Resolution Dependence of Atmosphere–Ocean Interactions and Water Mass Transformation in the North Atlantic

Water mass transformation (WMT) in the North Atlantic plays a key role in driving the Atlantic Meridional Overturning Circulation (AMOC) and its variability. Here, we analyze subpolar North Atlantic WMT in high- and low-resolution versions of the Community Earth System Model version 1 (CESM1) and investigate whether differences in resolution and climatological WMT impact low-frequency AMOC variability and the atmospheric response to this variability. We find that high-resolution simulations reproduce the WMT found in a reanalysis-forced high-resolution ocean simulation more accurately than low-resolution simulations. We also find that the low-resolution simulations, including one forced with the same atmospheric reanalysis data, have larger biases in surface heat fluxes, sea-surface temperatures, and salinities compared to the high-resolution simulations. Despite these major climatological differences, the mechanisms of low-frequency AMOC variability are similar in the high- and low-resolution versions of CESM1. The Labrador Sea WMT plays a major role in driving AMOC variability, and a similar North Atlantic Oscillation-like sea-level pressure pattern leads AMOC changes. However, the high-resolution simulation shows a pronounced atmospheric response to the AMOC variability not found in the low-resolution version. The consistent role of Labrador Sea WMT in low-frequency AMOC variability across high- and low-resolution coupled simulations, including a simulation which accurately reproduces the WMT found in an atmospheric-reanalysis-forced high-resolution ocean simulation, suggests that the mechanisms may be similar in nature.