The Regional Importance of Oxygen Demand and Supply for Historical Ocean Oxygen Trends

Ocean deoxygenation is an emerging hazard for marine ecosystems and a fingerprint of anthropogenic change. Interior ocean oxygen concentrations respond to changes in ventilation that supply oxygen and the demand of biogeochemical processes that consume oxygen. A better understanding of their regional importance would improve confidence in Earth System Model projections, which underpin ecosystem risk and vulnerability assessments. Using a hindcast reanalysis simulation, we find that oxygen trends between 1975 and 2014 in low-oxygen zones along eastern margins are strongly affected by biogeochemical processes that alter oxygen demand, while oxygen-rich regions of the open ocean are driven by ventilation changes. A similar regional distinction emerges among CMIP6 Earth System Models. Therefore, while biogeochemical functioning is an important source of uncertainty in low-latitude, low-oxygen regions, uncertainty in global trends is due to insufficient physical supply changes in Earth System Models, which is confirmed using repeat hydrographic section data in the southern mid-latitudes. Plain Language Summary Measurements of dissolved oxygen (O-2) in the ocean since the 1960s have revealed a multidecadal decrease in ocean O-2 levels that is a major concern. It represents a fingerprint of human-induced climate change and is also an ecological hazard in the highly productive upwelling systems of the lower latitudes that host present-day low-O-2 zones. Unfortunately, Earth System Models that are used to predict future changes fail to reproduce the magnitude of recent deoxygenation and are inconsistent in low-O-2 regions. This undermines our confidence in both reconstructions and future projections that form the basis of decision making. In this work, we use a suite of model simulations and observations over the 1975-2014 period to demonstrate that the biogeochemical response is important in the low-O-2 waters of the tropics and highly productive eastern margins, while physical supply is dominant elsewhere. Reducing our uncertainty in the low-O-2 regions will therefore rely partly on constraining the contribution of biogeochemical processes, while large-scale trends are dependent on how models represent physical processes, which are chronically underestimated by models. Key Points Observed and modeled mesopelagic oxygen trends are driven (85%) by apparent oxygen utilization that integrates oxygen supply and demand Biogeochemical processes shaping oxygen demand are important along eastern margins and some tropical areas Large-scale oxygen trends are driven by changing supply, and weak model trends are linked to misrepresented physical processes

Automated summary

Ocean deoxygenation is a growing concern for marine ecosystems, influenced by human activities. Changes in oxygen concentrations are affected by both oxygen supply and demand, with biogeochemical processes shaping oxygen demand in certain regions while ventilation changes dominate in others. Improved understanding and representation of these processes in Earth System Models is crucial for accurate projections of ecosystem risk and vulnerability.