Northern Annular Mode

Time-height cross-sections of the Northern Annular Mode (NAM) calculated following the zonal-mean method of Baldwin and Thompson (2009). The zonal-mean approach is favoured in part because the leading empirical orthogonal function (EOF) in the free troposphere resembles a mix of the NAM and Pacific-North American (PNA) patterns, but it is also much less computationally expensive.

The time series are computed using daily zonal-mean 00Z ERA5 data from 1 January 1959 – 31 December 2021 poleward of 20°N on a 1.5° grid (2022 to follow soon). The mean climate is subtracted from each day, and then data are linearly detrended. The leading EOF is computed from this dataset at each pressure level (29 levels between 1000 and 1 hPa), and its principal component (PC) time series is standardised by dividing by the square-root of its eigenvalue (i.e., dividing by the standard deviation of the entire timeseries at each level).

The figures here show the evolution of the NAM between 1 November and 30 April to focus on stratosphere-troposphere coupling and the period when NAM variance is greatest. Note that the time series are re-computed for all years whenever new data are added, which may result in incremental changes to previous years.

Winter 2020-2021

NAM

Previous years (currently 1959 onwards)

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 1980 1979 1978 1977 1976 1975 1974 1973 1972 1971 1970 1969 1968 1967 1966 1965 1964 1963 1962 1961 1960 1959

Vertical structure of the NAM: (left) Linear regression of the standardised NAM time series at each level with zonal-mean geopotential height anomalies over 1959-2021. (Right) Fraction of the total variance of the daily zonal-mean geopotential height anomalies explained by the NAM at each level.