On models of bomb C-14 in the North Atlantic

Coarse resolution general circulation ocean models show a tendency to accumulate an excess of bomb C-14 On the eastern margin of the subtropical gyres, while observations suggest an excess is found in the west. Here simulations of the bomb C-14 transient in the North Atlantic Ocean are made using coarse resolution (1.0 degrees X 1.2 degrees) and ''eddy-permitting'' (1/3 degrees x 2/5 degrees) tracer transport models. The former employs large horizontal diffusivities to parameterize eddy transfer. Both models employ the same air-sea exchange parameterization, with a specified tropospheric history of Delta(14)C. The coarse resolution model accumulates C-14 evenly over the whole subtropical gyre, while the eddy-permitting model has a pronounced maximum column inventory in the western part, consisent with the observed patterns. The prescence of explicit eddies enhances the ventilation of density surfaces in the recirculation zone on the western margin of the gyre, suggesting that the zonal gradient hinted at in the observations is intimately tied to properties of geostrophic eddy dynamics which are not captured parametrically in the coarse resolution model.