Isopycnal mixing by mesoscale eddies significantly impacts oceanic anthropogenic carbon uptake

Anthropogenic carbon dioxide uptake varies across Earth System Models for reasons that have remained obscure. When varied within a single model, the lateral eddy mixing coefficient A(Redi) produces a range of uptake similar to the modeled range. The highest uptake, resulting from a simulation with a constant A(Redi) of 2400m(2)/s, simulates 15% more historical carbon uptake than a model with A(Redi)=400m(2)/s. A sudden doubling in carbon dioxide produces a 21% range in carbon uptake across the models. Two spatially dependent representations of A(Redi) produce uptake that lies in the middle of the range of constant values despite predicting very large values in the subtropical gyres. One-dimensional diffusive models of the type used for integrated assessments can be fit to the simulations, with A(Redi) accounting for a substantial fraction of the effective vertical diffusion. Such models, however, mask significant regional changes in stratification and biological carbon storage.