The Importance of Accounting for the North Atlantic Oscillation When Applying Observational Constraints to European Climate Projections

Variability in the North Atlantic Oscillation (NAO) has contributed to the recent multidecadal trends observed in European climate, especially to trends in winter precipitation over Northern Europe. However, the current generation of coupled climate models struggle to reproduce the NAO's contribution to multidecadal trends, which has important implications for deriving constraints based on the comparison of observed and modeled trends. An observational constraint based on attribution results, both with and without the contribution of variability associated with the NAO, is applied to projections of Northern European precipitation and temperature, and observed NAO variability is shown to lead to a constraint that overestimates future forced changes. Only after removing the NAO variability is the observed climate change consistent with model simulations, and a tighter, unbiased observational constraint based on the forced signal (without the NAO) can be applied to future projections.

Automated summary

Variability in the North Atlantic Oscillation (NAO) plays a crucial role in European climate trends, particularly in winter precipitation over Northern Europe. However, current climate models struggle to accurately capture the NAO's contribution to these trends, impacting the comparison between observed and modeled trends. By removing the variability associated with the NAO, the observed climate change aligns with model simulations and allows for a more precise and unbiased constraint for future projections.