Kyung-Il Chang

Low-Frequency Current Observations in the Korea/Tsushima Strait*

Abstract High resolution, continuous current measurements made in the Korea/Tsushima Strait between May 1999 and March 2000 are used to examine current variations having time periods longer than 2 days. Twelve bottom-mounted acoustic Doppler current profilers provide velocity profiles along two sections: one section at the strait entrance southwest of Tsushima Island and the second section at the strait exit northeast of Tsushima Island. Additional measurements are provided by single moorings located between Korea and Tsushima Island and just north of Cheju Island in Cheju Strait. The two sections contain markedly different mean flow regimes. A high velocity current core exists at the southwestern section along the western slope of the strait for the entire recording period. The flow directly downstream of Tsushima Island contains large variability, and the flow is disrupted to such an extent by the island that a countercurrent commonly exists in the lee of the island. The northeastern section is marked b...

Connectivity of the Taiwan, Cheju and Korea Straits

Insight into the circulation of the East China Sea and origin of the Tsushima Current are investigated through direct, concurrent measurements of velocities through the Taiwan, Cheju, and Korea Straits. Current data are obtained from six bottom-mounted acoustic Doppler current profilers (ADCPs) arrayed along a section spanning the Korea Strait, a single bottom-moored ADCP in the Cheju Strait, and four bottom-moored ADCPs along a section spanning the Taiwan Strait. Mass transports are computed for the October–December, 1999 time period. In addition, temperature and salt transports are examined in conjunction with climatological values of temperature and salinity. Average volume transport is 0.14 Sverdrups (Sv) through the Taiwan Strait, 0:59 Sv for the Cheju Strait, and 3:17 Sv for the Korea Strait. Salt and temperature transport through the Korea Strait and into the Japan/East Sea are 110:48 � 10 6 kg=s and 0:24 � 10 15 watts (W), respectively. Heat loss in the East China Sea is approximately 200 W=m 2 : Winds affect the transports in each of the straits. Most noticeable wind effects are observed in the Taiwan Strait where strong north wind events force flow into the South China Sea. The main source for the Tsushima Current and its flow into the Japan/East Sea is clearly the Kuroshio for fall, 1999. Published by Elsevier Science Ltd.

Transition to Aperiodic Variability in a Wind-Driven Double-Gyre Circulation Model

Multiple equilibria as well as periodic and aperiodic solution regimes are obtained in a barotropic model of the midlatitude ocean’s double-gyre circulation. The model circulation is driven by a steady zonal wind profile that is symmetric with respect to the square basin’s zonal axis of north‐south symmetry, and dissipated by lateral friction. As the intensity of the wind forcing increases, an antisymmetric double-gyre flow evolves through a pitchfork bifurcation into a pair of steady mirror-symmetric solutions in which either the subtropical or the subpolar gyre dominates. In either one of the two asymmetric solutions, a pair of intense recirculation vortices forms close to and on either side of the point where the two western boundary currents merge to form the eastward jet. To the east of this dipole, a spatially damped stationary wave arises, and an increase in the steady forcing amplifies the meander immediately to the east of the recirculating vortices. During this process, the transport of the weaker gyre remains nearly constant while the transport of the stronger gyre increases. For even stronger forcing, the two steady solution branches undergo Hopf bifurcation, and each asymmetric solution gives rise to an oscillatory mode, whose subannual period is of 3.5‐6 months. These two modes are also mirror-symmetric in space. The time-average difference in transport between the stronger and the weaker gyre is reduced as the forcing increases further, while the weaker gyre tends to oscillate with larger amplitude than the stronger gyre. Once the average strength of the weaker gyre on each branch equals the stronger gyre’s, the solution becomes aperiodic. The transition of aperiodic flow occurs through a global bifurcation that involves a homoclinic orbit. The subannual oscillations persist and stay fairly regular in the aperiodic solution regime, but they alternate now with a new and highly energetic, interannual oscillation. The physical causes of these two oscillations—as well as of a third, 19-day oscillation—are discussed. During episodes of the high-amplitude, interannual oscillation, the solution exhibits phases of either the subtropical or subpolar gyre being dominant. Even lower-frequency, interdecadal variability arises due to an irregular alternation between subannual and interannual modes of oscillation.

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.

Currents in Korea‐Tsushima Strait during summer 1999

Results are presented from continuous current measurements across Korea-Tsushima Strait between May and October 1999. The data are from eleven bottom-mounted Acoustic Doppler Current Profilers that recorded full-depth profiles of currents along two lines, one at each end of the Strait. The two sections show markedly different mean flow regimes. At the southern entrance, the cross-section flow varies smoothly across the channel, showing a broad maximum at mid-channel. The northern section is marked by strong spatial variability but in the mean consists of two streams, one on each side of the strait. Between the two is a regime of highly variable flow with a weak mean, presumably indicating the wake from Tsushima Island. Flow variability in time is described by statistical measures and by representative snapshots.

Regime-Dependent Nonstationary Relationship between the East Asian Winter Monsoon and North Pacific Oscillation

AbstractThe East Asian winter monsoon (EAWM) and the North Pacific Oscillation (NPO) constitute two outstanding surface atmospheric circulation patterns affecting the winter sea surface temperature (SST) variability in the western North Pacific. The present analyses show the relationship between the EAWM and NPO and their impact on the SST are nonstationary and regime-dependent with a sudden change around 1988. These surface circulation patterns are tightly linked to the upper-level Ural and Kamchatka blockings, respectively. During the 1973–87 strong winter monsoon epoch, the EAWM and NPO were significantly correlated to each other, but their correlation practically vanishes during the 1988–2002 weak winter monsoon epoch. This nonstationary relationship is related to the pronounced decadal weakening of the Siberian high system over the Eurasian continent after the 1988 regime shift as well as the concomitant positive NPO-like dipole change and its eastward migration in tropospheric circulation over the N...

Hydrography around Dokdo

CTD data taken in the Ulleung Basin between 1996 and 2001 are analyzed to understand the hydrography around Dokdo. Major features occurring in the Ulleung Basin such as the path variability of the East Korean Warm Current (EKWC), the location and size of the Ulleung Warm Eddy (UWE) and the position of the Offshore Branch along the Japanese coast all influence the hydrography around Dokdo. The Dokdo area frequently lies in the eastern part of the meandering EKWC and the UWE that results in a filting of isolines sloping upwards to Dokdo in the Ulleung Interplain Gap (UIG) between Ulleungdo and Dokdo. Subsurface water near Dokdo then becomes colder and less saline than water near Ulleungdo. Two cases that are opposite to this general trend are also identified when the Dokdo area is directly affected by the EKWC and by a small scale eddy ffd by the Offshore Branch. High salinity cores and warm waters are then found near Dokdo with isolines sloping upwards to Ulleungdo. Freshening of the East Sea Intermediate Water was observed in the UIG when neither the EKWC nor the UWE was developed in the Ulleung Basin during June-November 2000.

Summer primary productivity and phytoplankton community composition driven by different hydrographic structures in the East/Japan Sea and the Western Subarctic Pacific

The East/Japan Sea (EJS) is a highly productive marginal sea in the northwest Pacific, consisting of three basins (Ulleung Basin: UB, Yamato Basin: YB, and Japan Basin: JB). To find causes of the reportedly high primary productivity in summer in the EJS, especially in the UB, we measured primary productivity, phytoplankton composition, and other environmental variables. The water column was strongly stratified in the EJS compared with the Western Subarctic Pacific (WSP). Integrated primary productivity was two times higher in the EJS (612 mg C m−2 d−1) than in the WSP (291 mg C m−2 d−1). The vertical distributions of physicochemical and biological factors confirmed that production in the subsurface chlorophyll maximum layer in the study regions was an important factor regulating primary productivity within the water column. While picoplankton (<2.7 µm) dominated in the WSP, JB, and YB, micro/nanoplankton (≥2.7 µm) dominated in the UB. Contribution by picoplankton to total biomass and primary productivity in the UB was significantly lower than in the other regions. CHEMTAX analysis using marker pigments showed that diverse phytoplankton groups inhabited the study regions. Cluster and canonical correspondence analyses showed high correlation between the spatial variation in phytoplankton assemblages with the water mass properties mainly represented by water temperature and nitrate concentration. Overall, our results suggest that the hydrographic structure of water column in the study region is an important controlling factor of the biomass and productivity of phytoplankton as well as their diversity in size and taxonomic groups.

Oceanography of the East Sea (Japan Sea)

Introduction.- History.- Geography.- Physical Oceanography.- Introduction.- Circulation.- Water masses.- Inflow-outflow system.- Air-Sea Interaction.- High frequency variation.- Remote Sensing.- Chemical Oceanography.- Introduction.- Nutrients and dissolved oxygen.- Carbon cycle.- Trace elements.- Tracer geochemistry.- Sediment Geochemistry.- Mass Balance and Box Model.- Biology & Fisheries.- Introduction.- Phytoplankton & Primary Production.- Microbiology.- Zooplankton.- Benthic Algae.- Fish & Fisheries.- Benthos & Benthic Process.- Mammals.- Food Web.- Marine Geology.- Introduction.- Physiography.- Geophysics.- Stratigraphy.- Paleoceanography.- Sedimentary environments.

Multidecadal wind‐driven shifts in northwest Pacific temperature, salinity, O2, and PO4

The North Pacific gyre boundaries are characterized by stark contrasts in physical and biogeochemical properties. Meridional movement of gyre boundaries, influenced by climate change, can therefore exert a large influence not only on marine ecosystems but also on climate. We examine the evidence for wind-driven southward shifts in subsurface temperature, salinity, PO4, and O2 within the northwest Pacific from the 1950s to the 2000s. Gyre boundary shifts can explain 30–60% of temperature and salinity trends zonally averaged in the northwest Pacific and observed PO4 and O2 trends along the 137°E and 144°E meridians. The close tie between the wind-driven shifts in gyre boundaries and the tracer distributions is further supported by results from an eddy-resolving (0.1° × 0.1°) Geophysical Fluid Dynamics Laboratory climate model, suggesting that the physical and biogeochemical properties averaged within the northwest Pacific gyre boundaries closely follow the latitude changes of the zero Sverdrup stream function with lags of 0 to 3 years. The gyre shift effect on tracer distribution is poorly represented in a coarse resolution (1° × 1°) model due partly to poor representations of fronts and eddies. This study suggests that future changes in northwest Pacific PO4 and O2 content may depend not only on ocean temperature and stratification but also on the ocean gyre response to winds.