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Applying the adjoint method for biogeochemical modeling: Export of particulate organic matter in the World Ocean

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Schlitzer, R. (2000): Applying the adjoint method for biogeochemical modeling: Export of particulate organic matter in the World Ocean , Inverse methods in biogeochemical cycles ( P Kasibhata, ed) AGU Monograph 114, pp. 107-124 .
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Abstract:

The oceanic distributions of oxygen, dissolved nutrients and carbonare strongly affected by the production of particulate material nearthe ocean surface and its subsequent remineralization during sinkingor after deposition on the sea-floor. Dissolved nutrient data canthus be used to derive the rate constants of biogeochemicalprocesses responsible for the observed fields using inversemodeling. Here, a global ocean circulation model is presented thatexploits the existing large sets of hydrographic, oxygen, nutrientand carbon data and determines rates of export production andvertical particle fluxes that are compatible with the concentrationdata. The model is fitted to the property concentration data bysystematically varying circulation, air-sea fluxes, production andremineralization rates simultaneously. The adjoint method is appliedas an efficient tool for the iterative optimization procedure andproduces simulated property fields that are in very good agreementwith measurements. The globally integrated export flux ofparticulate organic matter necessary for the realistic reproductionof nutrient observations is significantly larger than exportestimates derived from primary productivity maps. Discrepancies arelargest in oligotrophic, open-ocean areas, where the model exportfluxes are up to a factor 6 higher than values based on primaryproductivity estimates of Berger [1989]. This modelresult is in line with a recent investigation in the subtropicalNorth Pacific that also revealed high open-ocean fluxes. Modelexport production values are in much closer agreement with estimatesbased on satellite pigment data in most regions except in theSouthern Ocean, where the satellite derived export fluxes seem tounderestimate the carbon export significantly.

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