Award: OCE-1948718

Objective: This project aimed at improving our understanding of how dissolved oxygen (O2) structure and variability in the tropical Pacific is shaped by eddy (10-100km) circulation and large-scale phenomena associated with El Niño Southern Oscillation (ENSO). We developed and analyzed a range of modeling and observational tools and products including a high resolution global (1/10º) and coarse (1º) configuration of the NSF NCAR Community Earth System Model (CESM), a state estimate of the tropical Pacific Ocean coupled to biogeochemistry (B-TPOSE), Lagrangian particle tracking simulations, and machine learning-based estimates of O2 variability from BGC Argo floats.

Research Outcomes We found that eddies play a critical role in oxygenating the northern upper equatorial Pacific from summer through winter when they form and propagate towards the west. This oxygenation occurs primarily through the injection of oxygenated lawyers through the 3-D eddy circulation, deepening the depth of hypoxia and expanding ecosystem habitats from summer through winter. 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 identified the main sources of O2 supply in the equatorial Pacific, focusing on the role of regional processes such as eddies and the equatorial current system. We found that O2  supply here is driven both by lateral transport by the equatorial current system as well as by mixing by turbulence that arise from the high shear induced by the equatorial currents. We also 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 into a publication titled “Eddy-Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific” in the Journal of Geophysical Research Oceans. 

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

These findings were broadly disseminated to the public through numerous oral and poster presentations including several invited and keynote talks. The modeling activities supported by this project led to the development of a regional ocean state estimate of the tropical Pacific, and a long-term simulation of a high resolution simulation of CESM. These activities also contributed to a multi-institution study on the representation of observed O2 trends and distributions in Earth system models titled “Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models,“ and a special issue on Ocean Deoxygenation in Frontier in Marine Science. 

Broader Impacts Outcomes This project contributed to three key broad impacts: 
Informing observing network and modeling The model simulations, analysis, and process understanding of ocean biogeochemical and physical variability at regional scales developed under this project was key in informing key phenomena, parameters, and critical scales of biogeochemical variability towards the recent renovation of the tropical Pacific Observing System (TPOS). Specifically, the PIs contributed significantly in advancing BGC observing needs through TPOS workshop leadership, authorship of the “TPOS Final Report”, the editorial of a US CLIVAR variation issue on “Needs and prospects for advancing tropical Pacific observations of the ocean and atmosphere”, a US CLIVAR review on “Upwelling and Mixing in the Equatorial Pacific Cold Tongue: Biogeochemical Implications, Dynamics, and Observing Needs”, and a study “Toward an integrated pantropical ocean observing system.”
Advancing education and training:The PIs also collaborated with local teachers on the development of two 12-week long lesson plans for 7th and 8th grade classes on the oxygen and carbon cycles, which 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 and summer research of three undergraduate students that led to key development of their data analysis and scientific investigative skills, and 2 PhD graduate students who developed parts of their thesis using the scientific and modeling tools developed in this project.   
Career and Professional Development: 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 to develop his research agenda and transition into a research faculty position with a growing research group.

Last Modified: 07/07/2025
Modified by: Matthew Long