In-water microstructure

Oceans 2025 Theme 8, work package 8.5a

Turbulence in the water column is a control on many processes especially air-sea momentum transfers; vertical fluxes of nutrients; patchiness in relation to shelf topography; mixing where internal waves are energetic; algal functional groups; phytoplankton photo-physiology and their mixing through light and nutrient gradients; SPM aggregation. The following aim to extend our ability to measure microstructure and turbulence in the water column.

Thermal and density structures, and internal waves, are imaged extensively (10s-100s km) and in detail (10-50m) in 2-D seismic sections of the water column. The promise of seismics is being tested in the EU-NEST-Adventure Geophysical Oceanography project GO (2006-09), comparing inferred waves and mixing in contemporary seismic and in situ measurements (in the Gulf of Cadiz; Mediterranean Water descends the slope and mixes).

For horizontal resolution of thermocline and chlorophyll structures, in conjunction with vertical profiling, we are developing a towed temperature-fluorometer chain. This will improve on present ~ 1 km horizontal resolution (from Scanfish, Seasoar etc.) and resolve the vertical to ~1m. Interpretation of the temperature and chlorophyll fields will be helped by currents from shipboard ADCP data at similar resolution. A compact fluorometer to include in a line is now available.

Microstructure can be profiled vertically by a commercially available free-fall instrument. To sample complementary horizontal patchiness, we propose to mount microstructure sensors, with fluorometry to support dye-release experiments, on a small AUV and later on a glider. Microstructure has been measured from a large AUV (e.g. Autosub). However, a small AUV or glider is easier to deploy; POL has recovered a glider from a RIB, but a (propelled) AUV might be necessary for speed in the water.

Glider deployment and track in Liverpool Bay

Air-sea momentum transfer measurements from ships would be enhanced by a microstructure probe on a "stinger" ~ 1m below the sea surface on the bow. Successful precedents are: i) a short pitot tube and pressure gauge on the front of a catamaran sponson and ii) micro-structure and pressure sensors on a bow-mounted frame. Issues to tackle are flow distortion, analysis separating waves and turbulence, allowance for varying sensor depth in waves with ship pitching. The "stinger" will be designed to be removable (bolted to a permanent fixing) yet robust.