Effects of mesoscale eddies on global ocean distributions of CFC-11, COsub2/sub, and Δsup14/supC

摘要

Global-scale tracer simulations are typically made at coarseresolution without explicitly modelling eddies. Here we ask what roledo eddies play in ocean uptake, storage, and meridional transport oftransient tracers. We made global anthropogenic transient-tracersimulations in coarse-resolution (2°cosφ×2°, ORCA2) and eddy-permitting (?°cosφ×?°, ORCA05) versions of theocean general circulation model OPA9. Our focus is onsurface-to-intermediate waters of the southern extratropics whereair-sea tracer fluxes, tracer storage, and meridional tracer transport arelargest. Eddies have little effect on global and regional bombΔ14C uptake and storage. Yet for anthropogenicCO₂ and CFC-11, refining the horizontal resolution reducedsouthern extratropical uptake by 25% and 28%, respectively. There isa similar decrease in corresponding inventories, which yields betteragreement with observations. With higher resolution, eddiesstrengthen upper ocean vertical stratification and reduce excessiveventilation of intermediate waters by 20% between 60° S and40° S. By weakening the residual circulation, i.e., the sum ofEulerian mean flow and the opposed eddy-induced flow, eddies reducethe supply of tracer-impoverished deep waters to the surface near theAntarctic divergence, thus reducing the air-sea tracer flux. Thus inthe eddy permitting model, surface waters in that region have moretime to equilibrate with the atmosphere before they are transportednorthward and subducted. As a result, the eddy permitting model'sinventories of CFC-11 and anthropogenic CO₂ are lower in thatregion because mixed-layer concentrations of both tracers equilibratewith the atmosphere on relatively short time scales (15 days and 6months, respectively); conversely, bomb Δ14C'sair-sea equilibration time of 6 years is so slow that, even in theeddy permitting model, there is little time for surface concentrationsto equilibrate with the atmosphere, i.e., before surface waters aresubducted.