On the origin of North Atlantic Oscillation longitudinal displacements

Previous studies have shown that the centers of action of the North Atlantic Oscillation (NAO), leading mode of regional atmospheric variability, shifted eastward in the mid-1970s compared to the preceding decades of the mid-twentieth century. They pointed at increasing greenhouse gas forcing and/or low-frequency oceanic variability as potential players. Here, the possible driving mechanisms of this shift are further investigated using long-term reanalysis data (NOAA 20CR, ERA 20CR) and two 500-year climate simulations performed with EC-EARTH 3.3 (CMIP6 version) keeping fixed the radiative forcing at present-day conditions [year 2000], in atmosphere-only and coupled ocean-atmosphere configurations. 30-year running empirical orthogonal functions (EOFs) of sea level pressure anomalies in the North Atlantic-European region are computed to track the longitudinal displacements of the NAO pattern, first showing that a similar eastward shift happened at the beginning of the observational record. Recurrent eastern and western locations of the NAO pattern are properly simulated by the model, with marginal differences between the atmosphere-only and coupled configurations. Moreover, the fraction of variance explained by the running EOFs varies with time displaying a comparable range (~30% to 60%) in observations and model simulations. Composites of eastern and western NAO patterns, based on upper and lower terciles, are built to show that the longitudinal shifts appear linked to changes in the variability of the regional eddy-driven zonal wind. The similarity in the NAO displacements between observations, atmosphere-only and coupled simulations suggests that the observed eastward shift could have occurred previously and might not be due to anthropogenic or oceanic forcing but to pure internal variability.