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Dispersion and Mixing Caused By Near-Inertial Internal Waves in Lake Michigan

Near-inertial internal waves are ubiquitous features in the ocean and large lakes. In the coastal ocean, near-inertial internal waves often exist as horizontally-propagating vertical modes, but their presence and importance is usually overshadowed by more energetic internal tides. However, in large lakes, the stratified coastal environment is dominated by modal near-inertial waves because energetic tides are absent. The study of these features in large lakes is therefore motivated because the lake setting provides a unique laboratory in which one can study inertial internal waves without interference from other energetic processes. Additionally, in large lakes, inertial internal waves are of first order importance for coastal mixing and dispersion because other mechanisms are absent and stratification is strong. Intellectual Merit: This project is based on the hypothesis that near-inertial internal waves play a significant role in vertical mixing and horizontal dispersion in Lake Michigan and other Great Lakes during the heavily stratified, weakly-forced summer period. Many Great Lakes circulation and dispersion studies filter out processes on timescales of inertial period or smaller, and it is further hypothesized that this filtering can have important consequences for inferences drawn about vertical mixing and horizontal dispersion of heat, biota, and pollutants. The objectives of this study are twofold. First, the project will quantify vertical mixing in coastal Lake Michigan during the stratified period, in order to determine the magnitudes of baseline turbulent mixing and the magnitudes of cross-thermocline mixing caused by episodic near-inertial waves; newly-developed oceanic mixing parameterizations for these waves will be tested. Secondly, the project will directly measure horizontal dispersion with dye release studies and numerical modeling for both quiescent and inertial-wave dominated conditions to explicitly resolve the role of near-inertial waves in supposedly heightened horizontal dispersion. Two types of experiments will be conducted. In the first set of experiments, which target vertical mixing, scheduled microstructure measurements will complement fixed moorings of velocity and temperature to determine the vertical and cross-shelf variability of turbulent dissipation and its relationship to resolvable internal wave parameters. Episodic microstructure measurements during inertial wave events will attempt to quantify the variability of this turbulence over the inertial wave cycle. In the second type of experiment, which targets horizontal dispersion, dye release experiments will be performed in stratified waters during inertial wave events. The dye dispersion will be quantified by Purdue's autonomous underwater vehicle (AUV), which will be vessel-supported. Additional dispersion work will be performed using a validated, high-resolution numerical model of Lake Michigan and analysis based on idealized representations of near-inertial waves in the coastal environment. Broader Impacts: The broader impacts of this project are centered on collaboration with the National Parks Service's Indiana Dunes National Lakeshore. Purdue's autonomous underwater vehicle is a natural, proven outreach tool that will be utilized in a set of beach-based outreach activities organized with the National Parks Service, including demonstrations and an educator workshop. The educator workshop will focus on training K-12 educators on how to incorporate elements of Great Lakes limnology to inspire future scientists. Additionally, the investigator has an established record of mentoring undergraduate researchers through the Purdue Summer Undergraduate Research Fellowship (SURF) program, and this project will support several undergraduate researchers. Finally, the project will form the basis of a graduate student's Ph.D. dissertation, and support an early-career faculty member.

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