The date of the major warming is calculated for each reanalysis product using daily-mean zonal-mean zonal winds at 60N and 10 hPa, where the winds must return to westerly for 20 consecutive days between events, and for at least 10 days prior to April 30, following Charlton and Polvani (2007).
The El Niño-Southern Oscillation (ENSO) phase (E = El Niño, L = La Niña, N = Neutral) is defined following the convention of the NOAA NCEP Climate Prediction Center, which is based on the DJF Oceanic Niño Index (ONI) using ERSSTv5 data with a +/-0.5C threshold.
The Quasi-Biennial Oscillation (QBO) phase (E = easterly, W = westerly) is defined using the DJF-mean equatorial QBO time series at 50 hPa from the Freie Universität Berlin.
The frequency of SSWs for each reanalysis is calculated for the 1958-2023 period for NCEP-NCAR, JRA-55, and ERA5; for the 1958-2002 period for ERA-40; for the 1979-2019 period for ERA-interim; and for the 1980-2023 period for MERRA-2.
| Event Name | NCEP-NCAR | ERA40 | ERA-Interim | JRA-55 | MERRA2 | ERA5 | ENSO | QBO 50mb |
|---|---|---|---|---|---|---|---|---|
| JAN 1958 | 30-Jan-58 | 31-Jan-58 | 30-Jan-58 | 8-Feb-58 | E | W | ||
| NOV 1958 | 30-Nov-58 | **** | **** | **** | E | E | ||
| JAN 1960 | 16-Jan-60 | 17-Jan-60 | 17-Jan-60 | 17-Jan-60 | N | W | ||
| JAN 1963 | **** | 28-Jan-63 | 30-Jan-63 | 27-Jan-63 | N | E | ||
| MAR 1965 | 23-Mar-65 | **** | **** | **** | L | W | ||
| DEC 1965 | 8-Dec-65 | 16-Dec-65 | 18-Dec-65 | 16-Dec-65 | E | E | ||
| FEB 1966 | 24-Feb-66 | 23-Feb-66 | 23-Feb-66 | 22-Feb-66 | E | E | ||
| JAN 1968 | **** | 7-Jan-68 | 7-Jan-68 | 7-Jan-68 | L | W | ||
| NOV 1968 | 27-Nov-68 | 28-Nov-68 | 29-Nov-68 | 28-Nov-68 | E | E | ||
| MAR 1969 | 13-Mar-69 | 13-Mar-69 | **** | 13-Mar-69 | E | E | ||
| JAN 1970 | 2-Jan-70 | 2-Jan-70 | 2-Jan-70 | 2-Jan-70 | E | W | ||
| JAN 1971 | 17-Jan-71 | 18-Jan-71 | 18-Jan-71 | 18-Jan-71 | L | E | ||
| MAR 1971 | 20-Mar-71 | 20-Mar-71 | 20-Mar-71 | 20-Mar-71 | L | E | ||
| JAN 1973 | 2-Feb-73 | 31-Jan-73 | 31-Jan-73 | 31-Jan-73 | E | E | ||
| JAN 1977 | **** | 9-Jan-77 | 9-Jan-77 | 9-Jan-77 | E | E | ||
| FEB 1979 | 22-Feb-79 | 22-Feb-79 | 22-Feb-79 | 22-Feb-79 | 22-Feb-79 | N | W | |
| FEB 1980 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | 29-Feb-80 | E | E |
| FEB 1981 | **** | **** | **** | 6-Feb-81 | **** | **** | N | W |
| MAR 1981 | **** | 4-Mar-81 | 4-Mar-81 | 4-Mar-81 | **** | 4-Mar-81 | N | W |
| DEC 1981 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | 4-Dec-81 | N | E |
| FEB 1984 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | 24-Feb-84 | L | W |
| JAN 1985 | 2-Jan-85 | 1-Jan-85 | 1-Jan-85 | 1-Jan-85 | 1-Jan-85 | 1-Jan-85 | L | E |
| JAN 1987 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | 23-Jan-87 | E | W |
| DEC 1987 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | 8-Dec-87 | E | W |
| MAR 1988 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | 14-Mar-88 | E | W |
| FEB 1989 | 22-Feb-89 | 21-Feb-89 | 21-Feb-89 | 21-Feb-89 | 21-Feb-89 | 21-Feb-89 | L | W |
| DEC 1998 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | 15-Dec-98 | L | E |
| FEB 1999 | 25-Feb-99 | 26-Feb-99 | 26-Feb-99 | 26-Feb-99 | 26-Feb-99 | 26-Feb-99 | L | E |
| MAR 2000 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | 20-Mar-00 | L | W |
| FEB 2001 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | 11-Feb-01 | L | W |
| DEC 2001 | 2-Jan-02 | 31-Dec-01 | 30-Dec-01 | 31-Dec-01 | 30-Dec-01 | 30-Dec-01 | N | E |
| FEB 2002 | **** | 18-Feb-02 | **** | **** | 17-Feb-02 | 17-Feb-02 | N | E |
| JAN 2003 | 18-Jan-03 | 18-Jan-03 | 18-Jan-03 | 18-Jan-03 | 18-Jan-03 | E | W | |
| JAN 2004 | 7-Jan-04 | 5-Jan-04 | 5-Jan-04 | 5-Jan-04 | 5-Jan-04 | N | E | |
| JAN 2006 | 21-Jan-06 | 21-Jan-06 | 21-Jan-06 | 21-Jan-06 | 21-Jan-06 | L | E | |
| FEB 2007 | 24-Feb-07 | 24-Feb-07 | 24-Feb-07 | 24-Feb-07 | 24-Feb-07 | E | W | |
| FEB 2008 | 22-Feb-08 | 22-Feb-08 | 22-Feb-08 | 22-Feb-08 | 22-Feb-08 | L | E | |
| JAN 2009 | 24-Jan-09 | 24-Jan-09 | 24-Jan-09 | 24-Jan-09 | 24-Jan-09 | L | W | |
| FEB 2010 | 9-Feb-10 | 9-Feb-10 | 9-Feb-10 | 9-Feb-10 | 9-Feb-10 | E | W | |
| MAR 2010 | 24-Mar-10 | 24-Mar-10 | 24-Mar-10 | 24-Mar-10 | 24-Mar-10 | E | W | |
| JAN 2013 | 7-Jan-13 | 6-Jan-13 | 7-Jan-13 | 6-Jan-13 | 6-Jan-13 | N | E | |
| FEB 2018 | 12-Feb-18 | 12-Feb-18 | 12-Feb-18 | 12-Feb-18 | 12-Feb-18 | L | W | |
| JAN 2019 | 2-Jan-19 | 2-Jan-19 | 2-Jan-19 | 2-Jan-19 | 1-Jan-19 | E | E | |
| JAN 2021 | 5-Jan-21 | 5-Jan-21 | 5-Jan-21 | 5-Jan-21 | L | W | ||
| FEB 2023 | 16-Feb-23 | 16-Feb-23 | 16-Feb-23 | 16-Feb-23 | L | W | ||
| Total Events | 39 | 29 | 26 | 41 | 27 | 42 | ||
| Frequency: Events/Decade | 5.9 | 6.4 | 6.3 | 6.2 | 6.1 | 6.4 |
Table prepared by Dr. A.H. Butler for the SSWC. Updated September 2023.