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

Models and observations of atmospheric potential oxygen (APO similar or equal to O-2 + 1.1 * CO2) are used to investigate the influence of El Nino-Southern Oscillation (ENSO) on air-sea O-2 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 Nino3.4 index, indicating anomalous ocean outgassing of APO during El Nino. 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, O-2 accounts for most APO flux variations. Detailed analysis in CESM shows that the O-2 response is driven primarily by ENSO modulation of the source and rate of equatorial upwelling, which moderates the intensity of O-2 uptake due to vertical transport of low-O-2 waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally driven O-2 exchange. During El Nino, shallower and weaker upwelling leads to anomalous O-2 outgassing, whereas deeper and intensified upwelling during La Nina drives enhanced O-2 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 O-2 cycle to climate variability that is significantly (> 50%) underestimated in magnitude by ocean models.