CGD's Dr. Keith Lindsay

Abstract

A global ocean biogeochemical model is used to quantify the sensitivity of marine biogeochemistry and air-sea CO₂ exchange to variations in dust deposition over decadal timescales. Estimates of dust deposition generated under four climate states provide a large range in total deposition with spatially realistic patterns; transient ocean model experiments are conducted by applying a step-function change in deposition from a current climate control. Relative to current conditions, higher dust deposition increases diatom and export production, nitrogen fixation and oceanic net CO₂ uptake from the atmosphere, while reduced dust deposition has the opposite effects. Over timescales less than a decade, dust modulation of marine productivity and export is dominated by direct effects in high-nutrient, low-chlorophyll regions, where iron is the primary limiting nutrient. On longer timescales, an indirect nitrogen fixation pathway has increased importance, significantly amplifying the ocean biogeochemical response. Because dust iron input decouples carbon cycling from subsurface macronutrient supply, the ratio of the change in net ocean CO₂ uptake to change in export flux is large, 0.45-0.6. Decreasing dust deposition and reduced oceanic CO₂ uptake over the next century could provide a positive feedback to global warming, distinct from feedbacks associated with changes in stratification and circulation.

Figure caption: Sea-air CO₂ flux is plotted versus sinking particulate organic carbon export (POC) relative to Current era dust control simulations for each dust-forcing scenario. For the Future and Last Glacial Maximum simulations, results are shown with and without nitrogen fixation, in each case relative to the corresponding Current era dust control.

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Mikaloff Fletcher, S. E., Gruber, N., Jacobson, A. R., Gloor, M., Doney, S. C., Dutkiewicz, S., Gerber, M., Follows, M., Joos, F., Lindsay, K., Menemenlis, D., Mouchet, A., Muller, S. A., Sarmiento, J. L., 2007: Inverse estimates of the oceanic sources and sinks of natural CO₂ and the implied oceanic carbon transport, Global Biogeochemical Cycles, Vol. 21, GB1010.

Abstract

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.

Figure caption: Global map of the transport (shown above or below arrows) of natural CO₂ (Pg C yr^-1) based on the inverse flux estimates (bold). The values shown are the weighted mean estimates and their weighted standard deviation. The transport estimates include only that component of DIC that reflects exchange with the atmosphere. The width of the arrows are only qualitatively proportional to the transports.

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Najjar, R. G., Jin, X., Louanchi, F., Aumont, O., Caldeira, K., Doney, S. C., Dutay, J.-C., Follows, M., Gruber, N., Joos, F., Lindsay, K., Maier-Reimer, E., Matear, R. J., Matsumoto, K., Monfray, P., Mouchet, A., Orr, J. C., Plattner, G.-K., Sarmiento, J. L., Schlitzer, R., Slater, R. D., Weirig, M.-F., Yamanaka, Y., Yool, A., 2007: Impact of circulation on export production, dissolved organic matter, and dissolved oxygen in the ocean: Results from Phase II of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2), Global Biogeochemical Cycles, Vol. 21, GB3007.

Abstract

Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1σ) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr^-1. Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

Figure caption: Relationship between global particle export and (a) radiocarbon content of Circumpolar Deep Water (CDW), (b) radiocarbon content of North Pacific Deep Water (NPDW), (c) global CFC-11 inventory in 1994, and (d) mean anthropogenic CO₂ uptake in the 1990s. Model radiocarbon is from Matsumoto et al. [2004], model CFC uptake is from Dutay et al. [2002], and model CO₂ uptake is from Watson and Orr [2003]. The bars show observation-based estimates: CO₂ uptake is from Mikaloff Fletcher et al. [2006], CFC inventory is from Willey et al. [2004], radiocarbon is from Matsumoto et al. [2004], and particle export encompasses the range of satellite-based [Gnanadesikan et al., 2004] and inverse [Schlitzer, 2002] estimates.