Variations in Ocean Deoxygenation Across Earth System Models: Isolating the Role of Parameterized Lateral Mixing

Modern Earth system models (ESMs) disagree on the impacts of anthropogenic global warming on the distribution of oxygen and associated low‐oxygen waters. A sensitivity study using the GFDL CM2Mc model points to the representation of lateral mesoscale eddy transport as a potentially important factor in such disagreement. Because mesoscale eddies are smaller than the spatial scale of ESM ocean grids, their impact must be parameterized using a lateral mixing coefficient A REDI . The value of A REDI varies across modern ESMs and nonlinearly impacts oxygen distributions. This study shows that an increase in atmospheric CO 2 results in a decline in productivity and a decrease in ventilation age in the tropics, increasing oxygen concentrations in the upper thermocline. In high latitudes global warming causes shallowing of deep convection, reducing the supply of oxygen to the deep. The net impact of these processes depends on A REDI , with an increase in hypoxic volume yet smaller total deoxygenation in the low‐mixing models, but a decrease in hypoxic volume yet larger total deoxygenation in the high‐mixing models. All models show decreases in suboxic volume, which are largest in the low‐mixing models. A subset of Coupled Model Intercomparison Project Phase 5 models exhibits a similar range of responses to global warming and similar decoupling between total deoxygenation and change in hypoxic volume. Uncertainty in lateral mixing remains an important contributor to uncertainty in projecting ocean deoxygenation.