Upper Ocean Control on the Solubility Pump Of
Total Page:16
File Type:pdf, Size:1020Kb
Journalof MarineResearch, 61, 465–489, 2003 Upper ocean controlon the solubility pump of CO 2 byTakamitsu Ito 1 andMichael J. Follows 1 ABSTRACT Wedevelop and test a theoryfor therelationship of atmospheric pCO2 andthe solubility pump of CO2 inan abioticocean. The solubility pump depends on thehydrographic structure of theocean and thedegreeof saturation of thewaters. The depth of thermoclinesets the relative volume of warmand coldwaters, which sets the mean solubility of CO 2 intheocean. The degree of saturation depends on thesurfaceresidence time of the waters. Wedevelop a theorydescribing how atmosphericCO 2 varieswith diapycnal diffusivity and wind stressin asimple,coupled atmosphere-ocean carbon cycle, which builds on established thermocline theory.We consider two limit cases for thermocline circulation: the diffusive thermocline and the ventilatedthermocline. In the limit of a purelydiffusive thermocline (no wind-driven gyres), 1 /3 atmospheric pCO2 increasesin proportionto the depth of thermoclinewhich scales as k , where k isthe diapycnal mixing rate coef cient. In the wind-driven, ventilated thermocline limit, the 1 / 2 ventilatedthermocline theory suggests the thickness of thethermocline varies as we k . Moreover, surfaceresidence times are shorter, and subducted waters are undersaturated. The degree of undersaturationis proportional to the Ekman pumping rate, we k ,formoderate amplitudes of we k . 3 / 2 Hence,atmospheric pCO2 varies as we k formoderate ranges of surface wind stress. Numerical experimentswith an ocean circulation and abiotic carbon cycle model con rm these limit case scalingsand illustrate their combined effect. The numerical experiments suggest that plausible variationsin thewind forcing and diapycnal diffusivity could lead to changesin atmospheric pCO2 ofasmuchas 30ppmv.The deep ocean carbon reservoir is insensitiveto changesin thewind, due to compensationbetween the degree of saturation and the equilibrium carbon concentration. Conse- quently,the sensitivity of atmospheric pCO2 towind-stressforcing is dominated by thechanges in theupper ocean, in direct contrast to the sensitivity to surface properties, such as temperature and alkalinity,which is controlled by thedeep ocean reservoir. 1.Introduction Thepartitioning of carbonbetween oceanic and atmospheric reservoirs and its depen- denceon thecirculation and climate of theoceans has been studied extensively due to the likelyrole of theoceans in modulatingatmospheric CO 2 onglacial-interglacialtimescales (reviewedby Sigmanand Boyle,2000) andthe current oceanic sink of fossilfuel CO 2 from theatmosphere (e.g. Gruber et al., 1996).While it seems clear that the oceans play a signicant role with regard to bothof theseissues, much remains to beunderstood. 1.Program in Atmospheres, Oceans andClimate, Massachusetts Instituteof Technology, Cambridge, Massachusetts 02139,U.S.A. email:[email protected] 465 466 Journalof MarineResearch [61, 4 Theinventory of carbons in the ocean is largely controlled by two important mecha- nisms:the solubility and biological pumps (Volk and Hoffert, 1985).The solubility pump re ectsthe temperature dependence of solubilityof CO 2 andthermal strati cationof the ocean.Cold deep waters aregenerally enriched in dissolvedinorganic carbon (hereafter, C) inpartdue to theirhigher solubility. For areviewof oceancarbonate system, see Stumm andMorgan (1996). The biological pump involves production and dissolution of organic molecules.Photosynthesis occurs in the euphotic layer, converting inorganic carbon and nutrientsto organic matter. Some organic matter leaves the euphotic layer as sinking particles,and it isremineralized in the oceaninterior. Both solubility and biological pumps tendto increase the deep water C concentration,creating a verticalgradient of C. Boxmodel studies in the 1980 ’ssupportedthe notion of adominantrole for thehigh latitudeoceans in controllingatmospheric pCO2 (Sarmientoand Toggweiler, 1984; Knox andMcElroy, 1984; Siegenthaler and Wenk, 1984), largely through the biological pump. Recentstudies with three-dimensional ocean circulation and biogeochemistry models (Bacastow,1995; Broecker et al., 1999;Archer et al., 2000)suggest that atmospheric pCO2 alsoshows a greatersensitivity to low latitude surface ocean properties than had previouslybeen accepted. Archer et al. (2000)suggest that this might be due to the parameterizationsof subgrid-scalemixing and diapycnal mixing in the ocean. Toggweiler et al. (2003)found that the sensitivity of atmospheric CO 2 tolow latitude surface propertiesin box models depends on the representation of deep water formation, and showedthat the boxmodelscan be sensitiveto low latitudes by an adjustment in theareaof highlatitude surface ocean. Alternatively, Follows et al. (2002),using an idealized general circulationand abiotic carbon cycle model (Fig. 1), showed that the wind-driven circula- tionenhances the sensitivity of atmospheric pCO2 tolow latitude surface properties by creatinga poolof relativelywarm waters intheventilated thermocline which inherit their propertiesfrom themid-latitude surface. The model, which conserves the total amount of carbonin ocean and atmosphere, also illustrated a strongsensitivity of atmospheric pCO2 tothepresence or absence of wind forcing. InFigure 1, we illustrateresults from thatnumerical model, details of which may be foundin Follows et al. (2002).Brie y,theexperiments are performedwith the MIT ocean model con gured in a 60° longitudeby 90 ° latitudebasin at 3 ° 3 3° resolution,with 15 verticallevels. It isoverlainby anabioticcarbon cycle model which is coupledto asimple atmosphericreservoir of CO 2.Carbonatechemistry is explicitly solved and the air-sea exchangeof CO 2 isparameterizedwith a uniformgas transfercoef cient.In Follows et al. (2002),we compareexperiments with thermohaline forcing only to those with both thermohalineand wind forcing. The meridional overturning circulation and thermal structureof the model in these two cases is shown in Figure 2. The model resolves the ventilatedthermocline reasonably well. When wind forcing is present, the ocean gyres spin upandthe warm lensof thesubtropical thermocline develops. Though atmospheric pCO2 increasedwith the introduction of the wind forcing, the mean ocean temperature (and therefore,solubility) did not change signi cantly.The increase in atmospheric CO 2 was 2003] Ito& Follows:Solubility pump of CO 2 467 Figure1. Abiotic sector model of ocean-atmosphere carbon cycle: (a) Surface currents and differencein partial pressure of CO 2 acrossthe sea surface in the sector model without wind forcing.(b) Surfacecurrents and difference in partial pressure of CO 2 acrossthe seasurface in the sectormodel with strong wind forcing. (c) Difference in zonally averaged C concentration (mol m2 3 )betweenthe model with strong wind forcing and the model with no windforcing. attributableto thewaters ofthesubtropical thermocline becoming signi cantlyundersatu- rated.The surface currents are slow in the model without wind-forcing (Fig. 1a), the residencetime of water parcelsat the surface is long, and the surface ocean is close to equilibriumwith the atmosphere. In the wind-driven model the surface currents are swift (Fig.1b), the air-sea equilibration time scale and the residence time of surface waters are comparable.Waters of theventilated thermocline are signi cantlyundersaturated due to thestrong cooling in thewestern boundary current. Since the thermocline is depletedin C, bymass conservation,carbon must be transferredto theatmosphere. Figure 1c illustrates thedifference in C intheupper ocean between the two cases. Thesenumerical experiments suggested a potentiallysigni cantrole for theventilated thermoclinein modulating atmospheric CO 2 ondecadal and longer timescales, but motivatesthe broad questions: What are the physical controls on the solubility pump of carbonin the oceans? Can we providea generaltheory of itsdependence on wind-stress 468 Journalof MarineResearch [61, 4 Figure2. Meridional overturning circulation (Sv) and potential temperature ( °C)forthe sector model(a, c) without wind forcing and (b, d) with strong wind forcing (maximum wind-stress 0.2 N m22 ). forcingand diapycnal mixing rates? How mightthe solubility pump respond to climatic change,through changes to these physical properties? Inthis study we seekto understand how the physical structures and processes of the oceancontrol the solubility pump of CO 2.For simplicityand clarity we focuson an abiotic ocean.We developscalings which relate the storage of carbon in the ocean and, thus, atmospheric pCO2 todiapycnal mixing and Ekman pumping rates. We demonstratethat thesescalings accurately predict the behavior of an idealized, three-dimensional, ocean- atmospherecarbon cycle model (Follows et al., 2002).Over areasonablerange of diapycnaldiffusivity and wind-stress, atmospheric pCO2 variesby as much as 30ppmv. Thissuggests a possiblysigni cantrole for thewind-driven gyres in modulating atmo- spheric CO2 ondecadaltimescales, and even with regard to glacial-interglacialchange. It alsoprovides the basis for adeeperunderstanding of the differences between more complexnumerical simulations of theoceans. InSection 2 we reviewthe mass balanceof carbon in a simpli ed,abiotic ocean- atmospheresystem. In Section 3 we brie yreviewsome relevant aspects of thetheory of 2003] Ito& Follows:Solubility pump of CO 2 469 thermoclinethickness. We outlinethe scaling arguments which relate atmospheric pCO2, diapycnalmixing and Ekman pumping in Section