Subinertial Currents on the Southern Califoinia Shelf

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  • 1 Woods Hole Oceanographic Institution, Woods Hole, MA 02543
  • | 2 Scripps Institution of Oceanography, La Jolla, CA 92093
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Abstract

Subinertial currents on the southern California shelf are investigated using observations from a current meter array deployed near San Diego during the summer, fall and winter of 1978/79. This region is characterized by weak winds (order 1–2 m s−1) and thus other driving mechanisms for the subinertial currents may be important. A simplified depth-averaged longshore momentum equation consisting of a balance between current accelerations, the adjusted sea level (ASL) gradient and the wind and bottom stress is examined. The ASL gradient is estimated using sea level and atmospheric pressure observations separated by 350 km. A linear parameterization of the bottom stress is used with a drag coefficient of 5 × 10−4 m s−1. During fall and winter, the first-order balance of terms in the longshore momentum equation over the inner-shelf (15 m depth) is between the wind and bottom stress. Over the outer-shelf (60 m) the primary balance is between the ASL gradient and the bottom stress. At midshelf (30 m) both driving mechanisms are important. An estimate of the depth-average longshore velocity in turns of wind stress and the ASL gradient reproduce the major features of the observed depth-averaged velocity (correlations from 0.59 to 0.85). These simple dynamics fail to describe the summer observations, except during one strongly forced event. The vertical structure in summer is complicated by the presence of a thermocline and strong shears over the shelf. Examination of observed and estimated winds within the Southern California Bight and to the south along Baja California indicates that winds along Baja California (∼500 km south of San Diego) may play an important role in generating the ASL gradient fluctuations in the Southern California Bight.

Abstract

Subinertial currents on the southern California shelf are investigated using observations from a current meter array deployed near San Diego during the summer, fall and winter of 1978/79. This region is characterized by weak winds (order 1–2 m s−1) and thus other driving mechanisms for the subinertial currents may be important. A simplified depth-averaged longshore momentum equation consisting of a balance between current accelerations, the adjusted sea level (ASL) gradient and the wind and bottom stress is examined. The ASL gradient is estimated using sea level and atmospheric pressure observations separated by 350 km. A linear parameterization of the bottom stress is used with a drag coefficient of 5 × 10−4 m s−1. During fall and winter, the first-order balance of terms in the longshore momentum equation over the inner-shelf (15 m depth) is between the wind and bottom stress. Over the outer-shelf (60 m) the primary balance is between the ASL gradient and the bottom stress. At midshelf (30 m) both driving mechanisms are important. An estimate of the depth-average longshore velocity in turns of wind stress and the ASL gradient reproduce the major features of the observed depth-averaged velocity (correlations from 0.59 to 0.85). These simple dynamics fail to describe the summer observations, except during one strongly forced event. The vertical structure in summer is complicated by the presence of a thermocline and strong shears over the shelf. Examination of observed and estimated winds within the Southern California Bight and to the south along Baja California indicates that winds along Baja California (∼500 km south of San Diego) may play an important role in generating the ASL gradient fluctuations in the Southern California Bight.

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