Use of a simple model for studying oceanic tracer distributions and the global carbon cycle

We have studied, based on work of Shaffer and Sarmiento (1992), a model for simulating the transport of CO₂ and tracers in the ocean (HILDA, for High-Latitude Exchange/Interior Diffusion-Advection Model) that combines features of box models and of the box-diffusion model. It is latitudinally divided into two zones; in the low latitudes, transport into the deep ocean occurs by eddy diffusion, while the high-latitude zone consists of two boxes (surface and deep ocean). We compare different ways of calibration and find that in order to reproduce the distributions of natural ¹⁴C as well as of bomb-produced ¹⁴C, the vertical eddy diffusivity K must decrease with depth. The concept of eddy diffusion is discussed by calculating apparent eddy diffusivities from 3-D model tracer simulations. The depth dependence of K is qualitatively confirmed by these calculations, reflecting the fact that the water circulation is more vigorous near the surface than at depth. We find that eddy diffusivities derived from ¹⁴C are not appropriate for representing the large-scale vertical temperature distribution, because the latitudinal distribution of temperature differs in a systematic way from that of ¹⁴C and also of anthropogenic CO₂. Oceanic uptake of anthropogenic CO₂, biospheric CO₂ emissions and isotopic perturbations are calculated, based on the observed atmospheric CO₂concentration history. The results indicate an oceanic uptake of 1.9 Gt C yr^-1 in 1980 and a near-zero net contribution from the biota in the past several decades. The HILDA model is compared with other models, and we find that its response to atmospheric CO₂ perturbations is rather similar to that of a 3-D ocean carbon cycle model of Sarmiento et al. (1992).