Kuril Straits Research Articles

Tidal Exchange through the Kuril Straits

The tidal exchange between the Okhotsk Sea and the North Pacific Ocean is studied numerically with particular emphasis on the predominant K1 barotropic component. The calculated harmonic constants of the K1 tide in and around the Okhotsk Sea agree well with those obtained from extensive tide gauge observations. The features of the simulated tidal fields are similar to those reported in the literature. Since the K1 tide is subinertial in the Okhotsk Sea, topographically trapped waves are effectively generated, contributing to strong tidal currents with a maximum amplitude of over 1.5 m s−1 in the Kuril Straits. The structures of tide-induced mean flows in most passages of the straits are characterized by “bidirectional currents” (in which the mean flow exhibits a reversal in direction across the passages). This feature is clearly indicated in NOAA infrared imagery. The mean transport shows significant net exchange of water via several straits in the Kuril Islands. A transport of about 5.0 Sv (1 Sv ≡ 106 m3 s−1) toward the North Pacific is produced by the K1 tide, primarily through the Bussol, Kruzenshterna, and Chetverty Straits. Analysis reveals that the bidirectional mean currents at shallow passages are produced through the well-known process of tidal rectification over variable bottom topography, whereas in deep passages such as Bussol Strait, propagating trapped waves along the islands are essential for generating the bidirectional mean currents. Particle tracking clearly demonstrates these features. The tidal current is therefore thought to play a major role in water exchange processes between the Okhotsk Sea and the North Pacific.

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

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.

Concentration of pelagic organisms at mesoscale fronts in the western subarctic Pacific: small fish on long waves

ABSTRACTAcoustical backscatter observations conducted off the Kuril Straits (Boussole and Vries Straits) in the western subarctic Pacific show increased concentrations of mesopelagic fish (suspected to be fish of the family Myctophidae) associated with long (4–40 km), high‐amplitude (about 100 m) internal waves. Such wave packets are thought to be generated by tidally driven flows through the deep Kuril Island straits. Because myctophids are a small, subarctic fish that must remain in cold waters, it is assumed that their increased concentration is due to an ability to sense flow direction and take advantage of the horizontal flow convergence in these long waves. Because vertical motions in waves are weak, small fish can easily maintain their preferred depth of habitat, and their concentrations tend to increase along horizontal convergence lines formed by wave orbital velocities.

An Okhotsk Sea water anomaly: implications for ventilation in the North Pacific

An unusually cold, fresh and oxygenated layer of water centered at a pressure of 800 dbar and σθ of 27.4 was found at a CI'D station in the western Pacific at 43°5'N. I 53°20'E in August 1985. The anomaly was part of a larger pattern of less dramatic but nevertheless higher variance at densities up to 27.6 σθ in the mixed water region of the Oyashio and Kuroshio. south of the Bussol' Strait, which connects the Sea of Okhotsk and the open North Pacific. Isopycnal maps indicate that the source of the anomaly, which was emhedded in a cyclonic flow, was the Okhotsk Sea. Surface properties in the Okhotsk Sea, based on all available NODC observations, and isopycnal maps indicate that the layer probably did not originate at the sea surface in open water. Instead, the principal modifying influences at densities of 26.8–27.6 σθ in the North Pacific are sea-ice formation and vertical mixing. the latter primarily in the Kuril Straits. A simple calculation shows thaI most of the low salinity influence al these densities in the North Pacific can originate in the Okhotsk Sea and that vertical mixing in the open North Pacific may be much less important than previously thought.