Uncertainty in future open-ocean deoxygenation reflects the imbalance between ventilation and remineralization

Classic box model studies show that once preformed nutrient is determined, the oxygen deficit relative to saturation (i.e. Apparent Oxygen Utilization, AOU) is also set, independent of the rates of ocean ventilation or export production. The balance between ocean vertical exchange and the export and remineralization of organic matter facilitate this remarkable stability. Under the global warming scenario, however, this balance may be violated, at least regionally. Increased stratification is thought to cause a deoxygenation of subsurface waters due to the weakened ocean ventilation and reduced O₂ supply. We analyze the results from the CMIP-5 and a suite of sensitivity experiments using an ocean biogeochemistry model to quantify the competing effects of ventilation and remineralization using the thermocline O₂ as a metric. We perform a suite of sensitivity experiments systematically changing vertical tracer diffusivity and evaluate its impact on the mean state as well as the deoxygenation under a warming climate. We separate simulated changes in AOU into two parts: a component driven by ventilation and another component driven by remineralization. We find that the two components generally compensate with one another, qualitatively consistent with the box model. However, there are significant regional imbalances that control the evolution of the thermocline AOU. One of the prominent regions of this imbalance is the tropical oxygen minimum zone. Highly diffusive thermocline is more susceptible to thermally driven deoxygenation due to stronger heat uptake. AOU change plays more important roles under a low diffusivity. In our calculation, the ventilation strength as well as upwelling nutrients supply and surface nutrient limitation status control the ventilation-remineralization imbalance. Predicting future deoxygenation in tropical thermocline is highly uncertain as it likely reflects relatively small residual among multiple physical and biogeochemical processes.