Effects of eddies on global distributions of transient tracers

Global-scale tracer simulations are typically made at coarse resolution without explicitly modeling eddies. Here we ask what role do eddies play in ocean uptake, storage, and meridional transport of transient tracers. We made global anthropogenic transient-tracer simulations in non-eddying (2 • cos ϕ×2 • , ORCA2) and eddying (1 2 • cos ϕ× 1 2 • , 5 ORCA05) versions of the ocean general circulation model OPA9. We focus on the Southern Ocean where tracer air-sea fluxes are largest. Eddies have little effect on global and regional bomb ∆ 14 C uptake and storage. Yet for anthropogenic CO 2 and CFC-11, increased eddy activity reduces southern extratropical uptake by 28% and 25% respectively. There is a similar decrease in corresponding inventories, which pro-10 vides better agreement with observations. With higher resolution, eddies strengthen upper ocean vertical stratification and reduce excessive ventilation of intermediate waters by 20% between 60 • S and 40 • S. By weakening the Residual Circulation, i.e., the sum of Eulerian mean flow and the opposed eddy-induced flow, eddies reduce the supply of tracer-impoverished deep waters to the surface near the Antarctic divergence, 15 thus reducing the air-sea tracer flux. Consequently, inventories for both CFC-11 and anthropogenic CO 2 decrease because their mixed layer concentrations in that region equilibrate with the atmosphere on relatively short time scales (15 days and 6 months, respectively); conversely, the slow air-sea equilibration of bomb ∆ 14 C of 6 years, gives surface waters little time to exchange with the atmosphere before they are subducted.