Research

The coupling between ocean biogeochemical and physical dynamics sets the large scale structure of ocean biogeochemical tracers and marine ecosystem habitats from diurnal to multi-decadal timescales. The oxygen minimum zones (OMZ) of the tropical Pacific, for instance, are shaped by microbial consumption and poor ventilation at depth. The intense tropical Pacific flux of carbon – the largest oceanic source of carbon into the atmosphere – is set by equatorial upwelling of carbon-rich thermocline waters to the surface. This upwelling also entrains nutrients and feeds plankton growth along the equatorial Pacific cold tongue, sustaining highly biodiverse ecosystems and productive fisheries. Changes in ocean circulation driven by the seasons and climate modes (e.g. El Niño Southern Oscillation) regulate the variability of these biogeochemical cycles, e.g. initiating Arctic plankton blooms and controlling the intensity of carbon outgassing in the tropical Pacific. This coupling also dictates the severity of climate change impacts on marine ecosystems, as ocean warming and increased stratification modulate the intensity and spatial patterns of the OMZ, carbon flux, and primary productivity. Using models and observations, my research aims to unravel the mechanisms governing these interactions from sub-seasonal to multidecadal timescales, and from the mesoscale to global scales.

1. Ocean Biogeochemical Dynamics at the Mesoscale

A major gap in our understanding of ocean biogeochemical-physical interactions concerns the role of mesoscale (10-100km) circulation in setting O2 distributions and ventilation, particularly in the eastern tropical Pacific where direct O2 supply by the subtropical gyres is suppressed. I am currently leading an NSF-funded project with colleagues at CU Boulder, Scripps, and NCAR to investigate how mesoscale eddies and the equatorial zonal jets regulate the structure of the OMZs, O2 supply, and variability in the upper equatorial Pacific using a hierarchy of models of different resolutions and hydrographic observations. A recent finding from this work is that tropical instability vortices (TIVs), which dominate the eddy kinetic energy field in this region, oxygenate the upper equatorial Pacific, modulating the vertical structure and seasonal variability of the tropical Pacific OMZs (Eddebbar et al. 2021). We are currently exploring the impacts of mesoscale circulation on the equatorial Pacific O2 balance, supply pathways, and biogeochemical feedbacks associated with nutrient transport and changes in productivity, all of which remain very poorly represented by coarse climate models.

More reading: Eddebbar et al. 2021 | Frenger et al 2018 | Resplandy et al 2018

Collaborators: Aneesh Subramanian (CU Boulder), Ariane Verdy (Scripps), Dan Whitt (NASA Ames), Matthew Long (NCAR), Mark Merrifield (Scripps)

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2. Internal variability and predictability of ocean biogeochemistry

The ocean biogeochemical response to anthropogenic forcing can be blurred by “unforced” natural climate variability. Oceanic O2 and carbon can vary naturally at interannual-to-multidecadal timescales due to the long memory imparted by ocean circulation and climate variability, making it difficult to distinguish between anthropogenic and internally generated trends. I am interested in quantifying the response of the oceanic O2 and carbon cycles to various sources of climate variability, and have focused recently on tropical processes, including El Niño Southern Oscillation (ENSO), the dominant climate mode. Using observations from the Scripps atmospheric O2 and CO2 networks and a reanalysis-forced simulation of CESM, my research showed that El Niño events induce a net outgassing of O2, driven by a large-scale redistribution of O2 associated with reduced upwelling in the equatorial Pacific. This work pointed to a substantial underestimate in the O2 cycle sensitivity to climate variability in models, which is likely driven by poor representation of mesoscale eddies and equatorial circulation in models. On-going work aims at exploring these unforced interactions focusing on decadal timescales and the impacts of high latitude modes on O2 ventilation and carbon transport. The ocean biogeochemical response to climate modes has major implications for informing the near-term predictability of marine ecosystems and extreme events in the ocean on timescales that are relevant to conservation and fisheries management.

More reading: Eddebbar et al. 2017 | Ito and Deutsch 2013 | Duteil et al 2018 | Oschlies et al 2018

Collaborators: Matthew Long (NCAR), Ralph Keeling (Scripps), Laure Resplandy (Princeton), Christian Rodenbeck (Max Planck Institute – Jena).

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3. Forced drivers of climate and biogeochemical variability

In addition to “unforced” variability induced by climate modes (e.g. ENSO), external drivers, e.g. volcanic eruptions, induce major changes in climate with poorly known effects on ocean biogeochemistry, and may serve as useful analogs for the unintended consequences of geoengineering on ocean ecosystems. Using the CESM Large Ensemble (LENS) framework, we showed a major signature of volcanic aerosol forcing on the oceanic O2 cycle, outlined by a substantial uptake of O2 at high latitudes that counteracts the effects of ocean deoxygenation due to anthropogenic warming (Eddebbar et al. 2019). This work further outlined an El Niño-like ocean circulation response to volcanic radiative cooling, and isolated — for the first time — a “forced” signal in the interannual-to-decadal variability of carbon fluxes through suppressed equatorial upwelling. I continue to explore these volcanic effects in more detail through improved simulations of external drivers of climate, including geoengineering, and recent advances in the LENS framework which will enables us to explore i) better estimates of “time of emergence” of anthropogenic impacts, ii) improved attribution and mechanistic understanding of anthropogenic, volcanic, vs. internally generated variability, and iii) greater statistical power in evaluating projected changes in extreme events.