Quantifying the Response of the North Atlantic Oscillation to a Wide Range of CO2 Forcing

The North Atlantic Oscillation (NAO) is the principal mode of atmospheric variability in the North Atlantic region, modulating European weather and climate during both winter and summer. While Earth System Models project a more positive NAO under the high-emission 21st century scenarios, uncertainties persist regarding the exact response of the NAO in a warmer world. In this study, we investigate the NAO response to CO₂ forcing in a wide range of CO₂ perturbations from one-eighth to eight times the pre-industrial values. By defining the NAO as simply the difference in mean sea level pressure between the Azores and Iceland, we find that NAO becomes generally more positive with increasing CO₂ concentrations. However, an additional complexity arises from shifts to the spatial pattern of the winter NAO at higher CO₂ concentrations: the Icelandic Low weakens and shifts eastward into the Norwegian Sea, while the Azores High also shifts eastward. As a result, the winter NAO becomes more similar to the summer NAO at high CO₂ forcing. This structural shift cautions against using stationary NAO metrics that may not capture such changes. Furthermore, we diagnose how the persistence of the NAO on daily timescales varies as a function of CO₂ forcing. Lastly, we examine how the NAO response relates to the North Atlantic SSTs and AMOC state. Our results indicate that the response of the NAO to CO₂ forcing may be more complicated than previously thought due to changes to the structure of variability in the North Atlantic region, with large implications for European weather and climate.