More positive and less variable North Atlantic Oscillation at high CO2 forcing

The North Atlantic Oscillation (NAO) is the principal mode of atmospheric variability over the North Atlantic, modulating the weather and climate of neighboring regions in both winter and summer. While Earth System Models generally project a more positive NAO under 21st century high-emission scenarios, uncertainties persist as to the precise response of the NAO to increased CO2 levels, owing to large internal variability. In this study we investigate the response of the NAO to a wide range of CO2 forcings, from two to eight times the preindustrial values. Analyzing a large sample of present-generation climate models, we find that the NAO likely becomes more positive with increasing CO2 concentrations. Moreover, we find a reduction in NAO variability. This leads to a smaller increase in the likelihood of extremely positive NAO events than would be expected based solely on the shift in the mean. On the other hand, we also find a reduction in extremely negative NAO events, which is attributable to both the shift toward more positive values and the decrease in variance. Finally, our analysis reveals that the distribution of the NAO response at high CO2 forcing is negatively skewed. This fact partially offsets the decrease in extremely positive NAO events associated with reduced variability. Ultimately, our results suggest a greater increase in positive NAO events compared to the decrease in extremely negative NAO events at higher CO2 forcing.