5. Summary
The Okhotsk Sea has been thought of as the origin of low salinity in the North Pacific intermediate layer, whose manifestation is the North Pacific Intermediate Water (NPIW). To confirm this interpretation, we need to clarify the mechanism responsible for intense vertical mixing in the Kuril Straits and its effect on water modification, especially on freshening of the intermediate layer. To clarify the mechanism of this vertical mixing, we numerically investigated internal waves generated through the interaction between the tidal currents (predominant in the Kuril Straits) and the sill and their effect on mixing, using a nonhydrostatic f-plane model.
The model results reveal the following. Internal waves at the tidal frequency (internal tides) generated by the K1 flow are trapped to the sill, because the K1 tide is subinertial in the Kuril Straits. Since the strong barotropic flow is further intensified near the bottom by the trapped internal tides, large-amplitude short free-propagating internal waves are repeatedly generated and break around the sill top, thus producing relatively vertically-uniform water over and around the sill top. On the other hand, in the M2 case, most of the generated internal waves propagate away as 1st-mode internal tides, conveying energy away from the sill. Moreover, because the barotropic flow amplitude is small and its period is short, their amplitudes do not become so large (thus wave breaking does not occur). As a result, the M2 tidal current does not have the potential to cause strong vertical mixing enough to explain the freshening in the Kuril Straits.
Our theoretical consideration identifies the large-amplitude short waves in the K1 case as unsteady lee waves, whose existence has been neglected in previous oceanic internal wave theories. Because the intrinsic frequencies of unsteady lee waves are higher than the tidal frequency, the excitation of freely-propagating waves (superinertial) by a subinertial K1 flow is naturally explained. Moreover, these unsteady lee waves, particularly those excited at the break on the downstream side of the sill, are effectively trapped and amplified. Describing the propagation of unsteady lee waves, we showed that the waves are effectively trapped at the excitation region to form large-amplitude internal waves. Furthermore, the superposition of a propagating unsteady lee wave and a newly generated lee wave over the sill generates a large-amplitude internal wave, which gradually causes wave breaking and induces intense vertical mixing over the sill top.
Although internal tides have a secondary effect on mixing in the Kuril Straits at high latitudes, they are expected to induce significant vertical mixing in the subtropical and tropical regions, where the superinertial semidiurnal tides dominate. In such regions, not only unsteady lee waves but also internal tides are likely to be effectively amplified. The latter is especially so when the condition of the critical slope (e.g., Wunsch . 1969) and/or the critical Froude number(Hibiya 1986) is satisfied. These effects are interesting problems for future theoretical and observational studies.
References
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Craig, P. D., 1987: Solutions for internal tidal generation over coastal topography. J. Marine. Res., 45, 83-105.
