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The Latitudinal Dependence of Shear and Mixing in the Pacific Transiting the Critical Latitude for PSI

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  • 1 Scripps Institution of Oceanography, La Jolla, California
  • | 2 Applied Physics Laboratory, and School of Oceanography, University of Washington, Seattle, Washington
  • | 3 Scripps Institution of Oceanography, La Jolla, California
  • | 4 University of Victoria, Victoria, British Columbia, Canada
  • | 5 Applied Physics Laboratory, University of Washington, Seattle, Washington
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Abstract

Turbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2–4 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

Corresponding author address: J. A. MacKinnon, Scripps Institution of Oceanography, 8622 Kennel Way, La Jolla, CA 92037. E-mail: jmackinn@ucsd.edu

Abstract

Turbulent mixing rates are inferred from measurements spanning 25°–37°N in the Pacific Ocean. The observations were made as part of the Internal Waves Across the Pacific experiment, designed to investigate the long-range fate of the low-mode internal tide propagating north from Hawaii. Previous and companion results argue that, near a critical latitude of 29°N, the internal tide loses energy to high-mode near-inertial motions through parametric subharmonic instability. Here, the authors estimate mixing from several variations of the finescale shear–strain parameterization, as well as Thorpe-scale analysis of overturns. Though all estimated diffusivities are modest in magnitude, average diffusivity in the top kilometer shows a factor of 2–4 elevation near and equatorward of 29°N. However, given intrinsic uncertainty and the strong temporal variability of diffusivity observed in long mooring records, the meridional mixing pattern is found to be near the edge of statistical significance.

Corresponding author address: J. A. MacKinnon, Scripps Institution of Oceanography, 8622 Kennel Way, La Jolla, CA 92037. E-mail: jmackinn@ucsd.edu
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