Effect of horizontal grid resolution on simulations of oceanic CFC-11 uptake and direct injection of anthropogenic CO2

Abstract

We simulate direct injection of CO2 and uptake of CFC-11 in a global, three-dimensional ocean general circulation model using two model resolutions: a coarse resolution of 4° in longitude by 2° in latitude and a finer resolution of 1° in both longitude and latitude. We assess the impact of resolution on the relative effectiveness of ocean carbon sequestration for four different injection sites: New York at 710 and 3025 m depths and San Francisco at 710 and 3025 m depths. Results show that deep injection is generally effective, with relatively small differences in retention, transport, and fluxes between the two resolutions. Results for the change in ocean pH due to CO2 injection show that resolution does limit the details at sufficiently small scales, with the finer resolution showing greater maximum pH changes. Model predictions of CFC-11 uptake generally have shallower penetration than is seen in observations, and the differences between the model resolutions are much smaller than the differences between either simulation and the observations. There is no persuasive evidence of improvement of large-scale results with globally higher horizontal resolution in these non-eddy-resolving simulations to justify the computational expense. However, when local details are the primary interest, the use of higher resolution may be justified. We suggest that the best approach to improving the results of coarse-resolution ocean models is not to globally increase horizontal resolution outside of the eddy-resolving regime, but rather to pursue other approaches such as improved numerical methods, better parameterizations of sub-grid-scale processes, better forcing data, or perhaps local resolution increases.