Unparalleled EA-like leading mode of variability in the early 20th century highlights the need for understanding non-stationarity in the North Atlantic climate system

Abstract

The dominant winter modes of large-scale atmospheric variability in the North Atlantic are the North Atlantic Oscillation (NAO), East Atlantic (EA), and Scandinavian (SCA) patterns. This study examines their multi-decadal variability over the Instrumental Period. We identify stages through which interactions between NAO and both EA and SCA phases alter westerly winds and precipitation anomalies in Europe, which would be overlooked if only NAO phases were considered. Additionally, we show periods where EA becomes the leading mode, likely driven by natural ocean-atmosphere coupling variability, which can enhance climate reconstructions and projections. This study also provides new insights into shifts in NAO action centers. The recent shift (post-1980s) is notably distinct within the Instrumental Period. Unlike previous NAO pattern variations, EA and SCA intensify simultaneously, contributing to an expansion of the Azores High, combined with the Atlantic Ridge and/or Scandinavian Blocking. This results in drier conditions in southern Europe and wetter conditions in the north. The simultaneous intensification of NAO, EA, and SCA supports hypotheses of the Azores High expanding towards Central Europe. While this study does not attribute the shift to global warming, the eastward displacement of the northern NAO center post-1980s suggests some anthropogenic forcing through atmospheric and oceanic warming. We thus propose keeping this issue open, despite recent NAO reconstructions not identifying a singular signal in the latter half of the 20th century.