EAGER: Langmuir Turbulence Measurements at 35-40m Depth off Cape Hatteras in Fall 2015

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

Langmuir circulation is a well-known response of the upper ocean to wind and wave forcing and is often visible as a series of windrows where foam and other floating material is aggregated by converging surface flow. The main sense of circulation is on a vertical plain perpendicular to these rows, where the water circulates down under the rows, out to either side at some depth and back up again. Wind forcing tends to grow, merge and deepen the Langmuir Cells, while density stratification or the presence of denser layers below tends to limit their downward extent. Under the right conditions, supercells are observed going the full depth of the water column. These cells are very efficient at mixing the water vertically and lifting sediments and other material from the bottom into the water column. This project will deploy an acoustic Doppler current meter (VADCP) specially designed to accurately resolve the vertical motion as an addition to an existing experiment separately funded by Office of Naval Research and run by the Naval Research Laboratory (NRL). These detailed Doppler measurements will be used to study the development and evolution of the Langmuir Cells at a location significantly deeper than any of the previous measurements and under a wider range of stratification. The data about Langmuir Cells will help better interpret the extensive NRL measurements, while those measurements provide a spatial context for the analysis of the Langmuir dynamics. The most critical element in the deployment of the VADCP is the precise alignment of the vertical axis. The deployment will be at 35-40m, which is at the edge of safe deployment with divers, but other means of deployment will also be developed and tested, possibly leading to deeper deployments in the future. The NRL experiment is an intensive 30-day campaign in October 2015 near Cape Hatteras. It includes two aircraft carrying a multichannel synthetic aperture radar, infrared and visible hyperspectral imagers, and air-sea fluxes of momentum and heat, directional surface wave spectra from wave buoys, basic air-sea measurements of temperature, humidity, and wind using wave gliders and drifting buoys, sub-surface profiles of the mixed layer from gliders with turbulence microstructure probes, and multi-color dye releases surveyed by aircraft, and an AUV with optical/microstructure probes. This project will add a VADCP and taut-line conductivity-temperature string at 35-40m depth, between 36o and 36.5oN in August through October, to measure storm events during and after destruction of the strong Mid-Atlantic Bight summertime pycnocline. Aircraft and mooring/glider spatial coverage will reveal the range of cell horizontal scales; whether cells are wind-elongate and wind-aligned as expected, and whether temporal variability in the VADCP records represents spatial variation in a statistically frozen-field ocean response, recorded as cells drift by the stationary VADCP. Horizontal spatial structure of surface fluxes, winds and wave fields will inform how the forcing regimes may evolve in the cross-shelf direction with increasing wind and wave forcing, competing with evolving buoyancy fluxes. Glider microstructure measurements can be used to cross-validate TKE dissipation rates from the gliders with the large eddy approach from the VADCP, as done previously for freefall probes and VADCP in tidal flows. VADCP velocity fields will significantly enhance the interpretation of the NRL group's optical data. Still, the most compelling motivation is the project's potential to broaden our view of turbulence in the coastal ocean. There is growing evidence the full depth supercells occur often over the continental shelf. This deployment will extend existing VADCP measurements at 15m and 27m depth. If Langmuir supercells can be verified with direct measurements, their effects will need to be considered over a much wider range of shelf settings than appreciated before. This will require reassessment of current understanding and modeling of seawater and sediment fluxes under wind/wave forcing in the coastal ocean.