Coupled climate effects of eddy rich model resolution in and south of the Agulhas

In this study, we compare global coupled climate simulations (1950s and abrupt 4xCO2) with different ocean resolution in the Atlantic sector of the Southern Ocean (ASO), including the Agulhas, with parameterised and explicitly simulated eddies respectively. We find that the eddy-rich 1950s simulation has a reduced South Atlantic warm bias, because of a more defined Agulhas retroflection, and coupled climate effects are observed outside the region with increased ocean resolution where e.g. equatorial precipitation changes markedly. The Agulhas leakage plays a key role in connecting the Indian and the Atlantic oceans, with mesoscale eddies carrying heat and salt into the South Atlantic. In most state-of-the-art coupled climate models, the ocean resolution is insufficient to explicitly simulate those eddies, and they are instead represented through a parameterisation of the eddy induced flow. We use the coupled climate model FOCI, which combines a NEMO3.6 ocean with an ECHAM6 atmosphere, LIM2 sea ice, and a JSBACH land module, via an OASIS coupler. Through AGRIF nesting, we increase the ocean resolution from 1/2 to 1/10 in the Atlantic sector of the Southern Ocean. The eddy-rich 1950s simulation exhibits a reduced warm bias in the South Atlantic compared to the simulation without it. The bias reduction is a result of a more defined Agulhas retroflection which reduces ocean heat transport into the South Atlantic while increasing heat transport poleward. This change in ocean temperature distribution is anticipated from previous studies with ocean-only models. However, we also see coupled climate effects extending to the equatorial region, well outside the region with increased ocean resolution. We observe changes in precipitation and surface wind fields over both the tropical Atlantic and tropical/South Pacific. The changes over the tropical Atlantic are likely linked to a direct response to changes in sea surface temperature that extend across the South Atlantic. The eddy-rich 1950s simulation also shows significant reduction of surface air temperature (SAT) biases, mostly in the Northern Hemisphere, and winds in the Southern Hemisphere, w.r.t. observationally based reanalysis products. In the strong warming scenario (abrupt 4xCO2), the eddy-rich simulation shows less SAT increase over the Atlantic and a larger seasonality in the response of the westerly wind fields over the Southern Ocean. In conclusion, increased resolution of the ASO, allowing for explicit simulation of mesoscale eddies e.g. in the Agulhas, leads to reduction of model biases and coupled climate effects.