Master´s thesis defense by Mads Bruun Poulsen – Niels Bohr Institute - University of Copenhagen

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Master´s thesis defense by Mads Bruun Poulsen

The Sensitivity of the Southern Ocean Circulation to Local Wind Stress Forcing in a Coarse Resolution Coupled GCM
The intense southern hemisphere westerlies are believed to exert a strong control on the slope of the Southern Ocean isopycnals and the strength of the associated Antarctic Circumpolar Current (ACC). However, the absence of a response in the ocean density structure to the observed ongoing intensification of these winds has motivated a reassessment of the Southern Ocean dynamics. Recent studies have emphasized the compensating role of mesoscale eddies on the circulation, and the sensitivity of the circumpolar transport to wind stress variability is presently debated.

This thesis uses the output from a 1000 yr control integration of the fully coupled coarse resolution Community Climate System Model 4 to investigate this dynamical relationship. Four different measures of the wind stress are evaluated to understand in what way the winds drive the circulation. Moreover, the response of the baroclinic circumpolar transport to wind stress variability is addressed by developing a measure of the large-scale isopycnal slope through the use of the baroclinic potential energy.

It is here shown that the zonal wind stress over the path of the ACC is a useful forcing metric, and that it correctly predicts the relevant ocean Ekman transports. Using this metric, it is found that the isopycnal slope and the baroclinic transport of the ACC responds significantly to changes in the wind stress. This conclusion is supported by a detailed study in the three strongest multi-decadal internal wind stress trends of the model run.

However, recent studies have questioned the capability of coarse resolution models in adequately representing the effect of the mesoscale eddies. This work finds that the model eddies only marginally compensate the variability in the wind-driven circulation, which conflicts with the output from eddy resolving models. This points at an overestimation of the ocean response to wind stress variability that is presented in this thesis. The shortcoming of the model in correctly capturing the dynamical response is partly ascribed to the present formulation of the eddy transfer coefficient in the implemented eddy mixing parameterization.