The impact of horizontal resolution on the representation of thermal air-sea interaction of the Agulhas system in coupled climate-models

The Agulhas Current is a vigorous western boundary current that flows off the southeastern coast of Africa, linking the Indian Ocean to the Atlantic Ocean as part of a broader inter-ocean current system. The Agulhas Current is characterized by intense heat fluxes between the ocean and atmosphere, as well as complex multiscale ocean dynamics. Dynamically unstable currents in this region generate mesoscale eddies, which propagate into the South Atlantic and interact with the Benguela Upwelling system and the Atlantic Meridional Overturning Circulation. These unique features make the Agulhas region an ideal site for studying air-sea interactions in climate models and examining the relative roles of atmospheric and oceanic weather in driving upper-ocean variability. In this study, we investigate how horizontal resolution in climate models affects their ability to represent thermal air-sea interactions over the Agulhas Current region, by comparing several simulations of state-of-the-art models. We identify ocean- and atmosphere-driven regimes using a covariance analysis of sea surface temperatures and turbulent heat fluxes. Our findings suggest that a minimum ocean model resolution of approximately 25 km is necessary to capture the signature of ocean dynamics, leading to a better alignment with theoretical and observational results. Furthermore, we identify a transition scale between ocean-driven and atmosphere-driven regimes within the 2°–5° range: when this critical length scale is exceeded, the ocean-driven behavior is filtered out and the atmosphere-driven regime, which becomes relevant at larger scales, dominates. While both oceanic and atmospheric resolution play a role, we find that increasing the horizontal resolution in the ocean component yields a comparatively larger improvement in the representation of air-sea flux variability.