Model Resolution Sensitivity of the NAO-AMOC-Impacts Causal Chain

The Atlantic Meridional Overturning Circulation (AMOC) is thought to play a key role in driving decadal surface climate variations in the pan-Atlantic region and beyond, while also providing a key source of ocean memory underpinning decadal climate predictions. Model results show that consecutive years of strong wintertime North Atlantic Oscillation (NAO) forcing drive decadal AMOC fluctuations that produce decadal sea surface temperature (SST) changes resembling the observed Atlantic Multidecadal Variability (AMV) pattern. Observations suggest there is a surface climate response to AMV, including possible feedbacks on the NAO, but the fidelity of models in representing these linkages is questionable. We explore the NAO-AMOC-Impacts causal chain in a set of idealized coupled ensemble experiments in which observation-based NAO heat flux is added as an ocean forcing perturbation during the first 10 winters of 20-year long simulations. The experiment is performed using two different configurations of the Community Earth System Model (CESM) that differ primarily in terms of horizontal resolution: low-resolution (LR; ~1° in all components) and high-resolution (HR; ~0.1° ocean coupled to ~0.25° atmosphere). In both configurations, persistent NAO+ buoyancy forcing induces a positive AMOC and subpolar Atlantic SST response. However, the atmospheric response to SST change is much stronger in HR than in LR and is characterized by a high signal-to-noise delayed NAO- response together with a shift of the North Atlantic Jet. The results suggest that current LR decadal prediction systems may significantly underestimate the predictability of North Atlantic climate.