Projected Centennial Oxygen Trends and Their Attribution to Distinct Ocean Climate Forcings

Abstract

We explore centennial changes in tropical Pacific oxygen (O2) using numerical models to illustrate the dominant patterns and mechanisms under centennial climate change. Future projections from state-of-the-art Earth System Models exhibit significant model to model differences, but decreased solubility and weakened ventilation together deplete thermocline O2 in middle to high latitudes. In contrast, the tropical thermocline O2 undergoes much smaller changes or even a slight increase. A suite of sensitivity experiments using a coarse resolution ocean circulation and biogeochemistry model show that ocean warming is the leading cause of global deoxygenation in the thermocline across all latitudes with secondary contributions from changes in hydrological cycles and wind stress modulating regional changes in O2. The small O2 changes in the tropical Pacific thermocline reflect near-complete compensation between the solubility decrease due to warming and reduction in apparent oxygen utilization (AOU). We further quantified the changes in AOU due to contributions from changes in water mass age and biological remineralization from the sensitivity experiments. The two effects almost equally contribute to the reduction of AOU in the tropical Pacific thermocline (43% for physical circulations and 57% for biology). Our results suggest that better understanding of water mass changes in the tropical oceans is key to improving projections and reducing the uncertainties of future O2 changes.