The influence of eddy parameterizations on the transport of the Antarctic Circumpolar Current in coupled climate models

The transport of the AntarcticCircumpolarCurrent (ACC) varies strongly across the coupled GCMs (general circulation models) used for the IPCC AR4. This note shows that a large fraction of this across-model variance can be explained by relating it to the parameterization of eddy-induced transports. In the majority of models this parameterization is based on the study by Gent and McWilliams (1990). The main parameter is the quasi-Stokes diffusivity κ (often referred to less accurately as ”thickness diffusion”). The ACC transport and the meridional density gradient both correlate strongly with κ across those models where κ is a prescribed constant. In contrast, there is no correlation with the isopycnal diffusivity κiso across the models. The sensitivity of the ACC transport to κ is larger than to the zonal wind stress maximum. Experiments with the fast GCM FAMOUS show that changing κ directly affects the ACC transport by changing the density structure throughout the water column. Our results suggest that this limits the role of the wind stress magnitude in setting the ACC transport in FAMOUS. The sensitivities of the ACC and the meridional density gradient are very similar across the AR4 GCMs (for those models where κ is a prescribed constant) and among the FAMOUS experiments. The strong sensitivity of the ACC transport to κ needs careful assessment in climatemodels.

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