Inverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport
Figure S2: Comparison between the inverse estimates of the air-sea flux of natural CO2 from Gloor et al.  and this study (Pg C yr−1). (374.9Kb)
Figure S4: Meridional pattern of the air-sea flux and meridional transport of natural carbon. (744.1Kb)
Figure S5: The natural carbon air-sea flux and transport integrated with longitude and depth over the Atlantic (left) and Pacific (right) ocean basins. (1.152Mb)
Figure S6: The average of the residuals between the observed gas exchange tracer and the inverse gas exchange tracer estimates (umol/kg) for the 10 participating OGCMs plotted verses their model skill score. (427.4Kb)
Figure S7: Histogram of the residuals between the observed gas exchange tracer and the inverse gas exchange tracer estimates divided by the observed gas exchange tracer concentrations (dimensionless). (17.32Kb)
Figure S8: Comparison between the zonally averaged residuals between the observed and modeled gas exchange tracer in umol/kg (shaded area) and the spatial footprint of the three basis functions in the Southern Ocean. (2.153Mb)
Figure S9: Meridional section of the zonal mean of the temperature (degrees C) and salinity residuals. (777.7Kb)
Figure S10: Sensitivity of the inverse estimates of the natural CO2 flux (Pg C yr−1) to errors in the anthropogenic carbon estimates used in the calculation of the gas exchange tracer. (706.1Kb)
Table S1: Evaluation of model skill based on comparisons between results from natural radiocarbon simulations and observationally based estimates of natural radiocarbon. (337bytes)
Mikaloff Fletcher, Sara E.
Jacobson, Andrew R.
Doney, Scott C.
Follows, Michael J.
Muller, Simon A.
Sarmiento, Jorge L.
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We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO2, i.e., the component of the CO2 flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate ΔC gasex , i.e., the component of the observed DIC that is due to the gas exchange of natural CO2. We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model's simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO2 characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO2 flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed ΔC gasex , and that it is relatively insensitive to the choice of OGCM or potential biases in ΔC gasex . Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr−1, most of which occurs in the Atlantic. This is considerably smaller than the 1 Pg C yr−1 of Northern Hemisphere uptake that has been inferred from atmospheric CO2 observations during the 1980s and 1990s, which supports the hypothesis of a Northern Hemisphere terrestrial sink.
Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 21 (2007): , doi:10.1029/2006GB002751.
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