Atmospheric pCO(2) sensitivity to the biological pump in the ocean

In models of the global carbon cycle, the pCO(2) of the atmosphere is more sensitive to the chemistry of the high-latitude surface ocean than the trop ical ocean. Because sea-surface nutrient concentrations are generally high in the high latitudes, pCO(2) sensitivity to high-latitude forcing also det ermines pCO(2) sensitivity to the biological pump globally. We diagnose hig h-latitude sensitivity of a range of ocean models using atmospheric pCO(2) above an abiotic ocean; cold high-latitude waters pull abiotic pCO(2) to lo w values. Box models are very high-latitude sensitive, while most global ci rculation models are considerably less so, including a two-dimensional over turning model, two primitive equation models, the Hamburg class of large sc ale geostrophic (LSG) general circulation models (GCMs), and the MICOM isop ycnic GCM. High-latitude forcing becomes more important in a depth-coordina te GCM when lateral diffusion is oriented along isopycnal surfaces, rather than horizontally, following Redi [1982]. In two different GCMs (a primitiv e equation model and LSG), addition of the Gent and McWillams [1990] isopyc nal thickness diffusion scheme had only minor impact on high-latitude sensi tivity. Using a simplified box model, we show that high-latitude sensitivit y depends on a high-latitude monopoly on deep water formation. In an attemp t to bridge the gap between box models and GCMs, we constructed a simple sl ab overturning model with an imposed stream function which can be discretiz ed at arbitrary resolution from box model to GCM scale. High-latitude sensi tivity is independent of model resolution but very sensitive to vertical di ffusion. Diffusion acts to break the high-latitude monopoly, decreasing hig h-latitude sensitivity. In the isopycnal GCM MICOM, however, high-latitude sensitivity is relatively insensitive to diapycnal diffusion of tracers suc h as CO,. This would imply that flow pathways in MICOM take the place of ve rtical diffusion in the slab model. The two nominally most sophisticated oc ean models in the comparison are the isopycnal model MICOM and the depth-co ordinate GCM with Redi [1982] and Gent cmd McWilliams [1990] mixing. Unfort unately, these two models disagree in their abiotic CO2 behavior; the depth -coordinate isopycnal mixing GCM is high-latitude sensitive, in accord with box models, while MICOM is less so. The rest of the GCMs, which have histo rically seen the most use in geochemical studies, are even less high-latitu de sensitive than MICOM. This discrepancy needs to be resolved. In the mean time, the implication of the MICOM/traditional GCM result would be that box models overestimate high-latitude sensitivity of the real ocean. This woul d eliminate iron dust fertilization of the ocean as an explanation for the glacial pCO(2) range of 180-200 mu atm [Archer et nl., 2000].