Using bottom pressure to quantify tidal form drag on a sloping headland
Warner, Sally Jane
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Bottom pressure is used to quantify and understand form drag induced by tidal currents as they flow over and around Three Tree Point (TTP), a sloping headland in Puget Sound, WA that is typical of topography found in high latitude, coastal regions. By dividing the bottom pressure field into parts that are due to different physical processes, it can be determined how inertia, internal waves and eddies all contribute to form drag. In chapter 2, idealized numerical models of vertical-walled headlands are combined with theory to show how the oscillatory nature of the flow can increase the magnitude of the form drag, but cannot increase the magnitude of the tidally averaged work done on the flow. In chapter 3, an array of seafloor pressure sensors is deployed across the topography to directly measure total form drag for the first time in the ocean. It is found that form drag is much larger than frictional drag, and that a linear wave drag law is a better parameterization of form drag than a bluff body drag law. The form drag is estimated to convert 0.2 W m<super>-2</super> of tidally averaged power away from the barotropic tides. In chapter 4, a numerical model of TTP is used to investigate the tilted eddies and internal lee waves that are generated at this site. It is found that the sea surface and isopycnal perturbations tend to counteract each other within the eddy, but work in tandem in the internal lee wave. Therefore, despite the large sea surface depression in the eddy, the eddies and the internal lee waves remove the same amount of energy from the barotropic tides. Through this thesis, it is shown that form drag -- not frictional drag -- is the dominant mechanism for removing energy from the tides at TTP. Steps are made toward betters parameterizations of drag that may be implemented into larger scale models that do not resolve the scales important to form drag.
- Oceanography