The Impact of Turbulence and Convection on Transport in the Southern Ocean

Volume transport in the presence of fully resolved convection and turbulence is investigated in a reentrant channel model of the Southern Ocean. The response of the meridional overturning and zonal transport to variations in wind and buoyancy forcing is quantified. Our simulations show two overturning cells—a buoyancy-driven lower cell and a wind-driven upper cell. The lower overturning cell is much larger in meridional extent and magnitude than the upper cell. The mean component of the overturning transport is smaller than the fluctuating component in the lower overturning cell, indicating that transport is dominated by eddies and/or turbulent convective flow. In contrast, the upper cell can be primarily described as a mean flow (indicating minimal eddy compensation). Both cells strengthen with increasing winds, with the upper cell being more sensitive to increasing wind intensity than the lower cell. Scaling for the mean upper overturning is also derived and matches previous theories which predict a linear sensitivity of the upper cell to wind. Zonal transport remains insensitive to increasing wind stress, suggesting that the system is eddy saturated when turbulence and eddies are resolved. These results suggest that fine-scale flows control zonal transport and abyssal meridional overturning in the Southern Ocean, while wind stress drives the upper meridional overturning transport, highlighting the importance of accurately characterizing turbulence and convection in large-scale models.