The Sensitivity of Future Ocean Oxygen to Changes in Ocean Circulation

A decline in global ocean oxygen concentrations has been observed over the twentieth century and is predicted to continue under future climate change. We use a unique modeling framework to understand how the perturbed ocean circulation may influence the rate of ocean deoxygenation in response to a doubling of atmospheric CO2 and associated global warming. These simulations suggest that much of the oxygen decline under warming is due to changes in ocean mixing and O-2 solubility. However, in our model, the large-scale ocean circulation response to CO2 doubling slows the pace of future oxygen loss by 20%. Oxygen concentration changes are most sensitive to circulation perturbations in the Southern Ocean. A small stabilizing effect on oxygen arises from the reduction of export productivity and associated respiration in the ocean interior. A slowdown of the Atlantic Meridional Overturning Circulation increases the residence time of the deep Atlantic Ocean but does not cause a major oxygen decline at the time of CO2 doubling, because respiration is slow at these depths. The simulations show that the decrease in O-2 solubility associated with ocean warming is greater than the realized decrease in preformed O-2, particularly at high latitudes, where circulation changes reduce the proportion of undersaturated waters sinking into the ocean interior. Finally, in the tropical Pacific oxygen minimum zone, a predicted weakening of the Walker Circulation slows the regional upwelling of nutrients and the associated export productivity and respiration, preventing the intensification of hypoxia there. Plain Language Summary The ocean has been losing oxygen over the previous century, a trend that is predicted to continue under global warming and is likely to stress many marine animals. We use numerical models under a climate change scenario to understand the role of shifting ocean currents in this oxygen loss. Much to our surprise, the perturbed ocean circulation stabilizes global ocean oxygen concentrations by 20% relative to a model in which the ocean circulation is artificially kept in its unperturbed state. In other words, global warming causes the ocean to lose oxygen as expected, but shifting ocean currents slow the rate at which this oxygen is lost. This stabilizing effect is realized despite a simulated slowing of the large-scale overturning circulation often referred to as the great ocean conveyor belt. The conveyor belt slowdown does correspond to a slightly slower delivery of oxygenated waters to the deep Atlantic, but respiration is so slow on these layers that the perturbed circulation does little to change oxygen concentrations over the course of our 70-year simulations.