06 Dec 2023
 | 06 Dec 2023

Changes in the North Atlantic Oscillation over the 20th Century

Stephen Outten and Richard Davy

Abstract. The North Atlantic Oscillation explains a large fraction of the climate variability across the North Atlantic from the eastern seaboard of North America across the whole of Europe. Many studies have linked the North Atlantic Oscillation to climate extremes in this region, especially in winter, which has motivated considerable study of this pattern of variability. However, one overlooked feature of how the North Atlantic Oscillation has changed over time is the explained variance of the pattern. Here we show that there has been a considerable increase in the percentage variance explained by the NAO over the 20th century from 32 % in 1930 to 53 % by the end of the 20th century. Whether this change is due to natural variability, a forced response to climate change, or some combination remains unclear. However, we found no evidence for a forced response from an ensemble of 50 CMIP6 models. These models did all show substantial internal variability in the strength of the North Atlantic Oscillation, but it was biased towards being too high compared to the reanalysis and with too little variation over time. Since there is a direct connection between the North Atlantic Oscillation and climate extremes over the region, this has direct consequences for both the long term projection and near term prediction of changes to climate extremes in the region.

Stephen Outten and Richard Davy

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2832', Anonymous Referee #1, 15 Dec 2023
    • AC1: 'Reply on RC1', Stephen Outten, 20 Dec 2023
  • RC2: 'Comment on egusphere-2023-2832', Anonymous Referee #2, 29 Jan 2024
    • AC2: 'Reply on RC2', Stephen Outten, 19 Feb 2024
Stephen Outten and Richard Davy
Stephen Outten and Richard Davy


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Short summary
The North Atlantic Oscillation is linked to wintertime weather events over Europe. One feature often overlooked is how much the climate variability explained by the NAO has changed over time. We show that there has been a considerable increase in the percentage variance explained by the NAO over the 20th century, and that this is not reproduced by 50 CMIP6 climate models, which are generally biased too high. This has implications for projections and prediction of weather events in the region.