Measurements of Concurrent Turbulent Mixing and Velocity Shear in the Ocean Surface Boundary Layer with Gliders

Jan-Torben Witte, University of Southampton, Ocean and Earth Science, Southampton, United Kingdom
Within shelf seas, further understanding of processes that control the advection and dissipation of turbulent kinetic energy (TKE) in the pycnocline and upper mixed layer is important to improve biochemical and hydrodynamic models. Belcher et al. (2012) stressed that turbulence triggered by waves can dominate vertical turbulent velocity production and that Langmuir circulation might play a crucial role in affecting the turbulent field down to the pycnocline.

As fine-scale measurements are still challenging, much disparity remains between the parametrization of the turbulent dissipation rate through field measurements and the level of accuracy that models require to correctly predict turbulence in the ocean environment. For the first time, we collected an extensive dataset of concurrent measurements of TKE dissipation, velocity shear and buoyancy production throughout a period from end of September to end of November 2018 in the North Sea with the modified glider Kelvin which was equipped with a microstructure profiler and an ADCP.

Our dataset will assist with the closure of energy and momentum budgets for the ocean surface boundary layer (OSBL), providing an accurate quantification of the energy and momentum exchange at the sea surface as well as through the pycnocline. Furthermore, our study provides more evidence for the calculations of dissipation rates using steady-state or dynamical flight modules (Merckelbach et al. 2010, 2019). CFD simulations of the flow around the glider and its sensors help to estimate the bias in the velocity flow measurements. The results demonstrate the suitability of gliders as platform for turbulence measurements and indicates an underestimation of measured flow speeds which would feed back to the calculation of turbulent dissipation rates.