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Norihisa Imasato

Abstract

To study the relation between the water-volume exchange rates E v and the salt-exchange rates E z of the Akashi and the Naruto Straits in the Seto Inland Sea of Japan, we use an observed salt distribution near the straits, the calculated Eulerian velocity of the M2 current and the calculated location of about 20 000 labeled particles tracked during an M2 period. The salt-exchange rate E z was about one-third and one-twelfth that of the water-volume exchange rate E v for the Akashi Strait and the Naruto Strait, respectively. In general, the exchange rates of E z and E v do not equal each other, because E z largely depends on the spatial distribution of salt near a strait. Therefore, E z is not an adequate measure for the tidal exchange. We found that the salt distribution near the straits and the salt-exchange rates E z were dependent on the deformation of a water column and that this deformation is produced not only by the velocity shear of the nonlinear tidal current but also by tide-induced transient eddies.

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Norihisa Imasato

Abstract

We carried out a numerical experiment to study the velocity field of a two-dimensional tidal current in a simple model basin with a narrow strait. It was found that the tide-induced transient eddy (TITE) originated from the low pressure area that is generated downstream behind a headland by the nonlinearity or the centrifugal force of the tidal current flowing with a large curvature through a narrow channel. The transient eddy is maintained during certain phases of the tide, and therefore the Eulerian tide-induced residual current is the result of the averaging process of transient phenomena; the Eulerian residual current is only a mathematical representation of the tide-induced transient eddy and has no physical reality. We should abandon the concept of residual velocity.

The lifetime of TITE depends on the magnitude of vorticity and its dissipation. In an inner basin with large bottom friction, the eddy diminishes within a short time (one or two hours) after the generation, and the pressure gradient of the eddy is smaller than the pressure gradient of tidal flow in the strait at the time of high-tide stack water. In this case, TITE is swept away by the ebbing tidal current which flows outward through the strait and soon disappears. On the other hand, in a basin with small bottom friction, because the eddy grows so strong at the time of the start of the ebb tide that the pressure gradient becomes larger than that of the ebbing tide, it is maintained by the ebbing tidal current which runs around the eddy toward the strait. In the latter case, vorticity dissipation caused by horizontal eddy viscosity is larger than that due to bottom friction because of a large horizontal velocity shear near the eddy.

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Norihisa Imasato and Bo Qiu

Abstract

Low-salinity water masses were occasionally observed in spring and summer on the surface of the Kuroshio, south of Japan. Many of the masses were accompanied by excessive discharge of fresh water from major rivers in southern Japan and were observed 20–40 days after the discharges. The salinity difference between the low-salinity water mass and the Kuroshio was closely proportional to the river water discharge.

To specify their origin, the behavior of a water mass seen on 10 August 1979 was studied in detail. A large number of labeled particles were deployed in the water mass and were tracked numerically by the Euler-Lagrangian method. The fresh water forming the low-salinity water mass in the Kuroshio was concluded to originate in the Seto-Inland Sea and the Tosa Bay, and 58% of the river water discharge was sufficient to form the low-salinity water mass. When a particle in the Kuroshio flows along the southern coast of Japan, it receives coastal water which is discharged in excess due to snow melting in spring and heavy rainfall such as that caused by a stagnating front, a cyclone or a typhoon. The parcel of coastal water is estimated to move from the river mouth to the continental shelf region at a mean speed of 5–10 cm s−1.

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Toshiyuki Awaji, Kazunori Akitomo, and Norihisa Imasato

Abstract

The barotropic response of the shelf and coastal regions south of Japan to short-term variations in the Kuroshio was studied numerically with an inflow–outflow model. The onshore–offshore movements of the stream axis of the Kuroshio due to changes in the upstream volume transport have an important effect on shelf and coastal circulations off the coast of Japan. When the Kuroshio comes near the shelf south of Japan, topographic eddies of about 1.0 × 105 m diameter are produced on the shelf behind the capes by the separation of the Kuroshio from the subsurface tip of spurs at a depth of about 200 m projecting from the capes into the sea. On the other hand, when the Kuroshio moves away from the shelf, the eddies disappear. This implies that the periodic formation and disappearance of the eddies takes place on the shelf due to the combined effect of the short-term onshore–offshore movements of the Kuroshio axis and the irregular topography of the continental margin. Phenomena similar to the present experimental results are frequently detected in satellite imagery. This combined effect is one of the most important factors which control the shelf and coastal currents south of Japan. Tracking numerous labeled particles in the calculated velocity field by the Euler–Lagrangian method clearly showed that a large amount of the Kuroshio water is trapped in the eddy on the shelf. The net transport of the Kuroshio water to the shelf region over one event of the onshore–offshore movements was estimated to be 6 × 1012 m3 from the particle distributions. The net volume is as much as 20% of the water volume of the shelf and coastal region and shows that an effective water exchange takes place between the shelf and the Kuroshio. This is one of the major processes of water exchange between the shelf region south of Japan and the Kuroshio.

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Toshiyuki Awaji, Norihisa Imasato, and Hideaki Kunishi

Abstract

In order to investigate the mechanism of tidal exchange through a strait, we numerically track the Lagrangian movement of water particles over a full cycle of the M2 tide. As a result, it is found that the spatially rapid changes of the amplitude and the phase lag of the M2 current in the vicinity of the strait cause the exchange of an extremely large amount of water through the strait. The tidally-induced residual circulation in the vicinity of the strait also plays an important role in the water exchange. The calculated exchange ratio over one tidal cycle is ∼87%, i.e., the greater part of the outer water entering into the inner basin through the strait stays in the inner basin while an equal amount of basin water is drawn out after a cycle of the M2 tide. This fact also suggests that the major part of the water exchange through a strait is generated, not by turbulent diffusion, but by the dynamic process of the tidal current.

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Kazunori Akitomo, Norihisa Imasato, and Toshiyuki Awaji

Abstract

We studied numerically the frontogenesis of shallow sea fronts such as are observed in the Kii Channel, Japan, during winter, under conditions of sea surface cooling and buoyancy influx from coast and open ocean. Numerical experiments were carried out in a vertically two-dimensional basin with a new model (NH-model), without using the hydrostatic approximation and the convective adjustment.

Considering the vertical acceleration term in the momentum equation, intermittent gravitational convections with a large aspect ratio were produced in the frontal region to intensify the horizontal convergence and to strengthen the horizontal density gradient. Consequently, a front in the tracer distribution had a sharpness comparable to the observed front in the Kii Channel and 3.6 times the sharpness in an H-Model, using the hydrostatic approximation and the convective adjustment. In the present model situation, this effect of gravitational convections on sharpening a front in the NH-model is equivalent to that of the 10-time cooling rate in the H-model on the time average. Moreover, the convections intermittently intensified the sharpness of the front up to 2.4 times the time-averaged value: In the H-model, such a sharp front could not be formed even if the cooling rate was increased 10 times.

Further, we discussed the effect of eddy viscosity and diffusivity on frontogenesis.

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