Constraining a North Atlantic Ocean general circulation model with chlorofluorocarbon observations

Measurements of anthropogenic tracers such as chlorofluorocarbons and triti um must be quantitatively combined with ocean general circulation models as a component of systematic model development. The authors have developed an d tested an inverse method, using a Green's function, to constrain general circulation models with transient tracer data. Using this method chlorofluo rocarbon-11 and -12 (CFC-11 and -12) observations are combined with a North Atlantic configuration of the Miami Isopycnic Coordinate Ocean Model with 4/3 degrees resolution. Systematic differences can be seen between the observed CFC concentrations and prior CFC fields simulated by the model. These differences are reduced by the inversion, which determines the optimal gas transfer across the air- sea interface, accounting for uncertainties in the tracer observations. Aft er including the effects of unresolved variability in the CFC fields, the m odel is found to be inconsistent with the observations because the model/da ta misfit slightly exceeds the error estimates. By excluding observations i n waters ventilated north of the Greenland-Scotland ridge (sigma (0) < 27.8 2 kg m(-3); shallower than about 2000 m), the fit is improved, indicating t hat the Nordic overflows are poorly represented in the model. Some systemat ic differences in the model/data residuals remain and are related, in part, to excessively deep model ventilation near Rockall and deficient ventilati on in the main thermocline of the eastern subtropical gyre. Nevertheless, t here do not appear to be gross errors in the basin-scale model circulation. Analysis of the CFC inventory using the constrained model suggests that th e North Atlantic Ocean shallower than about 2000 m was near 20% saturated i n the mid-1990s. Overall, this basin is a sink to 22% of the total atmosphe re-to-ocean CFC-11 flux-twice the global average value. The average water m ass formation rates over the CFC transient are 7.0 and 6.0 Sv (Sv = 10(6) m (3) s(-1)) for subtropical mode water and subpolar mode water, respectively .