Turbulence Observations in the Upper Ocean from Autonomous Platforms

Anneke ten Doeschate, Dalhousie University, Oceanography, Halifax, NS, Canada, Alex E Hay, Dalhousie University, Halifax, NS, Canada and Brian Ward, National University of Ireland, Galway (NUIG), School of Physics, Galway, Ireland
Abstract:
Turbulent dynamics in the near-surface layer of the ocean drive air-sea exchange of momentum, heat and gasses through surface renewal, and affects the distribution of biogeochemical components in the surface mixed layer. It also plays an important role in the vertical fluxes between this layer and the deeper ocean interior.

To observe and quantify turbulent mixing in the first meters is challenging, due to the dynamics of the sea surface and the potential disturbance of the vertical structure by a research vessel. Autonomous platforms provide a means to obtain undisturbed measurements and, because they are operated remotely, are especially suitable for investigating surface layer processes during energetic weather conditions.

We present results from observational experiments during both calm, stratified sea states, as well as during storm conditions. A high resolution five-beam ADCP was mounted on an autonomous, wave-propulsed surface platform. This setup was used to measure velocity, turbulent mixing parameters and waves in the near-surface layer of the North Atlantic, off the coast of Nova Scotia (Canada). Methods used to correct the velocity measurements for the platform’s motion will be discussed.

Via satellite communication (Iridium) the platform’s position was kept close to that of a simultaneously deployed, semi-Lagrangian and autonomous, vertical turbulence profiler, of which the position was followed via Iridium as well. Turbulence dissipation rates derived from the ADCP’s along-beam velocity measurements are compared to those derived from the shear probes on this profiler.