Collaborative Research: Submesoscale Dynamics over the Continental Shelf: Drivers and Implications for Across-Shelf Exchange

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This project studies the mechanisms by which small scale (2-10km) coastal oceanic flows, over the continental shelf can stir the water masses and contribute to the transport material between the coast and the open ocean. A unique array of high frequency (HF) radar systems will be deployed to measure surface currents at high spatial resolution. These measurements will be able to resolve eddies with sizes ranging from 2 ? 20 km that are present over the New England Shelf (NES). The shelf area to be covered extends 30 to 80 km offshore, with water depths of 30 to 70 m. These surface current measurements will be augmented by simultaneous underwater measurement using a submerged instrumentation, while autonomous surface vehicles and drifters will be used to measure mid-shelf velocity structure, fine-scale horizontal gradients, and drifter trajectories. The analysis of the data collected will increase our understanding of exchange of material between the coast and the open ocean over a broad continental shelf. This exchange has implications for biogeochemistry, shelf productivity, fisheries, transport of pollutants, as well as for determining heat and freshwater budgets; this is a crucial step in diagnosing shelf-wide ecosystem variability, improving its predictability and thus contributing to better discriminating between anthropogenic and natural ecosystem changes. The proposed HF radar measurements will feed into the National High Frequency Radar (HFR) Network operated by NOAA and the data will be available for data assimilation into coastal ocean models as well as trajectory parameterizations that support search and rescue, and pollution responses. The project will contribute to education by providing the research opportunities for summer student fellows at the Woods Hole Oceanographic Institution and enable undergraduate students from the US Coast Gard Academy, an undergraduate teaching-focused institution, to participate in the field work and incorporate research activities into their senior projects. By measuring the surface velocity fields at about 2 km resolution over 1,200 square kilometers of the New England Shelf (NES) through an entire annual cycle the project will characterize the dynamical variability and seasonality of the eddy field. This work would identify processes and parameters that lead to submesoscale current variability and the across-shelf exchange of water masses and properties. The study hypothesizes that eddy variability is driven by baroclinic instability, but is intrinsically linked with wind and bottom stress, which generate spatially and temporally varying Ekman currents and shear-induced mixing that can oppose or reinforce frontal circulations. The observational results will be used to identify how eddy dynamics are modulated by the seasonal cycle of lateral and vertical buoyancy gradients within the NES. The role of submesoscale dynamics in across-shelf exchange will be assessed using tracers and particles to calculate dispersion and eddy fluxes both within the observed surface velocities, while drifters will be used to gain a direct measure in the field. All estimates will be synthesized with our dynamical characterization of the eddies to gain an understanding of the processes leading to across-shelf transport in different seasons and their importance relative to larger-scale exchange mechanisms.