Impacts of ENSO on air-sea oxygen exchange: Observations and mechanisms

Models and observations of atmospheric potential oxygen (APO ≃ O₂ + 1.1 * CO₂) are used to investigate the influence of El Niño–Southern Oscillation (ENSO) on air-sea O₂ exchange. An atmospheric transport inversion of APO data from the Scripps flask network shows significant interannual variability in tropical APO fluxes that is positively correlated with the Niño3.4 index, indicating anomalous ocean outgassing of APO during El Niño. Hindcast simulations of the Community Earth System Model (CESM) and the Institut Pierre-Simon Laplace model show similar APO sensitivity to ENSO, differing from the Geophysical Fluid Dynamics Laboratory model, which shows an opposite APO response. In all models, O₂ accounts for most APO flux variations. Detailed analysis in CESM shows that the O₂ response is driven primarily by ENSO modulation of the source and rate of equatorial upwelling, which moderates the intensity of O₂ uptake due to vertical transport of low-O₂ waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally driven O₂ exchange. During El Niño, shallower and weaker upwelling leads to anomalous O₂ outgassing, whereas deeper and intensified upwelling during La Niña drives enhanced O₂ uptake. This response is strongly localized along the central and eastern equatorial Pacific, leading to an equatorial zonal dipole in atmospheric anomalies of APO. This dipole is further intensified by ENSO-related changes in winds, reconciling apparently conflicting APO observations in the tropical Pacific. These findings suggest a substantial and complex response of the oceanic O₂ cycle to climate variability that is significantly (>50%) underestimated in magnitude by ocean models.