Effect of horizontal resolution on the Southern Ocean circulation in the new GFDL OM4 model

The Southern Ocean (SO) plays a pivotal role in the climate through two major circulation systems. The Antarctic Circumpolar Current (ACC) is the world largest current and propagates climatic anomalies between three major oceanic basins. The SO meridional overturning circulation (MOC) connects the surface with the deep ocean hence playing a major role in the ventilation and sequestration of heat and carbon. Therefore, the quality of climate models is highly dependent upon the accuracy of the SO simulation, and in particular upon the representation of mesoscale eddies which are known for their key contribution in restratifying the upper ocean and opposing the wind-driven circulation.

The Geophysical Fluid Dynamics Laboratory (GFDL) has recently developed OM4, an ocean/sea-ice model that participates in the Coupled Model Intercomparison Project version 6. Here we evaluate the SO major circulation features of three OM4 configurations: OM4p25 (1/4°), OM4p5n (1/2°), and OM4p5 (1/2° with parameterized mesoscale eddy transport and stirring). These configurations are forced by atmospheric reanalyses following the interannual Coordinated Ocean-sea ice Reference Experiments (CORE) protocol over five cycles.

We find that the ACC transport of both OM4p25 and OM4p5 stabilizes after three cycles, while that of OM4p5n continues to drift for several centuries. Under increasing wind stress, both the ACC transport and MOC of OM4p5 show twice the sensitivity of that of OM4p25, presumably because of the explicit (though incomplete) representation of mesoscale eddies in OM4p25. The mesoscale eddy parameterization in OM4p5 greatly reduces the mixed layer depth relative to OM4p5n, and inhibits the intense deep convection occuring around the Antarctic continent in OM4p5n, thus strongly affecting the overall structure of the MOC.