Young-Gyu Kim

Mean flow and variability in the southwestern East Sea

Abstract The Ulleung Basin is one of three deep basins that are contained within the East/Japan Sea. Current meter moorings have been maintained in this basin beginning in 1996. The data from these moorings are used to investigate the mean circulation pattern, variability of deep flows, and volume transports of major water masses in the Ulleung Basin with supporting hydrographic data and help from a high-resolution numerical model. The bottom water within the Ulleung Basin, which must enter through a constricted passage from the north, is found to circulate cyclonically—a pattern that seems prevalent throughout the East Sea. A strong current of about 6 cm s −1 on average flows southward over the continental slope off the Korean coast underlying the northward East Korean Warm Current as part of the mean abyssal cyclonic circulation. Volume transports of the northward East Korean Warm Current, and southward flowing East Sea Intermediate Water and East Sea Proper Water are estimated to be 1.4 Sv (1 Sv =10 −6 m 3 s −1 ) , 0.8 Sv , and 3.0– 4.0 Sv , respectively. Deep flow variability involves a wide range of time scales with no apparent seasonal variations, whereas the deep currents in the northern East Sea are known to be strongly seasonal.

The East Sea (Japan Sea) in change : A story of dissolved oxygen

Dissolved oxygen (DO) is one of the most important oceanographic parameters measured for understanding various physicochemical processes in the ocean. This situation has been particularly true for the East Sea study ever since the first extensive investigation in the area during the 1930s (Uda, 1934). Uda found very high and uniform concentrations of DO, around 250 μM (5.6 ml/l), for waters below a few hundred meters over entire basins, and assumed that a very fast ventilation system was operating in the East Sea. The Circulation Research of the East Asian Marginal Seas (CREAMS), Japan-Korea-Russia international cooperative studies on the East Sea have provided a unique opportunity to investigate the entire East Sea for the first time since Udas study. A spectrophotometrically modified Winkler method (Pai et al., 1993) and a DO sensor (Sea Bird Model SBE 13) were tested successfully during the CREAMS studies for improving the precision and accuracy of DO measurement. The study further confirmed an earlier observation by Gamo et al. (1986) that DO structures in the East Sea have been changing drastically in such a way that the DO minimum depths have deepened by more than 1000 meters during the last 30 years. While the causes for these changes are not known at the present time, the analysis of DO profiles strongly suggests that the mode of deep water ventilation system in the East Sea has shifted from bottom water formation in the past to intermediate water formation at the present time (Kim and Kim, 1996). Studies of precise and accurate DO monitoring, along with other chemical tracers, deserve the highest priority for future research in the East Sea.

Connectivity among straits of the northwest Pacific marginal seas

[1]xa0The connectivity among straits of the northwest Pacific marginal seas is investigated with a primitive-equation ocean circulation model simulated for 10 years from 1994 to 2003. Over the simulation interval the temporal and spatial means and variations of the model sea surface temperature are comparable to those of the satellite sea surface temperature. The model transport through the straits shows good agreement with the available observations and a high seasonality in the Taiwan Strait, the Korea Strait, and the Soya Strait but relatively low seasonality in the Tsugaru Strait. The Kuroshio and Taiwan Warm Current (TWC) are two sources of water flowing through the Korea Strait. The volume transport in the Korea Strait is dominated by the Kuroshio in winter (83%) and by the TWC in summer (66%). Relative to the transport through the Korea Strait, the transport percentages of the Tsugaru Strait connecting to the northwest Pacific Ocean are 79% in winter and 65% in summer. The seasonality of the Korea Strait transport is positively correlated with the cross-strait wind stress. The drifter experiments show that it takes about 4 months for most of the drifters deployed in the Taiwan Strait to enter the Korea Strait and more than 2 months to travel from the Korea Strait to the Tsugaru and Soya straits.

Sea level variability at Ulleung Island in the East (Japan) Sea

[1]xa0Monthly adjusted sea level (ASL) was analyzed to examine the characteristics of sea level variation at Ulleung Island in the East (Japan) Sea from 1979 to 1992. It is remarkable that the nonseasonal variation is comparable with the mean seasonal variation in amplitude at Ulleung Island, whereas the mean seasonal variation is dominant along the eastern coast of the Korean peninsula. At Ulleung Island the variation in the mean seasonal, interannual, and short-term component variability is 54, 13, and 30% of the total ASL variance, respectively. ASL variability is highly correlated with the upper layer heat content around Ulleung Island. There are two factors that contribute to large nonseasonal variation of the ASL around the island. One is the location of the polar front that divides the northern cold water region and the southern warm water region. The other is the presence of a warm eddy that frequently forms near Ulleung Island. When the northern cold water region extends southward to Ulleung Island, the heat content of the upper layer decreases, and ASL around the island falls. In contrast, when the southern warm water region extends northward to the island, the heat content increases, and ASL rises greatly. The movement of the front with a period of 1–2 years causes interannual variation of ASL. When a warm eddy approaches Ulleung Island, the heat content increases, and ASL rises. The short-term variation in ASL is caused primarily by the migration of the warm eddy around Ulleung Island.

Diagnosing long-term trends of the water mass properties in the East Sea (Sea of Japan)

[1]xa0Steady changes of temperature and oxygen concentration since the 1950s have been observed in the East Sea. In addition to temperature and oxygen, this study extends the analysis to salinity observations. Salinity has increased between 300 and 1000 m and decreased below 1500 m with a trend of about 0.06 and −0.02 psu/century, respectively. A simple diagnostic inverse model indicates that the changes in the intermediate depth (300–1000 m) were primarily due to a change in salinity at the surface outcrop, and those in the deep and bottom water were due to not only a warming of the surface source water but also a circulation change in the deep and bottom water, with similar magnitudes of contributions from the two factors. Both the observations and model consistently suggest that the water mass structures have changed during the last few decades.

Application of Real-Time Monitoring Buoy Systems for Physical and Biochemical Parameters in the Coastal Ocean around the Korean Peninsula

The development of real time ocean monitoring buoys and their application in the coastal ocean around the Korean Peninsula provide implications for future directions of buoy monitoring. In order to increase payload capacity allowing additional sensors to be incorporated, power supply and data need to be enhanced to a maximum. With regards to two- way telemetry, communication utilizing larger quantities of data, commands, and responses needs to be pursued in the future.Other efforts toward the improvement of mooring design would open the possibility of integrating additional sensors for the seafloor and the earth below, combining the advantages of a subsurface mooring with the capacities of a surface mooring.

Typhoon‐induced, highly nonlinear internal solitary waves off the east coast of Korea

[1]xa0Highly nonlinear internal solitary waves (ISWs) propagating both onshore and offshore were detected in a SAR image taken approximately 19 hours after typhoon MAEMIs passage across the east coast of Korea. Analysis of ocean buoy data suggests that the ISWs were generated by near-inertial waves in response to typhoon wind. The near-inertial waves can propagate seaward due to a downwelling coastal jet (positive relative vorticity offshore of the jet), and interact with sharply varying topography, producing the ISWs. The area of sharply varying topography approximately 28 km off the coast is suggested as a source region for the ISWs. Simple calculations of wave speed based on the two-layered Korteweg-deVries (KdV) equation with upper layer thickness and densities at both layers fixed indicate that the ISWs were generated 6 hours prior to the time of the acquisition of the SAR image (approximately 13 hours after the typhoon passage), consistent with simultaneous buoy measurements.

A Research on the Magnitude/Phase Asymmetry Measurement Technique of the RF Power Amplifier Based on the Predistortive Tone Cancellation Technique

This paper proposes a novel memory effect measurement technique in RF power amplifiers(PAs) using a two-tone intermodulation distortion(IMD) signal with a very simple and intuitive algorithm. Based on the proposed predistortive tone cancellation technique, the proposed measurement method is capable of measuring the relative phase and magnitude of the third-order and fifth-order IMDs, as well as the fundamental signal. The measured relative phase between the higher and lower IMD signal for specific tone spacing can be interpreted as the group delay(GD) information of the IMD signal concerned. From the group delay analysis, we can conclude that an adaptive control of GD as well as the magnitude and phase is a key function in increasing the linearization bandwidth and the dynamic range in a predistortion(PD) technique.

Water Masses and Their Long-Term Variability

We here review previous studies of spatiotemporal variability of surface and subsurface water properties in the East Sea (Japan Sea), including (1) sea surface temperature (SST) and mixed layer depth (MLD), (2) water masses, and (3) their long-term variability. Intra-annual, annual, and interannual variability of SSTs and SST-front structures are introduced together with a description of regionally adapted SST calibration methods. The MLD has distinct spatial patterns in its seasonal variability, which is determined by atmospheric forcing as well as advection effects. Water masses in the East Sea are listed in a comprehensive manner, covering not only subsurface water masses (e.g. Intermediate Waters) formed in the northern part of the sea but also near-surface water masses (e.g. Tsushima Warm Water) mostly originating from outside the sea. Where previous studies are available, we attempt to describe the definition, formation, and fate of the various water masses. Also, in terms of climate change in the East Sea, long-term variability found in the water masses or water properties is presented and its possible relationship with interannual to decadal variability of atmospheric forcing and heat content in the North Pacific is also considered.

A Study on Ray Tracing Method for Wave Propagation Prediction with Acceleration Methods

In this paper, we proposed an improved ray tracing method with an amelioration of visible tree structure, a visible face determination method, and non-uniform random test point method. In a proposed visible tree structure, it reduces tree nodes by means of merging similar nodes. In a visible face determination method, it shows that a ray hit test with a packet ray method can reduce a test time. A ray tracing method involving with a packet ray hit test method can improve a tree construction time up to 3.3 times than a ray tracing method with a single ray hit test method. Furthermore, by seeding a non-uniform and random test point on a face, tree construction time is improved up to 1.11 times. Received powers from the proposed ray tracing results and measured results have good agreement with 1.9 dB RMS error.