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Abstract

Global coupled ocean-atmosphere models are a valuable tool to study climate variability and to project future changes. Many of the present global coupled models have an ocean component with a low horizontal resolution that does not permit ocean mesoscale eddies. The ocean mesoscale is not only important for the ocean dynamics but can also have an imprint on the atmosphere. Increasing the horizontal resolution of the ocean model component is therefore crucial to improve simulations of the coupled climate system. In this study, a newly developed version of Australia’s ACCESS-CM2 climate model with a higher-resolution ocean-sea ice component (1/4°) is evaluated under present climate conditions against (i) the previous coarser (1°) version and (ii) against the ocean-only counterparts of the ACCESS-OM2 ocean-sea ice model suite. The 1/4° ACCESS-CM2 overall improves the ocean state compared to the 1° version but inhibits a large decadal variability in the upper ocean heat content that is not seen in any of the other models. The signal originates in the North Atlantic, can be traced to the Southern Hemisphere and dominates the global mean. Another notable aspect of the 1/4° ACCESS-CM2 is the large Drake Passage transport, a metric that many models do not simulate accurately, and which is underestimated in the ACCESS-OM2 models but represented reasonably well in the 1° ACCESS-CM2 version compared to observations.