Internal variability of the winter North Atlantic Oscillation longitudinal displacements

Abstract

The winter North Atlantic Oscillation (NAO), one of the leading modes of atmospheric variability in the Northern Hemisphere and key driver of surface climate anomalies, was long considered to be spatially stable. Yet, its northern center-of-action – the Icelandic Low (IL) – shifted eastward in the late 1970s compared to the preceding decades of the mid-20th century. The responsible processes are still uncertain, particularly after the decline of the positive NAO trend in the 21st century. Here, we present observational and model evidence that the NAO-IL moves naturally alternating between two preferential locations, west/east of Iceland, with no need for changes in anthropogenic forcing or low-frequency oceanic variability. These recurrent longitudinal displacements of the NAO pattern appear linked to zonal changes in the fluctuations (not mean-state) of transient-eddy activity, emphasizing the relevance of internal atmospheric variability, and could represent a major source of uncertainty in regional climate prediction and projection.