Mid-depth Ocean Stratification: Southern Ocean eddies vs interior vertical diffusivity

Abstract

The mid-depth ocean stratification was fitted by Munk (1966) to an exponential profile and shown to be consistent with a vertical advective-diffusive balance. However, tracer release experiments show that vertical diffusivity in the mid-depth ocean is an order of magnitude too small to explain the observed 1 km exponential scale. Alternative mechanisms suggested that the overturning is mostly adiabatic, that interior diapycnal upwelling is negligible, and that nearly all mid-depth water upwells adiabatically in the Southern Ocean (SO). In this picture, SO eddies and wind set isopycnal slopes in the SO and therefore determine a non-vanishing mid-depth interior stratification even in the adiabatic limit. The effect of SO eddies on SO isopycnal slopes can be understood via either a marginal criticality condition or via a near-vanishing residual overturning conditions in the adiabatic limit. We examine the role of SO eddies vs interior mixing in setting the mid-depth stratification by using eddy-permitting numerical simulations, in which we artificially change the diapycnal mixing only away from the SO. We find that SO isopycnal slopes change in response to changes of the interior diapycnal mixing even when the wind forcing is constant, consistent with previous studies. However, in the limit of small interior mixing, the interior stratification is far from exponential, suggesting that SO processes alone do not lead to the observed stratification. The results suggest that while SO eddies contribute to the non-vanishing mid-depth interior stratification, the exponential shape of the stratification must also involve interior diapycnal mixing. Both SO eddies and interior diapycnal mixing are therefore important in determining the interior mid-depth exponential stratification.