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Journal of Oceanography, Vol. 64, pp. 721 to 735, 2008

Review

Review of Recent Findings on the Water Masses and Circulation in the East Sea (Sea of Japan) K UH KIM 1*, KYUNG-IL CHANG1, DONG-J IN KANG1, Y OUNG HO KIM 2 and JAE-HAK LEE 2 1

Research Institute of Oceanography/School of Earth and Environmental Sciences, Seoul National University, Seoul 151-742, Korea 2 Climate Change and Coastal Disaster Research Department, Korea Ocean Research and Development Institute, Ansan 425-600, Korea (Received 27 December 2007; in revised form 24 March 2008; accepted 26 March 2008)

Recent findings on water masses, biogeochemical tracers, deep currents and basinscale circulation in the East/Japan Sea, and numerical modeling of its circulation are reviewed. Warming continues up to 2007 despite an episode of bottom water formation in the winter of 2000–2001. Water masses have definitely changed since the 1970s and further changes are expected due to the continuation of warming. Accumulation of current data in deep waters of the East/Japan Sea reveals that the circulation in the East/Japan Sea is primarily cyclonic with sub-basin scale cyclonic and anticyclonic cells in the Ulleung Basin (Tsushima Basin). Our understanding of the circulation of intermediate water masses has been deepened through high-resolution numerical studies, and the implementation of data assimilation has had initial success. However, the East/Japan Sea is unique in terms of the fine vertical structures of physical and biogeochemical properties of cold water mass measured at the highest precision and their rapid change with the global warming, so that full understanding of the structures and their change requires in-depth process studies with continuous monitoring programs.

Keywords: ⋅ Sea of Japan, ⋅ water mass, ⋅ biogeochemical tracers, ⋅ warming, ⋅ circulation, ⋅ numerical modeling.

symposia were held regularly to exchange data and new knowledge, leading to the subsequent publication of a large number of papers in major journals. Preliminary results of CREAMS were published as special issues in Journal of Oceanography (1999, Vol. 55, No. 2) and Journal of Marine Technology Society (1999, Vol. 33, No. 1). Results from further analysis of CREAMS data were later published in special issue of Progress in Oceanography (2004, Vol. 61, Issues 2–4). The success of CREAMS was followed by the second phase of CREAMS, lasting another five years from 1998 to 2002 as the Office of Naval Research of the U.S.A. initiated a five-year in-depth observation program, known as JES, in 1998, covering the entire EJS with various instruments which had never been used in the EJS before. The scale and scope of JES were unprecedented, and a wide variety of data were collected using new instruments during this period, allowing detailed studies of currents and circulation, providing new insights into the spatial and temporal variability of currents and circulation, particularly in the southern region of the EJS. Spe-

1. Introduction Extensive oceanographic observations have been carried out in the East/Japan Sea§ (EJS§ hereafter) recently, employing advanced techniques to measure currents and water properties and delivering major changes in our understanding of the circulation in the EJS. In particular, CREAMS (Circulation Research of the East Asian Marginal Seas) opened a new era in 1993 as a comprehensive observation program organized by an international team of scientists from Japan, Korea, and Russia. CREAMS aimed to obtain for the first time high-precision physical and chemical data for detailed water mass analysis and current data from long-term moorings over a period of five years. During CREAMS, workshops and * Corresponding author. E-mail: [email protected] §

The Editor-in-Chief does not recommend the usage of the term “East Sea”, “East/Japan Sea”, or “Japan/East Sea” in place of “Sea of Japan” or “Japan Sea”. Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer

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Fig. 1. (a) Locations of the four representative CTD stations occupied in the eastern Japan Basin, the western Japan Basin, the Ulleung Basin ¶ and the Yamato Basin, denoted by E, W, U and Y and the isobath of 2000 m. Dots indicate stations reported in Watanabe et al. (2001). T-S diagrams in (b), (c) and (d) show temperature-salinity curves taken at four representative stations in three different ranges and resolutions to magnify the variation of salinity at the highest precision.

cial issue of Deep-Sea Research, Part II (2005, Vol. 52, Issues 11–13) and Oceanography (2006, Vol. 19) contain major achievements of the JES. The North Pacific Marine Science Organization, known as PICES, has become interested in the EJS as new findings from CREAMS and JES indicate that the EJS has undergone a rapid change in the structure of physical and chemical variables such as temperature and dissolved oxygen. PICES introduced an interdisciplinary CREAMS/PICES program in 2004 to further understand the relationships between climate change and ecosystems. This program expands the scope of previous physical and chemical research to include biological and ecological aspects of marine sciences. Since the new results from the ten-year CREAMS program together with JES are so vast, as published in several special issues of major journals mentioned above, the first author of this paper was invited to present a summary of recent studies at the 14th PAMS/JECSS Workshop. This paper is based upon this presentation, focusing on the large scale circulation in the EJS; no attempt is made to review other types of current variability. New findings on water mass structure and currents also make it necessary to develop advanced numerical modeling of the circulation in the EJS to simulate new features. A separate section therefore reviews the results of new models. 722

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A cautionary note should be sounded, though: our understanding is evolving for two reasons. First, as new data become available, we gain a deeper understanding of the state of the EJS. Secondly, it is now well known that the EJS itself is evolving so rapidly in time that our understanding cannot be static. 2. New Water Masses and Water Formation 2.1 New water masses in the East/Japan Sea Until CREAMS it had been believed that the EJS was made up of three distinct waters basically, as reviewed by Moriyasu (1972). Uda (1934) found that the EJS was mostly filled with a water mass colder than 1°C with a very high concentration of dissolved oxygen; this he named “Proper Water” (hereafter PW). Moriyasu (1972) mentioned two more water masses in addition to the PW; the Tsushima Warm Water (hereafter TWW) of high salinity which occupies the southern half of the EJS above the permanent thermocline at about 200 m, and intermediate waters between the TWW and the PW. Later, Sudo (1986) found a discontinuity in the profile of the dissolved oxygen which is associated with the potential temperature of 0.1°C and introduced a mode water called the Upper Portion of the PW (UPPW) with a concentration of dissolved oxygen as high as 290 µmol/l.

Temperature and salinity data taken during CREAMS, JES, and subsequent cruises are of the highest precision and accuracy, so they provide an opportunity to re-examine water masses and their vertical structures. T-S diagrams of CREAMS CTD data taken in summer of 1994 show that the T-S relationship for the scale of 0–25°C and 33.50–34.90 psu varies in three ranges, divided by temperatures of 5°C and 1°C, as shown in Fig. 1(b). The salinity of water warmer than 5°C is low in the Japan Basin, but high in the Ulleung ¶ and Yamato Basins, reflecting the influence of the Tsushima Warm Current which carries saline water into this region. Below 5°C salinity in the western Japan and the Ulleung Basins is lower than that in the eastern Japan and the Yamato Basins. This low salinity was previously identified by Kim and Chung (1984) as the major characteristic of the East Sea Intermediate Water (ESIW), together with the high concentration of dissolved oxygen. Recently, Kim and Kim (1999) found that distinctly saline water (>34.07 psu) was prevalent in the eastern Japan Basin, which has a high concentration of dissolved oxygen (>250 µ mol/l) like the ESIW, implying renewal in the preceding winter, and newly named this water as High Salinity Intermediate Water (HSIW). Analyzing the CTD data taken east of St. E in Fig. 1(a) on January 7–18, 1997, Watanabe et al. (2001) found a tongue of high salinity and dissolved oxygen on the surface of sigma-t = 27.2 as an extension of the HSIW as indicated by an arrow in Fig. 1(a). Below 1°C, Fig. 1 shows that four T-S curves merge into one with salinity around 34.07, which is consistent with our historical knowledge of the PW since Uda (1934). However, expansion of the range 0–1°C and 34.05– 34.09 psu close to the highest resolution of 0.001°C and 0.001 psu in Fig. 1(c) revealed a T-S relationship which had never been observed in the EJS before CREAMS. Four T/S curves representing the eastern and western Japan basins, the Ulleung Basin and the Yamato Basin are clearly divided into two groups: two curves for the northern basin and the other two for the southern basins. The T-S relationship of the two curves for the Japan Basin is very tight, particularly in the range of 0.1–0.7°C, as if it were the same water. On the other hand, the other two curves for the Ulleung Basin and Yamato Basin do not show such a tight T/S relationship; the salinity in the southern basins is significantly lower than that in the Japan Basin. It is also apparent that the slope of temperature against salinity is positive in the range 0.1–1.0°C in the Japan Basin, whereas salinity varies little with temperature in the southern basins. It is most intriguing in Fig. 1(c) that the four T-S curves merge into one as temperature approaches about ¶ The Editor-in-Chief does not recommend the usage of the term “Ulleung Basin” in place of “Tsushima Basin”.

Fig. 2. Comparison of potential temperature taken at four different times at locations indicated in insert. Data for 1969 and 1979 are from hydrographic stations and those for 1996 and 2007 are from CTD stations.

0.1°C. As temperature decreases further, salinity increases from less than 34.068 psu to 34.070 psu, which is evident in Fig. 1(d). Salinity thus has a distinct minimum in the Japan Basin at about 1500 m, as shown in figure 3 of Kim et al. (2004). Kim et al. (1996) introduced a new water mass located above this salinity minimum, noting that the concentration of the dissolved oxygen has increased since the 1970s in the depth range of 500–1500 m, which was named the East Sea Central Water (ESCW). Merging of T-S curves below 0.1°C justifies naming the water below this particular temperature as the East Sea Deep Water (ESDW). Below the ESDW a vertically homogeneous layer exists above the bottom, which is naturally called the East Sea Bottom Water (ESBW). 2.2 Long-term change of temperature, dissolved oxygen and salinity Recent warming in the EJS was first unambiguously confirmed by the CREAMS group (Kim et al., 1996), and it now has a significant effect on the water mass structure, as described above. Hydrographic and CTD stations along 132°E have been occupied annually and have become a baseline for continuous monitoring in the EJS since CREAMS. Comparison of the potential temperatures taken at St. W in Fig. 1(a) in 1996 and 2007 indicates that the potential temperature has increased further since 1996 by about 0.05°C in 500–1000 m and 0.013°C in 2000–3000 m in Fig. 2. It is very clear that the warming continues in deep waters below the permanent thermocline. Over a longer period, between 1969 and

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2007, the change below 2000 m is about 0.05°C. It should be recognized that this rate is substantially larger than the global warming of 0.037°C between 1955 and 1998, which was reported by Levitus et al. (2005). During this warming period vertical profiles of dissolved oxygen have also undergone a major change. Gamo et al. (1986) reported for the first time the decrease of dissolved oxygen concentration in deep water, which is associated with the deepening of the depth of its minimum. Later, Kim et al. (2001) revealed a further decrease in dissolved oxygen concentration until recently. Chen et al. (1999) even claimed that the EJS might become anoxic within the next 200 years if the oxygen consumption in the bottom water continues to exceed the replenishment of oxygen by deep-water formation. On the other hand, Kang et al. (2004a) predicted that the EJS may remain well-oxygenated through the modification of the deep water ventilation system by shrinking of its oxygen-depleted deeper waters and an expansion of its oxygen-rich upper waters over the next few decades. One very important piece of information is that during the warming period salinity increased between 300 and 1000 m and decreased below 1500 m, according to Kwon et al. (2004). Utilizing a simple diagnostic model, Kwon et al. (2004) also suggested a change in the vertical mode of ventilation during the past few decades, i.e., from deep and bottom water formation in the past to intermediate water formation at present, and pointed to the resulting water mass structure changes as the main cause of the observed changes in the dissolved oxygen and salinity in the EJS. The diagnostic inverse model also suggested that not only diabatic changes at the surface outcrops but also adiabatic circulation changes could explain a significant portion of the observed changes in the deep and bottom waters. 2.3 Water formation in winter It is most surprising that sudden bottom water formation occurred during the severe winter of 2000–2001, as documented by Kim et al. (2002), Senjyu et al. (2002), Tsunogai et al. (2003), and Talley et al. (2003). An increase in dissolved oxygen and CFC-11 concentration, as well as a decrease in temperature and nutrient concentration for the bottom waters, provides unequivocal evidence that cold, oxygen-rich, and nutrient-depleted surface waters were injected directly to the bottom south of Vladivostok, in the path of cold continental air outbreaks. Since ventilation into the intermediate depth rather than the bottom has been suggested as above, this observation clearly demonstrates that episodic convection may reach the bottom, no matter how small the amount of new bottom water is formed, and this contributes to the spin-up of the thermohaline circulation. Clayson and Luneva (2004) reproduced the 2000 event in a three-dimensional

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numerical model, demonstrating that the confluence of warm and cold water masses at the front south of Vladivostok is the preferred location of deep convection (Seung and Yoon, 1995). A series of recent papers on the sea surface temperature and thermal fronts in the EJS shed some light on the region of water formation in winter. Park, K.-A. et al. (2004) revealed that the Subpolar Front in the central part of the EJS splits into two branches at its eastern and western ends in winter. In a following paper, Park et al. (2005) showed that early cooling and the large annual amplitude of SST change are due to the differential wind pattern off the coast of North Korea and Russia, and a seamount south of Vladivostok plays an important role in cooling after the stratification breaks down in winter. Finally Park et al. (2007) noted a very strong, stationary front extending from the border between North Korea and Russia to the southeast in winter, which is referred to as the Northwestern Branch of the Subpolar Front, and they clearly showed that the extreme heat loss associated with cold air outbreaks off Vladivostok contributes to this front’s genesis. This structure is consistent with the observation of sudden bottom water formation in the winter of 2000–2001. It should be also noted that the surface temperature west of the Northwestern Branch is less than 5°C, but higher than 1°C in January, February and March (see figure 6e in Park et al., 2007), implying that the ESIW is formed in this area off North Korea (see figure 2 in Park et al., 2007). 3. Biogeochemical Tracers and Carbon Cycle 3.1 Biogeochemical tracers CREAMS studies found that the EJS has undergone dramatic changes during the last 50–60 years as the result of a shift in its ventilation system from bottom water formation to intermediate water formation (Kim and Kim, 1996). Some studies (Chen et al., 1999; Gamo, 1999; Kim et al., 2001) confirmed that the bottom water formation of the past, at least during last 50–60 years, has been dramatically reduced. Recently, Kang et al. (2003b) developed a moving-boundary box model (MBBM) to quantify these changes. This model was calibrated with chlorofluorocarbon-11 (CFC-11) and tritium distribution, and showed that the bottom water formation of ~0.02 Sv (1 Sv = 106 m3/s) in magnitude in the past slowed down and came to a complete halt in the mid-1980s and late1990s. Furthermore, the MBBM predicts that the Bottom Water will disappear completely in 2040 as Central Water penetrates deeper. Using the MBBM, Kang et al. (2004b) estimated the levels of several conservative chemical tracers (CFCs, tritium, SF6, 137Cs) and bioactive tracers (oxygen and phosphate) in the deep water masses of the EJS, comparing these with available historical data,

Fig. 3. Vertical profiles of mean CFC-11 concentrations observed in the EJS§ during the 1995 Lavrentyev Expedition (circles), 1996 CREAMS Summer Expedition (inverted triangles), and 1999 JES Expeditions (triangles). Data variation ranges are shown as 1 standard deviation from mean values (after Min and Warner, 2005).

and making predictions for the near future. They also estimated about 4% replenishment of the Bottom Water with fresh surface water in the 2000/2001 winter, which is consistent with others’ results: 8% (Kim et al., 2002) and 3% (Tsunogai et al., 2003). JES provided an epochal opportunity for biogeochemical tracer studies in the EJS. During the JES expedition in 1999, CFC tracers were also observed to study its circulation and ventilation (Min and Warner, 2005). The CFCs penetrate throughout the water column, and the CFC concentrations in the Bottom Water have increased over measurements made in 1995 and 1996 (Fig. 3), indicating a continuous weak renewal of deep waters in the EJS during this recent period. Postlethwaite et al. (2005) observed oxygen isotopes and noble gas to understand the ventilation of water masses in the EJS. In particular, noble gases and oxygen isotopes are indicators of brine rejection. The oxygen isotope data presented indicate that both thermally driven convection and brine rejection have played significant roles in deep water formation but that brine rejection is unlikely to be a significant contributor at present (Postlethwaite et al., 2005). A box model, calibrated with tritium and helium-3 measurements, performed better when a significant decrease of dense-water formation rates in the mid-1960s was incorporated. However, the model calculations suggest that the ESIW formation is still occurring. Subduction of sea-ice melt water may be a significant ventilation mechanism for this water mass, based on an argon saturation minimum at the recently venti-

Fig. 4. Schematic of the water mass ventilation in the EJS§ in 1969 and 1999 inferred from tracer data (after Postlethwaite et al., 2005).

lated salinity minimum in the northwestern sector of the EJS. Oxygen isotope also indicated convection through cooling of water masses from the south of the Subpolar Front (Fig. 4). Oxygen isotopes are used not only to study the ventilation, but also to trace the origin of the Tsushima Warm Current (Kim et al., 2005). These authors suggested that there were two water sources of the Tsushima Warm Current with different paths: the depleted oxygen isotope water originating from the Taiwan Strait, and the enriched oxygen isotope water originating directly from the Kuroshio (Kim et al., 2005). 3.2 Carbon cycle The carbon cycle studies in the EJS have been carried out as part of the CREAMS studies (Kang et al., 2003a). However, they focused on the solubility pump (Oh et al., 1999) among various pumping mechanisms in the ocean, such as the solubility pump at the air-sea interface, the biological pump in the surface waters, the carbonate pump associated with carbonate deposition at the sea floor, and the dynamic pump associated with ocean circulation. In order to arrive at a more accurate estimation of the solubility pump in the EJS, Hahm et al. (2003) attempted to explore the spatial and temporal variations of gas transfer velocity (k) in the EJS, which is primarily controlled by wind stress on the air-sea interface. Thus all parameterizations of k involve wind speed, a rough indicator of wind stress, as one of the independent variables. For three different parameterizations employed, a


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Fig. 6. Plot of phosphate vs. nitrate data obtained from JES cruise in the EJS§ (Talley et al., 2004). A dashed line, the Redfield ratio of 16:1, is added to compare N:P ratio of the EJS§ (solid line).

Fig. 5. Map of anthropogenic CO 2 column inventory (mol C/m 2) in the EJS§. The anthropogenic CO 2 inventory for waters in the northwestern EJS § near North Korea was estimated by extrapolating anthropogenic CO2 concentrations from adjacent waters. Circles represent sampling locations (after Park et al., 2006).

threefold difference of k emerged, depending on the choice of parameterization. The net annual CO2 flux was estimated using the value of k described above and the monthly ∆CO2 values from Oh et al. (1999), which range from 0.034 to 0.11 Gt-C/yr. Recently, Park et al. (2006) reported a basin-wide inventory of anthropogenic CO 2 in the EJS determined from high-quality total alkalinity (TA), chlorofluorocarbon, and nutrient data collected during JES in 1999 and total dissolved inorganic carbon data calculated from pH and TA measurements. Their calculation yields a total anthropogenic CO2 inventory in the EJS of 0.40 ± 0.06 Pg-C as of 1999 (Fig. 5), which is comparable with the distributions found in other major basins such as the North Atlantic, where deep water formation occurs. Anthropogenic CO 2 has already reached the bottom of the EJS, largely owing to the effective transport of anthropogenic CO2 from the surface to the ocean interior via deep water formation in the northern EJS. The biological pump in the EJS was estimated utilizing the stoichiometric ratio between carbon and phosphorus (Kim et al., 2003). A simple phosphate budget model is constructed based on the seawater and dissolved oxygen box model, which can simulate the recent struc-

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tural change in deep water circulation of the EJS. Currently it exports about 0.016 Pg C/yr, which corresponds to 35% of the carbon introduced into the seawater by the air-sea exchange. Consequently, the biological sequestration of anthropogenic carbon is expected to increase with time. The estimated biological uptake of the anthropogenic carbon in the EJS since the Industrial Revolution is estimated as 0.025 Pg C. Water, salt, phosphorus, and nitrogen budgets of the EJS have been calculated by box model analyses using historical data (Yanagi, 2002). The input and output of salt and phosphorus in the EJS are nearly balanced. On the other hand, Yanagi found a significant discrepancy of the nitrogen budget in the EJS, and suggested that this may be due to denitrification. Other studies also suggest the possibility of denitrification in the EJS (Talley et al., 2001; Lee et al., 2007). The N:P ratio values lower than the Redfield ratio observed in most of surface water masses of the EJS (Fig. 6) indicate the possibility of denitrification in the water column. However, the occurrence of in situ denitrification in the EJS is still hard to explain with conventional wisdom due to the high oxygen concentration in the deep waters (Kim et al., 1999; Talley et al., 2006). 4. Direct Observations of Deep Currents 4.1 Observed deep currents in the East/Japan Sea The deep water masses discussed in the previous section are found below about 400 m depth in the EJS. The water column below the 400 m is fairly weakly stratified with potential temperature ranging from 0.07°C to 1.0°C. Hence, it is difficult to quantify deep circulation

and currents using hydrographic data only. It had long been believed that the deep currents in the EJS are very weak before data from direct deep current measurements became available for the Ulleung and Yamato Basins (Kitani, 1987; Lie et al., 1989), and in the Japan Basin after research conducted between 1993 and 1996 as part of the CREAMS Program (Takematsu et al., 1999a). The pioneering moored current measurements in the Japan Basin showed that subsurface current fluctuations below 500 m are mainly barotropic, and seasonal variations of deep currents are dominant with strong currents (>10 cm/s for low-passed currents) in winter. Later, Senjyu et al. (2002) suggested the occurrence of strong, deep currents off Vladivostok in the Japan Basin can be triggered by the wintertime convection based on moored current measurements made at the time when the renewal of bottom water occurred after the severe winter of 2000– 2001 (Kim et al., 2002). The deep water masses found south of the subpolar front can neither be formed by local convection, since surface temperature always exceeds 1°C, nor originate from the inflow through the Korea Strait, since the sill depth is less than 200 m. Hence, the deep water found in the southern basins is thought to originate from the Japan Basin. In particular, the deep water found below 1500 m depth enters the southern basins through deep channels. In the Ulleung Basin, the Ulleung Interplain Gap† (UIG) serves as a unique conduit for the deep water exchange below 1500 m between the Ulleung Basin and the Japan Basin. The UIG is about 2300 m deep, 75 km wide, and 90 km long (Fig. 7). Except for the UIG, the Ulleung Basin is surrounded by shallow topographic features: the Korea Plateau to the north, Korea Strait to the south, and Oki Bank to the east. The Korea Plateau consists of Gangwon Plateau and Ulleung Plateau, and the Usan Trough lies between them. The Usan Trough is about 95 km long and its southern opening is connected with the UIG northeast of Ulleungdo, hence it is another deep passage connecting the Ulleung Basin and the Japan Basin. The Ulleung Plateau is shallower than the Gangwon Plateau, and extends from 2000 m depth at the base to 1000 m depth at its shallowest part. Park et al. (2004) reported a mean circulation at 500~700 m depth range in the Ulleung Basin based on 1,381 displacement data from 24 profiling floats (PALACE, APEX, and PROVOR) obtained between 1998 and 2004. The mean circulation at this intermediate depth level is topographically controlled and characterized by a cyclonic circulation along the periphery of the Ulleung Basin from the east coast of Korea to the west of the Yamato Rise via the southern boundary of the Ulleung Basin. It was conjectured that the southward flow along †

The “Ulleung Interplain Gap” corresponds to the “Oki Gap”.

Fig. 7. Bottom topography of the Ulleung Basin¶. Isobaths are 200, 500, 1000, 1500, 2000, and 2500 m.

the east coast of Korea is a continuation of the North Korean Cold Current (NKCC) originating from the Japan Basin (e.g., Yun et al., 2004). Another notable feature of the intermediate level circulation is an anticyclonic flow around the Ulleung Plateau. The anticyclonic circulation is like a circulation around the Taylor column caused by a vortex column shrinking over the Ulleung Plateau, and only one float crossed the plateau among the 24 floats deployed. Northward flows were found over the Gangwon Plateau. The mean circulation pattern at 500~700 m depth range revealed by the profiling floats could well be extended to deeper depths, considering the barotropic nature of deep flows due to the weak vertical stratification. The abyssal flow in the Ulleung Basin, however, would not turn anticyclonically to flow northward over the Gangwon Plateau, because an anticyclonic turning should imply substantial vertical motion, since the Korea Plateau is about 500 m shallow than the Ulleung Basin. Mean deep flows and variability in the EJS have been investigated on the basis of directly measured current data. Chang et al. (2002) mapped the mean flow vectors below 600 m depth in the entire EJS using moored current data which are longer than 2 months collected between 1986 and 2000, and suggested that a deep cyclonic circulation exists in the Ulleung Basin. Later, Senjyu et al. (2005) updated the data comprising moored current measurements at 69 locations in the EJS collected between 1986 and 2003. According to their analysis, the distribution of mean deep flow vectors shows cyclonic circulations in each of three deep basins with relatively strong flows along the basin periphery and weak flows in the interior regions. Senjyu et al.’ paper did not include results from


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Fig. 8. Mean deep currents from near-bottom current meters and their standard deviation ellipses in the Ulleung Basin¶. Red shows mean currents based on data obtained before 2002, and blue those obtained between 2002 and 2004. Isobaths are 200, 1000, 1500, 2000, and 2500 m, as in Fig. 7.

an extensive moored current observation at 16 locations in the Ulleung Basin during JES. A more detailed abyssal flow field in the Ulleung Basin was reported by Chang et al. (2004a) and Teague et al. (2005) based on these 16 moorings and also moored bottom pressure measurements at 23 locations extending over two years. According to this extensive moored observation, mean abyssal currents ranges from 1 to 4 cm/s with horizontal correlation scales of about 40 km and with integral time scales ranging from 5 to 20 days. The deep circulation of the Ulleung Basin determined by dynamic pressure fields is cyclonic, with an additional sub-basin-scale cyclonic cell over the continental slope off the east coast of Korea and an anticyclonic cell over the Korea Plateau. Understanding and quantification of deep flows through the UIG will be important in understanding the deep circulation and its variability in the Ulleung Basin. An inflow into the Ulleung Basin through the UIG has been evidenced by direct current measurements at a single position (mooring EC1 in Fig. 8) in the middle of the southwestern UIG section (Chang et al., 2002, 2004a). Chang et al. (2002) suggested that the net inflow through the UIG would be balanced by an upwelling interior of the UB, requiring a high rate of vertical diffusion of approximately 10 –3 m 2/s. Results from a fine-resolution numerical model, however, predicted an inflow-outflow structure, which consists of southward deep flows in the western UIG and northward flows in the eastern UIG (Hogan and Hurlburt, 2000). Teague et al. (2005) also 728

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Fig. 9. Major circulation features in the EJS§ reproduced by the HYCOM with 1/25° horizontal grid. Color scale denotes mean sea surface height (after Hogan and Hurlburt, 2006).

conjectured the existence of the outflow through the UIG by examining observed bottom pressure maps. The outflow, however, has not yet been revealed by direct current measurements until recently. 4.2 Mean deep currents in the Ulleung Basin In this paper we further update the map of mean deep flows in the Ulleung Basin by adding some recent moored observations, made after 2002, to the existing data from various sources. Figure 8 shows mean abyssal current vectors together with standard deviation ellipses measured at around 20–200 m from the seabed in the Ulleung Basin. Current data shown in red in Fig. 8 were obtained before 2002, and their characteristics have been reported elsewhere (e.g., Teague et al., 2005). Mean currents based on data obtained between 2002 and 2004 are shown in blue. Moorings U1, U2, EC1, U4, and U5 in the UIG were deployed concurrently between November 2002 and April 2004 to quantify the deep water exchange through the UIG (Chang et al., 2004b). Mooring EC1 has been jointly maintained for longer than 10 years by various institutions in Korea since 1996, and the mooring is still in operation. We include EC1 data here only for the period when it was deployed simultaneously with other moorings in the UIG. According to Fig. 8, broad (~70 km) and weak (~1.2 cm/s) mean abyssal flows are directed into the Ulleung

Basin through the UIG (moorings U1, U2, EC1), and a narrow (~20 km) and strong (2.3–5.2 cm/s) northward flow exists at moorings U4 and U5 in the eastern UIG near Dokdo‡ (indicated as D in Fig. 7). Hence, the abyssal flow in the UIG is characterized by a two-way asymmetric flow structure consisting of a broad, weak inflow into the Ulleung Basin in the western part and a narrow, strong northward outflow in the eastern gap. The numerical model results reported by Hogan and Hurlburt (2000) underestimated the strength of the outflow and showed no asymmetry in the two-way flow structure. Within the Ulleung Basin, abyssal flows are weak in the interior of the basin (moorings M2-2 and EC3) and in the southern part of the basin (moorings M4-1, EC2, M43n, M4-3s, J1). These moorings are characterized by high temporal variability (Teague et al., 2005). The direction of abyssal currents in the southern Ulleung Basin is mainly east and northeast, and the mean abyssal flow is directed northward at the southeastern corner of the basin (mooring J1). Although the abyssal flows are weak and variable at these southern moorings, the standard error for dominant components of mean flows is less than the mean values of those components, indicating that the mean values are statistically reliable. The mean abyssal flows at moorings M2-2 and EC3 are directed to the northwest and southeast, respectively. The standard error is lower than the mean for both components of the mean current at mooring M2-2, while the standard error is higher than the mean at mooring EC3 located in the center of the Ulleung Basin. The mean flow at mooring EC3 is not so well defined due to the high variability of abyssal flow. Strong southward near-bottom flows were observed in the western part of the Ulleung Basin off the east coast of Korea (moorings EC4, M3-1). The two moorings were deployed at locations deeper than 1,250 m. Mean current speeds at these moorings range from 2.5 cm/s to 5.3 cm/s. The observed southward flows in a depth range of 1250–2200 m originate both from the mean cyclonic abyssal circulation in the Ulleung Basin and from farther north, connected with a southward flow observed at mooring M1-1. It has long been suggested that the NKCC flows southward along the east coast of Korea below the northward flowing East Korean Warm Current (EKWC) (e.g., Kim and Kim, 1983). A zonal velocity section running from the east coast of Korea passing moorings EC4 and M3-1 to mooring EC3 showed a southward undercurrent hugging the continental slope region with a maximum southward speed of about 12 cm/s (Chang et al., 2002). The core of the southward flow in 200–400 m depth range corresponds to a layer of subsurface salinity minimum, which defines the North Korean Cold Water carried by ‡ The Editor-in-Chief does not recommend the usage of the term “Dokdo” or “Dok Island” in place of “Take Shima”.

the NKCC. The southward flow extends down to the abyssal plain deeper than 2000 m. Over the Gangwon Plateau, the abyssal current flows to the south at mooring M1-1, while northward flows were found offshore at moorings M1-2, M2-1, and M1-3. The mean current speed at mooring M1-2 is 3.66 cm/s. The northward flows observed over the Gangwon Plateau correspond to the western rim of the anticyclonic subsurface circulation found in a depth range of 500–800 m by subsurface floats (Park et al., 2004). 5.

Numerical Models and Data Assimilation in the East/Japan Sea

5.1 Three-dimensional circulation models In an earlier phase of the numerical studies in the EJS, simple numerical models have been used to investigate the circulation dynamics (Yoon, 1982a, b, c). With the advent of high-performance computers, three-dimensional and high-resolution numerical models with real topography and time-varying meteorological forcing have been widely used (see Table 1). These three-dimensional models have not only reproduced the general circulation in the EJS but also contributed to our understanding of sophisticated phenomena occurring in the EJS, which could not be investigated through the simple numerical models and observational studies. Kim and Seung (1999) as well as Kim and Yoon (1999) reproduced the surface and intermediate circulation of the EJS by three-dimensional numerical models. The z-coordinate level model of Kim and Yoon (1999) with 1/6° horizontal resolution was based on the GFDLMOM2 (Geophysical Fluid Dynamics Laboratory Modular Ocean Model version 2), forced by the seasonally varying in- and outflow, monthly mean meteorological wind stresses, Haney-type heat flux, and the restoring condition for the freshwater flux. Kim and Seung (1999) used the MICOM (Miami Isopycnic Coordinate Ocean Model) of 1/5° horizontal resolution and 10 layers, forced by the constant in- and outflow, monthly mean meteorological wind stresses, and the restoring condition for the surface heat and freshwater fluxes. They resolved the branching of the Tsushima Warm Current and the separation of the EKWC. Furthermore, they reproduced the formation of the intermediate waters such as the North Korean Cold Water and the ESIW by winter convection off Vladivostok, which flowed southward through the intermediate layer into the Ulleung Basin. Yoon and Kawamura (2002) utilized the same model as Kim and Yoon (1999) but using the more realistic reanalyzed wind forcing of ECMWF (European Center for Medium-range Weather Forecasts) to clarify the formation and circulation of the ESIW and the UPPW precisely. In their analysis of the model results, they suggested that


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Table 1. Previous numerical studies of the circulation in the East/Japan Sea§.

Seung and Kim (1997) Kim and Yoon (1999) Kim and Seung (1999) Yoshikawa et al. (1999) Hogan and Hurburt (2000) Lee et al. (2003) Yoon and Kawamura (2002) Hogan and Hurburt (2006) Kim et al. (2007)

Horizontal resolution Grid system

Vertical resolution (Grid. No.)

Vertical coordinates (Model name)

1/2° × 1/2° Arakawa B 1/6° × 1/6° Arakawa B 1/5° × 1/5° Arakawa B 1/4° × 1/4° Arakawa B 1/8°~1/64° Arakawa C 1/6° × 1/6° Arakawa B 1/6° × 1/6° Arakawa B 1/25° × 1/25° Arakawa C (0.06~0.1°) × 0.1° Arakawa B

4 layers

Isopycnal coordinate (MICOM) z-coordinate (MOM2) Isopycnal coordinate (MICOM) z-coordinate with nudging Isopycnal coordinate (NLOM) z-coordinate (RIAMOM and MOM2) z-coordinate (MOM2) forced by ECMWF wind Hybrid coordinate (HYCOM) z-coordinate (MOM3) with 3D-Var

the UPPW is formed in the region southeast off Vladivostok between 41°N and 42°N, west of 136°E, and penetrates into the layer below the ESIW, and the origin of the ESIW is the low salinity coastal water along the Russian coast originating from the Amur River. Recently, Lee et al. (2003) compared model performances between the RIAMOM (RIAM Ocean Model) and GFDL-MOM1 in terms of the surface and intermediate circulation of the EJS. The models with 1/6° horizontal resolution and 19 variable vertical levels were forced by the monthly varying in- and outflow, Haney type heat flux, and the restoring condition for the freshwater flux. Their comparison allowed them to suggest that the RIAMOM has produced more rectified flows on the coastal region, that is, the narrower and stronger NKCC/Liman Cold Current. The finer-resolution NLOM (Naval Research Laboratory’s Layered Ocean Model) and HYCOM (Hybrid Coordinate Ocean Model), configured onto Arakawa’s C grid, have been recently introduced to simulate the EJS circulation. Using NLOM, Hogan and Hurlburt (2000) examined the effects of horizontal and vertical grid resolutions on the circulation of the EJS by varying the horizontal grid size from 1/8° to 1/64° and the number of vertical layers from 1.5 to 4. They also investigated the impacts of the bottom topography and the surface wind forcing. Based on 17 simulations, they suggested that the high resolution model with horizontal grid size of at least 1/ 32° would be required to generate sufficient baroclinic 730

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15 to 600 m (19) 10 layers 20 to 500 m (20) 1.5~4 layers 15 to 600 m (19) 15 to 600 m (19) 15 layers 2.5 to 450 m (42)

instability, which then reproduces eddy-driven cyclonic deep mean flows and the realistic separation of the EKWC. They also proposed that the separation of the EKWC is sensitive to the horizontal grid size, realistic wind stress forcing, and bottom topography. Hogan and Hurlburt (2006) used the HYCOM model with a horizontal grid size of 1/25° and 12 layers to reproduce intrathermocline eddies reported by Gordon et al. (2002) in the EJS (Fig. 9), and they identified at least three mechanisms for the formation of the intrathermocline eddies in the EJS based on the model results. 5.2 Data assimilation The establishment of ocean forecast systems in the EJS has grown recently as substantial amounts of data from satellites and various observation programs such as ARGO and CREAMS programs have been accumulated and computing power has increased. A prerequisite for forecast system development is to devise a data assimilation technique for the model initialization with a sufficient amount of data available for data assimilation (see Table 1). Yoshikawa et al. (1999) incorporated a simple nudging method for potential temperature and salinity into a numerical model to investigate the formation and circulation processes of intermediate water in the EJS. From the model results, they calculated the formation rates for each potential density surface and proposed two major areas where intermediate waters are formed. Even with

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assimilation of temperature and salinity into their model, it could not reproduce the seasonal variation of the NKCC and, correspondingly, the salinity minimum layer lies in a deeper depth range than the observed values. Hirose et al. (1999) applied a layered model equipped with data assimilation based on an approximate Kalman Filter to assimilate the TOPEX/POSEIDON altimeter data. They showed that the assimilation improved the model results, especially south of the Subpolar Front. The approximate Kalman Filter was also applied to the RIAM EJS operational ocean prediction system (http:// oops.riam.kyushu-u.ac.jp/vwp), which is the first operational ocean forecast system in the EJS (Hirose et al., 2007). Kim et al. (2007) applied the three-dimensional variational data assimilation (3D-Var) technique to the EJS Regional Ocean Model based on the GFDL-MOM3 to assimilate the temperature profiles, and satellite-derived SST, and sea surface height anomaly. From the comparison between the model results and observations, they showed that the assimilation system simulated the high variability of the Ulleung Warm Eddy and Dok Cold Eddy as well as the Tsushima Warm Current, and also reproduced the strong southward NKCC in summer and its retreat in winter (Fig. 10), which is consistent with the observational results (Kim and Kim, 1983). While their model well reproduced the meso-scale variability in the Ulleung Basin and the seasonal trend of the NKCC, the current speed of the NKCC and abyssal currents were still weaker than the observed values. 6. Summary and Discussion There is little doubt now that the EJS is different from what it was in the 1930s when Uda (1934) carried out an historical survey of the entire EJS. Measurement of salinity at the highest precision during CREAMS in 1993–1997 and subsequent cruises revealed new water

masses such as the East Sea Central Water (ESCW) and High Salinity Intermediate Water (HSIW). The discovery of the salinity minimum at about 1500 m across the entire Japan Basin raises an interesting question about its origin. No one has yet shown that this minimum can be traced to any surface process. It remains to be shown that this minimum represents a water mass such as the Antarctic Intermediate Water. Comparison of the most recent temperature data taken in April, 2007 with the historical data clearly indicates that the warming is continuing in deep waters below the permanent thermocline in the EJS, as shown in Fig. 2. It is remarkable that salinity has increased between 300 and 1000 m and decreased below 1500 m during the recent warming period, as shown recently by Kwon et al. (2004). A simple diagnostic model by Kwon et al. (2004) merits our special attention, since it provides an adequate explanation for the widespread confusion between the overlapping names and definitions of the water masses in the EJS. For example, the UPPW, ESCW, and HSIW occupy similar spatial and depth ranges, as shown in figure 12 of Kim et al. (2004), but UPPW was defined based on the 1960s observations (Sudo, 1986), while the Central Water and HSIW have been defined from the CREAMS observations in the 1990s (Kim et al., 1996). Thus, these definitions could reflect different water mass structures in the 1960s and 1990s, as we now know that the physical and chemical properties of the EJS itself have changed rapidly. Formation of the bottom water during the winter of 2000–2001, which was reported by several authors, has an important implication for the future of the EJS. Although no trace of this water was found in subsequent years, this unanticipated episode suggests that the chance of convective overturning might be increasing, as deep waters become lighter over time with warming, and the salinity will decrease if the warming continues at the present rate in the EJS. Therefore, the water mass structure might be changed further in future due to the nonlinearity of overturning processes and it is a matter of interest whether the water mass structure will be changed, as predicted by Kang et al. (2004b). Our understanding of the biogeochemistry of the EJS has been significantly improved thanks to the CREAMS and JES campaigns. The biogeochemical tracer studies conducted as part of CREAMS and JES have confirmed that the EJS has undergone dramatic changes during the last 50–60 years. However, there are issues that must be addressed in the future. The age of the water mass which can be determined by biogeochemical tracers only is poorly understood as yet. Information on the annual variation of nutrient concentrations is insufficient to understand the response of the ecosystem to the variability of physical forcing, and processes at the sediment-water in-


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terface are as yet unknown. The more accurate carbon cycle in the EJS can be understood by quantifying the various CO2 pumps. A study of the dissolved organic carbon, which is the biggest reservoir of carbon in the ocean, is in its initial stages. Even so, we have less information on the lateral transport of organic mater through the straits into the EJS. The basin-wide distribution of primary production and an estimate of export production (i.e. sediment trap study etc.) are essential if we are to understand the biological pump of the EJS. Data acquisition in the EJS is greatly hampered by the national interests of surrounding countries. Good international collaboration is needed for success, as exemplified by the first the CREAMS program and the following multinational efforts. Investigation of the deep circulation and interbasin water exchanges in the EJS is important to an understanding of its thermohaline circulation, and hence its climate variability and change. Direct observations of deep currents have revealed that the basin-scale deep circulation in three deep basins of the EJS is primarily cyclonic with relatively strong flows along the basin periphery and weak flows in the interior regions. Thank to extensive observations, sub-basin scale cyclonic and anticyclonic cells off the east coast of Korea and around Ulleung Plateau, respectively, have been further evidenced in the Ulleung Basin. The abyssal cyclonic circulation of the Ulleung Basin is connected with a broad, sluggish inflow in the western Ulleung Interplain Gap and a narrow, strong outflow in the eastern Ulleung Interplain Gap. The deep inflow into the Ulleung Basin also occurs along the continental slope off the east coast of Korea and has its origin in the western Japan Basin. Reports have been published on the variability of deep currents in the EJS ranging from a few days (Senjyu et al., 2005) to 20–50 days (Chang et al., 2004a) and to seasonal scale (Takematsu et al., 1999a). The observed deep flow variability could arise from thermohaline forcing (Senjyu et al., 2002) and/or local excitation due to energetic upper layer eddy processes (Takematsu et al., 1999b; Senjyu et al., 2005). This has not yet been clarified due to a lack of long-term current measurements that can reveal whether the thermohaline circulation in the EJS is changing substantially in accordance with drastic changes in the deep water and a reduction of bottom water formation. One current mooring at a location in the Ulleung Interplain Gap (mooring EC1 in Fig. 8) has been in operation since 1996, and analyses of data from this mooring will elucidate any long-term change of deep currents in the EJS. Recent numerical studies have extended our understanding of the circulation of intermediate water masses in the EJS, such as the ESIW and UPPW. The computing

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(a)

(b)

Fig. 11. (a) Observed salinity section map along the PM line averaged for 12 years from 1972 to 1983 (Maizuru Marine Observatory, 1985), and (b) simulated salinity section map (after Yoon and Kawamura, 2002).

power available to us today enables the high-resolution numerical model in the EJS to represent the meso-scale eddies as well as the realistic major current system, such as the branching of the Tsushima Warm Current and the separation of the EKWC. The high-resolution HYCOM model of Hogan and Hurlburt (2006) reproduced the intrathermocline eddies, and the high-resolution numerical model with the 3D-Var technique used by Kim et al. (2007) simulated the Dok Cold Eddy and its high variability, which has been recently reported by Mitchell et al. (2005). However, the previous numerical models leave some room for improvement in the intermediate and deep circulations. Most of them have reproduced the southward NKCC only in winter with a very low current speed, while the observed coldest North Korean Cold Water in summer implies the strongest NKCC in summer (Kim and Kim, 1983; Cho and Kim, 1994). Correspondingly, the simulated Salinity Minimum Layer depths are too deep compared with the observed depth. Figure 11 shows the observed and simulated salinity sections along the PM line (Yoon and Kawamura, 2002). The model reproduced a deep Salinity Minimum Layer that was too deep by a few hundred meters compared with the observed one. A recent numerical study with data assimilation (Kim et al., 2007) succeeded in reproducing the seasonal trend of the NKCC, but the current speed was still weaker than the observed value. Moreover, previous numerical models could not resolve the strong current in the abyssal layer, which has been reported by current observations as a part of CREAMS program in the Japan Basin (Takematsu et al., 1999a) and in the Ulleung Basin (Chang et al., 2002) (see Fig. 8). The coarse spatial resolution and excessive computational diffusion due to the tracer advection

scheme have been considered one of the reasons for the poor representation of the intermediate and deep circulation in the previous models (Hogan and Hurlburt, 2000; Yoon and Kawamura, 2002). Numerical modeling in the EJS has extended its area of application from reproduction of oceanic features to operational use for the now/forecast in the EJS by adapting various data assimilation techniques. In fact, the RIAM EJS operational ocean prediction system is in operation, and Kim et al. (2007) have successfully reproduced the observed 100 m temperature fields in the Ulleung Basin with a correlation coefficient of 0.79 using a data-assimilative numerical model. Acknowledgements The authors appreciate deeply the encouragement given by Prof. Arata Kaneko to prepare this paper and are thankful to Yun-Bae Kim and Hanna Na for organizing publications and drawing figures. This paper was written with support from the Korean EAST-I Program of CREAMS/PICES in grants from the Ministry of Land, Transport and Maritime Affairs of Korea (Kuh Kim, Kyung-Il Chang and Dong-Jin Kang), KORDI’s research project PG46000 (Young Ho Kim), and KORDI’s in-house project E97604 (Jae-Hak Lee). References Chang, K.-I., N. Hogg, M.-S. Suk, S.-K. Byun, Y.-G. Kim and K. Kim (2002): Mean flow and variability in the southwestern East Sea. Deep-Sea Res. I, 49, 2261–2279. Chang, K.-I., W. J. Teague, S. J. Lyu, H. T. Perkins, D.-K. Lee, D. R. Watts, Y.-B. Kim, D. A. Mitchell, C. M. Lee and K. Kim (2004a): Circulation and currents in the southwestern East/Japan Sea: Overview and review. Prog. Oceanogr., 61, 105–156. Chang, K.-I., Y.-B. Kim, K. Kim and J. C. Lee (2004b): Deep water flux through the Ulleung Interplain Gap in the southwestern East/Japan Sea. Proceedings of 2nd Workshop of PEACE, Kyushu Univ., Japan, 25–26 November 2004. Chen, C. T. A., A. S. Bychkov, S. L. Wang and G. Yu. Pavlova (1999): An anoxic Sea of Japan by the year 2200? Mar. Chem., 67, 249–265. Cho, Y.-K. and K. Kim (1994): Two modes of the salinity minimum layer in the Ulleung Basin. La mer, 32(3), 271–278. Clayson, C. A. and M. Luneva (2004): Deep convection in the Sea of Japan: A modeling perspective. Geophys. Res. Lett., 31, L17303, doi:10.1029/2004GL020497. Gamo, T. (1999): Global warming may have showed down the deep conveyor belt of a marginal sea of the northwestern Pacific: Japan Sea. Geophys. Res. Lett., 26, 3137–3140. Gamo, T., Y. Nozaki, H. Sakai, T. Nakai and H. Tsubota (1986): Spatial and temporal variations of water characteristics in the Japan Sea bottom layer. J. Mar. Res., 44, 781–793. Gordon, A. L., C. F. Giulivi, C. M. Lee, H. H. Furey, A. Bower and L. D. Talley (2002): Japan/East Sea thermocline eddies. J. Phys. Oceanogr., 32, 1960–1974.

Hahm, D., T. S. Rhee, D.-J. Kang and K.-R. Kim (2003): Influence of gas transfer velocity parameterization on air-sea CO2 exchange in the East (Japan) Sea. J. Korean Soc. Oceangr., 38, 135–142. Hirose, N., I. Fukumori and J.-H. Yoon (1999): Assimilation of TOPEX/POSEIDON altimeter data with a reduced gravity model of the Japan Sea. J. Oceanogr., 55, 53–64. Hirose, N., H. Kawamura, H. J. Lee and J.-H. Yoon (2007): Sequential forecasting of the surface and subsurface conditions in the Japan Sea. J. Oceanogr., 63, 467–481. Hogan, P. J. and H. E. Hurlburt (2000): Impact of upper oceantopography coupling and isopycnal outcropping in Japan/ East Sea models with 1/8° to 1/64° resolution. J. Phys. Oceanogr., 30, 2535–2561. Hogan, P. J. and H. E. Hurlburt (2006): Why do intrathermocline eddies form in the Japan/East Sea?; A modeling perspective. Oceanography, 19, 134–143. Kang, D.-J., K.-E. Lee and K.-R. Kim (2003a): Recent developments in chemical oceanography of the East (Japan) Sea with an emphasis on CREAMS findings: A review. Geosci. Journal., 7, 179–197. Kang, D.-J., S. Park, Y.-G. Kim, K. Kim and K.-R. Kim (2003b): A moving-boundary box model (MBBM) for oceans in change: An application to the East/Japan Sea. Geophys. Res. Lett., 30, 1299, doi:10.1029/2002 GL016486. Kang, D.-J., J.-Y. Kim, T. Lee and K.-R. Kim (2004a): Will the East/Japan Sea become an anoxic sea in the next century? Mar. Chem., 91, 77–84. Kang, D.-J., K. Kim and K.-R. Kim (2004b): The past, present and future of the East/Japan Sea in change: a simple moving-boundary box model approach. Prog. Oceanogr., 61, 175–191. Kim, C.-H. and K. Kim (1983): Characteristics and origin of the cold water mass along the east coast of Korea. J. Oceanol. Soc. Korea, 18, 73–83. Kim, C.-H. and J.-H. Yoon (1999): A numerical modeling of the upper and the intermediate layer circulation in the Eat Sea. J. Oceanogr., 55, 327–345. Kim, J.-Y., D.-J. Kang, E. Kim, J. H. Cho, C. R. Lee, K.-R. Kim and T. Lee (2003): Biological Pump in the East Sea Estimated by a Box model. The Sea, J. Korean Soc. Oceanogr., 8, 295–306 (in Korean with English abstract). Kim, K. and J. Y. Chung (1984): On the salinity-minimum and dissolved oxygen-maximum layer in the East Sea (Sea of Japan). p. 55–65. In Ocean Hydrodynamics of the Japan and East China Seas, ed. by T. Ichiye, Elsevier Science Publishers, Amsterdam. Kim, K., K.-R. Kim, J.-Y. Chung, B.-H. Choi, S.-K. Byun and G. H. Hong (1996): New findings from CREAMS observations: water masses and eddies in the East Sea. J. Korean Soc. Oceanogr., 31, 155–163. Kim, K., K.-R. Kim, D. H. Min, Y. Volkov, J. H. Yoon and M. Takematsu (2001): Warming and structural changes in the East (Japan) Sea: A clue to future changes in global oceans? Geophys. Res. Lett., 28, 3293–3296. Kim, K., K.-R. Kim, Y.-G. Kim, Y.-K. Cho, D.-J. Kang, M. Takematsu and Y. Volkov (2004): Water masses and decadal variability in the East Sea (Sea of Japan). Prog. Oceanogr., 61, 157–174.


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Kim, K. J. and Y. H. Seung (1999): Formation and movement of the ESIW as modeled by MICOM. J. Oceanogr., 55, 369– 382. Kim, K.-R. and K. Kim (1996): What is happening in the East Sea (Japan Sea)?: Recent chemical observations from CREAMS 93–96. J. Korean Soc. Oceanogr., 31, 164–172. Kim, K.-R., K. Kim, D.-J. Kang, S. Y. Park, M.-K. Park, Y.-G. Kim, H. S. Min and D. Min (1999): The East Sea (Japan Sea) in change: A story of dissolved oxygen. MTS Journal, 33, 15–22. Kim, K.-R., G. Kim, K. Kim, V. Lobanov, V. Ponomarev and A. Salyuk (2002): A sudden-bottom water formation during the severe winter 2000–2001: The case of the East/Japan Sea. Geophys. Res. Lett., 29, 1234, doi:10.1029/ 2001GL014498. Kim, K.-R., Y.-K. Cho, D.-J. Kang and J.-H. Ki (2005): The origin of the Tsushima Current based on oxygen isotope measurement. Geophys. Res. Lett., 32, L03602, doi:10.1029/ 2004GL021211. Kim, Y.-G. and K. Kim (1999): Intermediate waters in the East/ Japan Sea. J. Oceanogr., 55, 123–132. Kim, Y. H., K.-I. Chang, J. J. Park, S. K. Park, S.-H. Lee, Y.-G. Kim, K.-T. Jung and K. Kim (2007): Comparison between a reanalyzed product by 3-dimensional variational assimilation technique and observations in the Ulleung Basin of the East/Japan Sea. J. Mar. Syst. (submitted). Kitani, K. (1987): Direct current measurements of the Japan Sea proper water. Rep. Japan Sea Nat. Fish. Res. Inst., 341, 1–6 (in Japanese). Kwon, Y.-O., K. Kim, Y.-G. Kim and K.-R. Kim (2004): Diagnosing long-term trends of the water mass properties in the East Sea (Sea of Japan). Geophys. Res. Lett., 31, L20306, doi:10.1029/2004GL020881. Lee, H. J., J. H. Yoon, H. Kawamura and H.-W. Kang (2003): Comparison of RIAMOM and MOM in modeling the East Sea/Japan Sea circulation. Ocean and Polar Res., 25, 287– 302. Lee, T., I.-N. Kim, D.-J. Kang, D.-J. and D. Kim (2007): Implications of deep nitrite in the Ulleung Basin. The Sea, J. Oceanol. Soc. Korea, 12, 239–243. Levitus, S., J. Antonov and T. Boyer (2005): Warming of the world ocean, 1955–2003. Geophys. Res. Lett., 32, L02604, doi:10.1029/2004GL021592. Lie, H.-J., M.-S. Suk and C. Kim (1989): Observations of southeastward deep currents off the East Coast of Korea. J. Oceanol. Soc. Korea, 24, 63–68. Maizuru Marine Observatory (1985): Climatology of Hydrographic and Chemical Properties of the Japan Sea. 51 pp. Min, D.-H. and M. J. Warner (2005): Basin-wide circulation and ventilation studying the East Sea (Sea of Japan) using chlorofluorocarbon tracers. Deep-Sea Res. II, 52, 1580– 1616. Mitchell, D. A., W. J. Teague, M. Wimbush, D. R. Watts and G. G. Sutyrin (2005): The Dok Cold Eddy. J. Phys. Oceanogr., 35, 273–288. Moriyasu, S. (1972): The Tsushima Current. p. 353–369. In Kuroshio, ed. by H. Stommel and K. Yoshida, University of Tokyo Press.

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