Cuddy Decay: Observation of Subthermocline Eddy Spindown and Property Exchange

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This is a project to study the decay of a subthermocline eddy as it travels from near its generation site in the California Undercurrent (CU) over the continental slope into offshore waters. The CU, a prominent element of the California Current System (CCS), transports relatively warm, saline, anoxic Pacific Equatorial Water north along the North American west coast. Temperature and salinity generally decrease both poleward and offshore, while dissolved oxygen increases, as the CU spins off long lived eddies transporting its water properties away from the CCS into the eastern Pacific. These eddies, nicknamed cuddies for their origin, have been tracked by solitary floats for many months and surveyed hydrographically on occasion, but their structural evolution as they decay has yet to be observed. They take the form of submesoscale coherent vorticies and are largely invisible to remote sensing techniques due both to their small size and subsurface intensification. The extent to which cuddies contribute to broad scale property gradients in the CCS and beyond is tied to the evolution of their radial-vertical structure.This study will use a pair of extended-range Seagliders to frequently survey a cuddy over many months as it travels. A cuddy will be identified and located for study by an ongoing Seaglider repeat transect, separately supported by NOAA and maintained by University of Washington colleagues. With improvements to performance efficiency, Seagliders are projected to be capable of operating as long as a year while transmitting slanting profiles of upper-ocean structure ashore in near real time. Simultaneous transects at 4-6 km resolution along nearly perpendicular paths repeated every week or less over the course of ten months appear feasible, with an additional two months of mission time devoted to reaching from and returning to the coastal region for launch and recovery. The sequence of approximately 50 three-dimensional surveys of a cuddy will be used to observe its decay rate, associated frequency and intensity of fine scale stratification features, and changes in vertical-radial structure.The observed structural evolution of a subsurface eddy will be used to infer the exchange rates of physical, chemical, and biological properties with the surrounding waters. While cuddies are widely suspected to be important agents of property exchange in the CCS, the observations proposed will be the first to describe a cuddy in four dimensions over an appreciable portion of its lifetime.This study will contribute to understanding of eddy transport in general and the role of submesoscale coherent vorticies in particular in transmitting eastern boundary region properties to the ocean interior. Implications of eddy transport are important not only to physical distributions in the ocean, but to chemical and biological ones as well. The project will train a graduate student to be expert in an emerging observational technique as well as in the role of eddies in ocean circulation.