The generation of large-amplitude unsteady lee waves by subinertial K-1 tidal flow: A possible vertical mixing mechanism in the Kuril Straits

Numerical experiments with a two-dimensional nonhydrostatic model are perfo rmed to investigate tidally generated internal waves in the Kuril Straits a nd their effect on vertical mixing. The results show that sill-scale intern al waves at the K-1, tidal frequency are confined to the sill slopes becaus e the K-1, tide is subinertial in the Kuril Straits. In contrast to previou s theories, the authors show that intense short internal waves generated at the sill breaks by the subinertial K-1, tidal current can propagate upstre am as the tidal current slackens. Theoretical considerations identify these short waves as unsteady lee waves, which tend to be trapped at the generat ion region and:row into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagatin g unsteady lee wave and a newly generated lee wave over a sill causes signi ficant wave breaking lending to a maximum vertical diffusivity of similar t o 10(3) cm(2) s(-1). This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M-2, tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as first-mode internal tides and be cause the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of lee waves through interactions between the K-1, current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the form ation of the NPIW.