Award: OCE-1948281

Objectives: This project aimed at improving our understanding of how dissolved oxygen (O2) structure and variability in the tropical Pacific is shaped by regional-to-basin scale circulation. Specifically, we explored the following questions:  

1. How does mixing by eddies (i.e. coherent vortices) and turbulence influence the distribution and seasonal cycle of O2 in this region? 

2. What physical, biological and biogeochemical mechanisms explain these effects?

3. What controls the year-to-year variability of O2 in the tropical Pacific?

To address these questions, we developed and analyzed a range of modeling and observational products including global high resolution simulations of the Community Earth System Model, a state estimate of the tropical Pacific Ocean, and observational estimates of O2 variability based on BGC Argo floats. 

Research Outcomes 

We found that eddies play a critical role in oxygenating the upper equatorial Pacific from summer through winter (Figure 1). This oxygenation occurs primarily through the injection of oxygenated waters into the ocean interior through the 3-D eddy circulation, deepening the depth of hypoxia and expanding ecosystem habitats. These findings were summarized into a publication titled “Seasonal Modulation of Dissolved Oxygen in the Equatorial Pacific by Tropical Instability Vortices” in the Journal of Geophysical Research: Oceans. 

Next, we found that O2  supply in the equatorial Pacific interior is driven both by lateral transport by the equatorial current system as well as by mixing by turbulence, with the latter dominating in the upper ocean (50-150m), while the latter dominates deeper (Figure 2). We found that the passage of eddies amplify this turbulent mixing of O2, highlighting the importance of representing these regional circulation features in Earth system models. These findings were summarized in a study titled “Eddy-Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific” published in the Journal of Geophysical Research Oceans. 

We also learned that El Niño Southern Oscillation (ENSO) exerts a strong control on the interannual variability of the O2 content and structure throughout the tropical Pacific basin (Figure 3). El Niño conditions were found to lead to higher O2 content in the eastern Pacific despite reduced O2 supply by mixing and the equatorial current system due to large compensating contributions from vertical  transport and reduced microbial consumption of O2. These findings were summarized into a manuscript titled “ENSO-driven Variability of Oxygen Content and Distribution in the Tropical Pacific” to be submitted shortly to Journal of Climate. 

The modeling activities supported by this project led to the development of a regional ocean state estimate of the tropical Pacific and a 60-year global high resolution simulation of CESM. These activities also contributed to a multi-institution study on the representation of observed O2 trends and distributions in models titled “Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models,“ and the editing of a special issue on Ocean Deoxygenation, both in Frontiers in Marine Science. This project also supported the investigation of how eddies influence oxygen in the Southern Ocean, summarized in a study titled “Effects of Mesoscale Eddies on Southern Ocean Biogeochemistry” published in AGU Advances. Findings from this project were broadly disseminated to the public through numerous conferences, seminars, and workshop presentations. 

Broader Impacts:

The model simulations and process understanding developed under this project contributed to improving monitoring of ocean oxygen and informing the renovation of the tropical Pacific Observing System (TPOS). The PIs contributed to this end through contributing to the “TPOS Final Report”, organizing TPOS workshops, editing a US CLIVAR variation issue on “Needs and prospects for advancing tropical Pacific observations of the ocean and atmosphere”, leading a US CLIVAR review article on “Upwelling and Mixing in the Equatorial Pacific Cold Tongue: Biogeochemical Implications, Dynamics, and Observing Needs”, and contributing to a study “Toward an integrated pantropical ocean observing system” in Frontiers in Marine Science.

The PIs also collaborated with local teachers on the development of two 12 week-long lesson plans for 7th and 8th grade classes centered on the oxygen and carbon cycles and their coupling to ocean and climate. These lesson plans were disseminated at the California Association of Science Educators conference and the California Science Teachers Association conference for broad adoption across the state. This project also supported the mentorship of three summer undergraduate research students and the career development of an associate scientist at NSF NCAR, which advanced their data and scientific analysis skills, and supported their transition to PhD programs. The project also supported the mentorship of 2 PhD graduate students who developed parts of their PhD research using the scientific and modeling tools developed in this project. Finally, this project contributed substantially to the intellectual independence and career development of the lead PI, an early career scientist who led the submission of this grant as a postdoc, and leveraged this project’s tools and scientific inquiry to develop his research agenda and build a research group.

Last Modified: 06/30/2025
Modified by: Aneesh C Subramanian