Distribution of fine‐scale shear in the deep waters of the North Pacific obtained using expendable current profilers

Recent numerical experiments [ Hibiya et al. , 1996, 1998, 2002] predicted that the energy cascade across the internal wave spectrum down to small dissipation scales was under strong control by parametric subharmonic instabilities that transfer energy from the low vertical wave number, double‐inertial frequency wave band to a high vertical wave number, near‐inertial frequency wave band. To test whether or not the numerically predicted energy cascade process is actually dominant in the real deep ocean, 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 wave number (vertical wavelength ∼25 m) shear. At midlatitudes, 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 internal tides, the Aleutian Ridge, where the semidiurnal tidal frequency is less than twice the local inertial frequency. We find that the spatial distribution of the intensity of the 25 m vertical shear correlates very well with that of the low vertical wave number, double‐inertial frequency internal wave energy numerically predicted by Nagasawa et al. [2000] and Niwa and Hibiya [2001a, 2001b]. 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.