13C constraints on ocean carbon cycle models

Abstract

The sensitivity of oceanic δ13C fields to overturning and gas exchange is investigated in a suite of ocean general circulation models. The deep and oceanic mean δ13C in the models was sensitive to the balance between deep waters forming in the North Atlantic and the Southern Ocean. Increasing the Southern Ocean deep water formation rate to improve deep sea 14C and AOU fields was detrimental to model-data δ13C fidelity. A concurrent increase in North Atlantic Deep water would be needed to match the observed 14C and δ13C, constraining both the rate and schematics of model deep water formation, respectively, and improving sensitivity to future perturbations. Inter-basin trends in δ13C were sensitive to the rate of overturning in the models, with high mixing model configurations matching the observations best. Models' anthropogenic δ13C changes, used as a diagnostic of model CO2 uptake, were in agreement with the observations, except at high southern latitudes (<50S), where the model δ13C changes were greater than observed. There were predictive relationships among models' uptake of anthropogenic CO2 and depth-integrated δ13C changes. Model relationships between model anthropogenic CO2 uptake and the air-sea δ13C disequilibrium, and the sea surface δ13C, depend on preindustrial riverine fluxes of terrestrial organic carbon, and on the wind field used to drive the model circulation, respectively. Among the models tested, the relations among anthropogenic CO2 uptake and δ13C changes in the ocean are biased by the OCMIP practice of driving model momentum with one wind field, and gas exchange rates with another. © 2012 by the American Geophysical Union.