Effect of Model Resolution on Air-Sea CO2 Equilibration Timescales

Marine Carbon Dioxide Removal (mCDR) will likely play a role in efforts to keep global warming below 2°C. mCDR methods create a deficit in dissolved seawater CO₂ relative to the unperturbed counterfactual. This seawater CO₂ deficit induces either an uptake of atmospheric CO₂ or reduced CO₂ outgassing into the atmosphere. The immediate climatic benefit of mCDR depends on air-sea CO₂ equilibration before the CO₂ depleted seawater deficit in the surface ocean loses contact with the atmosphere through water mass ventilation. Air-sea CO₂ equilibration occurs over vast ocean regions, which are too large to constrain equilibration with current observational methods. As such, numerical modeling is needed to evaluate the spatial and temporal scales of air-sea CO₂ equilibration. This study employs the ACCESS-OM2 model at three resolutions (0.1°, 0.25°, and 1°) to evaluate the dependency of simulated equilibration timescales on model resolution. Results indicate that model resolution has limited influence on equilibration timescales in the tropics but exerts a more significant effect in polar regions. The main reason for the simulated differences is that different resolutions advect CO₂-deficient seawater into different locations (horizontally and vertically) where air-sea exchange can occur at different rates. The comparison of our results with simulations made with other ocean models further suggests that differences due to model resolution are smaller than differences between different models of similar resolutions. Our results are one step forward in evaluating the robustness of model-based assessments of air-sea CO₂ equilibration timescales.