AMOC Discrepancies across Reanalyses and Models Limit Dynamical Understanding

Abstract

The Atlantic meridional overturning circulation (AMOC) influences global climate, yet its multiscale variability and the processes governing it remain uncertain across models and observations. Using four ocean reanalyses and forced ocean–sea ice simulations from three model families [Energy Exascale Earth System Model Model for Prediction Across Scales (MPAS), Parallel Ocean Program (POP), and Nucleus for European Modelling of the Ocean (NEMO)], each run in both a coarse [low resolution (LR)] and a mesoscale-permitting or eddy-resolving [high resolution (HR)] configuration, we assess the consistency of their representation of AMOC strength, overturning freshwater transport, salt-advection feedback, and gyre circulation around 58°N, 26°N, and 34°S over the past 50–70 years. Across the three model families, two LR–HR pairs simulate a stronger mean overturning at higher resolution, while one shows weak sensitivity. Across reanalyses, larger overturning amplitudes coincide with higher nominal resolution, but these products differ also in forcing, model formulation, and assimilation scheme. At 26°N, a robust negative coupling between freshwater transport and AMOC strength emerges across datasets, consistent with the salt-advection feedback. This relationship is absent at 58°N and inconsistent across datasets at 34°S, suggesting that the linear coupling between overturning and freshwater transport breaks down outside the subtropical North Atlantic. Among gyre diagnostics, only the mean strength of the North Atlantic subtropical gyre measured by its barotropic streamfunction exhibits a coherent and significant positive linear relationship with mean AMOC strength across all products and resolutions. No analogous signal emerges for the subpolar gyre or the South Atlantic subtropical gyre. This robustness points to the North Atlantic subtropical gyre as a consistent indicator of mean overturning strength across model strategies.