Major Contribution of Reduced Upper Ocean Oxygen Mixing to Global Ocean Deoxygenation in an Earth System Model

We present a quantitative analysis of deoxygenation drivers applied to an Earth System Model and easily transposable to large model ensembles. The preindustrial ocean breathes in oxygen in polar regions and in subtropical gyres, and breathes out oxygen in the equatorial band and in subpolar gyres. Under a high-CO₂ emission scenario for the 21st century, small deviations of these large natural oxygen fluxes cause global deoxygenation. We attribute half of this trend to a decrease in oxygen solubility. The other half is explained by negative trends in subduction and respiration, which largely cancel out each other. Moreover, 75% of the subduction decrease occurs through changes in mixing across the mixed-layer base. Our analysis also highlights strong modulations of subduction at the regional scale linked to shifts in wind patterns and associated Ekman pumping.