Enhanced eddy activity along the Subantarctic Front under intensified westerly winds

The westerlies in the southern hemisphere have intensified and shifted southward since the middle of the twentieth century. Previous studies have indicated that the expected increase in isopycnal slopes and acceleration of the Antarctic Circumpolar Current (ACC) is considerably weakened by the strengthening of mesoscale eddies and that this “eddy saturation” occurs mainly downstream of the major bottom topographic features such as the Kerguelen Plateau. Such eddy “hotspots” are thus considered to regulate the ACC responses to changes in external forcing. To improve our understanding of the ACC response to intensified winds, a sensitivity study is conducted using an eddy-resolving quasi-global ocean general circulation model named “OFES.” The reference run is driven by a climatological atmospheric forcing and the sensitivity run is driven by artificially intensified climatological westerlies. Our new finding is that the baroclinic energy pathway is enhanced over the Subantarctic Front (SAF) as well as over the hotspots identified by previous studies. A linear stability analysis indicates that the spin-up of the subtropical gyres north of the SAF and the enhanced Ekman upwelling south of the SAF by the intensified wind stress curl increase the vertical shear of zonal velocity along the SAF, enhancing baroclinic instability. We have also performed the same stability analysis comparing the 1985–2018 and 1955–1984 periods of a hindcast run of OFES, confirming the result from the climatological sensitivity study. These results suggest that the SAF is another eddy hotspot when the wind stress curl keeps increasing.