On evaluating ocean models with atmospheric potential oxygen

We used observed and simulated atmospheric potential oxygen (APO) to evaluate simulated air-sea flux fields from 11 ocean global carbon cycle models. APO is defined in terms of atmospheric CO2, O-2 and N-2 so as not to depend on terrestrial photosynthesis and respiration. Hence, it is in principal suited to evaluate simulated air-sea fluxes of these gases. We forced two different atmospheric transport models, TM2 and TM3, with simulated air-sea fluxes from each of the 11 ocean models, and we compared resulting simulated latitudinal and seasonal variations in APO with observations. Differences between the two atmospheric transport models, which offer a first estimate of uncertainty due to atmospheric transport, are similar in magnitude to the average model-data differences and to the spread between the ocean models. Simulated annual mean meridional APO profiles qualitatively resemble the observations, although at individual stations there remain substantial differences between models and observations. The simulated amplitude of the seasonal APO variability was generally less than observed. We conclude that it is difficult to validate ocean models based on APO because shortcomings in atmospheric transport models and problems with data representativity cannot be distinguished from ocean model deficiencies.