A Comparison of Low-Frequency Current Observations off British Columbia with Coastal-Trapped Wave Theory

T. Yao Department of Oceanography, University of British Columbia, Vancouver, B.C., V6T 1W5

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Howard J. Freeland Institute of Ocean Sciences, P.O. Box 6000, Sidney, B.C., V8L 4B2

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Lawrence A. Mysak Department of Oceanography and Mathematics, University of British Columbia, Vancouver, B.C., V6T 1W5

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Abstract

Six current-meter mooring were deployed in a line approximately 600 km in length along the continental shelf of British Columbia. Analysis of the low frequency (periods exceeding a day) fluctuations in current for the winter 1981–82 period is discussed. Alongshore currents off Vancouver Island are mutually correlated with time lag less than a day. The region of mutual correlation does not extend north of Vancouver Island, across Queen Charlotte Sound. Coherence is observed between currents south and north of Queen Charlotte Sound only in a frequency band where there is mutual coherence with local wind. A comparison is made between observation and free coastal-trapped wave theory. Off northern Vancouver Island, where the shelf is narrower than off southern Vancouver Island, there is increased vertical shear, a feature of the second coastal-trapped wave mode. A consistency test is applied using the cross spectral matrix of alongshore components of velocity. In the dominant energy-containing frequency bands (periods ≳10 days), the structure of alongshore currents off Vancouver Island is consistent with the two lowest free coastal-trapped wave modes locked in phase.

Abstract

Six current-meter mooring were deployed in a line approximately 600 km in length along the continental shelf of British Columbia. Analysis of the low frequency (periods exceeding a day) fluctuations in current for the winter 1981–82 period is discussed. Alongshore currents off Vancouver Island are mutually correlated with time lag less than a day. The region of mutual correlation does not extend north of Vancouver Island, across Queen Charlotte Sound. Coherence is observed between currents south and north of Queen Charlotte Sound only in a frequency band where there is mutual coherence with local wind. A comparison is made between observation and free coastal-trapped wave theory. Off northern Vancouver Island, where the shelf is narrower than off southern Vancouver Island, there is increased vertical shear, a feature of the second coastal-trapped wave mode. A consistency test is applied using the cross spectral matrix of alongshore components of velocity. In the dominant energy-containing frequency bands (periods ≳10 days), the structure of alongshore currents off Vancouver Island is consistent with the two lowest free coastal-trapped wave modes locked in phase.

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