TESTING GLOBAL OCEAN CARBON-CYCLE MODELS USING MEASUREMENTS OF ATMOSPHERIC O-2 AND CO2 CONCENTRATION

We present a method for testing the performance of global ocean carbon cycle models using measurements of atmospheric O-2 and CO2 concentrat ion. We combine these measurements to define a tracer, atmospheric pot ential oxygen (APO approximate to O-2 + CO2), which is conservative wi th respect to terrestrial photosynthesis and respiration. We then comp are observations of APO to the simulations of an atmospheric transport model which uses ocean-model air-sea fluxes and fossil fuel combustio n estimates as lower boundary conditions. We present observations of t he annual-average concentrations of CO2, O-2, and APO at 10 stations i n a north-south transect. The observations of APO show a significant i nterhemispheric gradient decreasing towards the north. We use air-sea CO2, O-2, and N-2 fluxes from the Princeton ocean biogeochemistry mode l, the Hamburg model of the ocean carbon cycle, and the Lawrence Liver more ocean biogeochemistry model to drive the TM2 atmospheric transpor t model. The latitudinal variations in annual-average APO predicted by the combined models are distinctly different from the observations. A ll three models significantly underestimate the interhemispheric diffe rence in APO, suggesting that they underestimate the net southward tra nsport of the sum of O-2 and CO2 in the oceans. Uncertainties in the m odel-observation comparisons include uncertainties associated with the atmospheric measurements, the atmospheric transport model, and the ph ysical and biological components of the ocean models. Potential defici encies in the physical components of the ocean models, which have prev iously been suggested as causes for anomalously large heat fluxes out of the Southern Ocean, may contribute to the discrepancies with the AP O observations. These deficiencies include the inadequate parameteriza tion of subgrid-scale isopycnal eddy mixing, a lack of subgrid-scale v ertical convection, too much Antarctic sea-ice formation, and an overe stimation of vertical diffusivities in the main thermocline.