|Title||Impacts of ENSO on air-sea oxygen exchange: Observations and mechanisms|
|Publication Type||Journal Article|
|Year of Publication||2017|
|Authors||Eddebbar YA, Long MC, Resplandy L, Rödenbeck C, Rodgers KB, Manizza M, Keeling RF|
|Journal||Global Biogeochemical Cycles|
|Keywords||0414 Biogeochemical cycles, processes, and modeling; 1616 Climate variability; 4215 Climate and interannual variability; 4273 Physical and biogeochemical interactions; 4805 Biogeochemical cycles, processes, and modeling; air-sea O2 flux; Climate variability; El Niño–Southern Oscillation (ENSO); ocean biogeochemical dynamics; ocean deoxygenation; oxygen cycle|
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.
The impacts of ENSO on air-sea O2 exchange are significant and complex, involving interactions between biogeochemical and physical processes. The atmospheric inversion and ocean model simulations presented here indicate that in an anomalous sense, the upper ocean loses O2 to the atmosphere during El Niño and gains O2 during La Niña. In CESM, these anomalies are driven by significant modulation of the O2 content in the upper equatorial Pacific by coupled ocean-atmosphere dynamics. During El Niño, the deepening of thermocline waters in the eastern equatorial Pacific and the weakening of upwelling lead to significant reductions in the ventilation of O2-deficient waters, driving anomalous O2 outgassing. El Niño is also associated with diminished biological productivity and net ocean heat loss, which drives anomalous uptake of O2. Conversely, during La Niña, intensified upwelling of shoaling thermocline waters strongly reinvigorates the ventilation of low-O2 waters while weakly enhancing the biological production and thermal outgassing of O2.