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

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 ?g ≡ FE?1 is estimated. For a single free wave, observed group speed |?g| 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 |?g| 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 |?g| values consistent with the passage of events generated at various latitudes, with implied frequencies ? ≈ 1.05?1.25 ? f, as typically observed in frequency spectra.