What explains the interannual variability of O2 content and distribution in the tropical Pacific?

Marine ecosystems and fisheries in the tropical Pacific exhibit substantial year-to-year variability associated with El Niño Southern Oscillation (ENSO). While much of this ecosystem variability has been attributed to ENSO-driven changes in temperature and primary productivity due to changes in nutrient supply, little is known about the role and variability of other key ecosystems drivers such as dissolved oxygen (O₂), a critical component of metabolic activity for predators and lucrative fisheries such as tuna. In this region, models have suffered from large biases in representing the distribution of O₂, while basin scale O₂ observations have long been severely under-sampled.

To address these gaps, Eddebbar et al. (2026) employed high-resolution (~0.1º) and low-resolution (~1º) global model simulations of ocean circulation and biogeochemistry along with a machine learning-based estimate of O₂ derived from BGC Argo floats. Both observations-based and modeling products reveal that El Niño and La Niña events significantly modulate the O₂ content and distribution in the tropical Pacific. During El Niño, the eastern tropical Pacific experiences an excess of O₂, while the western Pacific exhibits less O₂, and vice versa during La Niña events. The ENSO-driven O₂ variability in the eastern tropical Pacific is particularly of interest due to the presence of oxygen minimum zones in this region where O₂ variability is found to be governed by opposing physical and biological drivers. During El Niño, reduced upwelling of low-O₂ waters and weaker O₂ consumption at depth prevail over the reduction in O₂ ventilation by weaker turbulent mixing and suppressed lateral supply by the Equatorial Undercurrent. During La Niña, O₂ ventilation by turbulent mixing and lateral advection is intensified, but enhanced upwelling of low-O₂ nutrient rich waters along with intensified consumption of O₂ at depth due to an increase in surface productivity overcompensate for the ventilation changes, leading to a net reduction of O₂ in the eastern tropical Pacific. Thus while ENSO has a leading order effect on O₂ ventilation in the eastern tropical Pacific, changes in upwelling and biological activity play a more dominant role in setting the O₂ content and distribution in this region.

Future changes in the tropical Pacific O₂ are thus likely to be driven not only by the large-scale reduction on O₂ ventilation from higher latitudes due to ocean warming, but may also be highly sensitive to local changes in equatorial Pacific circulation and productivity associated with coupled ocean-atmosphere interactions.