No 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 K 1 tidal frequency are confined to the sill slopes because the K 1 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 K 1 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 ϳ10 3 cm 2 s Ϫ1 . 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 because 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 formation of the NPIW.

Microstructure measurements were made in the Mixed Water Region of the Oyashio/Kuroshio/Tsugaru currents system where both turbulence and double diffusion are involved in mixing. While intense turbulence is observed near the front between the Oyashio and the Tsugaru Current, double diffusion occupies a noticeable fraction in both the Tsugaru Water and the Mixed Water between the Oyashio and the Kuroshio. After determining a criterion to distinguish double diffusion from turbulence, vertical diffusivities and buoyancy fluxes are estimated using microstructure data. When turbulence is weak, double diffusion is observed around temperature and salinity anomalies, partly due to interleaving, and dominates the buoyancy flux. Vertical diffusivities due to double diffusion are parameterized as a function of the 10-m-scale density ratio. The 10-m-scale diffusivity estimates are consistent with the microstructure data when an appropriate criterion to reproduce a probability density function for the Turner angle is applied. A weighted-average diffusivity model is proposed to account properly for turbulence and double diffusion simultaneously.

No abstract