NOC Liverpool report for the Wirewalker underwater profiler deployment for the RidgeMix research programme

This report provides a review of a ‘wirewalker’ underwater profiler that was used for high resolution water quality and turbulence measurements as part of a mid-Atlantic ocean based scientific survey. The profiler utilised wave energy and buoyancy to generate automated, cyclic underwater vertical measurements. A drifting supporting surface buoy and mooring configuration was used that moved horizontally in response to underwater currents, surface wind and waves, while generating repeated sub-surface vertical scientific measurements. The profiler instrumentation payload included a high-resolution chlorophyll-a fluorimeter, a fast sampling, precision conductivity, temperature and depth sensor (CTD) and a fragile, high resolution oceanic microstructure and turbulence sensor. A bespoke guard was used to help protect the fragile microstructure and turbulence sensors from damage. The mooring surface buoy instrumentation included telemetered GPS at nominally 30 minute intervals, a backup telemetered position indication, a VHF radio beacon based position locator and a solar powered night time light. The experiment was designed to resolve time-variability of upper-ocean mixing and chlorophyll-a fluorescence over the Mid-Atlantic underwater Ridge, an internal wave generation hotspot. Typically more than three wave driven descents to 200 metres and subsequent buoyancy driven ascents to close to the sea surface were achieved per hour during a 22 day deployment, whilst continuously sampling. This represents the first time the UK National Oceanography Centre (NOC) have used this type of underwater profiling system for high resolution oceanic microstructure and turbulence measurements to support a scientific campaign. Information to support a series of developing operational best practices for the use of this delicate and precision sensing arrangement with a wirewalker underwater profiler is provided. Recommendations relating to the future development of this versatile measurement system are also discussed.