  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.
