CHLOROFLUOROCARBON UPTAKE IN A WORLD OCEAN MODEL .1. SENSITIVITY TO THE SURFACE GAS FORCING

The uptake and redistribution of chlorofluorocarbons (CFCs) CFC-11 and CFC-12 are studied in a series of world, ocean model experiments. In part 1 of this study the sensitivity of the simulated CFC distribution s to the model parameterization of air-sea CFC fluxes is examined with in a control experiment. The control experiment represents a low-resol ution ocean model with global coverage and a proper seasonal cycling i n surface thermohaline and wind stress conditions. The specification o f a surface ocean CFC concentration that is instantaneously in saturat ed equilibrium with the atmosphere is found to flux too much CFC into the model. Signatures of CFC-11 are found to be grossly overestimated in regions of deep and bottom water formation, both in the surface mix ed layer and at depth. The use of a classical air-sea gas exchange for mula (even one with a simplified gas transfer velocity that is indepen dent of wind speed) is seen to greatly improve the CFC simulations at depth. In addition, the model reproduces many of the observed trends i n surface CFC concentrations; namely, undersaturation in regions of de ep convective overturn and near-surface upwelling and supersaturation in the summer mixed layer. In further sensitivity experiments, we cons ider the effect of sea ice cover in limiting air-sea gas exchange in p olar waters. It is found that bottom water in the Arctic Ocean and aro und the Antarctic continent is significantly reduced in CFC content on ce regions covered with sea ice are limited to fractional air-sea gas exchange. This more physically meaningful framework is found to furthe r reduce the spurious uptake of CFC-11 and CFC-12 found under a ''satu rated surface'' assumption. In a final sensitivity experiment the gas exchange rate is parameterized using a complete wind speed and Schmidt number dependence. The wind speed dependent gas forcing increases the surface CFC equilibration rate under the subpolar westerlies. On the other hand, the polar and tropical oceans witness reduced CFC uptake u nder a wind speed dependent flux regime. Simulated ocean CFC concentra tions are compared directly with observational data in certain key are as for deep and bottom water formation. It is found that a reasonable representation of oceanic CFC is achieved in the convected water colum n in the Weddell and Labrador Seas. In contrast, deep waters that have left the convective area with the model ocean currents are found to b e deficient in CFC-11 in the North Atlantic Ocean. This is because the model advective timescale for North Atlantic Deep Water (NADW) outflo w across the equator is too long compared with observed ocean estimate s. The long timescale is not due to unrealistically sluggish deep curr ents. Rather, the path of NADW outflow includes a loop eastward from t he Labrador Sea into the Northeastern Atlantic Basin, effectively incr easing the required outflow journey by around 4000 km. This ages the w ater mass by almost 10 years, thereby yielding significantly lower CFC concentrations in the NADW extension. In addition, the outflow signat ure spreads too far into the eastern North Atlantic, presumably becaus e the advective process is too broad and the horizontal diffusion too strong at depth. Contrasting the North Atlantic, bottom water CFC vent ilation in the Southern Ocean is found to be too strong, even when sig nificant levels of surface undersaturation are simulated in polar wate rs. CFC-tagged waters flowing into the deep South Atlantic basin (from the Weddell Sea formation zone) are too enriched in CFC11, even when the deep signatures adjacent to the Antarctic shelf remain close to ob servations. This suggests that the advective timescale for bottom wate r ventilation is too rapid in the Southern Ocean. In addition, too muc h convective overturn persists in the Southern Ocean at 55 degrees S-7 0 degrees S, with unrealistically deep CFC-11 penetration noted at par ticular longitudes. This is because not enough older (CFC-deprived) wa ter recirculates and upwells into the Southern Ocean. For example, mor e upwelled circumpolar deep water in the Southern Ocean would weaken t he CFC-11 concentrations by contributing to a lower CFC mixture and by suppressing the convective activity in the region. Bottom and deep le vel CFC signatures are broad and diffuse compared with the real ocean. The broadness of the CFC imprint is due, in part, to the model resolu tion, which gives any convective event a spatial extent of at least 3. 75 degrees longitude by 4.5 degrees latitude and a bottom level CFC si gnal thickness in excess of 800 m. An important finding of our study i s that the vertical convection of unstable waters acts as the efficien t tracer ventilator of the ocean system. This has significant implicat ions for numerical studies of the world's climate, since the meridiona l overturning has traditionally been considered the reason for the oce an's moderating influence during global warming scenarios. Our study s uggests that the vertical convection would play a much greater role ov er the typical timescale for anthropogenic climate change.