Global Eddy‐Driven Transport Estimated From In Situ Lagrangian Observations

This project will examine fluid transport by long-lived coherent mesoscale eddies in the global ocean, including volumes within their coherent cores, transiently trapped fluids in eddy peripheries, and stirring effects in the ambient water-masses due to eddy influence therein. The project would rely on a novel eddy-identifying analysis technique (developed in prior work by the PIs) applied to in-situ measurements from global surface drifter dataset and the historical set of acoustically-tracked subsurface floats. This is a departure from the usual approach of eddy detection in gridded satellite products, relying instead on the adaptation of signal processing techniques to float trajectory data. Prior studies based on such gridded products significantly underestimate numbers of eddies, and overestimate eddy sizes and transport of water trapped within them. Data analysis will be supplemented by theoretical idealized and realistic numerical modeling. This work will address what observed ubiquitous coherent eddies actually accomplish in terms of their effect on the large-scale flow. This is a question of societal importance because of its relevance for the development of accurate subgrid-scale parameterizations for general circulation models. The project will advance the boundaries of the viable use of Lagrangian data, and thus provide new tools for eddy examination to the community. The project will support and inform free online courses in fundamental and advanced oceanographic data analysis, so that that these state-of-the-art methodologies will be broadly accessible to the next generation of researchers. The project supports an early career latino scientist, who will develop an undergraduate-level teaching module related to this project.

This project will produce a definitive study on the role of coherent eddies in driving fluid transport, taking significant eddy detections from in situ Lagrangian observations as the starting point. The detection method, called vortex signal extraction, recovers time-varying oscillatory signal components from Lagrangian trajectories, without a requirement for the oscillations to be strictly periodic. Available data include approximately 24,000 global surface drifter trajectories plus another 3,000 subsurface trajectories from an historical set of eddy-resolving floats, both NOAA datasets. Data analysis will be complemented by idealized and ultra-high-resolution realistic modeling. These components will be used to explore the subtleties of observing the eddy field from the Lagrangian perspective, to examine the theoretical properties of the eddy detection methods, and to investigate the dynamics of the transport processes of interest. The project will proceed in three branches: (i) dynamics of direct and indirect eddy-driven transport, (ii) the vortex observability problem, and (iii) global estimates. Anticipated products will be new global estimates of coherent eddy properties, populations, and boundaries through statistical modeling informed by an improved understanding of the physics of long-term and transitory trapping. The proposed work will further provide a calibration process by which remotely-sensed features can be more accurately mapped onto fluid structures, and a hydrographic analysis will convert areal transport estimates into mass transports.