Ocean carbon transport in a box-diffusion versus a general circulation model

International audienceWe have compared vertical transport of temperature, anthropogenic CO:, natural radiocarbon (14C), and bomb 14C in a global box-diffusion model (B-D) and a three-dimensional (3-D) ocean general circulation model from the Geophysical Fluid Dynamics Laboratory. Our main objectives were (1) to test the eddy diffusion parameterization of large-scale vertical transport in ocean box models and (2) to assess the utility of bomb-produced and natural 14C observations to validate ocean models used to estimate anthropogenic CO: uptake. From the 14 14 3-D model's distributions and fluxes of natural C, bomb C, and anthropogenic CO:, we have calculated apparent diffusivities (Kap) vertically over the global ocean that range mostly between 4000 and 8000 m: yr-1. These Kap agree quantitatively with diffusivities found by fitting B-D models to observed distributions of natural and bomb 14C. We then used these sets of Kap in different runs of a global B-D model. Results from all B-D models runs matched to within 13% those from the 3-D model for global uptake of anthropogenic CO: and bomb-14C penetration depth. Although Kap from 3-D simulations for bomb 14C vary with time, those from 3-D runs for anthropogenic CO: are essentially constant. Still, we found nearly the same results with the B-D model when Kap from 3-D bomb 14C simulations are approximated as time invariant. The best agreement (within 3%) between 3-D CO: simulations and B-D model runs was found when applying Kap derived from bomb 14C in the surface and from natural 14C in the deep. Agreement was worse when using Kap from 3-D simulations for anthropogenic CO: itself, mostly because in this case deeper Kap could only be extrapolated from higher surface values. We have found it appropriate to study global oceanic uptake of anthropogenic CO: with B-D model and to validate anthropogenic carbon uptake models using natural and bomb 14C observations. For bomb 14C in the 3-D model, convective transport was most important during 1955-1964 while atmospheric levels were rising; afterward, atmospheric levels drop, and advective overturning dominates as for natural 14C. Thus 14C seems less than ideal to validate the convective scheme of general circulation models