Increasing model resolution improves but overestimates global mid-depth circulation simulation.

Increasing the spatial resolution in climate models has significantly improved the simulation of global upper-layer ocean circulation. However, the ability of high-resolution models to accurately reproduce mid-depth circulation, in terms of strength and direction, still remains uncertain. An analysis of 17 climate models with varying resolutions reveals that both low and high-resolution models depict weaker current speeds compared with observations. High-resolution models demonstrate improved simulations of current speed and flow direction, except in the Southern Ocean. The performance of high-resolution models in regions with strong currents is generally better than in regions with weak flows. Dynamically, increasing the model resolution enhances the representation of temporal variations in mid-depth circulation by effectively capturing mesoscale processes. However, this also results in an overestimation of their intensity by approximately 65% on average across the global ocean.