This study addresses the response of the Southern Ocean to high-frequency wind forcing, focusing on the impact of several barotropic modes on the circumpolar transport. A suite of experiments is performed with an unstratified model of the Southern Ocean, forced with a stochastic wind stress that contains a large range of frequencies with synoptic time scales.
The Southern Ocean adjustment displays a different character for frequencies below and above 0.2 cpd. The low-frequency range is dominated by an `almost-free-mode' response, in the region where contours of $f/H$ are obstructed by only a few bathymetric features; the truly free-mode only plays a minor role. Topographic form stress, rather than friction, is the dominant decay mechanism of the Southern Mode. It leads to a spin-down time scale of the order of 3 days.
For the high-frequency range, the circumpolar transport is dominated by the resonant excitation of oscillatory modes. The `active' response of the ocean leads to strong changes and even discontinuities in the phase relation between transport and wind stress. Full text (pdf)