The Generation of Large-Amplitude Unsteady Lee Waves by SubinertialK1Tidal Flow: A Possible Vertical Mixing Mechanism in the Kuril Straits

Abstract

Numerical experiments with a two-dimensional nonhydrostatic model are performed to investigate tidally generated internal waves in the Kuril Straits and their effect on vertical mixing. The results show that sill-scale internal waves at the K1 tidal frequency are confined to the sill slopes because the K1 tide is subinertial in the Kuril Straits. In contrast to previous theories, the authors show that intense short internal waves generated at the sill breaks by the subinertial K1 tidal current can propagate upstream as the tidal current slackens. Theoretical considerations identify these short waves as unsteady lee waves, which tend to be trapped at the generation region and grow into large-amplitude waves, eventually inducing vigorous mixing along their ray paths. In particular, superposition of a propagating unsteady lee wave and a newly generated lee wave over a sill causes significant wave breaking leading to a maximum vertical diffusivity of ∼103 cm2 s−1. This quite intense mixing reaches down to the density layer of the North Pacific Intermediate Water (NPIW). In contrast, the M2 tidal current does not cause such strong vertical mixing, because most of generated internal waves propagate away as first-mode internal tides and because the barotropic flow amplitude is small. The authors therefore suggest the possibility that generation of lee waves through interactions between the K1 current and the bottom topography of the Kuril Straits contributes to the observed modification of the Okhotsk Sea water required in the formation of the NPIW.