Observations of Energetic High-Wavenumber Internal Waves in the Kuroshio

Intense and spatially coherent shear layers were detected in and under the Kuroshio in an April 2000 survey in the East China Sea. The sloping layers, revealed by shipboard Doppler sonars on the R/V Roger Revelle, appeared to cross isopycnal surfaces. Except in a small region near the Kuroshio shelfbreak front, the rms finescale shear associated with the layers significantly exceeded the geostrophic shear. An April 2002 follow-on cruise was organized to establish whether these motions were propagating internal waves. Shipboard and lowered ADCPs were both operated from the R/V Melville. In addition to CTD-sonar transects, a 30-h time series of currents and shear was obtained in the core of the Kuroshio near the island of Kyushu, Japan. The shear structures were indeed found to be propagating, with both up- and down-going internal wave motions present. In comparison with the nearby open ocean, the finescale (<160 m) vertical shear variance is increased by a factor of 3 in the Kuroshio and by a factor of 6 in the region between the shelf break and the Kuroshio. If energy dissipation indeed scales as shear variance squared, very high values of dissipation (over 30 times the open-ocean value) can be anticipated in this region. It is conjectured that the geostrophic vorticity associated with the Kuroshio acts as a barrier, impeding the seaward propagation of internal waves generated at the shelf break onshore of the Kuroshio front.