Dynamics and mixing of stratified rotating bottom boundary layers
Final Report Abstract
The overall goal of this small project was the investigation of the dynamics of the bottom boundary layer (BBL) on the continental shelf in collaboration with project partners from the College of Earth, Ocean, and Atmospheric Sciences (CEOAS) at the Oregon State University (OSU) in Corvallis, OR, USA. DFG provided funding for a research visit of PI Umlauf at OSU, which took place between 03 July and 28 November 2013. The work done during this visit constitutes the main component of the project. Together with project partners W. D. Smyth and J. N. Moum from OSU, a numerical model was developed, and simulation results were analyzed and compared to field data by the OSU group. This model was able to reproduce the generation of gravitationally unstable BBLs found in field observations from the Oregon shelf that originally motivated this project. Using a nondimensional version of the governing equations, these results were discussed on a more general framework, describing the dynamics and energetics of Ekman layers near sloping topography. We identified the key non-dimensional parameters of the problem, and investigated the energetics over the oceanographically relevant parameter space. These investigations revealed, surprisingly, that the amount of kinetic energy converted to available potential energy (APE) by Ekman-induced cross-slope advection of isopycnals was of the same order of magnitude as the energy lost to turbulent dissipation. Cumulative mixing efficiencies inside the BBL were found to be larger for steeper slopes and stronger interior stratification but did generally not exceed the threshold of 5 percent, except for very steep slopes, where efficiencies of up to 20 percent were reached. During the work in this project, we became unexpectedly aware of a related data set, recently obtained by Japanese colleagues during a research cruise on the shelf of the East China Sea. Based on highresolution moorings and densly-spaced turbulence microstructure profiles in a tidal current, they found a periodic destabilization of the BBL for which they had no viable explanation. Tidal straining in regions of freshwater influence, usually taken as an explanation for this effect, could be excluded because the observation site was located on the open shelf, far from any freshwater source. A closer investigation, however, revealed that this phenomenon can be explained with the theory for shear-induced convection near sloping topography developed in our project. This finding implies that, different from classical tidal straining, a periodic shear-induced gravitational destabilization of the BBL near sloping topography may be expected in principle everywhere on sloping shelves, even outside any region of freshwater influence.
Publications
- Energetics of bottom Ekman layer during buoyancy arrest. J. Phys. Oceanogr., Volume 45 No. 12, 3099–3117, December 2015
Umlauf, L., W. D. Smyth, and J. N. Moum
(See online at https://doi.org/10.1175/JPO-D-15-0041.1)
