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

Abstract

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.

Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition

“…. More complex coupling is also possible on interannual time-scales 29 , such as the weaker lagged air-sea CO 2 flux compared to O 2 during El Nino events 30 . www.nature.com/scientificreports www.nature.com/scientificreports/ Atmospheric O 2 and CO 2 measurements have previously been applied to estimate global land and ocean CO 2 sinks, but relied on ocean heat content estimates and model-based oceanic O 2 -to-heat ratios to correct for climate-driven O 2 outgassing [31][32][33] .…”

In situ measurements of atmospheric O2 and CO2 reveal an unexpected O2 signal over the tropical Atlantic Ocean

“…More recently, the long O 2 and CO 2 data set of Tohjima et al [2015] over the western Pacific Ocean showed that the amplitude of the western Pacific equatorial APO bulge in this region was anticorrelated to El Niño-Southern Oscillation (ENSO), with a suppressed APO bulge over the western Pacific during El Niño conditions, which is mostly attributed to changes in atmospheric circulation associated with meridional shifts in the ITCZ (Intertropical Convergence Zone). The results of Tohjima et al [2015] are contradictory to those of a previous modeling study by Rödenbeck et al [2008], who found a significant positive correlation between APO anomalies over the tropical Pacific Ocean and ENSO variability; however, this discrepancy has recently been reconciled by Eddebbar et al [2017], who showed that the response of APO to ENSO variability differs significantly between the western and central/eastern Pacific. Model simulations suggest that an APO bulge of comparable shape and magnitude should exist over the Atlantic ], but these predictions have never been verified with observational data until now.…”

“…They also found, however, that up to~25% of the equatorial annual mean APO interannual variability might be caused by changes in equatorial APO outgassing fluxes in the western Pacific, which are also associated with the ENSO cycle. The modeling study by Rödenbeck et al [2008] also found a significant correlation between ENSO variability and APO anomalies, albeit in the opposite direction to that found by Tohjima et al [2015], and more recently, Eddebbar et al [2017] showed that the response of APO to ENSO variability differs between the western and central/eastern Pacific and confirmed that the suppression of the APO bulge over the western Pacific is mainly dominated by the weakening of trade winds.…”

Global carbon budgets estimated from atmospheric O<sub>2</sub>∕N<sub>2</sub> and CO<sub>2</sub> observations in the western Pacific region over a 15-year period

“…Conducting atmospheric inversion analyses based on the APO data from the Scripps observation network, Rödenbeck et al (2008) suggested anomalous outgassing of APO from the equatorial region during El Niño periods, while Tohjima et al (2015) found a suppressed equatorial peak during El Niño periods based on the western Pacific observations. Eddebbar et al (2017) reconciled these conflicting results by predicting the existence of a zonal dipole-like El Niño-Southern Oscillation (ENSO) response in the equatorial Pacific based on several ocean process-based models and an atmospheric transport model. These results suggest that enhanced zonal coverage of the atmospheric observations in the equatorial Pacific is needed to constrain the full basin-scale ENSO response.…”