Collaborative Research: Internal Swash zones and boundary-interior exchange: High-accuracy modeling and field observations

Western Washington University, Bellingham WA

Investigators

Abstract

This project will examine how strongly non-linear internal waves (i.e., waves developed at the interface of two fluids with different density) induce mixing near the bed and at the interface of the two fluids. In-situ observations will be carried out in a north-south oriented finger-lake (Lake Cayuga, NY), a natural laboratory with conditions analogous to those found at the edge of the outer continental shelf near the slope during summertime (i.e., warm surface layer overlying a colder layer of water). The analysis of the experimental data combined with sophisticated numerical experiments will provide information that will fill the gap between laboratory work and the most up-to-date oceanographic studies. This work may improve parameterization of boundary fluxes in the ocean, which are important to lateral dispersion of buoyancy, heat, and potentially of organic carbon offshore of productive shelf systems. The work may advance interdisciplinary activities as it may improve communication between physical oceanography and physical limnology. In addition, results may also be pertinent to water quality issues in lakes and coastal areas. Lake Cayuga has been plagued by late-summer algal blooms, while supplying drinking water for the city of Ithaca NY, and for Cornell University. Although this problem is not the direct subject of the study, the data collected will be useful for addressing this societally important issue. The project will support the training of two graduate and one undergraduate student and provide research material that will be integrated in the classroom activities of the principal investigators. The project combines modeling and observations to investigate how nonlinear internal waves (NLIWs) affect near-boundary mixing and boundary-interior exchange events. The objectives are to: 1) explore, in the parameter space of incident internal wave nonlinearity, the mechanisms of NLIW instability, and assess the roles of bottom shear stress, convective/shear instability in the wave front, or bottom boundary layer separation in the wave footprint; 2) quantify and contrast the contribution of each instability type to diapycnal flux generation and boundary-interior exchange (intrusion generation) as a function of incident NLIW energy; 3) determine the role of temporal variability of the advective swash zone and the resulting cyclically incident NLIWs and potential wave-wave interactions, in establishing episodic turbulence, mixing, horizontal transport, and re-stratification of near-bed water. The numerical model is based on high-accuracy spectral element methods, to enable the high- resolution study of internal wave breaking over variable bathymetry. The southern slope and shelf of Lake Cayuga, New York, provides an exceptional natural laboratory for the project field studies, serving as a simpler analog system for the ocean. This lake mimics conditions seen on many summer continental shelves and slopes with a warm surface layer intersecting the shelf, overlying a stratified layer extending down over a steeper bathymetric slope. A low-frequency seiche advects the pycnocline back and forth across the shelf/slope, and hosts higher frequency NLIWs which break near boundaries, mixing the stratified fluid and contributing to expulsion of boundary layer fluid into a basin’s stratified interior. Better understanding of the basic physics will be relevant to a wide range of fields and contribute to better parameterization of the relevant processes. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.