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Yang-Ki Cho and Kuh Kim

Abstract

Hydrographic studies show the seasonal variation of the East Korean Warm Current (EKWC), which is a branch of the Tsushima Current along the Korean coast. To understand the dynamics of the branching mechanism of the TC in the Korea Strait, a hydraulic model with two active layers was investigated in a rectangular strait with varying depth. When the lower cold water flows southward in a shallow meridional channel from the deep northern basin, it separates from the eastern boundary because of the sloping bottom to conserve potential vorticity. After separation, the lower layer hugs the western boundary as the channel becomes shallow. In a region where the lower layer is absent due to separation, the northward flow in the upper layer has a positive relative vorticity to conserve potential vorticity because the bottom topography becomes deeper from south to north. The northward velocity has its maximum on the eastern boundary. This mechanism may explain the formation of the branch along the Japanese coast. The upper layer along the western boundary experiences shrinking of its water column because of the presence of the lower layer, and negative relative vorticities are induced to conserve potential vorticity. The negative relative vorticity intensifies the northward flow of the upper layer near the western boundary. This is believed to be the causal mechanism of the EKWC. If the top of the lower layer in the basin is deep, such as it is in winter, the lower layer cannot reach the strait since the Bernoulli potential of the lower layer is small. This may explain why the EKWC is absent in winter.

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Sang Ho Lee and Kuh Kim

Abstract

The scattering solution around a small cylindrical island in a shelf sea of uniform depth is derived for Sverdrup, right-bounded Poincare, and Kelvin waves, which includes linear bottom friction, and the solution is extended to the subinertial frequency range. Effects of scattering on the amplitude and phase vary, depending on the type of incident waves. A Sverdrup wave scattering near the inertial frequency produces large amplitude and phase differences around the island due to singularity effect. However, the singularity effect does not happen for Poincare and Kelvin waves, even though the amplitude and phase variation depends on bottom friction and wave frequency. For an observer looking down the direction of wave propagation around the island, the maximum amplitude due to Sverdrup wave scattering occurs on the left-hand side, and the phase difference increases more than twice that by incident wave propagation. Scattering of Poincare waves at a superinertial frequency for an island located at a fixed distance from the straight coast produces its maximum amplitude on the right-hand side and at a subinertial frequency on the leeward coast. In the case of Kelvin wave scattering, the amplitude attenuates by frictional damping along the direction of wave propagation around the island and phase difference increases as much as twice that by incident wave propagation. Application of these theoretical results to tides around Cheju Island, off the south coast of Korea, suggests that the amplitude and phase variations of the M 2 and O 1 tides are due to the scattering of those tides comprised of Sverdrup and Kelvin waves having superinertial and subinertial frequencies.

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Jong Jin Park, Kuh Kim, Brian A. King, and Stephen C. Riser

Abstract

Subsurface ocean currents can be estimated from the positions of drifting profiling floats that are being widely deployed for the international Argo program. The calculation of subsurface velocity depends on how the trajectory of the float while on the surface is treated. The following three aspects of the calculation of drift velocities are addressed: the accurate determination of surfacing and dive times, a new method for extrapolating surface and dive positions from the set of discrete Argos position fixes, and a discussion of the errors in the method. In the new method described herein, the mean drift velocity and the phase and amplitude of inertial motions are derived explicitly from a least squares fit to the set of Argos position fixes for each surface cycle separately. The new method differs from previous methods that include prior assumptions about the statistics of inertial motions. It is concluded that the endpoints of the subsurface trajectory can be estimated with accuracy better than 1.7 km (East Sea/Sea of Japan) and 0.8 km (Indian Ocean). All errors, combined with the error that results from geostrophic shear and extrapolation, should result in individual subsurface velocity estimates with uncertainty of the order of 0.2 cm s−1.

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Kuh Kim, Sang Jin Lyu, Young-Gyu Kim, Byung Ho Choi, Keisuke Taira, Henry T. Perkins, William J. Teague, and Jeffrey W. Book

Abstract

Voltage induced by the Tsushima Current on an abandoned submarine telephone cable between Pusan, Korea, and Hamada, Japan, has been measured since March 1998 in order to monitor the volume transport through the Korea Strait. Voltage has a good linear relationship with the transport measured by bottom-mounted acoustic Doppler current profilers (ADCPs) along a section spanning the Korea Strait. The linear conversion factor is estimated to be Λ0 = (8.06 ± 0.63) × 106 m3 s−1 V−1 with the reference voltage of V 0 = 0.48 ± 0.07 V. The voltage-derived transport reveals various temporal variations that have not been known previously. Measurement of the cable voltage provides a reliable means for continuous monitoring of the volume transport of the Tsushima Current, which determines the major surface circulation and hydrography in the East Sea.

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Sok Kuh Kang, Young Ho Seung, Jong Jin Park, Jae-Hun Park, Jae Hak Lee, Eun Jin Kim, Young Ho Kim, and Moon-Sik Suk

Abstract

Trajectories of Argo floats deployed in the East/Japan Sea from 2001 to 2014 reveal that the middepth gyral circulation pattern of the Japan basin, the central part of the East/Japan Sea, undergoes a seasonal variation. The middepth circulation of the Japan basin is found to be characterized usually by the gyres trapped to the east of the Bogorov Rise (E-gyres) and those extending farther westward into the whole basin (BW-gyres). The E-gyre trajectories are generally associated with the turning of the floats toward deeper regions off the isobaths. This occurs in winter either on the northern or eastern side of the eastern Japan basin. It seems that the upstream part of the otherwise BW-gyre is subject to a strong negative wind stress curl in winter, and there the circulating water columns are driven toward the deeper region, thus triggering the formation of the E-gyre. The topographic effect associated with the Bogorov Rise seems to interfere thereafter in the process of determining the passage of the E-gyre. Otherwise, the water columns continue to flow along the isobaths, hence maintaining the BW-gyre. To the knowledge of the authors, this is the first observational evidence of seasonal variability in the middepth gyral circulation pattern in the East/Japan Sea. It suggests that oceanic middepth circulation, usually known to be quasi steady or slowly varying on climatological time scales, might also undergo a significant seasonal variation as it does in the East/Japan Sea.

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