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Kimio Hanawa
and
Yasushi Yoshikawa

Abstract

By using the accumulated datasets of CTD-XBT comparison experiments since 1985, the depth errors for both T-7 and T-6 probes were reexamined. All the XBT probes used here were manufactured by the Tsurumi-Seiki Company, Limited, Japan. The same method as that of Hanawa and Yoritaka was adopted for the detection of depth error. The empirical depth-time equation for T-7 probes newly obtained from an average of all datasets was very similar to that by Hanawa and Yoritaka: depth difference between the corrected and uncorrected data was about 25 m at 800 m. The new equation for T-6 probes based on a single dataset also showed that the depth diference between the corrected and uncorrected data was greater than 10 m at 500 m. It was confirmed that the free-fall velocity estimated by the XBT manufacturer considerably underestimates the actual velocity for both T-7 and T-6 probes.

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Yasushi Fujiwara
and
Yutaka Yoshikawa

Abstract

Wave-resolving simulations of monochromatic surface waves and Langmuir circulations (LCs) under an idealized condition are performed to investigate the dynamics of wave–current mutual interaction. When the Froude number (the ratio of the friction velocity of wind stress imposed at the surface and wave phase speed) is large, waves become refracted by the downwind jet associated with LCs and become amplitude modulated in the crosswind direction. In such cases, the simulations using the Craik–Leibovich (CL) equation with a prescribed horizontally uniform Stokes drift profile are found to underestimate the intensity of LCs. Vorticity budget analysis reveals that horizontal shear of Stokes drift induced by the wave modulation tilts the wind-driven vorticity to the downwind direction, intensifying the LCs that caused the waves to be modulated. Such an effect is not reproduced in the CL equation unless the Stokes drift of the waves modulated by LCs is prescribed. This intensification mechanism is similar to the CL1 mechanism in that the horizontal shear of the Stokes drift plays a key role, but it is more likely to occur because the shear in this interaction is automatically generated by the LCs whereas the shear in the CL1 mechanism is retained only when a particular phase relation between two crossing waves is kept locked for many periods.

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Ming Feng
,
Humio Mitsudera
, and
Yasushi Yoshikawa

Abstract

Four years of mooring array measurements in Tokara Strait, south of Kyushu, Japan, from 1992 to 1996 are used to analyze the structure and temporal variability of the Kuroshio Current. The mean Kuroshio current in Tokara Strait shows a nearly permanent subsurface double-core structure, possibly due to topographic blockage effects.

The dominant variations of the Kuroshio in Tokara Strait are separated into long-term variability (typically 100-day period) and short-term variability (10 days to 1 month) according to spectrum and wavelet analysis. The long-term variability has a large horizontal scale across the strait, with a striking twofold banded structure in spatial correlations. This is due to the north–south Kuroshio axis shift that advects the double-core structure of the mean current. The axis shift can be indexed with the northeastward current velocity at the northernmost station; a composite analysis using this index shows well-defined northward and southward axis shift structures of the Kuroshio current. From the composite of the TOPEX/Poseidon sea level anomaly in terms of this index, the Kuroshio axis shift and the current structure change are associated with a dipole-shape sea level anomaly east of Tokara Strait. On the other hand, the short-term variability of high kinetic energy only has a small horizontal scale within the northern part of the current, which is related to frontal variability.

There exists a deep southwestward undercurrent below 600 m in the northern part of Tokara Strait, flowing along the isobaths. The undercurrent becomes stronger during the northward shift of the Kuroshio axis, while it almost disappears during the southward shift.

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Yasushi Fujiwara
,
Yutaka Yoshikawa
, and
Yoshimasa Matsumura

Abstract

Fujiwara et al. explicitly simulated Langmuir circulations using a wave-resolving simulation (WRS) technique and found that the residual wave effect on vorticity was well represented by the vortex force of the Craik–Leibovich (CL) equation, at least in the simulated situation. In response to the simulation results, Mellor has proposed a view that ubiquitous applicability of the CL formulation is still questionable and that the three-dimensional radiation stress (3DRS) formulation that he has derived encompasses both of the vortex force effect and an effect that is lower order in terms of wave steepness. Here, these opinions are discussed in terms of the approximations used in the wave-averaged formulations. The asymptotic expansion of the Eulerian-averaged momentum equation allows the separate discussion of two different wave effects: pressure correction and torque. It is argued that the approximation adopted in Mellor’s 3DRS formulation is presumably not accurate enough to properly parameterize the wave torque effect, and possible approaches to examine its performance are proposed. We agree with the view that the applicability of the CL formulation needs further investigation. WRS will be a helpful tool for this purpose.

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Humio Mitsudera
,
Bunmei Taguchi
,
Takuji Waseda
, and
Yasushi Yoshikawa

Abstract

This paper discusses the role of the Izu Ridge in blocking the Kuroshio large meander from propagating eastward across the ridge. It is shown that a combination of the sloping bottom with baroclinicity in the Kuroshio flow is important for blocking of the large meander. It produces a cyclonic torque over the western slope of the ridge when the large meander impinges upon it. That is, the cyclonic torque is formed ahead of the large meander, which results in blocking and amplification of the meander upstream of the Izu Ridge. The baroclinicity of the Kuroshio over the ridge is caused by baroclinic topographic Rossby waves generated when the large meander encounters the ridge.

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Yasushi Fujiwara
,
Yutaka Yoshikawa
, and
Yoshimasa Matsumura

Abstract

The present study performs a wave-resolving simulation of wind-driven currents under monochromatic surface gravity waves using the latest nonhydrostatic free-surface numerical model. Here, phase speed of the waves is set much greater than the current speed. Roll structures very similar to observed Langmuir circulations (LCs) appear in the simulation only when both waves and down-wave surface currents are present, demonstrating that the rolls are driven by the wave–current interaction. A vorticity analysis of simulated mean flow reveals that the rolls are driven by the torque associated with wave motion, which arises from a correlation between wave-induced vorticity fluctuation and the wave motion itself. Furthermore, it is confirmed that the wave-induced torque is very well represented by the curl of the vortex force (VF), that is, the vector product of mean vorticity and Stokes drift velocity. Therefore, it is concluded that the simulated rolls are LCs and that the wave effects are well represented by the VF expression in the present simulation. The present study further revisits the scaling assumptions made by previous studies that derived VF formulation and shows that there is disagreement among the previous studies regarding the applicability of VF formulation when the wave orbital velocity (proportional to the amplitude times the frequency) is much smaller than the mean flow velocity. The result from the present simulation shows that the VF expression is still valid even with such small wave amplitudes, as long as phase speed of the waves is much greater than the current speed.

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Humio Mitsudera
,
Bunmei Taguchi
,
Yasushi Yoshikawa
,
Hirohiko Nakamura
,
Takuji Waseda
, and
Tangdong Qu

Abstract

In this paper, results of a high-resolution regional model of the Kuroshio–Oyashio confluence, where the mixed water region (MWR) forms off the northeastern coast of Japan, are discussed. The model simulates major characteristics of the Kuroshio and the Oyashio system well, such as the separation of the Kuroshio Extension from the Japanese coast and southward intrusion of the Oyashio. Further, potential temperature and salinity structures in the intermediate layer σ θ = 27.0 resemble those obtained from historical data. Upon the success of this simulation, the authors focus on the diagnosis of the Oyashio water pathways intruding into the subtropics. It is found that the pathways of the Oyashio water form in the vicinity of the Japanese coast, where warm core rings and the Oyashio intrusion are active. These pathways are shown to be primarily eddy driven. Of particular interest is the water that originates in the Sea of Okhotsk, characterized by low potential vorticity (PV). Impacts of the Okhotsk water are identified by conducting an experiment in which the exchange of waters between the Pacific Ocean and the Sea of Okhotsk is blocked. The impacts are striking. If the exchange were blocked, the pathways would not form in the MWR. Instead, a strong cyclonic recirculation, caused by separation of the Kuroshio from the Japanese coast, dominates the MWR and advects warm and salty Kuroshio water northwestward, letting it occupy the entire MWR. It is found that the low-PV flux from the subpolar region tends to reduce the cyclonic circulation in the MWR. As a result, a southward intrusion of the Oyashio is induced. Concurrently, this intrusion blocks the northward advection of the Kuroshio water, maintaining the Oyashio water pathways in the MWR.

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