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Anisotropic Response of Surface Currents to the Wind in a Coastal Region

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  • 1 Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
  • | 2 Climate, Atmospheric Science, and Physical Oceanography, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
  • | 3 Marine Physical Laboratory, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California
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Abstract

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind–ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42° ± 2° to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10° ± 4° (71° ± 3°) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield wind-driven currents with 3%–5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.

Corresponding author address: Sung Yong Kim, Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093-0213. Email: syongkim@mpl.ucsd.edu

Abstract

Analysis of coastal surface currents measured off the coast of San Diego for two years suggests an anisotropic and asymmetric response to the wind, probably as a result of bottom/coastline boundary effects, including pressure gradients. In a linear regression, the statistically estimated anisotropic response explains approximately 20% more surface current variance than an isotropic wind–ocean response model. After steady wind forcing for three days, the isotropic surface current response veers 42° ± 2° to the right of the wind regardless of wind direction, whereas the anisotropic analysis suggests that the upcoast (onshore) wind stress generates surface currents with 10° ± 4° (71° ± 3°) to the right of the wind direction. The anisotropic response thus reflects the dominance of alongshore currents in this coastal region. Both analyses yield wind-driven currents with 3%–5% of the wind speed, as expected. In addition, nonlinear isotropic and anisotropic response functions are considered, and the asymmetric current responses to the wind are examined. These results provide a comprehensive statistical model of the wind-driven currents in the coastal region, which has not been well identified in previous field studies, but is qualitatively consistent with descriptions of the current response in coastal ocean models.

Corresponding author address: Sung Yong Kim, Scripps Institution of Oceanography, 9500 Gilman Dr., La Jolla, CA 92093-0213. Email: syongkim@mpl.ucsd.edu

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