Development and Test of Subsurface ADCP Mooring for Near Surface Current Profiles in 200-300m Waters

The National Oceanic and Atmospheric Administration (NOAA) Center for Operational Oceanographic Products and Services (CO-OPS) and the East Carolina University Coastal Studies Institute (CSI) have collaborated to complete two successful, long-term field demonstrations of a new ADCP mooring, developed to address current survey needs. Recent NOAA National Current Observation Program (NCOP) field applications have required current measurements in the top 30m of a 200-300m water column, for a 60-90 day duration. Maximum near surface current speeds at sites of interest range from 2-2.5 m/s. Historical current observations at these CO-OPS sites only measure currents beneath 50m due to limitations of the sampling methods and platforms, so derived navigational support products are not representative of conditions near the surface. CO-OPS developed and field tested a subsurface, moored buoy system consisting of commercial-off-the shelf (COTS) components. As its top-most component, the mooring employs an ellipsoid shaped, syntactic foam buoy, with an upward looking 300 kHz acoustic Doppler current profiler (ADCP). A relatively long line is required to position the buoy with ADCP in the upper 100m of the 250-300m water column. A strong, light weight, low-drag wire rope with fairings along the top 60m was selected for this purpose. The two field demonstrations of the new ADCP mooring were conducted during May 2017- June 2018 and October 2020 - August 2021. Both field deployments took place offshore of Cape Hatteras, NC near the edge of the Gulf Stream, where 200-500m water depths are on the shelf slope in an area frequented by Gulf Stream incursions. Results from the first field deployment are reported in two conference proceedings papers (MTS/IEEE OCEANS18, Charleston and CWTMC19, San Diego). The focus of this paper is on the latest mooring design, deployment method enhancements and results from the second field demonstration.