Collaborative research: Coastal inertial-band dynamics: separating forced and free responses in a natural laboratory

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Surface winds globally impart about half a terawatt of energy to inertial oscillations in the surface mixed layer. The coastal ocean receives less wind work than the open ocean because it has less surface area. However, near-inertial wave generation is enhanced along coastlines, because the boundary induces large convergences and divergences in mixed-layer velocities. This process is dynamically significant because it transfers energy from forced surface-trapped motions to free motions, which can produce currents and turbulence in the stratified interior. Idealized theories and observations of the coastal ocean have confirmed the generation and propagation of near-inertial waves, but observational studies have not yet separated free and forced motions so that internal-wave generation, energy flux, and dissipation can be accurately quantified. Furthermore, the results of individual studies have not been synthesized to estimate the global significance or geography of coastal near-inertial wave generation. This project will improve the dynamical understanding and prediction of near inertial motions throughout the coastal ocean, in marginal seas, and in large lakes. The fieldwork will also contribute to our knowledge of the geophysical dynamics of Lake Superior, a valuable resource that is under-sampled with respect to physical measurements. These contributions include the first broad-scale turbulence measurements in the lake and the extension of a multi-year time series of density and currents. Coinciding measurements of chlorophyll fluorescence, turbidity, and oxygen will also illuminate the presently unknown relationships between turbulence and biogeochemistry in Lake Superior. The project will also develop tools and lessons for graduate oceanography and limnology courses, and support citizen science in Duluth, MN, via the development of a public drifter-building program in conjunction with the local office of the Environmental Protection Agency. The project will also train a PhD student and two undergraduate summer researchers.This project aims to quantify coastal kinetic-energy pathways from wind work to dissipation, and produce a detailed description of coastal inertial-band dynamics by extending existing theories of coastal near-inertial wave generation and definitively testing them. This will be done using realistic numerical simulations and intensive direct observations of wind work, mixed-layer and stratified turbulence, and near-inertial internal-wave generation, propagation, and dissipation along a coastline and over rough coastal topography. Specifically, extensive observations will collected in Lake Superior, which is dominated by near-inertial motions during summer and has negligible tides, weak mean circulation, and little river input, making it also an ideal laboratory of the coastal ocean. The observations will include broad-scale measurements of turbulence from ship-based surveys and Wirewalker wave-powered moored profilers. Three traditional moorings will also be deployed for four years, extending an existing time series that can be used to identify extreme events and long-term trends in near-inertial motions. The project also includes novel analyses of analytical and numerical models, which will aid in the collection and interpretation of observations and enable the results in Lake Superior to be extended throughout the coastal ocean using historical wind, stratification, and bathymetry data.