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Internal Tides Observed by an Acoustic Doppler Current Profiler

R. F. MarsdenDepartment of Physics, Royal Roads Military College, Victoria, British Columbia, Canada

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K. C. GreenwoodDepartment of Physics, Royal Roads Military College, Victoria, British Columbia, Canada

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Abstract

Previous studies of Knight Inlet, British Columbia, revealed the presence of a strong internal M2 tide. Most of the energy was found to be in either the first or second dynamic modes. Due to difficulties in sampling of the surface layer, objective estimates of the distribution of tidal energy and reflection coefficients, until now, have been impossible to obtain. During June and July 1989, an intense acoustic Doppler current profiler (ADCP) sampling of the inlet was conducted. These results are used to show the existence of a strong semidiurnal flow in the baroclinic field. The authors propose that the horizontal phase information inherent in the spatial sampling by the ADCP can be used to resolve the distribution of energy between dynamic modes. Through a least squares fit of the data to a simple free wave propagation model of the inlet, the authors arrive at objective estimates of the distribution of M2 internal tide energy. The fitting procedure is found to be sensitive to fluctuations in the basin width. When an accurate estimate of width is incorporated into the fit, the authors arrive at net energy fluxes of 0.44 × 106 W toward the mouth of the inlet at Protection Point and 1.17 × 106 W toward the head of the inlet at Tomakstum Island. It is shown that these results do not display the degeneracy inherent in other estimates and that they are in agreement with a recent numerical model of the inlet by Stacey and Pond.

Abstract

Previous studies of Knight Inlet, British Columbia, revealed the presence of a strong internal M2 tide. Most of the energy was found to be in either the first or second dynamic modes. Due to difficulties in sampling of the surface layer, objective estimates of the distribution of tidal energy and reflection coefficients, until now, have been impossible to obtain. During June and July 1989, an intense acoustic Doppler current profiler (ADCP) sampling of the inlet was conducted. These results are used to show the existence of a strong semidiurnal flow in the baroclinic field. The authors propose that the horizontal phase information inherent in the spatial sampling by the ADCP can be used to resolve the distribution of energy between dynamic modes. Through a least squares fit of the data to a simple free wave propagation model of the inlet, the authors arrive at objective estimates of the distribution of M2 internal tide energy. The fitting procedure is found to be sensitive to fluctuations in the basin width. When an accurate estimate of width is incorporated into the fit, the authors arrive at net energy fluxes of 0.44 × 106 W toward the mouth of the inlet at Protection Point and 1.17 × 106 W toward the head of the inlet at Tomakstum Island. It is shown that these results do not display the degeneracy inherent in other estimates and that they are in agreement with a recent numerical model of the inlet by Stacey and Pond.

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