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Saturation and compensation: Southern Ocean circulation in CMIP5 models

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We use thirteen climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5) to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. The circulation responses of these coarse resolution coupled models to surface buoyancy fluxes are compared with the theoretical wind-based predictions of eddy saturation (where the ACC transport is insensitive to increased surface wind stress) and eddy compensation (where eddies balance the increased wind-driven Eulerian overturning circulation). The change in the eddy induced overturning in both depth and density space is quantified. Under a future CMIP5 climate pathway the models robustly project reduced Southern Ocean density in the upper 2000~m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress, the projected ACC transports lie within 15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models project a strong increase in the surface heat and freshwater fluxes that weaken the ACC transport at the end of the 21st century. In contrast, the surface buoyancy flux changes in the northern half of the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian mean meridional overturning circulation.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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