Natural ocean carbon cycle sensitivity to parameterizations of the recyclingin a climate model

[摘要] Sensitivities of the oceanic biological pump within the GISS (Goddard Institute for Space Studies ) climate modelingsystem are explored here. Results are presented from twin control simulationsof the air–sea CO₂ gas exchange using two different ocean models coupledto the same atmosphere. The two ocean models (Russell ocean model and HybridCoordinate Ocean Model, HYCOM) use different vertical coordinate systems, andtherefore different representations of column physics. Both variants of theGISS climate model are coupled to the same ocean biogeochemistry module (theNASA Ocean Biogeochemistry Model, NOBM), which computes prognosticdistributions for biotic and abiotic fields that influence the air–sea fluxof CO₂ and the deep ocean carbon transport and storage. In particular, themodel differences due to remineralization rate changes are compared todifferences attributed to physical processes modeled differently in the twoocean models such as ventilation, mixing, eddy stirring and verticaladvection. GISSEH(GISSER) is found to underestimate mixed layer depthcompared to observations by about 55% (10%) in the Southern Oceanand overestimate it by about 17% (underestimate by 2%) in thenorthern high latitudes. Everywhere else in the global ocean, the two modelsunderestimate the surface mixing by about 12–34%, which prevents deepnutrients from reaching the surface and promoting primary production there.Consequently, carbon export is reduced because of reduced production at thesurface. Furthermore, carbon export is particularly sensitive toremineralization rate changes in the frontal regions of the subtropical gyresand at the Equator and this sensitivity in the model is much higher than thesensitivity to physical processes such as vertical mixing, vertical advectionand mesoscale eddy transport. At depth, GISSER, which has a significant warmbias, remineralizes nutrients and carbon faster thereby producing more nutrients andcarbon at depth, which eventually resurfaces with the global thermohalinecirculation especially in the Southern Ocean. Because of the reduced primaryproduction and carbon export in GISSEH compared to GISSER, the biologicalpump efficiency, i.e., the ratio of primary production and carbon export at75 m, is half in the GISSEH of that in GISSER, The Southern Ocean emerges asa key region where the CO₂ flux is as sensitive to biologicalparameterizations as it is to physical parameterizations. The fidelity ofocean mixing in the Southern Ocean compared to observations is shown to be agood indicator of the magnitude of the biological pump efficiency regardlessof physical model choice.

[学科分类] 地球化学与岩石