Atmospheric pCO₂ sensitivity to the biological pump in the ocean

Abstract

Abstract. 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 tropical ocean. Because sea-surface nutrient concentrations are generally high in the high latitudes, pCO 2 sensitivity to high-latitude forcing also determines pCO 2 sensitivity to the biological pump globally. We diagnose highlatitude sensitivity of a range of ocean models using atmospheric pCO 2 above an abiotic ocean; cold high-latitude waters pull abiotic pCO 2 to low values. Box models are very high-latitude sensitive, while most global circulation models are considerably less so, including a twodimensional overturning model, two primitive equation models, the Hamburg class of large scale geostrophic (LSG) general circulation models (GCMs), and the MICOM isopycnic GCM. Highlatitude forcing becomes more important in a depth-coordinate GCM when lateral diffusion is oriented along isopycnal surfaces, rather than horizontally, following Redi [1982]. In two different GCMs (a primitive equation model and LSG), addition of the Gent and McWillams [ 1990] isopycnal thickness diffusion scheme had only minor impact on high-latitude sensitivity. Using a simplified box model, we show that high-latitude sensitivity depends on a high-latitude monopoly on deep water formation. In an attempt to bridge the gap between box models and GCMs, we constructed a simple slab overturning model with an imposed stream function which can be discretized at arbitrary resolution from box model to GCM scale. High-latitude sensitivity is independent of model resolution but very sensitive to vertical diffusion. Diffusion acts to break the high-latitude monopoly, decreasing high-latitude sensitivity. In the isopycnal GCM MICOM, however, high-latitude sensitivity is relatively insensitive to diapycnal diffusion of tracers such as CO2. This would imply that flow pathways in MICOM take the place of vertical diffusion in the slab model. The two nominally most sophisticated ocean models in the comparison are the isopycnal model MICOM and the depth-coordinate GCM with Redi [ 1982] and Gent and McWilliams [1990] mixing. Unfortunately, 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 historically seen the most use in geochemical studies, are even less high-latitude sensitive than MICOM. This discrepancy needs to be resolved. In the meantime, the implication of the MICOM/traditional GCM result would be that box models overestimate high-latitude sensitivity of the real ocean.

Representation of the carbon cycle in box models and GCMs: 1. Solubility pump

“…The standard three-box model in this paper has an abiotic pCO 2 of 211 ppm. (The abiotic pCO 2 of 211 ppm determined here is lower than the 219 ppm figure given for the three-box model by Archer et al [2000] because the three-box model used here has no mixing between the lowand high-latitude surface boxes and no mixing between the low-latitude surface box and the deep box.) The surface to deep TCO 2 difference in this model is 143 mmol/kg, not unlike the result in Figure 2 derived with a 280-ppm atmosphere.…”

“…Bacastow [1996] and Archer et al [2000] argue that this kind of communication unduly favors the partitioning of CO 2 into cold deep water. GCMs, they claim, include circulation and mixing processes that allow the warm surface ocean and cold deep ocean to interact directly.…”

“…One of the box models and one of the GCMs examined in the Bacastow [1996], Broecker et al [1999], and Archer et al [2000] studies are examined here. The three carbon pumps are pulled apart and investigated separately.…”

“…[1] Bacastow [1996], Broecker et al [1999], and Archer et al [2000] have called attention recently to the fact that box models and general circulation models (GCMs) represent the thermal partitioning of CO 2 between the warm surface ocean and cold deep ocean in different ways. They attribute these differences to mixing and circulation effects in GCMs that are not resolved in box models.…”

Historical and idealized climate model experiments: an intercomparison of Earth system models of intermediate complexity

“…There are six soil carbon pools with varying temperature dependence of respiration: microbial, metabolic and structural litter; fast, intermediate, and slow soil. A three-box ocean carbon model including low latitude, high latitude, and deep ocean (Archer et al, 2000) is coupled to the terrestrial component through a fully mixed atmosphere. No ocean biology or sea ice is included in the model.…”