The Sensitivity of Future Ocean Oxygen to Changes in Ocean Circulation

Abstract

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 CO 2 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 CO 2 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 CO 2 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.