosphere into the troposphere over the subsequent 2 to 8 weeks, SSWs serve as one of the most important sources of subseasonal-to-seasonal predictability for winter cold-air outbreaks across North America, Europe, and northern Asia.

During a normal boreal winter, the Arctic stratosphere enters polar night, radiatively cools via infrared emission by CO₂ and O₃ to temperatures below 195 K (−78 °C), and forms a deep cyclonic low-pressure vortex encircled at ~60°N by the westerly polar night jet (with zonal-mean wind speeds ū of 30 to 60 m s⁻¹ at 10 hPa, ~32 km altitude). The World Meteorological Organization defines four distinct classes of wintertime stratospheric warming events:

In 1971, Taroh Matsuno demonstrated that Sudden Stratospheric Warmings are triggered from below by the upward propagation and irreversible breaking of tropospheric planetary-scale Rossby waves (quasi-stationary waves of zonal wavenumber k = 1 and k = 2, with horizontal wavelengths of 10,000–20,000 km):

1. Tropospheric Wave Generation and the Charney–Drazin Theorem: In the Northern Hemisphere, continental mountain ranges (the Rocky Mountains, Tibetan Plateau/Himalayas) and sharp land–sea thermal contrasts between Eurasia/North America and the Pacific/Atlantic oceans excite large-amplitude stationary planetary Rossby waves (k = 1, 2). By the Charney–Drazin theorem (1961), stationary Rossby waves (c = 0) can propagate vertically from the troposphere into the stratosphere only when the background stratospheric winds are westerly and weaker than a cutoff speed (0 < ū < U_c ∝ [k² + l²]⁻¹), which restricts vertical propagation to the winter season and to ultra-long wavenumbers k = 1 and k = 2. (Because the Southern Hemisphere has far less land mass and mountain topography at mid-latitudes, upward planetary wave fluxes in the austral winter are ~3× weaker, explaining why Southern Hemisphere SSWs are rare.)
2. Amplitude Growth and Wave Breaking ("The Stratospheric Surf Zone"): As a planetary Rossby wave propagates upward into the stratosphere while conserving wave-activity density, the exponential decrease of atmospheric air density with altitude (ρ(z) = ρ₀ exp(−z/H), with scale height H ≈ 7 km) causes the wave's velocity and displacement amplitudes to grow proportionally to ρ₀^−1/2 ∝ exp(+z / [2H]). Between 30 and 50 km altitude, the wave amplitude becomes so large that material contours of potential vorticity (PV) overturn irreversibly—creating the planetary-scale Rossby "surf zone" discovered by Michael E. McIntyre and Timothy N. Palmer (1983).
3. Eliassen–Palm (EP) Flux Convergence and Zonal Wind Reversal: In the Transformed Eulerian Mean (TEM) momentum equation, breaking planetary Rossby waves converge upward and poleward Eliassen–Palm (EP) flux (∇ · F < 0, equivalent to a northward eddy flux of negative potential vorticity v'q' < 0). This wave drag exerts an intense westward torque of −10 to −30 m s⁻¹ day⁻¹ on the stratospheric polar night jet, bringing the 50 m s⁻¹ westerly wind to a halt and reversing it to an easterly wind within days.
4. Adiabatic Downwelling Compression and Rapid Polar Warming: By Coriolis torque balance in the TEM equations, the westward wave drag (∇ · F < 0) drives a poleward and downward residual meridional circulation (w̄^* < 0) over the polar cap. As stratospheric air parcels sink downward by 3 to 6 kilometers in a few days, adiabatic compression warms the air at the dry adiabatic lapse rate (~9.8 K km⁻¹), raising polar stratospheric temperatures by +30 to +50 K despite the absence of winter sunlight.

Depending on whether zonal wavenumber k = 1 or k = 2 dominates the upward Eliassen–Palm wave flux, major SSWs exhibit two distinct geometric morphologies (Charlton & Polvani, 2007):

• Vortex Displacement Events (Wavenumber k = 1, ~55%–60% of SSWs): Amplification of the stratospheric Aleutian High pushes the intact cyclonic polar vortex off the North Pole toward northern Eurasia and Scandinavia (e.g., January 2006, January 2019, January 2021).
• Vortex Split Events (Wavenumber k = 2, ~40%–45% of SSWs): Symmetric amplification of both a Pacific ridge and an Atlantic/European ridge pinches the polar vortex across the pole and cleaves it into two separate daughter vortices—one positioned over Canada/North America and the other over Siberia/Eurasia (e.g., February 1979, January 2009, February 2018 "Beast from the East").

In a landmark 2001 study, Mark P. Baldwin and Thomas J. Dunkerton demonstrated that the circulation anomalies of a major SSW do not remain trapped in the stratosphere: when plotted as a time–height cross section of the Northern Annular Mode (NAM) index (colloquially known as a "dripping paint plot"), negative NAM (easterly) wind and positive geopotential-height anomalies propagate downward from 10 hPa to the Earth's surface over 10 to 60 days:

• Shift to Negative North Atlantic Oscillation (NAO−) and Greenland Blocking: Roughly two-thirds of major SSWs are followed by a persistent negative North Atlantic Oscillation (NAO−) regime lasting 4 to 8 weeks. High-latitude anticyclonic blocking over Greenland and the Arctic weakens and shifts the Atlantic jet stream southward toward Spain and the Mediterranean.
• Cold-Air Outbreaks Across Northern Eurasia and Eastern North America: With the tropospheric jet displaced equatorward, frigid continental Arctic and Siberian air masses spill across northern Europe, the United Kingdom, and northern Asia (producing cold waves of −4 to −10 °C below normal, such as the February–March 2018 cold wave following the February 12, 2018 vortex split), as well as across the midwestern and eastern United States (such as the February 2021 Texas freeze following the January 5, 2021 SSW), while Greenland, eastern Canada, and the Mediterranean experience abnormally mild and wet conditions.
• Suppression of Polar Stratospheric Clouds (PSCs) and Ozone Depletion: Because heterogeneous chlorine activation on polar stratospheric clouds (PSCs) requires stratospheric temperatures below 195 K (−78 °C), a mid-winter SSW warms the Arctic stratosphere well above the PSC threshold, shutting down catalytic ozone depletion for the remainder of that spring (as observed when the September 2002 Southern Hemisphere SSW split the Antarctic ozone hole in two and reduced its area by >50%, and again during the minor Southern Hemisphere warming of September 2019).

In 2020, North Atlantic–European weather regime analyses demonstrated that whether a major Sudden Stratospheric Warming produces surface cold-air outbreaks over Eurasia depends on the tropospheric blocking state over Greenland and Scandinavia during the downward propagation window (Domeisen et al., 2020).

In January 2021, a major Northern Hemisphere Sudden Stratospheric Warming reversed the 60°N, 10-hPa zonal winds to easterly on January 5 and displaced and split the lower-stratospheric polar vortex, contributing to persistent high-latitude blocking prior to the February 2021 North American cold-air outbreak (Baldwin et al., 2021).

In 2021, multi-dataset diagnostic assessments of the late-winter Southern Hemisphere stratospheric warming showed that upward-propagating wavenumber-1 planetary Rossby waves warmed the Antarctic polar stratosphere by more than 40 K, curtailed springtime polar stratospheric cloud ozone loss, and promoted negative Southern Annular Mode anomalies across eastern Australia (Lim et al., 2021).

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2. Matsuno, T. (1971). A dynamical model of the stratospheric sudden warming028<1479:ADMOTS>2.0.CO;2). Journal of the Atmospheric Sciences, 28(8), 1479–1494.
3. Baldwin, M. P., & Dunkerton, T. J. (2001). Stratospheric harbingers of anomalous weather regimes. Science, 294(5542), 581–584.
4. Butler, A. H., Seidel, D. J., Hardiman, S. C., Butchart, N., Birner, T., & Match, A. (2015). Defining sudden stratospheric warmings. Bulletin of the American Meteorological Society, 96(11), 1913–1928.
5. Domeisen, D. I. V., Grams, C. M., & Papritz, L. (2020). The role of North Atlantic–European weather regimes in the surface impact of sudden stratospheric warming events. Weather and Forecasting, 35(4), 1373–1388.
6. Baldwin, M. P., Ayarzagüena, B., Birner, T., Butchart, N., Butler, A. H., Charlton-Perez, A. J., et al. (2021). Sudden stratospheric warmings. Reviews of Geophysics, 59(1), e2020RG000708.
7. Lim, E.-P., Hendon, H. H., Butler, A. H., Thompson, D. W. J., Lawrence, Z. D., Scaife, A. A., et al. (2021). The 2019 Southern Hemisphere stratospheric polar vortex weakening and its impacts. Bulletin of the American Meteorological Society, 102(6), E1150–E1171.