Wind effects on sub-tidal currents in Puget Sound |
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| Authors: | Hiroshi Matsuura Glenn A Cannon |
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| Institution: | (1) School of Oceanography, University of Washington, 98195 Seattle, WA, USA;(2) Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, 7600 Sand Point Way N.E., 98115 Seattle, WA, USA;(3) JAMSTEC, 2-15, Natushima-cho, Yokosuka-shi, 237 Kanagawa, Japan |
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| Abstract: | Wind effects on sub-tidal currents are studied using current meter records obtained at six moorings across the main basin
of Puget Sound. High correlations between wind speeds and currents are found near the surface and at mid-depths of about 100
m. Empirical Orthogonal Function analysis applied to the axial currents in 1984 and 1985 shows that mode 1, containing over
60% of the variance, is highly correlated with wind speed even without any near surface current records. When near surface
stratification is strong, direct wind effects are limited to the upper 30 m with counter currents in the lower layer indicating
a baroclinic response. The transport in the lower layer almost balances the transport in the upper layer. When near surface
stratification is weak, direct wind effects on currents can be detected to about 100 m. In this case, there is no clear and
consistent depth at which one can separate the upper from the lower layer. Time series show that the acceleration in the surface
layer initially increases in the same direction as the wind when the wind starts blowing, but it reaches a maximum, starts
decreasing, and eventually changes to the opposite direction (decelerates) while the wind continues to blow in one direction.
Results of a continuously stratified normal mode model and estimations from the observations suggest that friction at solid
boundaries is a major cause of these phenomena. The model shows that modal currents of normal modes 2 and 3 are as important
as mode 1, although the resultant vertical structure of total current shows a two-layer type pattern with only one zero crossing.
The effect of the baroclinic pressure gradient is only apparent at low frequencies and among lower modes. |
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| Keywords: | Wind effect Normal mode model Fjord circulation EOF Puget Sound |
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