Abstract

The meridional overturning circulation and stratfication of the global ocean are shaped critically by processes in the Southern Ocean. The zonally unblocked nature of the Antarctic Circumpolar Current (ACC) confers the region with a set of special dynamics that ultimately results in the focussing therein of large vertical exchanges between layers spanning the global ocean pycnocline. These vertical exchanges are thought to be mediated by oceanic turbulent motions (associated with mesoscale eddies and small-scale turbulence), yet the vastness of the Southern Ocean and the sparse and intermittent nature of turbulent processes make their relative roles and large-scale impacts extremely difficult to assess. Here we address the problem from a new angle, and use measurements of the centimetre-scale signatures of mesoscale eddies and small-scale turbulence obtained during the DIMES experiment to determine the contributions of those processes to sustaining large-scale meridional overturning across the ACC . We find that mesoscale eddies and small-scale turbulence play complementary roles in balancing a meridional circulation of O(1 mm / s) across the Southern Ocean, and that their roles are underpinned by distinct and abrupt variations in the rates at which they mix water parcels. The physical controls behind these variations and their implications for our understanding of the Southern Ocean circulation will be discussed.