Picture: Ocean eddies around South Africa. Credit: NASA Goddard Space Flight Center Scientific Visualization Studio.

Wind stress at the surface of the ocean is an important driver of ocean currents. However, what sets up the strength of the currents remains puzzling.

The strongest ocean current flows around Antarctica: the Antarctic Circumpolar Current.

It is believed that the Antarctic Circumpolar Current is close to a so-called “eddy saturated” state, a regime in which changes in the strength of the wind stress forcing do not alter the strength of the mean current. Instead, the swirling oceanic eddy motions that accompany the current are enhanced.

Here, we investigate the physics and assess the relative importance of the two mechanisms proposed in the literature to explain this phenomenon: the most commonly invoked baroclinic mechanism and the recently proposed barotropic mechanism that crucially involves the interaction of the oceanic flow with bathymetric features.

The results suggest that the oftentimes ignored depth-averaged (barotropic) component of the ocean flow and its interaction with bathymetry play a dominant role in setting up the strength of the current in certain regimes.