Last updated 5 February 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.