Ocean Science www.ocean-sci.net 1812-0784 1812-0792 3 4 2007 10.5194/os-3-461-2007 http://www.ocean-sci.net/3/461/2007/ http://www.ocean-sci.net/3/461/2007/os-3-461-2007.html http://www.ocean-sci.net/3/461/2007/os-3-461-2007.pdf 461 482 2007-10-12 Effects of mesoscale eddies on global ocean distributions of CFC-11, CO₂, and Δ¹⁴C Z. Lachkar zouhair.lachkar@env.ethz.ch J. C. Orr J.-C. Dutay P. Delecluse LSCE/IPSL, Laboratoire des Sciences du Climat et de l'Environnement, CEA-CNRS-UVSQ, Gif-sur-Yvette, France Marine Environment Laboratories, International Atomic Energy Agency, Monaco now at: Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Zurich, Switzerland now at: Centre National de Recherches Météorologiques, Meteo-France, Paris, France Global-scale tracer simulations are typically made at coarse resolution without explicitly modelling 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 coarse-resolution (2°cosφ×2°, ORCA2) and eddy-permitting (½°cosφ×½°, ORCA05) versions of the ocean general circulation model OPA9. Our focus is on surface-to-intermediate waters of the southern extratropics where air-sea tracer fluxes, tracer storage, and meridional tracer transport are largest. Eddies have little effect on global and regional bomb Δ¹⁴C uptake and storage. Yet for anthropogenic CO₂ and CFC-11, refining the horizontal resolution reduced southern extratropical uptake by 25% and 28%, respectively. There is a similar decrease in corresponding inventories, which yields 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, thus reducing the air-sea tracer flux. Thus in the eddy permitting model, surface waters in that region have more time to equilibrate with the atmosphere before they are transported northward and subducted. As a result, the eddy permitting model's inventories of CFC-11 and anthropogenic CO₂ are lower in that region because mixed-layer concentrations of both tracers equilibrate with the atmosphere on relatively short time scales (15 days and 6 months, respectively); conversely, bomb Δ¹⁴C's air-sea equilibration time of 6 years is so slow that, even in the eddy permitting model, there is little time for surface concentrations to equilibrate with the atmosphere, i.e., before surface waters are subducted. 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