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Shuhei Masuda and Kazunori Akitomo

Abstract

The effects of stratification and bottom topography on the Kuroshio path transitions in a multiple equilibrium regime due to a short-term velocity increase are examined using a two-layer inflow–outflow model with simplified coastal geometry and bottom topography. For an imposed velocity increase on a straight-path state, the typical transition from a straight to a meandering path occurs with or without bottom topography through the same process as in a barotropic case with flat bottom. Thus, the geometrical effect of Kyushu is essential to this transition or to the formation of a small meander triggering the transition; stratification and bottom topography are somewhat secondary. Nevertheless, under the influence of stratification, a small meander significantly develops south of Kyushu during a decreasing phase of velocity through shoaling of the interface depth and triggers the transition with the amplification rate of velocity A mp of 1.5. The continental slope south of Japan prevents a small meander from developing and then the transition needs A mp of 2.0. With subcritical A mp, the transition to a C-type path occurs in almost all cases via similar eastward progression of a small meander. On the other hand, stratification and bottom topography cause significant differences from a flat-bottom barotropic case when a short-term increase in velocity is imposed on a meandering-path state. Stratification causes cyclonic eddy shedding from the tip of the meandering segment resulting in the reduction of its amplitude, and the formation of a distinct small meander, which progresses eastward to coalesce with the reduced meandering segment. The continental slope along the southern coast of Japan, on the other hand, enhances eddy shedding and suppresses development of a small meander. As a result, the reduced meandering segment finally changes into a C-type path with bottom topography, while an initial meandering path is recovered in a flat-bottom ocean. Similarities to observations are found to support the present results.

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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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Shuhei Masuda, Kazunori Akitomo, and Toshiyuki Awaji

Abstract

Numerical experiments are executed using a two-layer inflow–outflow ocean model with simplified geometry to investigate the effects of stratification and bottom topography on the path variation of the Kuroshio south of Japan. In a flat-bottom ocean, the dependence of the Kuroshio path selection on its inflow velocity V max is basically the same as in a barotropic ocean, that is, the Kuroshio takes a straight path at low V max (regime I), a meandering path at high V max (regime II), and both paths at intermediate V max (regime III: multiple equilibrium state). However, the range of regime III shifts to higher V max by 0.10∼0.30 m s−1. Stratification causes and maintains the offshore shift of the current path south of Kyushu through the conservation of potential vorticity. As a result, a small meander stagnates southeast of Kyushu, not developing into a large meander even for higher V max since the vorticity supply from the coast is reduced. For the same reason, higher V max is needed to maintain a meandering path. Bottom topographic features such as a continental slope and ridge significantly change the path selection. A straight path appears for the whole experimental range of V max and a meandering path only exists at intermediate V max. The continental slope along the southern coast of Japan captures the main flow to permit a straight path for all V max. Further, the Izu Ridge inhibits a meandering path hanging over the western flank of the ridge and narrows the range of V max in which the meandering state exists. The transition induced by monotonic changes of V max is significantly affected for a jump from a straight to a meandering path but not affected as much for a reverse jump. In a flat-bottom ocean, the enlargement of a small meander due to baroclinicity causes a quick transition from a straight to a meandering path with a large velocity change while the transition is impeded for a small velocity change. No transition occurs by a monotonic increase in V max with bottom topography since a straight path can exist for all V max.

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Yutaka Yoshikawa, Toshiyuki Awaji, and Kazunori Akitomo

Abstract

The formation and circulation processes of intermediate water in the Japan Sea have been investigated by study of the subduction of mixed layer water. To simulate realistic seasonal variations in the velocity and hydrographic structures, a numerical model with a nudging method for potential temperature and salinity, which reproduced the general features in the Japan Sea, is used. Close investigation of the subduction process reveals two major formation areas (A and B) of intermediate water. Area A (41°∼43°N, west of 135°E) corresponds to the region reported by recent observations, whereas Area B (40°∼43°N, east of 136°E) has not been reported so far. The mixed layer water subducted in Area A is advected southwestward and eventually its upper portion (above 200 m) reaches the eastern part of the Japan Basin, whereas the lower branch (below 200 m) reaches the Tsushima Basin. This indicates that the East Sea Intermediate Water originates from the mixed layer in Area A, and suggests that the East Sea Intermediate Water and the upper portion of the Japan Sea Proper Water represent the same type of intermediate water. In contrast, the water subducted in Area B is advected northward and some of it flows out through the Soya Strait, while another portion is reentrained into the mixed layer off the Primorye coast. Tracking of the subducted water particles clearly shows that the southward transport of the intermediate water takes a seasonally varying path: for example, a path along the continental coast in winter and one along the Japanese coast in summer. The total formation rate of the intermediate water is estimated to range between 0.48 and 0.69 (×106 m3 s−1) according to the strength of nudging terms, and the corresponding range in ventilation time is 20.3∼25.6 years.

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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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Yoichi Ishikawa, Tochiyuki Awaji, Kazunori Akitomo, and Bo Qiu

Abstract

A simultaneous assimilation model of drifting buoy and altimetric data is proposed to determine the mean sea surface height (SSH) as well as the temporal evolution of the surface circulation on synoptic scales. To demonstrate the efficiency of our assimilation model, several identical twin experiments for the double-gyre circulation system are performed using a 11/2-layer primitive equation model. An optimal interpolation for the multivariate is used for the assimilation scheme that assumes the geostrophic relationship between the error fields of the velocity and the interface depth. To identify the nature of the assimilation of the buoy-derived velocities into the dynamical ocean model, the authors first conduct the assimilation experiment using the drifting buoy data alone. The result shows that realistic buoy deployment (32 in a 40° square) can effectively constrain the model variables; that is, both the absolute (mean plus time varying) velocity and SSH (interface depth) fields are significantly improved by this buoy data assimilation. Moreover, in the case of denser buoy deployment in the energetic western boundary current regions, where the mean SSH is comparable to the time-varying part and the geoid error is relatively large, the assimilation provides a better determination of the absolute velocity and SSH. This is because significant changes in the mean SSH lead to an improvement along the extensive buoy trajectories associated with the strong current. It is worth noting that the assimilation of drifting buoy data is more effective than that of moored velocity data, thanks to the Lagrangian information content of the drifting buoys. Successive correction of the mean SSH is made with simultaneous assimilation of drifting buoy and altimetric data. Consequently, a better correction of the mean SSH is obtained: The initial error of the mean SSH is reduced by approximately 40% after the 1-year experiment. In contrast, the assimilation experiment of altimetric data alone corrects only the time-varying part, but yields little error reduction for the mean SSH in our model. These results clearly show that the simultaneous assimilation of drifting buoy and altimetric data into the dynamical model is a very useful tool for improving the model's realism.

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Tomohiro Nakamura, Toshiyuki Awaji, Takaki Hatayama, Kazunori Akitomo, and Takatoshi Takizawa

Abstract

The tidal exchange between the Okhotsk Sea and the North Pacific Ocean is studied numerically with particular emphasis on the predominant K 1 barotropic component. The calculated harmonic constants of the K 1 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 K 1 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 K 1 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.

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Tomohiro Nakamura, Toshiyuki Awaji, Takaki Hatayama, Kazunori Akitomo, Takatoshi Takizawa, Tokihiro Kono, Yasuhiro Kawasaki, and Masao Fukasawa

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 ∼103 cm2 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 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.

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