Is the Southern Ocean residual circulation eddy compensated?

The Southern Ocean wind stress has intensified over the past few decades due largely to the ozone hole and to anthropogenic climate change. Observations and model results are converging on the result that the Antarctic Circumpolar Current (ACC) transport is eddy saturated, meaning the baroclinic transport will not increase substantially with increased wind forcing. However, changes in the meridinoal overturning circulation (MOC), which cannot be directly observed, from numerical model results diverge significantly. Results are sensitive to the nature of the model, in particular, whether it is idealized or realistic in its geometry or if mesoscale eddies are permitted or parameterized. To address this point we have analyzed two experiments with a global coupled high-resolution version of the Community Earth System Model where mesoscale eddies are explicitly resolved (0.1^o ocean, 0.25^o atmosphere). The control experiment simulates 100 years at “present-day” (year 2000) greenhouse gas conditions. A wind perturbation (WP) experiment was integrated for 20 years with the Southern Ocean winds south of 30^oS increased by 50%. Results show that the ACC transport is nearly eddy saturated, but that the MOC is not eddy compensated. The overturning circulation cells do change in response to increased wind forcing by as much as 38% in the upper cell and 58% in the lower cell compared to the control. A striking result is that the transient eddy response is small and that any eddy compensation that takes place is achieved mainly by the standing component of the mean MOC, defined as the deviation from the time and zonal mean overturning circulation. The basin-wide eddy kinetic energy (EKE) steadily increased by 26% with no lag over the first 10 years. No significant trend in EKE was observed during the last 10 years of the simulation suggesting that the WP experiment had reached equilibrium. Variations in eddy compensation along the path of the ACC are also explored.