These are 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 1.5° ERA5 linearly-detrended 500 hPa geopotential height anomalies in the region 20-80°N, 90°W-30°E, from 1 December 1979 – 31 March 2022. Each day is then assigned to a regime based on the minimum Euclidean distance to a cluster centroid. The four regimes are:
- NAO- (alternatively, Greenland Blocking), occurring on ~20% of DJFM days.
- Atlantic Ridge (AR), occurring on ~21% of DJFM days.
- Scandinavian Blocking (SB), occurring on ~27% of DJFM days.
- NAO+ (alternatively, Zonal), occurring on ~32% 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+).
Further details on the weather regime index can be found here.
DJFM weather regimes 1979/80 – 2021/22
2022 2021 2020 2019 2018 2017 2016 2015 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1989 1988 1987 1986 1985 1984 1983 1982 1981 1980Proportion of DJFM days assigned to each regime per year
Simon H. Lee 