# Sudden stratospheric warming

A sudden stratospheric warming (SSW) is an event in which polar stratospheric temperatures rise by several tens of kelvins (up to increases of about 50 °C (90 °F)) over the course of a few days. The warming is preceded by a slowing then reversal of the westerly winds in the stratospheric polar vortex, commonly measured at 60 ° latitude at the 10 hPa level. SSWs occur about six times per decade in the northern hemisphere (NH), and about once every 20-30 years in the southern hemisphere (SH). In the SH, SSW accompanied by a reversal of the vortex westerly (which is soon after followed by a vortex recovery) was observed once during the period 1979–2024; this was in September 2002. Stratospheric warming in September 2019 was comparable to or even greater than that of 2002, but the wind reversal did not occur.

## History

The first continued measurements of the stratosphere were taken by Richard Scherhag in 1951 using radiosondes to take reliable temperature readings in the upper stratosphere (~40 km) and he became the first to observe stratospheric warming on 27 January 1952. After his discovery, he assembled a team of meteorologists at the Free University of Berlin specifically to study the stratosphere, and this group continued to map the NH stratospheric temperature and geopotential height for many years using radiosondes and rocketsondes.
When the weather satellites era began, meteorological measurements became far more frequent. Although satellites were primarily used for the troposphere, they also recorded data for the stratosphere. Today both satellites and stratospheric radiosondes are used to take measurements of the stratosphere.

## Classification and description

SSW is closely associated with polar vortex breakdown. Meteorologists typically classify vortex breakdown into three categories: major, minor, and final. No unambiguous standard definition of these has so far been adopted. However, differences in the methodology to detect SSWs are not relevant as long as circulation in the polar stratosphere reverses. "Major SSWs occur when the winter polar stratospheric westerlies reverse to easterlies. In minor warmings, the polar temperature gradient reverses but the circulation does not, and in final warmings, the vortex breaks down and remains easterly until the following boreal autumn". However, this classification is based on the NH SSWs, as no major SSW by this definition has been observed in the SH in winter.
Sometimes a fourth category, the Canadian warming, is included because of its unique and distinguishing structure and evolution.
"There are two main types of SSW: displacement events in which the stratospheric polar vortex is displaced from the pole and split events in which the vortex splits into two or more vortices. Some SSWs are a combination of both types".

### Major

These occur when the westerly winds at 60°N and 10 hPa reverse, i.e. become easterly. A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.
According to the World Meteorological Organization's Commission for Atmospheric Sciences:$^{:19}$ "a stratospheric warming can be said to be major if at 10 mb or below the latitudinal mean temperature increases poleward from 60 degree latitude and an associated circulation reversal is observed (that is, the prevailing mean westerly winds poleward of 60° latitude are succeeded by mean easterlies in the same area)."

### Minor

Minor warmings are similar to major warmings; however, they are less dramatic: the westerly winds are slowed but do not reverse. Therefore, a breakdown of the vortex before its final breakdown is never observed. All the SH SSWs observed since 1979 were minor warmings except for that in September 2002.
McInturff cites the WMO's Commission for Atmospheric Sciences: "a stratospheric warming is called minor if a significant temperature increase is observed (that is, at least 25 degrees in a period of week or less) at any stratospheric level in any area of winter time hemisphere. The polar vortex is not broken down and the wind reversal from westerly to easterly is less extensive."

### Final

The radiative cycle in the stratosphere means that during winter the mean flow is westerly and during summer it is easterly. A final warming occurs on this transition, so that the polar vortex winds change direction for the warming and do not change back until the following winter. This is because the stratosphere has entered the summer easterly phase. It is final because another warming cannot occur over the summer, so it is the final warming of the current winter. Most of the SH SSWs fall into this category as their onsets most commonly occur sometime in austral spring months, and the stratospheric wind and temperature anomalies tend to persist until early summer. In this sense, SH SSWs represent faster-than-normal seasonal march of the westerly polar vortex.

### Canadian

Canadian warmings occur in early winter in the stratosphere of the NH, typically from mid-November to early December. They have no counterpart in the SH.

## Dynamics

In a usual NH winter, several minor warming events occur, with a major event occurring roughly every two years. One reason for major stratospheric warmings in the NH is that orography and land-sea temperature contrasts are responsible for the generation of long (wavenumber 1 or 2) Rossby waves in the troposphere. These planetary-scale waves travel upward to the stratosphere and dissipate there, decelerating the westerly winds and warming the Arctic. This is the reason that major warmings are usually only observed in the NH, with an exception observed in September 2002. As the SH is largely an ocean hemisphere, the planetary-scale wave activity is much weaker, and the SH vortex westerly is much stronger in winter, which partly explains why major SSW has not been observed in the SH winter at least in the instrumental observation era.
At an initial time a blocking-type circulation pattern establishes in the troposphere. This blocking pattern causes Rossby waves with zonal wavenumber 1 and/or 2 to grow to unusually large amplitudes. The growing wave propagates into the stratosphere and decelerates the westerly mean zonal winds. Thus the polar night jet weakens and simultaneously becomes distorted by the growing planetary waves. Because the wave amplitude increases with decreasing density, this easterly acceleration process is not effective at fairly high levels. If the waves are sufficiently strong, the mean zonal flow may decelerate sufficiently so that the winter westerlies turn easterly. At this point planetary waves may no longer penetrate into the stratosphere). Hence, further upward transfer of energy is completely blocked and a very rapid easterly acceleration and the polar warming occur at this critical level, which must then move downward until eventually the warming and zonal wind reversal affect the entire polar stratosphere. This wave-mean flow interaction explains the stratosphere-troposphere downward coupling during the SH SSW events as well. The upward propagation of planetary waves and their interaction with the stratospheric mean flow is traditionally diagnosed via so-called Eliassen-Palm fluxes.
There is a link between sudden stratospheric warmings and the quasi-biennial oscillation (QBO): if the QBO is in its easterly phase, the atmospheric waveguide is modified in such a way that upward-propagating Rossby waves are focused on the polar vortex, intensifying their interaction with the mean flow. Thus, there exists a statistically significant imbalance between the frequency of sudden stratospheric warmings if these events are grouped according to the QBO phase (easterly or westerly). However, the QBO-polar vortex relationship is less statistically significant in the SH.

## Weather and climate effects

Although sudden stratospheric warmings are mainly forced by planetary-scale waves which propagate up from the lower atmosphere, there is also a subsequent return effect of sudden stratospheric warmings on surface weather and climate. Following a sudden stratospheric warming, the high altitude westerly winds reverse and are replaced by easterlies. The easterly winds progress down through the atmosphere, often leading to a weakening of the tropospheric westerly winds, resulting in dramatic reductions in temperature in Northern Europe. This process can take a few days to a few weeks to occur.
Similar downward processes are found in the SH in the austral late spring to early summer seasons. SH SSWs in austral spring tend to cause the Antarctic ozone concentration to be higher than normal from spring to early summer, and both weaker vortex and higher Antarctic ozone act to cause the tropospheric jet to shift equatorward, which is expressed as a negative phase of the Southern Annular Mode (SAM) in the SH extratropical geopotential height and surface pressure fields in the subsequent late spring to early summer seasons. SSWs in austral spring have been found to result in warmer and drier conditions over eastern Australia during late spring-early summer, increasing the risk of forest/bushfires, but cooler and wetter conditions over Patagonia. Also, austral spring to late spring SSWs influence the Antarctic sea-ice extent in the subsequent early summer season.

## Table of major mid-winter SSW events in reanalyses products

| Event name | NCEP-NCAR | ERA40 | ERA-Interim | JRA-55 | MERRA2 | ENSO | QBO 50mb |
| JAN 1958 | 30-Jan-58 | 31-Jan-58 | | 30-Jan-58 | | El Niño | West |
| NOV 1958 | 30-Nov-58 | **** | | **** | | El Niño | East |
| JAN 1960 | 16-Jan-60 | 17-Jan-60 | | 17-Jan-60 | | Neutral | West |
| JAN 1963 | **** | 28-Jan-63 | | 30-Jan-63 | | Neutral | East |
| MAR 1965 | 23-Mar-65 | **** | | **** | | La Niña | West |
| DEC 1965 | 8-Dec-65 | 16-Dec-65 | | 18-Dec-65 | | El Niño | East |
| FEB 1966 | 24-Feb-66 | 23-Feb-66 | | 23-Feb-66 | | El Niño | East |
| JAN 1968 | **** | 7-Jan-68 | | 7-Jan-68 | | La Niña | West |
| NOV 1968 | 27-Nov-68 | 28-Nov-68 | | 29-Nov-68 | | El Niño | East |
| MAR 1969 | 13-Mar-69 | 13-Mar-69 | | **** | | El Niño | East |
| JAN 1970 | 2-Jan-70 | 2-Jan-70 | | 2-Jan-70 | | El Niño | West |
| JAN 1971 | 17-Jan-71 | 18-Jan-71 | | 18-Jan-71 | | La Niña | East |
| MAR 1971 | 20-Mar-71 | 20-Mar-71 | | 20-Jan-71 | | La Niña | East |
| JAN 1973 | 2-Feb-73 | 31-Jan-73 | | 31-Jan-73 | | El Niño | East |
| JAN 1977 | **** | 9-Jan-77 | | 9-Jan-77 | | El Niño | East |
| FEB 1979 | 22-Feb-79 | 22-Feb-79 | 22-Feb-79 | 22-Feb-79 | | Neutral | West |
| FEB 1980 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | El Niño | East |
| FEB 1981 | **** | **** | **** | 6-Feb-81 | **** | Neutral | West |
| MAR 1981 | **** | 4-Mar-81 | 4-Mar-81 | 4-Mar-81 | **** | Neutral | West |
| DEC 1981 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | Neutral | East |
| FEB 1984 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | La Niña | West |
| JAN 1985 | 2-Jan-85 | 1-Jan-85 | 1-Jan-85 | 1-Jan-85 | 1-Jan-85 | La Niña | East |
| JAN 1987 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | El Niño | West |
| DEC 1987 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | El Niño | West |
| MAR 1988 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | El Niño | West |
| FEB 1989 | 22-Feb-89 | 21-Feb-89 | 21-Feb-89 | 21-Feb-89 | 21-Feb-89 | La Niña | West |
| DEC 1998 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | La Niña | East |
| FEB 1999 | 25-Feb-99 | 26-Feb-99 | 26-Feb-99 | 26-Feb-99 | 26-Feb-99 | La Niña | East |
| MAR 2000 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | La Niña | West |
| FEB 2001 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | La Niña | West |
| DEC 2001 | 2-Jan-02 | 31-Dec-01 | 30-Dec-01 | 31-Dec-01 | 30-Dec-01 | Neutral | East |
| FEB 2002 | **** | 18-Feb-02 | **** | **** | 17-Feb-02 | Neutral | East |
| JAN 2003 | 18-Jan-03 | | 18-Jan-03 | 18-Jan-03 | 18-Jan-03 | El Niño | West |
| JAN 2004 | 7-Jan-04 | | 5-Jan-04 | 5-Jan-04 | 5-Jan-04 | Neutral | East |
| JAN 2006 | 21-Jan-06 | | 21-Jan-06 | 21-Jan-06 | 21-Jan-06 | La Niña | East |
| FEB 2007 | 24-Feb-07 | | 24-Feb-07 | 24-Feb-07 | 24-Feb-07 | El Niño | West |
| FEB 2008 | 22-Feb-08 | | 22-Feb-08 | 22-Feb-08 | 22-Feb-08 | La Niña | East |
| JAN 2009 | 24-Jan-09 | | 24-Jan-09 | 24-Jan-09 | 24-Jan-09 | La Niña | West |
| FEB 2010 | 9-Feb-10 | | 9-Feb-10 | 9-Feb-10 | 9-Feb-10 | El Niño | West |
| MAR 2010 | 24-Mar-10 | | 24-Mar-10 | 24-Mar-10 | 24-Mar-10 | El Niño | West |
| JAN 2013 | 7-Jan-13 | | 6-Jan-13 | 7-Jan-13 | 6-Jan-13 | Neutral | East |
| FEB 2018 | 12-Feb-18 | | 12-Feb-18 | 12-Feb-18 | 12-Feb-18 | La Niña | West |
| JAN 2019 | 2-Jan-19 | | 2-Jan-19 | 2-Jan-19 | 2-Jan-19 | El Niño | East |
| JAN 2021 | | | | | | La Niña | West |