Oxygen in the Tropical Pacific responds to El Niño Forcing

El Niño and La Niña phases in the tropical Pacific Ocean are the major drivers of interannual variability in the region. This includes the oxygen (O₂) content of upper ocean waters, with El Niño resulting in higher O₂ in the eastern tropical Pacific due to a deepening of the thermocline and reduced upwelling of O₂-poor deepwater and lower O₂ in the west due to a large-scale shallowing of the thermocline; La Niña phases display the opposite pattern. New research led by Scripps Institution of Oceanography—with contributions from the University of Washington CICOES and NOAA's Pacific Marine Environmental Lab, along with NASA Ames, the University of Colorado Boulder, and UC Santa Barbara—leveraged two global ocean–sea ice simulations along with an observation-based data product, built using machine learning and in situ observational data including Argo floats, to examine variability in tropical Pacific oxygen content associated with El Niño–Southern Oscillation (ENSO).

The research demonstrated similar relationships between ENSO phase and O₂ content across both models and the observation-based data product. The authors explored the influence of model resolution on their conclusions, finding little difference in the amplitude of oxygen content changes between the high-resolution (0.1°) and low-resolution (~1°) models and the observational estimate, although regional differences in the relationship do arise (see figure). Broadly, the work shows that ENSO-driven oxygen content variability in the tropical Pacific is primarily controlled by vertical advective processes and changes in biological consumption, allowing for O₂ content to increase in the eastern tropical Pacific during El Nino despite a significant reduction in oxygen ventilation due to weaker mixing and lateral supply by the equatorial currents. Finally, the authors point out a positive relationship between heat and oxygen content in the tropical Pacific, opposite in sign to the well-established solubility-driven negative correlation between heat and oxygen content on a global scale.

The balance between competing processes driving oxygen content variability in the tropical Pacific has implications for the coming decades, as changes in local productivity and circulation will combine with variations in O₂ supply from the mid- and high-latitudes to impact biological oxygen availability. ENSO is key for predictability on interannual timescales, and understanding of the ENSO-related processes driving O₂ content in the tropical Pacific Ocean can inform future projections of ecosystem conditions for critically important fisheries in the region.