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 mechanism suggested that interior diapycnal upwelling is negligible, that all mid-depth water upwells in the Southern Ocean (SO), and that eddies and wind set isopycnal slopes in the SO and therefore determine the mid-depth interior stratification. We examine this hypothesis here by use numerical simulations of eddy-permitting resolution 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. This suggests that SO eddies do not control SO isopycnal slopes via a marginal criticality or a near-vanishing residual overturning conditions but are affected by ocean interior stratification. Furthermore, when the interior mixing is very weak (10 −5 m 2 s −1 ), the interior stratification is far from exponential, suggesting that SO eddies alone also do not lead to the observed stratification. The results suggest that the communication between the Southern Ocean and the interior is not one-way and that both SO eddies and interior diapycnal mixing are important in determining the interior mid-depth stratification.