OCEAN CARBON TRANSPORT IN A BOX-DIFFUSION VERSUS A GENERAL-CIRCULATION MODEL

We have compared vertical transport of temperature, anthropogenic CO2, natural radiocarbon (C-14), and bomb C-14 in a global box-diffusion m odel (B-D) and a three-dimensional (3-D) ocean general circulation mod el from the Geophysical Fluid Dynamics Laboratory. Our main objectives were (1) to test the eddy diffusion parameterization of large-scale v ertical transport in ocean box models and (2) to assess the utility of bomb-produced and natural C-14 observations to validate ocean models used to estimate anthropogenic CO2 uptake. From the 3-D model's distri butions and fluxes of natural C-14, bomb C-14, and anthropogenic CO2, we have calculated apparent diffusivities (K-ap) vertically over the g lobal ocean that range mostly between 4000 and 8000 m(2) yr(-1). These K-ap agree quantitatively with diffusivities found by fitting B-D mod els to observed distributions of natural and bomb C-14. We then used t hese sets of K-ap in different runs of a global B-D model. Results fro m all B-D models runs matched to within 13% those from the 3-D model f or global uptake of anthropogenic CO2 and bomb-C-14 penetration depth. Although K-ap from 3-D simulations for bomb C-14 vary with time, thos e from 3-D runs for anthropogenic CO2 are essentially constant. Still, we found nearly the same results with the B-D model when K-ap from 3- D bomb C-14 simulations are approximated as time invariant. The best a greement (within 3%) between 3-D CO2 simulations and B-E) model runs w as found when applying K-ap derived from bomb C-14 in the surface and from natural C-14 in the deep. Agreement was worse when using K-ap fro m 3-D simulations for anthropogenic CO2 itself, mostly because in this case deeper K-ap could only be extrapolated from higher surface value s. We have found it appropriate to study global oceanic uptake of anth ropogenic CO2 with B-D model and to validate anthropogenic carbon upta ke models using natural and bomb C-14 observations. For bomb C-14 in t he 3-D model, convective transport was most important during 1955-1964 while atmospheric levels were rising; afterward, atmospheric levels d rop, and advective overturning dominates as for natural C-14. Thus C-1 4 seems less than ideal to validate the convective scheme of general c irculation models.