Last updated 6 August 2026.
The zonal-mean zonal wind (i.e., the west-to-east wind averaged across longitudes) at 10 hPa and 60°N is a common diagnostic of the strength of the Arctic stratospheric polar vortex. On average, these winds turn westerly in late August, reach peak strength in December/January, and turn easterly in April/May.
A major sudden stratospheric warming (SSW) is commonly defined as a reversal of the daily-mean zonal-mean zonal winds at 10 hPa and 60°N during November to March (Charlton and Polvani 2007). A period of 20 consecutive days of westerly winds is then required before another SSW can be defined, as some SSWs oscillate between westerly and easterly winds. Events where westerly winds do not persistently return prior to the summer are considered final warmings. Butler and Gerber (2018) define this as the last date (of a July–June year) when the winds reverse to easterlies and do not return to westerly for more than 10 consecutive days.
Importantly, these criteria apply to daily-mean winds. There are instances of reversals of an instantaneous/sub-daily reversal, but these do not meet the established criteria for SSWs. Sub-daily reversals are not just dynamically less significant: they are more uncertain, inconsistent between reanalyses and their detection depends on the temporal resolution of the dataset.
In the below figures, daily-mean data are shown, constructed from 6-hourly data. The full ERA5 record is presented, but note that data prior to 1958 (start of more frequent high-altitude radiosondes) and especially prior to 1979 (start of the satellite era) are far more uncertain. It is still, however, interesting to see what ERA5 thinks happened.
Following the Charlton and Polvani (2007) definition, major SSWs occurred on the following dates:
12 January 1940 10 February 1940 7 February 1941 21 March 1942 19 March 1945 19 February 1946 19 March 1946 5 March 1950 22 February 1952 19 November 1952 18 December 1954 26 January 1955 4 February 1957 1 February 1958 17 January 1960 27 January 1963 16 December 1965 22 February 1966 7 January 1968 28 November 1968 13 March 1969 2 January 1970 18 January 1971 20 March 1971 31 January 1973 9 January 1977 22 February 1979 29 February 1980 4 March 1981 4 December 1981 24 February 1984 1 January 1985 23 January 1987 8 December 1987 14 March 1988 21 February 1989 15 December 1998 26 February 1999 20 March 2000 11 February 2001 30 December 2001 17 February 2002 18 January 2003 5 January 2004 21 January 2006 24 February 2007 22 February 2008 24 January 2009 9 February 2010 24 March 2010 6 January 2013 12 February 2018 1 January 2019 5 January 2021 16 February 2023 16 January 2024 4 March 2024 28 November 2025 4 March 2026
That yields a total of 59 events. There is no indication of a trend in the frequency of major SSWs in the reanalysis record.