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 CO2 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. Plain Language Summary Global warming is expected to change the amount of oxygen present in the oceans, both at the surface and at depth. Some of this is driven by oceanic warming due to an increase in atmospheric temperatures, as warmer water holds less oxygen. But changes in biological drawdown play an even bigger role. Because warmer water is less dense, it becomes more difficult for heavier deep waters rich in nutrients to surface. This reduces biological productivity and in turn means that less organic matter sinks into the deep ocean and rots. Because decomposition uses oxygen, a decrease in decomposition will result in more oxygen at depth. But as oxygen-rich surface waters get lighter, they do not flow as easily into the deep ocean and deliver oxygen to these depths. This causes oxygen to fall. The balance between these three processes determines whether increasing fractions of the deep ocean will become inhospitable to fish and other organisms. The Earth System Models used to project the impacts of climate change predict different balances between these processes. We demonstrate that an important reason for this disagreement is uncertainty in how to represent mixing associated with the oceanic storms known as mesoscale eddies.