Antarctic Circumpolar Current in coupled climate models

1 The transport of the Antarctic Circumpolar Current (ACC) varies strongly across the cou2 pled GCMs (general circulation models) used for the IPCC AR4. This note shows that a 3 large fraction of this across-model variance can be explained by relating it to the param4 eterization of eddy-induced transports. In the majority of models this parameterization is 5 based on the study by Gent and McWilliams (1990). The main parameter is the quasi6 Stokes diffusivity κ. The ACC transport and the meridional density gradient both correlate 7 strongly with κ across those models where κ is a prescribed constant. In contrast, there is no 8 correlation with the isopycnal diffusivity κiso across the models. The sensitivity of the ACC 9 transport to κ is larger than to the zonal wind stress maximum. Experiments with the fast 10 GCM FAMOUS show that changing κ directly affects the ACC transport by changing the 11 density structure throughout the water column. Our results suggest that this limits the role 12 of the wind stress magnitude in setting the ACC transport in FAMOUS. The sensitivities 13 of the ACC and the meridional density gradient are very similar across the AR4 GCMs (for 14 those models where κ is a prescribed constant) and among the FAMOUS experiments. The 15 strong sensitivity of the ACC transport to κ needs careful assessment in climate models. 16