spatial distribution of diapycnal mixing rates in the deep waters of the North Pacific

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The mechanical energy which drives the interior mixing in the oceans is believed to be originally supplied at large scales and then transferred across the deep ocean internal wave spectrum down to small dissipation scales. In recent numerical experiments (Hibiya et al., 1996, 1998, 2002), this energy cascade process was shown to be dominated by parametric subharmonic instabilities which transfer energy from the low vertical wavenumber, double-inertial frequency wave band to a high vertical wavenumber, near-inertial frequency wave band. The resulting enhanced, high vertical wavenumber, nearinertial current shear was shown to play a key role in controlling turbulent mixing processes.In this study, to test whether or not the numerically predicted energy cascade process is actually dominant in the real deep we deployed a total of 106 expendable current profilers over a large area in the North Pacific to examine the spatial distribution of high vertical wavenumber (vertical wavelength 〜25 m … More ) shear. At mid latitudes, significant enhancement of the 25-m vertical shear was found over prominent generation regions of semidiurnal internal tides such as the Hawaiian Ridge and the Izu-Ogasawara Ridge where the semidiurnal tidal frequency exceeds twice the local inertial frequency. At high latitudes, in contrast, no significant enhancement of the 25-m vertical shear was found to occur even over another prominent generation region of semidiurnal internaUtides, the Aleutian Ridge, where the semidiurnal tidal frequency is less than twice the local inertial frequency. We find that the observed spatial distribution of the intensity of the 25-m vertical shear correlates very well with that of the low vertical wavenumber, double-inertial frequency internal wave energy numerically predicted by Nagasawa et al. [2000] and Niwa and Hibiya [2001]. This is the first in-situ evidence for the dominant role of parametric subharmonic instability in transferring deep ocean internal wave energy down to small dissipation scales. This study provides a theoretical framework for future attempts to determine the large-scale structure of mixing over the world's oceans. Less