Yang-Ki Cho

Branching Mechanism of the Tsushima Current in the Korea Strait

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 ...

An evaluation of the thermal properties and albedo of a macrotidal flat

estimated heat capacity of the upper 0.1 m is 3.65 � 10 6 Jm � 3 K � 1 with a water content of 70%. Heat capacity decreases with depth to 2.96 � 10 6 Jm � 3 K � 1 at 0.4 m depth. Estimated thermal diffusivities are 0.47–0.63 � 10 � 6 m 2 s � 1 and 0.38–0.64 � 10 � 6 m 2 s � 1 in spring and summer, respectively. The calculated albedo is a strong function of the solar altitude and the atmospheric transmittance. Atmospheric transmittance is especially important to the albedo when the solar altitude is low. Seasonal mean albedos are 0.13 and 0.15 in spring and summer, respectively. The heat capacity and albedo values obtained above were verified by using them to make independent heat flux estimates at other stations. Estimates based on heat capacity were correlated to albedo-based heat flux estimates with an r 2 greater than 0.7.

Tidal Effects on Intermediate Waters: A Case Study in the East/Japan Sea

Although tides are believed to be the most important source for diapycnal mixing in the ocean, few studies have directly simulated open-ocean circulation including tides. Because the East/Japan Sea (EJS) has been considered to be a ‘‘miniature ocean,’’ tidal effects on the intermediate water of the EJS are investigated by using an eddy-resolving ocean general circulation model that can take account of M2 and K1 tides as well as oceanic flows. The simulated temperature and salinity in the intermediate layer are significantly improved by including tides. The improvement results from the combined effect of two internal tides. The M2 internal tide, propagating into the interior of the EJS, enhances vertical mixing and brings watermass characteristics closer to those observed. The K1 internal tide, trapped along the coast, induces southwestward flow along the Russian coast in the upper layer and thereby enhances the so-called Liman Current, which transports fresh waters with density conducive to the ventilation of intermediate waters in the EJS.

Year-to-year Variability of the Vertical Temperature Structure in the Youngsan Estuary

Long-term observations were conducted between 1997 and 2002 to examine the variability of the vertical temperature structure in the Youngsan Estuary, southwest Korea, in summer. The observed hydrographic data revealed that the temperature minimum layer in the middle depth persisted through the entire summer of 2000 but was rarely observed in other years. The variability in the vertical structure might be affected by the air temperature during the previous winter and the density difference between the open sea and the estuary. In 2000, the air temperature in the previous winter was lowest and the horizontal density difference during summer was largest. The large horizontal density difference probably produced more active driving of warm water along the bottom, which would have intruded into the Youngsan Estuary. Furthermore, the cold previous winter would have provided a better condition for maintaining cold temperatures in the middle water layer for a longer period.

Implementation of the Ensemble Kalman Filter into a Northwest Pacific Ocean Circulation Model

The Ensemble Kalman Filter (EnKF) was implemented to an ocean circulation modeling system of the Northwest Pacific Ocean. The study area includes the northwestern part of the Pacific Ocean, the East China Sea, the Yellow Sea and the East/Japan Sea. The numerical model used for the system was the Regional Ocean Model System, which is a 3-dimensional primitive-equation ocean circulation model. The performance of EnKF was evaluated by assimilating satellite-observed Sea Surface Temperature (SST) data into the numerical ocean model every 7 day for year 2003. SST data were obtained from 30 fixed points at a time. The number

Runoff-induced vertical thermal dynamics in a canyon-shaped reservoir during the summer monsoon

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Reproduction and population dynamics of Leptochela gracilis (Decapoda: Pasiphaeidae) on the western coast of Korea, Yellow Sea

of ensemble members used in this study was 16. Without localization of covariance matrix, ensemble spread (EnSP) drastically decreased due to rank deficiency and the large correlation between two distant state variables. To resolve the ensemble collapse, localization of covariance matrix was performed and EnSP did not collapse throughout the experiment. Root -mean-square (RMS) error of SST from the assimilative model (RMS error= 2.2°C) was smaller than that of the non-assimilative model (RMS error= 3.2°C). This work provides promising results that can be further explored in establishing operational ocean prediction systems for the Northwest Pacific including its marginal seas.

Tide‐induced residual circulation in a bay with laterally asymmetric depth

During the summer rainy season, double thermoclines were observed in a small canyon-shaped reservoir. The physical processes leading to thermocline evolution are examined from the vertical temperature profile observed along the reservoir before and after rain. Observations show that their evolution is related to the inflow of runoff, which is colder than the reservoir surface water and post-rain fair-weather conditions. Tongue-like distributions of turbidity, conductivity and nutrient concentrations downstream from the headwater clearly reveal the presence of runoff-induced intermediate inflows. In addition to supplying nutrients, the inflow provides the oxygen-deficient intermediate layer with a rich supply of dissolved oxygen. Concurrently, in the upper part of the reservoir runoff-induced inflows may drive the oxygen-deficient bottom water to shift downstream along the layer beneath the runoff-induced inflow. The water mass between the two thermoclines may operate as a source of nutrients for algal development in early autumn when the upper thermocline is destroyed by the convective overturn owing to the surface cooling.

ENSO Teleconnection to the Isopycnal Depth Fluctuations of the East/Japan Sea Intermediate Water in the Ulleung Basin during 1968–2002

Investigations were made on reproduction and population dynamics of Leptochela gracilis on the western coast of Korea, Yellow Sea, between May 2000 and October 2001. The egg volume was significantly larger at later egg stage than at early egg stage. Brood loss did not occur during the incubation period. Based on dry weight, the reproductive output (mass of incubating eggs/mass of female) averaged 0.18. The mature females first appeared in May, reached a peak in July and August, and then did not appear after September. The main breeding season was summer, although slightly different between the two years. A similar pattern could be found in monthly changes of gonadosomatic index (GSI), showing relatively higher GSI during the annual breeding season. A significant difference in ovarian dry weight between females with non-eyed eggs and eyed eggs indicates that ovarian maturation occurred during the embryonic development, suggesting that females were potentially consecutive breeders, capable of multiple spawning within a reproductive season. The size at which 50% of females were mature (CL 50 ) was estimated as 7.63 mm carapace length. Females grew faster and reached a larger size at age than males ( L ∞ =12.43 mm CL and K =0.90 y −1 for females, and L ∞ =12.22 mm CL and K =0.58 y −1 for males).

Water masses and decadal variability in the East Sea (Sea of Japan)

The tide-induced residual current is a crucial determinant of the distribution of sediment in a tidally dominated bay. There have been only a few studies of residual currents in bays with laterally asymmetric depth, although the residual circulation in bays with symmetric lateral depth variation has been studied extensively. Gomso Bay is a typical bay with laterally asymmetric depth. The bay is characterized by a deep channel to the north and a wide shoal to the south. This study reports the tide-induced residual current in Gomso Bay and explains its dynamics using numerical and analytical models. The results from the models are consistent with observations. Residual flows show a clockwise gyre inside the bay and counter-clockwise circulation near the mouth. Three physical causes induce residual circulation: 1) bottom friction coupled with a tidal height, 2) advection causing inward residual transport, and 3) a pressure gradient due to a surface slope induced by subtidal flow. The maximum inward residual current is shifted toward the wider shoal. This is caused by across-channel advection due to the laterally asymmetric depth.