Wind Stress Parameterisation in the Southern Ocean

Abstract

Wind stress is often parameterised in ocean-atmosphere coupled models by a quadratic function of atmosphere velocity. Strictly speaking, this drag law should be a quadratic function of the difference between the ocean and atmosphere velocities. The two schemes give very similar magnitudes of stress, however the power input is significantly reduced under the velocity difference stress. This work is the first to model this effect in the Southern Ocean. The velocity difference stress is found to reduce power input, but paradoxically the circumpolar transport is increased. The increase in transport is a consequence of two main factors: eddy saturation of the Antarctic Circumpolar Current, and eddy damping under the velocity difference stress. ¶ The wind stress parameterisation is also modified, by coupling the quadratic drag law to the temperature difference between the ocean and atmosphere. This simple representation is shown to be consistent with scatterometer observations, and the coupling constant is calibrated using known correlations between gradients of stress and sea surface temperature. Unlike the velocity difference stress, the temperature coupled stress is found to have a negligible effect on the mean flow in this model.