  The Meridional Overturning Circulation (MOC) in the Southern Ocean is
investigated using hydrographic observations combined with satellite
observations of sea-surface height. A three-dimensional (spatial and vertical)
estimate of the isopycnal eddy-diffusivity in the Southern Ocean is obtained
using the theory of Ferrari & Nikurashin (2010), that includes the influence of
suppression of the diffusivity by the strong, time-mean flows. It is found that
the eddy diffusivity is enhanced at depth, reaching a maximum at the "critical
layer" near 1000m. The estimate of diffusivity is used with a simple diffusive
parameterization to estimate the meridional eddy volume flux. Together with an
estimate of the meridional Ekman transport and the time-mean meridional
geostrophic transport, the eddy volume flux is used to reconstruct the
time-mean overturning circulation. By comparing the reconstruction with, and
without, suppression of the eddy diffusivity by the mean flow, the influence of
the suppression on the overturning is illuminated. It is shown that the
suppression of the eddy diffusivity results in a large reduction of interior
eddy transports, and a more realistic eddy induced overturning circulation.
Finally, a simple conceptual model is used to show that the MOC is influenced
not only by the existence of enhanced diffusivity at depth, but also by the
details of the vertical structure of the eddy diffusivity, such as the depth of
the critical layer.
