Last updated 21 April 2025.
The Northern Annular Mode (NAM), also called the Arctic Oscillation (AO), is the leading mode of atmospheric variability in the Northern Hemisphere, characterised by mass fluctuations between the middle and high-latitudes.
Various methods exist to define the NAM (some recent discussion in Lee and Polvani, 2024). Here, the NAM is defined for each pressure level as the standardised anomalies of 60–90°N average geopotential height, where the daily mean and standard deviation are computed over 1 January 1940 – 31 December 2024 and smoothed with a 30-day running mean. To reduce the influence of temperature trends, the daily global-mean geopotential height is subtracted prior to computing the index (similar to Gerber and Martineau, 2018).
The time series are computed using once-daily 00Z ERA5 data on a 1.0° grid. 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 (and generally inconsequential) changes to previous years.
Note that the earlier part of the record, particularly prior to 1958, is subject to much greater uncertainty due to reduced observational constraints on the reanalysis model. Please refer to Soci et al. (2024) for further details.