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Global Patterns of Low-Mode Internal-Wave Propagation. Part II: Group Velocity

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  • 1 Applied Physics Laboratory, and School of Oceanography, University of Washington, Seattle, Washington
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Abstract

Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity ĉgFE−1 is estimated. For a single free wave, observed group speed |ĉg| should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal |ĉg| closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered |ĉg| values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05–1.25 × f, as typically observed in frequency spectra.

Corresponding author address: M. H. Alford, Applied Physics Laboratory, 1013 NE 40th St., Seattle, WA 98105. Email: malford@apl.washington.edu

Abstract

Using a set of 80 globally distributed time series of near-inertial and semidiurnal energy E and energy flux F computed from historical moorings, the group velocity ĉgFE−1 is estimated. For a single free wave, observed group speed |ĉg| should equal that expected from linear wave theory. For comparison, the latitude dependence of perceived group speed for perfectly standing waves is also derived. The latitudinal dependence of observed semidiurnal |ĉg| closely follows that expected for free waves at all latitudes, implying that 1) low-mode internal tides obey linear theory and 2) standing internal-tidal waves are rare in the deep ocean for latitudes equatorward of about 35°. At higher latitudes, standing waves cannot be easily distinguished from free waves using this method because their expected group speeds are similar. Near-inertial waves exhibit scattered |ĉg| values consistent with the passage of events generated at various latitudes, with implied frequencies ω ≈ 1.05–1.25 × f, as typically observed in frequency spectra.

Corresponding author address: M. H. Alford, Applied Physics Laboratory, 1013 NE 40th St., Seattle, WA 98105. Email: malford@apl.washington.edu

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