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The Meridional Overturning Circulation (MOC) in the Southern Ocean isinvestigated using hydrographic observations combined with satelliteobservations of sea-surface height. A three-dimensional (spatial and vertical)estimate of the isopycnal eddy-diffusivity in the Southern Ocean is obtainedusing the theory of Ferrari & Nikurashin (2010), that includes the influence ofsuppression of the diffusivity by the strong, time-mean flows. It is found thatthe eddy diffusivity is enhanced at depth, reaching a maximum at the "criticallayer" near 1000m. The estimate of diffusivity is used with a simple diffusiveparameterization to estimate the meridional eddy volume flux. Together with anestimate of the meridional Ekman transport and the time-mean meridionalgeostrophic transport, the eddy volume flux is used to reconstruct thetime-mean overturning circulation. By comparing the reconstruction with, andwithout, suppression of the eddy diffusivity by the mean flow, the influence ofthe suppression on the overturning is illuminated. It is shown that thesuppression of the eddy diffusivity results in a large reduction of interioreddy transports, and a more realistic eddy induced overturning circulation.Finally, a simple conceptual model is used to show that the MOC is influencednot only by the existence of enhanced diffusivity at depth, but also by thedetails of the vertical structure of the eddy diffusivity, such as the depth ofthe critical layer.

How Does the Antarctic Circumpolar Current Affect the Southern Ocean Meridional Overturning Circulation?