North Atlantic Wintertime Weather Regimes

Last updated 28 April 2025.

Weather regimes are recurrent, persistent and quasi-stationary circulation patterns which represent large-scale, low-frequency variability. The wintertime North Atlantic regimes are here computed following a similar method to Cassou (2008): k-means clustering (with k = 4) is performed on the leading 14 principal components of daily 00Z December–March 1.0° ERA5 linearly-detrended 500 hPa geopotential height anomalies in the region 20–80°N, 90°W–30°E, from 1 January 1979 – 31 March 2025. Each day is then assigned to a regime based on the minimum Euclidean distance to a cluster centroid (a ‘no regime’ class is not used). The four regimes are:

  • NAO+ occurring on ~32% of DJFM days.
  • Scandinavian Blocking (SB) occurring on ~27% of DJFM days.
  • NAO– occurring on ~21% of DJFM days.
  • Atlantic Ridge (AR) occurring on ~20% of DJFM days.

Note that the regimes named after the two phases of the NAO are not equal-and-opposite, and are not to be confused with the NAO as defined as the leading EOF or the Azores-Iceland pressure difference. Days with a negative NAO index are not necessarily in the NAO– regime (and vice versa for NAO+).

North Atlantic wintertime weather regimes: Composite-mean 500 hPa geopotential height anomalies for all days assigned to each of the four North Atlantic wintertime weather regimes. The % of DJFM days in 1979/80 – 2024/25 assigned to each regime is also shown.

The weather regime index (WRI) is the standardised projection of each day’s Z500 anomalies onto the cluster mean (see, e.g., Michel and Rivière 2011, Lee and Messori 2024) and provides continuous information on the amplitude of each pattern. Although not applied here, one can use a WRI threshold to define a ‘no regime’ class (e.g., Grams et al. 2017). In the below, the year corresponds to the January–March period.

1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020 2021 2022 2023 2024 2025

Seasonal analyses

Seasonal frequency time series plot

Most NAO+ days: 2013/2014 (84 days)
Most Scandi Blocking days: 2002/2003 (65 days)
Most NAO– days: 2009/2010 (89 days)
Most Atlantic Ridge days: 1999/2000 (54 days)

Least NAO+ days: 1995/1996 (7 days)
Least Scandi Blocking days: 1999/2000 (14 days)
Least NAO– days: 2014/2015 (0 days)
Least Atlantic Ridge days: 1989/1990 (3 days)