The Influence of Natural Climate Variabilty on the Relation Between Air-sea Oxygen and Heat Exchange

Dissolved oxygen (O₂) is a powerful tracer of ocean biogeochemical and physical processes and is tightly linked to the oceanic heat content. As anthropogenic warming reduces gas solubility and ventilation of the ocean’s interior, the oceanic O₂ inventory is expected to decline. Natural variability, however, can mask or modulate this decline, challenging the detection and attribution of ocean deoxygenation and the use of O₂ as a tracer for ocean heat uptake. Continuous global measurements of atmospheric O₂, corrected for terrestrial and anthropogenic influences, show a decadal trend towards anomalous ocean uptake of O₂ since ~2000, though uncertainties remain. This observed trend contradicts the expected enhanced outgassing of O₂ due to increased ocean heat uptake driven by natural variability, the leading explanation for the hiatus in global mean surface warming. The coupling of heat and O₂ fluxes due to natural variability, however, is poorly understood, and reflects complex ocean-atmosphere interactions and internal ocean processes linked to water mass formation. In this study, we investigate how natural variability couples or decouples global and regional heat and O₂ fluxes, focusing on dominant modes of variability (namely SAM, NAO, ENSO, PDO, and AMO), using different configurations of the Community Earth System Model. We also investigate the impacts of volcanic eruptions on air-sea heat and O₂ fluxes and mechanisms driving their coupling. We find that modes of climate variability and volcanic events have unique and significant influence on the interannual to decadal exchange of heat and O₂, driven by region-specific thermal, biological, and dynamic processes. Implications for the hiatus and its imprints on ocean biochemical cycles are discussed.