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

We have compared vertical transport of temperature, anthropogenic CO 2 , natural radiocarbon ( 14 C), and bomb 14 C 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 14 C observations to validate ocean models used to estimate anthropogenic CO 2 uptake. Prom the 3‐D model's distributions and fluxes of natural 14 C, bomb 14 C, and anthropogenic CO 2 , we have calculated apparent diffusivities ( K ap ) vertically over the global ocean that range mostly between 4000 and 8000m 2 yr −1 . These K ap agree quantitatively with diffusivities found by fitting B‐D models to observed distributions of natural and bomb 14 C We then used these sets of K ap 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 2 and bomb‐ 14 C penetration depth. Although K ap from 3‐D simulations for bomb 14 C vary with time, those from 3‐D runs for anthropogenic CO 2 are essentially constant. Still, we found nearly the same results with the B‐D model when K ap from 3‐D bomb 14 C simulations are approximated as time invariant. The best agreement (within 3%) between 3‐D CO 2 simulations and B‐D model runs was found when applying K ap derived from bomb 14 C in the surface and from natural 14 C in the deep. Agreement was worse when using K ap from 3‐D simulations for anthropogenic CO 2 itself, mostly because in this case deeper K ap could only be extrapolated from higher surface values. We have found it appropriate to study global oceanic uptake of anthropogenic CO 2 with B‐D model and to validate anthropogenic carbon uptake models using natural and bomb 14 C observations. For bomb 14 C 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 14 C. Thus 14 C seems less than ideal to validate the convective scheme of general circulation models.