Altered NAO - North Atlantic SST Feedback in Mesoscale Resolving Simulations

Recent studies using mesoscale resolving simulations have shown a stronger and spatially different large-scale atmospheric response to North Atlantic (NA) sea-surface temperature (SST) anomalies compared to more coarse-resolution simulations. An idealized setup proved that moving from a horizontal resolution of 110-km of more classic general circulation models to 28-km and further to 14-km leads to distinct large-scale responses to Gulf Stream SST anomalies. 

Here, we investigate a new set of simulations using a variable resolution version of the Community Atmospheric Model (CAM6) and more realistic specified SST anomalies. The horizontal resolution is regionally refined in the NA domain from the global 110-km resolution to 28-km, which is not fully mesoscale resolving, and further to 14-km, capable of resolving weather fronts which are crucial features for ocean-atmosphere coupling. 
The specified SST anomaly forcing the simulations is created by regressing the observational North Atlantic Oscillation (NAO) index onto SSTs over the period 1958–2018, resulting in a cold-warm-cold tripole anomaly and enabling a comparison of the NAO - NA SST feedback between the different resolutions. 

We find that the resulting NA SST tripole anomaly feeds back positively onto the NAO in the 14-km simulations. This positive NAO–SST–NAO feedback is not present in the 28-km and 110-km simulations which show a distinct spatial structure and generally a weaker response.
With the overall push towards more expensive higher-resolution coupled simulations, our results will provide valuable insights into the required atmospheric resolution needed to correctly represent ocean-atmosphere coupling.