SEASONAL UPTAKE OF ANTHROPOGENIC CO2 IN AN OCEAN GENERAL-CIRCULATION MODEL

A model of the inorganic carbon cycle, embedded within an ocean general circulation model is described, and results from a seasonal simulation of the present-day ocean carbon cycle are presented. Whilst the circulation characteristics of the model appear reasonable, the model gives North Atlantic Deep Water rates that are too low compared with deep water of southern origin, which may result in an overestimate of the southern hemisphere uptake. A comparison of the model results with available observations of dissolved inorganic carbon and partial pressure of carbon dioxide reveals that a defect in the present model is the neglect of ocean biology, which causes the phase of seasonal variations in partial pressure in high latitudes to be incorrectly simulated. A series of experiments were performed in which CO2 was added to the atmosphere at a specific rate to simulate anthropogenic emissions. The model predicts an oceanic uptake of 44% of atmospheric emissions at the end of a 50-year run. The largest sinks in the model occur in the southern hemisphere and equatorial regions. A comparison between parallel runs made under conditions of seasonally varying and annual-mean forcing reveals that the use of seasonal forcing in the present model makes little difference to the estimate of global oceanic uptake of anthropogenic CO2. Varying the background coefficient of vertical diffusion, within commonly accepted limits, is found to have a much greater impact on CO2 uptake estimates.