Internal Tide and Waves on the Oregon Continental Shelf

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The goal of this research project is to improve our understanding of the generation, propagation and dissipation of the oceanic internal tide on the continental shelf. The internal tide is an internal wave generated when the stratified ocean is forced over bathymetric relief by the barotropic tide. The internal tide that propagates across the shelf toward the shore can become quite complex and often develops into packets of nonlinear internal waves. The internal tide is a major source of variability on continental shelves around the world and is potentially an important source of energy for generating the turbulence responsible for mixing scalars and momentum. It is important to determine the role the internal tide plays in the energy and momentum budgets to better understand and model coastal ocean circulation. It is also important to know the contribution of the internal tide to processes that lead to cross-shelf transport of mass, which includes nutrients, larvae, and sediment. The specific scientific objectives of this project are to: characterize the amplitude, energy flux, and vertical structure of the internal response near tidal frequencies on the shelf, describe the vertical structure and propagation characteristics of the nonlinear internal tide as it propagates across the shelf, quantify the internal wave background frequency continuum on the shelf, and assess the potential impact of the internal tide on cross-shelf transport of biota and sediment and parameterizations of mixing used in current numerical models of coastal circulation. The approach to accomplish these objectives is to analyze the wealth of observations made off the Oregon coast during three comprehensive experiments in summer 1999, summer 2001 and winter 2003. Previously funded analysis focused on the low-frequency wind-forced circulation. This project focuses on the internal tide and higher frequencies. A two-dimensional primitive equation numerical model of the internal tide will be used as a tool to guide the analysis. The purpose of the model is to help us understand the basic physical balances that are consistent with the observations.Broader Impacts: The internal tide contributes to the dissipation of energy and mixing, however, the tide is typically not explicitly included in circulation models. Research, such as that proposed here, will help determine the potential importance of internal tide, eventually leading to direct inclusion of the tide into models, or modification of the existing parameterization used for mixing. Nonlinear internal wave packets may be important in transporting biota, such as larvae, onshore, and sediment offshore. This study will provide some insight into these processes, including information on their spatial and temporal variability. There is much interest in understanding the impacts of cross-shelf transport on biological processes and the ecology of the Oregon shelf and coast. The data analysis methods and the numerical model developed here will be used directly in the education of graduate students in a class being offered by Levine on the internal tide at Oregon State University during the 2006-07 academic year. This class will be taken by graduate students pursuing PhDs in oceanography. The students will learn about the internal tide by running the model and analyzing observations for themselves.