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This study investigates the variability of coastal currents off the east coast of Korea, specifically in the Wangdolcho area of the East/Japan Sea, from June 2021 to October 2022. The observations revealed significant fluctuations in depth-averaged current velocity, with a peak of 0.81 m/s in August 2021 and a minimum of −0.05 m/s in August 2022. These year-to-year variations highlight the complex dynamics of coastal currents influenced by regional wind patterns and mesoscale eddies. In the summer of 2021, the development of offshore eddies, coupled with variable alongshore wind stress, led to increased current velocities and alternating upwelling and downwelling conditions, resulting in abrupt changes in current intensity. Conversely, in the summer of 2022, the negative vorticity in the Korea Strait and negative wind stress curl along the coast likely caused the East Korean Warm Current to shift further offshore, resulting in a weakened southward flow along the coast. This study emphasizes the need for long-term monitoring to better understand the coastal current dynamics and their environmental impacts. The anticipated completion of the Wangdolcho Ocean Research Station is expected to greatly improve monitoring capabilities, providing continuous and comprehensive data that will enhance our understanding of coastal currents and their broader impacts on the marine environment. [ABSTRACT FROM AUTHOR]

This study investigates the variability of coastal currents off the east coast of Korea, specifically in the Wangdolcho area of the East/Japan Sea, from June 2021 to October 2022. The observations revealed significant fluctuations in depth-averaged current velocity, with a peak of 0.81 m/s in August 2021 and a minimum of −0.05 m/s in August 2022. These year-to-year variations highlight the complex dynamics of coastal currents influenced by regional wind patterns and mesoscale eddies. In the summer of 2021, the development of offshore eddies, coupled with variable alongshore wind stress, led to increased current velocities and alternating upwelling and downwelling conditions, resulting in abrupt changes in current intensity. Conversely, in the summer of 2022, the negative vorticity in the Korea Strait and negative wind stress curl along the coast likely caused the East Korean Warm Current to shift further offshore, resulting in a weakened southward flow along the coast. This study emphasizes the need for long-term monitoring to better understand the coastal current dynamics and their environmental impacts. The anticipated completion of the Wangdolcho Ocean Research Station is expected to greatly improve monitoring capabilities, providing continuous and comprehensive data that will enhance our understanding of coastal currents and their broader impacts on the marine environment.

The interpretation of decline in dissolved oxygen (DO) in the oxygenated bottom water of the Ulleung Basin (UB), southwest of the East/Japan Sea has been challenging because of the integrated influence of various DO-consuming processes. Therefore, the stable oxygen isotopic fractionation of DO was investigated to enhance our understanding of the distinct DO consumption observed in the bottom layers of the center of the UB. We explored the relationship between DO and its oxygen isotope composition (δ18ODO) using data collected at a station located in the center of the UB in 2020, 2021, and 2022. An unforeseen decrease in δ18ODO in the bottom layer (> 1800 m) where DO was depleted was discovered. The overall DO consumption in the mesopelagic water layer (300-1000 m), primarily attributed to water column respiration, exhibited an isotopic fractionation factor (a) with 0.985 ± 0.001 in the δ18ODO/[O2] relationship. The consumptive isotope fractionation factor in the bottom waters near the sediments (approximately 2146 m) showed a value slightly higher (0.988 ± 0.002) than that in the mesopelagic water layer. This isotopic signature is likely due to a smaller fractionation in the bottom waters relative to the mesopelagic water. The isotopic evidence suggests the involvement of mineral oxidation associated with excess dissolved Mn and Fe in the bottom waters because mineral oxidation exhibits a smaller fractionation effect than respiration. Our study demonstrates that DO depletion results from multiple consumption processes, including respiration, mineral oxidation, and diffusive transport, and the isotopic behavior provides evidence that mineral oxidation significantly influences DO consumption. [ABSTRACT FROM AUTHOR]

Spatial patterns of the Primorye Current, as a band of cold water along the coast of Primorye (northwestern East/Japan Sea), are considered on the data of satellite images and standard sections obtained in 2000–2020. The data of AVHRR and MODIS scanners mounted on the satellites NOAA, Terra and Aqua are used with spatial resolution of 1.0–1.1 km. Temperature and salinity profiles in the upper 500 m layer were measured at the standard sections directed southward and east-southeastward from Vladivostok. The new data contradict a false supposition about the current origin from the Okhotsk Sea that is preserved in the name of “Liman Current”, still used sometimes. The cold-water area prolongs from the southern Tatar Strait to Peter the Great Bay but is definitely separated from the Amur Liman. Its core with the lowest SST is usually located on the external shelf of Primorye, outside of the Tatar Strait. Shape of the cold-water area is complicated and highly variable; it looks seldom as an alongshore flow, but more frequently as chains of eddies or filaments. In winter, the Primorye Current is presented usually as a wide uninterrupted alongshore belt with many intrusions to the warmer waters. This belt is distorted in spring by mesoscale eddies. In summer, the cold-water zone is narrowed and sometimes interrupted between 43 and 45°N; the cold waters interact with large anticyclonic eddies which transport the warm subtropic water close to the shore. The cold-water zone begins to extend again in fall season: large eddies and cold intrusions spread offshore, and upwellings appear at the coast. In November–December, summer stratification is destroyed and the warm water advection is weakened, so the solid band of cold water mixed by convection occupies a vast area of the northwestern East/Japan Sea off the coast of Primorye. Observed structural patterns show that the cold water at the sea surface along the coast of Primorye is produced on the shelf by upwellings or tidal mixing. Possible mechanisms of the cold spot inducing are discussed. [ABSTRACT FROM AUTHOR]

The northwestern part of the East/Japan Sea (EJS) is a region with large sea surface temperature (SST) variability and is known as a hotspot of marine heatwaves (MHW) stress for marine environments that peaked in boreal winter (January-February-March). This could have profound impacts on the marine ecosystems over the EJS. Here, we used a set of high-resolution satellite and reanalysis products to systematically analyze the spatiotemporal SST variations and examine their linkage to a large-scale mode of climate variability, such as the Arctic Oscillation (AO). The results show that AO-related wind forcing modulates the SST variability over the EJS via the oceanic dynamic adjustment processes. In particular, the abnormally warm SSTs in the northwestern part of the EJS are driven by the anomalous anticyclonic eddy-like circulation and Ekman downwelling during a positive AO phase. This physical linkage between a positive AO and the abnormally warm SST could be conducive to MHW occurrences in the EJS as in the extremely positive AO event during the winter of 2020. These results have implications that the MHW occurrences in the EJS could be amplified by natural climate variability along with long-term SST warming.

Oceanic mesoscale eddies continuously regulate the horizontal and vertical transport of mass, heat, salt, carbon, and nutrients throughout the ocean system owing to their ubiquity, three-dimensionality, and long-term persistence. Although satellites have been the main platforms used to observe mesoscale eddies and chlorophyll-a (Chla) distributions, they cannot support submesoscale physical–biological interactions. Contemporary satellite observations of Eulerian velocity fields are unable to resolve submesoscale processes that govern vertical migration and mixing, which are crucial for controlling the nutrients and light for phytoplankton in the surface layer. We explored the physical–biological interaction between the anticyclonic mesoscale eddy and the Chla secondary bloom that occurred after the spring bloom in the East/Japan Sea using the Geostationary Ocean Color Imager (GOCI). The GOCI currents were generated using GOCI Chla data and were used to map streamlines, vorticity, and divergence to characterize the surface current near the eddy. In the early spring bloom period, the eddy interior showed Chla depletion as the eddy was trapped externally. We found that the second bloom period coincided with a higher divergence or upwelling period in the eddy core, and a sharp Chla peak was observed when wind-induced Ekman suction was pronounced. This study describes the first satellite observation of surface layer divergence inside an anticyclonic mesoscale eddy with internal Chla blooms, utilizing a submesoscale-permitting GOCI-based surface current. [ABSTRACT FROM AUTHOR]

A total of 222 surface sediments were collected in the Ulleung Basin, southwestern East/Japan Sea, from the coast to the basin and analyzed for grain size, total organic carbon, biogenic silica and major and trace elements. The metal concentrations of the surface sediments were highly spatially variable, and their distributions were grouped into four types by factor analysis: waste dumping materials (Cr, Cu, Zn, Sn, Sb, and Pb); detrital materials (Al, Fe, and Cs); suboxic diagenetic materials (Mn, Mo, Co, and Ni); and anoxic diagenetic materials (Cd and U). From core samples collected near the dumping site, approximately the top 10 cm was well mixed in the 210Pb ex profiles and highly enriched in the above metals related to dumping materials. The enrichment factor of Sn indicates dumping materials well, and its distribution can be used to estimate the dispersion range of the dumped material in this basin. • From 1991, land-based wastes were dumped at two sites in the Ulleung Basin, Korea. • High concentrations of Cr, Cu, Zn, Sn, Sb, and Pb were observed at the dumping site. • Suboxic and anoxic diagenesis control the enrichment of other trace metals. • The distribution of Sn indicates the dispersion of dumping materials. [ABSTRACT FROM AUTHOR]

A substantial decrease in the dissolved oxygen (DO) concentration over the lower layers of the East/Japan Sea (EJS), including the deep and bottom waters (1500 to 2500 m and 2500 to 3500 m, respectively), has been reported during the last few decades. This study quantitatively explain the mechanisms underlying the long-term decline in the DO concentration of the EJS for the first time using a coupled physical-biogeochemical model. We demonstrated that the cessation of new water formation was a major factor underlying the long-term decline in the DO concentration over the lower layers, and its contribution exceeded the net decrease. In addition, our simulations confirmed that the enhanced primary production in the euphotic zone accompanying increases in organic matter exported to the lower layers accelerates the DO decline in a quantitative respect. It was accounted for 12% of the net DO change at the lower layers from 1980 to 2015. The enhanced primary production resulted from increases in available nutrients due to the intensified vertical mixing caused by the strengthened inflow transport through the Korea/Tsushima Strait. Physical activities such as vertical transport of oxygen from the central water increased the DO concentration of the lower layers, and offset 42% of the net decrease in DO concentration due to organic matter decomposition.

Walleye pollock (Gadus chalcogramma) caught in the Korean fishing area dramatically decreased in the late 1980s. To investigate the potential impact of the late 1980s climate regime shift on the collapse of the pollock catch, we developed a three-dimensional hydrodynamic model with data assimilation and a particle tracking model. Data-assimilated reanalysis showed that sea surface temperature increased by approximately 2°C in the spawning area of pollock in the late 1980s. The suitable spawning area in the East Korean Bay decreased due to warming in the late 1980s. Spawned eggs of walleye pollock were tracked using a particle tracking model for 30 days in January and February during 1983–1992. The number of individuals transported to the nursery within the Korean fishing area from the spawning area was reduced by 74% in the late 1980s. The intensified East Korean Warm Current (EKWC) could be responsible for the decreased number of individuals transported to the southern area in the late 1980s. Warming in the Korean fishing area could also cause a decrease in pollock. These oceanic changes might be linked to climate regime shifts in the late 1980s. The warming regime with positive Arctic Oscillation and weakened monsoon intensified the northward flow of the EKWC and accelerated the warming of the spawning and fishing areas in the late 1980s.

Sparse observations in the East/Japan Sea (EJS) suggested that open-ocean deep convection occurs south of Vladivostok; however, more recent observations suggest that deep convection occurs along the continental slope, resulting in bottom water formation in the EJS. We investigated the process of deep convection along the EJS continental slope using large-eddy simulation (LES), which demonstrated that dense water, formed by strong wintertime cooling in the shelf, flows down along the slope as a bottom Ekman current. The characteristics of the initial dense water were relatively well conserved on the continental slope during convection, but they changed rapidly by mixing with the surrounding waters in the open ocean. Accordingly, slope convection penetrated deeper compared to open-ocean convection under the same surface heat flux. Our numerical experiments showed that, under typical surface cooling during winter (i.e., 200 W m–2), slope convection reaches depths greater than 2,700 m, generating a potential ventilation process for deep- and bottom-water formations, whereas open-ocean convection reaches approximately 700 m depth, contributing to the intermediate- and central-water formations in the EJS. Various topography experiments revealed that downward speed was proportional to the continental-slope inclination; the initial characteristics remained relatively well conserved at a small continental-slope inclination. Increased salinity due to brine rejection in the shelf could accelerate the slope convection.

A subpolar front (SPF) generated between the East Korea Warm Current (EKWC) and the North Korea Cold Current (NKCC) in the western margin of the East/Japan Sea has shifted northward in recent decades. This study investigated the biomass and composition of the phytoplankton assemblage in relation to hydrological and biogeochemical features in the shallow shelf and slope off the Korean coast from January to June in 2016 and 2017, to determine the mechanistic effects of SPF on spring–summer phytoplankton bloom dynamics. Monthly average depth-integrated chlorophyll a (Chl a) levels and the contribution of phytoplankton classes revealed bimodal diatom blooms in early spring and summer in the frontal zone. Canonical correspondence analysis showed that the distribution of high Chl a was associated with cold, low-salinity NKCC water in March 2016. No Chl a peak was observed in March 2017 when the warm saline EKWC water mass invaded. These results suggest that the NKCC intrusion acts as a forcing mechanism leading to enhanced phytoplankton biomass in the frontal zone. In contrast, positive correlations of Chl a concentration with water density and nutrient concentrations suggest that summer blooms were fed by the subsurface chlorophyll maximum (SCM) driven by shoaling of the pycnocline and nitracline. Varying water-column stratification determined the thickness of the SCM layer, driving year-to-year variability in the magnitude of diatom blooms. These findings further suggest that seasonal/interannual variability in the timing of algal blooms affects regional trophodynamics and hence could be an important factor in explaining ecosystem changes in this region.

In this study, we developed a new high-resolution regional climatology (0.1° by 0.1° by 19 levels) in the East/Japan Sea. National Centers for Environmental Information and Korea Oceanic Data Center already released the regional climatology of East Asian seas including the East/Japan Sea with 0.1° by 0.1° resolution. It provides a reliable temperature and salinity structure compared to previous 1° or 0.25° climatologies. However, this study found an abnormal meridional temperature gradient problem when calculating geostrophic currents based on this new climatology. Geostrophic currents show a strong repetitive eastward flow along the 1° latitudinal band especially in the East/Japan Sea, which corresponds with abnormal meridional temperature gradients at the same areas. This problem could be related to the objective analysis procedure generating the high-resolution climatology. Here, we reproduced a high-resolution climatology without the abnormal meridional temperature gradient problem by using the optimal interpolation method. Results show that the meridional gradient problem can partly be attributed to both the use of the World Ocean Atlas background field on the 1° grid, and the spatial distribution of World Ocean database observation data; however, these are not the primary causes. We corrected the abnormal temperature gradient by increasing the meridional decorrelation length scale without losing the meso-scale feature in the East/Japan Sea, as shown by the wavelet analysis. Improvement of the new gridded field is also validated by using serial hydrographic data and the altimetry-derived surface current product.

The East Sea Intermediate Water (ESIW) is the subsurface salinity minimum layer in the depth range 200–400 m in the East/Japan Sea. The origin, variability, and pathways of the ESIW were investigated using a 4‐year‐long (2015–2018) high‐resolution (1/24°) ocean reanalysis and Lagrangian particle modeling. A backward particle tracking analysis revealed that the ESIW was formed south of Vladivostok and shows strong interannual variability. In 2018, owing to strong northwesterly winds, heat loss to the air intensified, and freshwater transport from the northern East/Japan Sea was simultaneously decreased by about 18% compared with the previous year. Furthermore, there was increased formation of water masses heavier than that of the typical ESIW, and the volume of the ESIW was reduced. A forward particle tracking analysis showed that the contribution of the particles originated from the northern end of the East/Japan Sea was about two times greater than those from the southern end. The major portion of the ESIW's freshwater came from the northern East/Japan Sea, where sea ice melted and the Amur River discharged. The remaining portion was from the Changjiang River, which came through the Korea Strait. The ESIW reached the Ulleung Basin over the course of a year mainly through the deep channel, the Ulleung Trough, located to the north of the basin. The diapycnal diffusivity of 1.0 × 10−4 m2 s−1 could erode away the low salinity core in about 27 years. Plain Language Summary: In the southwestern part of the East/Japan Sea (EJS), low salinity water called the East Sea Intermediate Water (ESIW) is found between depths of 200 and 400 m. We studied the origin, variability, and pathways of the ESIW through high‐resolution ocean reanalysis and Lagrangian particle tracking modeling. It was found that that the major source of the ESIW is the Amur River freshwater and the sea ice meltwater from the northern end of the EJS. The ESIW forms southward of Vladivostok and extends southward below the mixed layer. It moves southward, mostly through deep channels until it reaches the Ulleung Basin, which is located in the southwestern part of the EJS. The ESIW characteristics show interannual variation. In 2018, the salinity of the ESIW was higher than that in the other years, and it corresponded to strong atmospheric forcing and reduced the low salinity water transport from the northern end. The amount of the ESIW, thus, was reduced by about 18% compared with that for the previous year. Owing to vertical mixing, the low salinity core of the ESIW could be eroded away in about 27 years. Key Points: The main source of the East Sea Intermediate Water is the low salinity water at the northern end of the East/Japan SeaThe formation of the water mass was significantly decreased in 2018 due to enhanced cooling and reduced freshwater transportA major portion of the water mass reaches the Ulleung Basin through the Ulleung Trough over a period of one year [ABSTRACT FROM AUTHOR]

The current phytoplankton community structure is expected to change, with small phytoplankton becoming dominant under ongoing warming conditions. To understand and evaluate the ecological roles of small phytoplankton in terms of food quantity and quality, the carbon uptake rates and intracellular biochemical compositions (i.e., carbohydrates, CHO; proteins, PRT; and lipids, LIP) of phytoplankton of different sizes were analyzed and compared in two different regions of the western East/Japan Sea (EJS): the Ulleung Basin (UB) and northwestern East/Japan Sea (NES). The average carbon uptake rate by the whole phytoplankton community in the UB (79.0 ± 12.2 mg C m–2 h–1) was approximately two times higher than that in the NES (40.7 ± 2.2 mg C m–2 h–1), although the average chlorophyll a (chl a) concentration was similar between the UB (31.0 ± 8.4 mg chl a m–2) and NES (28.4 ± 7.9 mg chl a m–2). The main reasons for the large difference in the carbon uptake rates are believed to be water temperature, which affects metabolic activity and growth rate, and the difference in euphotic depths. The contributions of small phytoplankton to the total carbon uptake rate were not significantly different between the regions studied. However, the rate of decrease in the total carbon uptake with increasing contributions from small phytoplankton was substantially higher in the UB than in the NES. This result suggests that compared to other regions in the EJS, the primary production in the UB could decrease rapidly under ongoing climate change. The calorific contents calculated based on biochemical compositions were similar between the small (1.01 ± 0.33 Kcal m–3) and large (1.14 ± 0.36 Kcal m–3) phytoplankton in the UB, whereas the biochemical contents were higher in the large phytoplankton (1.88 ± 0.54 Kcal m–3) than in the small phytoplankton (1.06 ± 0.18 Kcal m–3) in the NES. The calorific values per unit of chl a were higher for the large phytoplankton than for the small phytoplankton in both regions, which suggests that large phytoplankton could provide a more energy efficient food source to organisms in higher trophic levels in the western EJS.

The current phytoplankton community structure is expected to change, with small phytoplankton becoming dominant under ongoing warming conditions. To understand and evaluate the ecological roles of small phytoplankton in terms of food quantity and quality, the carbon uptake rates and intracellular biochemical compositions (i.e., carbohydrates, CHO; proteins, PRT; and lipids, LIP) of phytoplankton of different sizes were analyzed and compared in two different regions of the western East/Japan Sea (EJS): the Ulleung Basin (UB) and northwestern East/Japan Sea (NES). The average carbon uptake rate by the whole phytoplankton community in the UB (79.0 ± 12.2 mg C m–2 h–1) was approximately two times higher than that in the NES (40.7 ± 2.2 mg C m–2 h–1), although the average chlorophyll a (chl a) concentration was similar between the UB (31.0 ± 8.4 mg chl a m–2) and NES (28.4 ± 7.9 mg chl a m–2). The main reasons for the large difference in the carbon uptake rates are believed to be water temperature, which affects metabolic activity and growth rate, and the difference in euphotic depths. The contributions of small phytoplankton to the total carbon uptake rate were not significantly different between the regions studied. However, the rate of decrease in the total carbon uptake with increasing contributions from small phytoplankton was substantially higher in the UB than in the NES. This result suggests that compared to other regions in the EJS, the primary production in the UB could decrease rapidly under ongoing climate change. The calorific contents calculated based on biochemical compositions were similar between the small (1.01 ± 0.33 Kcal m–3) and large (1.14 ± 0.36 Kcal m–3) phytoplankton in the UB, whereas the biochemical contents were higher in the large phytoplankton (1.88 ± 0.54 Kcal m–3) than in the small phytoplankton (1.06 ± 0.18 Kcal m–3) in the NES. The calorific values per unit of chl a were higher for the large phytoplankton than for the small phytoplankton in both regions, which suggests that large phytoplankton could provide a more energy efficient food source to organisms in higher trophic levels in the western EJS. [ABSTRACT FROM AUTHOR]

The summer/fall variability of the upper ocean circulations and properties of the East/Japan Sea occurring during the El Nino-Southern Oscillation (ENSO) period is investigated by use of the high-resolution HYCOM reanalysis data. From August to November of the La Niña period, the inflow through the Korea/Tsushima Strait is intensified, and so are the East Korean Warm Current (EKWC) and the Nearshore Branch of the Tsushima Warm Current (NBTWC) in the southern East/Japan Sea. The intensified EKWC induces an offshore current anomaly near 36°N, weakening the anticyclonic gyre circulation in Ulleung basin, and the intensified NBTWC leads to weakening of the Offshore Branch of the Tsushima Warm Current (OBTWC). The sea surface temperature begins to increase from the northeastern coast of Japan in August, and its positive anomalies expand to the largest area of southern East/Japan Sea in October and last until November. The positive anomalies of sea surface salinity are mainly observed along the south and east coast of Korea. The cyclonic gyre circulation anomaly in Ulleung Basin is accompanied by the negative SSH anomaly and shallowing of the upper ocean isopycnal lines, which leads to substantial cooling of the subsurface water. The reverse is true in the El Nino period. [ABSTRACT FROM AUTHOR]

Dramatic environmental changes have been recently reported in the Yellow Sea (YS), the South Sea of Korea (SS), and the East/Japan Sea (EJS), but little information on the regional primary productions is currently available. Using the 13C-15N tracer method, we measured primary productions in the YS, the SS, and the EJS for the first time in 2018 to understand the current status of marine ecosystems in the three distinct seas. The mean daily primary productions during the observation period ranged from 25.8 to 607.5 mg C m−2 d−1 in the YS, 68.5 to 487.3 mg C m−2 d−1 in the SS, and 106.4 to 490.5 mg C m−2 d−1 in the EJS, respectively. In comparison with previous studies, significantly lower (t-test, p < 0.05) spring and summer productions and consequently lower annual primary productions were observed in this study. Based on PCA analysis, we found that small-sized (pico- and nano-) phytoplankton had strongly negative effects on the primary productions. Their ecological roles should be further investigated in the YS, the SS, and the EJS under warming ocean conditions within small phytoplankton-dominated ecosystems.

Near‐inertial waves (NIWs), a fundamental oceanic response to wind forcing, play an important role in dynamical processes related to ocean mixing and hence have attracted sustained interest. Herein, we investigate temporally and spatially varying NIW distributions in the mixed and deep layers of the East/Japan Sea (EJS) using high‐resolution hourly‐wind‐forced data‐assimilated ocean model outputs. Temporally, the kinetic energy of NIWs in the mixed and deep layers is higher in fall and winter than in spring and summer, showing maxima in December, corresponding to wind forcings of both wind stress and wind‐current resonance. Spatially, the NIW energy in the mixed layer is higher on the northern side of the subpolar front (SPF), although there are no significant spatial differences in the wind forcings. Because of intensive background currents and their vorticities in the upper layer on the southern side of the SPF, vertical transfer of NIW energy is facilitated, shown by a shorter e‐folding decay time scale of NIWs in the mixed layer on the southern side of the SPF. The NIW kinetic energy in the deep layer of 400–1,000 m is higher on the southern side than on the northern side, an opposite spatial pattern to that in the mixed layer, but consistent with a previous observational study. Our results confirm that energetic anticyclonic circulations with negative relative vorticity in the upper layer on the southern side enable vertical penetration of NIW energy from the mixed layer to the deep layer more effectively. Plain Language Summary: Near‐inertial waves (NIWs) are an important source for vertical ocean mixing, which is associated with the maintenance of stratification and overturning circulation of the East/Japan Sea (EJS); however, the spatiotemporal variability of NIWs is unknown. We investigate the distribution of NIWs in the EJS focusing on the surface mixed layer and deep layer using a data‐assimilative numerical ocean model. Temporally, NIW energy in the mixed and deep layers shows seasonality, higher in fall and winter than in spring and summer, maxima in December and minima in June, due to wind forcings. Spatially, mixed‐layer NIWs are stronger (weaker) on the northern (southern) side of the subpolar front, whereas deep‐layer NIWs are stronger (weaker) on the southern (northern) side. Predominant clockwise mesoscale circulations on the southern side play a key role in energy distribution by facilitating vertical propagation of NIWs. Variation of the NIWs due to wind and meridionally different mesoscale circulation implies spatiotemporal differences of NIW‐induced ocean mixing. Key Points: Mixed‐layer near‐inertial waves are stronger (weaker) on the northern (southern) side of the subpolar front in the East/Japan SeaDeep‐layer near‐inertial waves are stronger (weaker) on the southern (northern) sideMesoscale background circulation governs near‐inertial wave distribution in the East/Japan Sea [ABSTRACT FROM AUTHOR]

In this study, a separation index for the East Korea Warm Current (EKWC) is defined by Lagrangian particle tracking using surface geostrophic currents based on satellite-observed absolute dynamic topography. After determining the main stream from the inflow through the western channel of the Korea Strait, the separation index is obtained by averaging the latitude of the main particle trajectories crossing the meridian along 130.5°E. Although the separation index is obtained from coastal velocity, it describes well the surface circulation patterns in the Ulleung Basin. When it is low (i.e., the EKWC separates at low latitude), the flow toward the Japanese coast is the strongest and the EKWC is weak. In the middle phase of the separation index, a large portion of the EKWC meanders and forms the Ulleung Warm Eddy-like feature. When the separation index is high, the EKWC stretches further northward to almost 40°N. Therefore, the separation index could be used as a simple representation of the surface circulation pattern in the East/Japan Sea. [ABSTRACT FROM AUTHOR]

The cellular macromolecular contents and energy value of phytoplankton as primary food source determine the growth of higher trophic levels, affecting the balance and sustainability of oceanic food webs. Especially, proteins are more directly linked with basic functions of phytoplankton biosynthesis and cell division and transferred through the food chains. In recent years, the East/Japan Sea (EJS) has been changed dramatically in environmental conditions, such as physical and chemical characteristics, as well as biological properties. Therefore, developing an algorithm to estimate the protein concentration of phytoplankton and monitor their spatiotemporal variations on a broad scale would be invaluable. To derive the protein concentration of phytoplankton in EJS, the new regional algorithm was developed by using multiple linear regression analyses based on field-measured data which were obtained from 2012 to 2018 in the southwestern EJS. The major factors for the protein concentration were identified as chlorophyll-a (Chl-a) and sea surface nitrate (SSN) in the southwestern EJS. The coefficient of determination (r2) between field-measured and algorithm-derived protein concentrations was 0.55, which is rather low but reliable. The satellite-derived estimation generally follows the 1:1 line with the field-measured data, with Pearson’s correlation coefficient, which was 0.40 (p-value < 0.01, n = 135). No remarkable trend in the long-term annual protein concentration of phytoplankton was found in the study area during our observation period. However, some seasonal difference was observed in winter protein concentration between the 2003–2005 and 2017–2019 periods. The algorithm is developed for the regional East/Japan Sea (EJS) and could contribute to long-term monitoring for climate-associated ecosystem changes. For a better understanding of spatiotemporal variation in the protein concentration of phytoplankton in the EJS, this algorithm should be further improved with continuous field surveys.

The sources and behaviors of particulate organic carbon (POC) and particulate iron and manganese (pFe and pMn, respectively) in the bottom nepheloid layer (or benthic nepheloid layer, BNL) of the southwestern East Sea, also known as the Japan Sea, along a transect from the shelf to the central Ulleung Basin were investigated. The fluxes of POC, pFe, and pMn from the BNL to the seafloor on the shelf and in the basin were determined based on thorium-234 (234Th). The influence of resuspended sediment on POC was quantified using radiocarbon isotope ratio of POC (Δ14C). Sources and behaviors of pFe and pMn in the BNL were investigated by comparison to those of particulate aluminum (pAl). The stable carbon isotope ratios mainly indicated the marine origin of the POC, and the Δ14C values indicated that the majority of the POC in the BNL (68% ± 22%) was supplied by sediment resuspension. pAl and pFe were lithogenic in origin, whereas pMn was mainly authigenic (89–100%). The deficiency of 234Th activity relative to that of 238U increased toward the seafloor in the BNL, implying the efficient removal of 234Th by adsorption to the resuspended sediment particles. The 234Th-based settling fluxes of POC, pAl, pFe, and excess Mn (pMn xs) to the seafloor in the central basin agreed with the results previously obtained from a sediment trap study in the Ulleung Basin. The settling flux of pAl and pFe in the central basin was 2–8% of the lateral transport from the Korea Strait, implying that the transported lithogenic particles mostly settled on the slope before reaching the central basin. In contrast, the settling flux of pMn in the central basin was much larger than that of pMn in the shelf, implying that pMn is further transported toward the central basin or supplied from the local sediments. • Resuspended sediment in benthic nepheloid layer was examined using 14C and 234Th. • Aged carbon from sediment accounted for ∼70% of particulate organic carbon. • Over 90% of particulate Mn was authigenic while Al and Fe were of lithogenic origin. • Deficiency of 234Th increased toward the seafloor due to the resuspended sediments. • Lithogenic particles from the shelf mostly settled down on the slope. [ABSTRACT FROM AUTHOR]

Recent global climate change often leads to poleward expansions of habitat range of marine organisms in response to increasing water temperature at high latitude. This study investigated latitudinal distribution patterns of Turbo sazae from 2009 to 2018 along the southern and eastern coasts of Korea to verify whether gradual increases in seawater temperature in the East Sea/Sea of Japan (hereafter East/Japan Sea) accelerate changes in the geographic distribution of T. sazae. Between 2009 and 2018, underwater SCUBA surveys were conducted at 19 subtidal rocky shore habitats from the southern and eastern coast of the Korean Peninsula, including Jeju Island. Additionally, long-term seawater temperature records over the last 40 years (between 1980s and 2010s) from the East/Japan Sea were analyzed to verify how changes of water temperature corresponded to geographical distributions of T. sazae. The habitat range of T. sazae was found to have extended from latitude 34°02′ N to latitude 37°06′ N from 2009 to 2018. Although seawater temperature has gradually increased since the 1990s in the East/Japan Sea, habitat expansion was particularly evident during the rapid rise of coastal seawater temperature in the 2010s. Because the strong northward expansion of the Tsushima Current can accelerate the rise of seawater temperature in the East/Japan Sea, studies of the effects of climate change on marine ecosystems of the Korean Peninsula should include data from monitoring the dynamics of the Tsushima Current.

As a part of Korean-Russian joint expeditions in the East/Japan Sea during 2012 and 2015, a set of total and small (3− and NH4+ uptake rate estimations were carried out. The study aimed to assess the spatio-temporal variations in dissolved inorganic nitrogen (DIN) assimilation by the total and small phytoplankton. The results show that the total NO3− uptake rates during 2012 varied between 0.001 and 0.150 μmol NL−1h−1 (mean ± SD = 0.034 ± 0.033) and that the total NH4+ uptake rates ranged between 0.002 and 0.707 μmol NL−1h−1 (mean ± SD = 0.200 ± 0.158). The total uptake rates during 2015 were ranged from 0.003 to 0.530 (mean ± S.D. = 0.117 ± 0.120 μmol NL−1h−1) for NO3− and from 0.008 to 1.17 (mean ± S.D. = 0.199 ± 0.266 NL−1h−1) for NH4+. The small phytoplankton NO3− and NH4+ uptake rates during 2015 ranged between 0.001 and 0.164 (mean ± S.D. = 0.033 ± 0.036) μmol NL−1h−1 and 0.010–0.304 (mean ± S.D. = 0.101 ± 0.073) μmol NL−1h−1, respectively. Small phytoplankton’s contribution to the total depth-integrated NO3− and NH4+ uptake rates ranged from 10.24 to 59.36% and from 30.21 to 68.55%, respectively. The significant negative relationship observed between the depth-integrated total NO3− and NH4+ uptake rates and small phytoplankton contributions indicates a possible decline in the DIN assimilation rates under small phytoplankton dominance. The results from the present study highlight the possibility of a reduction in the total DIN assimilation process in the East/Japan Sea when small phytoplankton dominate under strong thermal stratification due to sea surface warming. The present study’s findings agree with the model projections, which suggested a decline in primary production in the global warming scenario.

In recent years, the change of marine environment due to climate change and declining primary productivity have been big concerns in the East/Japan Sea, Korea. However, the main causes for the recent changes are still not revealed clearly. The particulate organic carbon (POC) to chlorophyll-a (chl-a) ratio (POC:chl-a) could be a useful indicator for ecological and physiological conditions of phytoplankton communities and thus help us to understand the recent reduction of primary productivity in the East/Japan Sea. To derive the POC in the East/Japan Sea from a satellite dataset, the new regional POC algorithm was empirically derived with in-situ measured POC concentrations. A strong positive linear relationship (R2 = 0.6579) was observed between the estimated and in-situ measured POC concentrations. Our new POC algorithm proved a better performance in the East/Japan Sea compared to the previous one for the global ocean. Based on the new algorithm, long-term POC:chl-a ratios were obtained in the entire East/Japan Sea from 2003 to 2018. The POC:chl-a showed a strong seasonal variability in the East/Japan Sea. The spring and fall blooms of phytoplankton mainly driven by the growth of large diatoms seem to be a major factor for the seasonal variability in the POC:chl-a. Our new regional POC algorithm modified for the East/Japan Sea could potentially contribute to long-term monitoring for the climate-associated ecosystem changes in the East/Japan Sea. Although the new regional POC algorithm shows a good correspondence with in-situ observed POC concentrations, the algorithm should be further improved with continuous field surveys.

The complete mitochondrial genome of the morotoge shrimp, Pandalopsis japonica was determined by the high-throughput sequencing technique. The mitogenome of P. japonica (15,909 bp) encoded the 13 protein-coding genes (PCGs), 22 tRNA genes, 2 rRNA genes as well as the non-coding control region (D-Loop). The mitochondrial genome of P. japonica showed an identical gene arrangement in its mitochondrial genome to its relative Pandalid shrimps including P. borealis and P. hypsinotus. Among 13 compared mitogenomes within order Suborder Pleocyemata, P. japonica was most closely related to P. hypsinotus with 86% sequence identity clustering together with three other shrimps in family Pandalidae.

The dynamics responsible for the East/Japan Sea throughflow are examined using the Finite Volume Coastal Ocean Model. Existing ocean data assimilation models (Δx ~10 km) present overestimates of the volume transport through the Tsugaru Strait, and the outflow partitioning between the Tsugaru and Soya/La Perouse Straits also differs depending on the models. We find that this overestimate occurs when the form drag that is induced by detailed bottom topographic features in the Tsugaru Strait is not adequately resolved. A range of experiments with different horizontal resolutions in the Korea/Tsushima, Tsugaru, and Soya/La Perouse Straits were conducted to examine how detailed bottom topographic features in these three straits may affect the throughflow. We find that the volume transport is highly sensitive to the spatial resolution at the Tsugaru Strait but not at the Korea/Tsushima and Soya/La Perouse Straits. The high‐resolution experiment showed a decreased outflow through the Tsugaru Strait and a reduction in the model bias. The outflow through the Soya/La Perouse Strait increased, while the inflow through the Korea/Tsushima Strait remained relatively unchanged. Additional experiments with modified topography further indicated that the abrupt change of the topography located at the upstream side of the Tsugaru Strait plays an important role in the outflow partitioning of the East/Japan Sea throughflow. Key Points: The dynamics responsible for the East/Japan Sea throughflow are examined using the Finite Volume Coastal Ocean ModelTopographically induced form drag in the Tsugaru Strait plays an important role in the outflow partitioning in the East/Japan SeaThe enhancement of form drag at the Tsugaru Strait is created by the rugged and sharp topographical features in the upstream/downstream region [ABSTRACT FROM AUTHOR]

The contributions of bottom cold water and planetary β-effect to the formation of the East Korean Warm Current (EKWC), the western boundary current in the East/Japan Sea (EJS), were evaluated using an idealized three-dimensional numerical model. The model results suggest that the bottom cold water and, to a lesser extent, the planetary β-effect both contribute to the formation of the EKWC. The cold water functions as the bottom of the upper layer, to control the EKWC via conservation of potential vorticity. It is known that cold waters, such as the North Korean Cold Water and Korea Strait Bottom Cold Water often observed during summer along the southwestern coast of the EJS, originate from the winter convection in the northern area. Observational studies consistently show that the EKWC strengthens in summer when the cold water extends further south along the western boundary. [ABSTRACT FROM AUTHOR]

The results of the present study prove that snowfall occurred due to the polar low (PL) in the Korean Peninsula and six cases of snowfall exceeding a snow depth of 2 cm over the past 16 years in Busan, South Korea. A strong northwesterly air current with a cold outbreak at the lower level passed through the Korean Peninsula and penetrated into the East/Japan Sea causing the generation and characteristics of a PL. However, a northeasterly air current due to a synoptic low (SL) in East Japan approached the east coast via the East/Japan Sea, which generated a wind field with mesoscale cyclonic circulation. In the center of this cyclone, a strong positive vorticity region was revealed from the lower level to the upper level. The air temperature in the center of the PL was warmer than the surrounding areas at the lower level. As the PL developed and the air temperature decreased, a rapid tropopause drop followed due to the effect of the cold core along with the cutoff low at the mid-level or the higher level. As a result, the stratification became more unstable. The PL moved into Busan as the cold core at the upper level rapidly moved to the lower latitudes, which formed an unstable region around Busan. The PL decayed because the cutoff low, the cold core, and the positive vorticity region at the upper level quickly moved to the east, thereby causing the stratification to stabilize. Also, because the approach to the Japanese Archipelago caused an increase in surface friction, the original structure could no longer be maintained. [ABSTRACT FROM AUTHOR]

The East Sea Intermediate Water (ESIW), defined as the salinity minimum in the East Sea (hereafter ES) (Sea of Japan), is examined with respect to its overall characteristics and its low salinity origin using historical Argo float data from 1999 to 2015. Our findings suggest that the ESIW is formed in the western Japan Basin (40–42°N, 130–133°E), especially west of the North Korean front in North Korean waters, where strong negative surface wind stress curl resides in wintertime. The core ESIW near the formation site has temperatures of 3–4 °C and less than 33.98 psu salinity, warmer and fresher than that in the southern part of the ES. In order to trace the origin of the warmer and fresher water at the sea surface in winter, we analyzed the data in three different ways: (1) spatial distribution of surface water properties using monthly climatology from the Argo float data, (2) seasonal variation of heat and salt contents at the formation site, and (3) backtracking of surface drifter trajectories. Based on these analyses, it is likely that the warmer and fresher surface water properties found in the ESIW formation site are attributed to the low-salinity surface water advected from the southern part of the ES in autumn. [ABSTRACT FROM AUTHOR]

The East/Japan Sea (EJS) is well ventilated to deep water via brine rejection from ice formations and thermohaline convection, resulting in a short overturning period in several decades. Due to these characteristics, the dissolved oxygen concentration in the EJS deep water is much higher (190–200 μg L −1 at 3000 m water depth) than that found at the same depths of the Northwestern Pacific (30 μg L −1 ) or anywhere in the Pacific Ocean. The total mercury (THg) and methylmercury (MeHg) distributions, and MeHg mass budgets were investigated to identify how the EJS's distinct circulation pattern affects Hg speciation. Whereas the THg concentration in the surface seawater (ranging from 0.20 to 1.2 pM, mean 0.59 ± 0.24 pM) showed no site variation between the Japan Basin and the Ulleung Basin, the MeHg concentration in the surface seawater was significantly higher (p < 0.05) in the Japan Basin (32 ± 24 fM) than in the Ulleung Basin (12 fM), with a south to north increasing gradient. This observation was supported by the mass budget estimation showing that upward diffusion as well as net methylation of Hg(II) was the primary source of MeHg in the surface seawater; the upward diffusion value was higher in the Japan Basin (3.2 nmol m −2 yr −1 ) than in the Ulleung Basin (1.9 nmol m −2 yr −1 ) due to the shallow thermocline depths in the Japan Basin. In contrast, the MeHg concentration in deep seawater (1000–3000 m) was similar between the Japan Basin (530 ± 87 fM) and the Ulleung Basin (610 ± 99 fM) and significantly (p < 0.05) higher than in the North Pacific (24 ± 40 fM) or North Atlantic (87 ± 96 fM) deep seawater. The Hg(II) methylation capacity, represented by the MeHg concentration normalized to apparent oxygen utilization, was also higher for the EJS deep water (0.0048) than the Northeastern Pacific (0.0030) and Northwestern Pacific (0.0025) intermediate waters, implying that the short overturning period of EJS may cause exclusively high MeHg concentrations in the deep water. [ABSTRACT FROM AUTHOR]

Significant wave height (SWH) data of nine satellite altimeters were validated with in-situ SWH measurements from buoy stations in the East/Japan Sea (EJS) and the Northwest Pacific Ocean. The spatial and temporal variability of extreme SWHs was investigated by defining the 90th, 95th, and 99th percentiles based on percentile analysis. The annual mean of extreme SWHs was dramatically increased by 3.45 m in the EJS, which is significantly higher than the normal mean of about 1.44 m. The spatial distributions of SWHs showed significantly higher values in the eastern region of the EJS than those in the western part. Characteristic seasonality was found from the time-series SWHs with high SWHs (>2.5 m) in winter but low values (<1 m) in summer. The trends of the normal and extreme (99th percentile) SWHs in the EJS had a positive value of 0.0056 m year −1 and 0.0125 m year −1 , respectively. The long-term trend demonstrated that higher SWH values were more extreme with time during the past decades. The predominant spatial distinctions between the coastal regions in the marginal seas of the Northwest Pacific Ocean and open ocean regions were presented. In spring, both normal and extreme SWHs showed substantially increasing trends in the EJS. Finally, we first presented the impact of the long-term trend of extreme SWHs on the marine ecosystem through vertical mixing enhancement in the upper ocean of the EJS. [ABSTRACT FROM AUTHOR]

Dietary niches and food resource partitioning can support the coexistence of many fishes in benthic marine systems, which can lead to the greater abundances of those species that can potentially support their fisheries. Diets and niche overlap among nine demersal fish species were investigated in the southern continental shelf of East/Japan Sea, Korea. Specimens were collected monthly from January to November 2007 on soft bottoms between 40 and 100 m depth using a bottom trawl. A total of 20 prey taxa were found in 1904 stomachs of the nine species. Comparison of the stomach contents among the nine species showed that inter-specific dietary compositions differed significantly. Although all fish species consumed similar types of prey items, their contributions to the diet of different species varied. Among prey taxa, carid shrimps contributed greatly to the diets of Amblychaeturichthys hexanema , Amblychaeturichthys sciistius , Coelorinchus multispinulosus , Lepidotrigla guentheri , and Liparis tanakae , whereas polychaetes and teleosts contributed to the diets of Callionymus lunatus and Lophius litulon , respectively. On the other hand, carid shrimps and teleosts together contributed to the diets of Pseudorhombus pentophthalmus . Non-metric multivariate analysis of the mass contributions of dietary categories for food resources emphasized visually that the dietary compositions of the nine species differed. Although C. multispinulosus , L . guentheri , L . litulon , and L . tanakae showed similar dietary compositions between small and large size classes, ontogenetic diet changes of the remaining six species were evident. Feeding relationships among the nine demersal species were complicated, but inter- and intra-specific differences in dietary composition among the species reduced potential competition for food resources within the fish community in the southern continental shelf of East/Japan Sea, Korea. [ABSTRACT FROM AUTHOR]

Korea-Russia joint expeditions have been conducted mainly in the less studied Russian sector of the East/Japan Sea to understand the physical and ecological structures. In this study, the carbon uptake rates of phytoplankton measured in 2012 (middle-late October) and 2015 (middle April-early May) were analyzed to understand seasonal and spatial distributions of phytoplankton production, using a 13 C- 15 N dual isotope tracer technique. The water columns in the euphotic layers were well mixed during our cruise periods in both years. The water column-integrated chl- a concentrations (mean ± S.D. = 2.28 ± 1.47 mg m −3 ) in 2015 was significantly higher (t-test, p < 0.01) than in 2012 (mean ± S.D. = 0.49 ± 0.29 mg m −3 ) because of different sampling seasons. Small phytoplankton (< 2 μm) were relatively dominant in 2012, whereas different sizes of phytoplankton were evenly distributed in 2015 although a spatial distribution of large phytoplankton (> 20 μm) was observed near the Russian coast. The daily carbon uptake rates in this study were 180.5 and 441.6 mg C m −2 d −1 in 2012 and 2015, respectively which are significantly (t-test, p < 0.01) lower than the averaged values previously reported in the East/Japan Sea (863 ± 679.6 mg C m −2 d −1 ). The potential reasons for the lower rate in this study are discussed. The small phytoplankton contribution (47.4%) averaged from the two different cruises in this study is consistent with the result (47%) reported in temperate regions. Moreover, a significantly (t-test, p < 0.01) lower contribution of small phytoplankton in total primary production than total phytoplankton biomass in this study is consistent with the results from other regions. Lower total primary production might be expected due to increasing contribution of small phytoplankton under warmer conditions. [ABSTRACT FROM AUTHOR]

The East/Japan Sea (EJS) where is surrounded by the Korean peninsula, the Japanese islands, and the Russian coast has been experiencing a large change in physicochemical properties. Based on biochemical composition analysis (carbohydrates, proteins, and lipids), the current qualitative status of phytoplankton was identified in the northern EJS from two different sampling seasons (fall and spring in 2012 and 2015, respectively). The average chlorophyll-a (chl-a) concentration integrated from the euphotic depths was significantly higher in 2015 (99.3 ± 69.2 mg m −2 ) than 2012 (21.5 ± 6.7 mg m −2 ). Large phytoplankton (> 2 µm) were predominant in 2015 accounting for 64.5 ± 19.7% whereas small-size phytoplankton (0.7–2 µm) were dominant (49.1 ± 17.5%) in 2012. The biochemical compositions of phytoplankton were predominated by lipids (42.6 ± 7.8%) in 2012 whereas carbohydrate composition largely contributed (53.2 ± 11.7%) to the total biochemical composition in 2015, which is mainly due to different nutrient availabilities and growth stages. Interestingly, the averaged FM concentrations and calorific values for phytoplankton based on the biochemical compositions had similar values between the two years, although the integrated chl-a concentrations were substantially different between 2012 and 2015. In terms of different cell sizes of phytoplankton, we found that small phytoplankton assimilate more FM and calorific energy per unit of chl-a concentration than total phytoplankton. Our results are meaningful for the understanding of future marine ecosystems where small phytoplankton will become dominant at a scenario of ongoing warmer oceans. [ABSTRACT FROM AUTHOR]

Three cruises were undertaken along a meridional transect in the East/Japan Sea (EJS) in spring (May 2007), summer (July 2009), and fall (October 2012) to determine the geographic variations in phytoplankton biomass and community composition. This study revealed a gradient of surface temperature and a fluctuation of hydrographic conditions along the transect. Although a subpolar front (SPF) formed between the warm- and cold-water masses (37–40°N), no significant differences in phytoplankton biomass and community composition were detected between the southern and northern parts of the EJS. These results disprove our initial hypothesis that different water masses may contain differently structured phytoplankton communities. In the present study, isothermal layers (≤ 12 °C) fluctuated over a depth of 50 m in both warm- and cold-water masses, depending on the SPF. In contrast, the nitracline (i.e. 2.5 μM nitrate isopleth) depth was recorded within a limited range of 20–40 m in spring, 30–50 m in summer, and 40–60 m in fall. The chlorophyll a concentrations at the subsurface chlorophyll maxima (SCM) were significantly higher in spring and summer (356 ± 233 and 270 ± 182 ng L –1 , respectively) than in fall (117 ± 89 ng L –1 ). The relative contributions of individual phytoplankton groups to the depth-integrated chlorophyll a concentration conformed to the composition of the phytoplankton community in the SCM layer, showing a dominance of diatoms (58 ± 19% in spring, 48 ± 11% in summer, and 30 ± 20% in fall). Canonical correspondence analysis revealed that the geographic structures of phytoplankton communities were strongly associated with the vertical structures of water temperature and nutrient concentration in the water column rather than with horizontal gradients of hydrographic conditions. Finally, our findings suggest that water column stability and light–nutrient availability in the euphotic zone play a key role in determining geographical consistency of the biomass and the community structure of phytoplankton in the EJS. [ABSTRACT FROM AUTHOR]

The macromolecular composition of phytoplankton communities and the proximate composition of zooplankton communities were measured monthly in the southwestern East/Japan Sea from April to November 2014 in order to identify seasonal changes in, and relationships among, the biochemical compositions in both phytoplankton and zooplankton. The carbohydrate content of phytoplankton was highest in June, whereas the protein content was highest in August and lipids were highest in April. Overall, carbohydrates were dominant (53.2 ± 12.5%) in the macromolecular composition of phytoplankton during the study period. This composition is believed to result from the dominance of diatoms and/or nutrient-depleted conditions. In comparison, the protein level of zooplankton was highest in November, whereas lipids were slightly higher in May than other months. Overall, proteins were the dominant organic compounds (47.9±8.6% DW) in zooplankton communities, whereas lipids were minor components (5.5±0.6% DW). The high protein content of zooplankton might be related to the abundance of copepods, whereas the low lipid content might be due to a relatively high primary production that could provide a sufficient food supply for zooplankton so that they do not require high lipid storage. A significant positive correlation (r=0.971, n=7, p <0.01) was found between the lipid compositions of phytoplankton and zooplankton during our study period with a time lag, which is consistent with the findings from previous studies. More detailed studies on the biochemical composition of phytoplankton and zooplankton are needed to better understand the East/Japan Sea ecosystem's response to the many environmental changes associated with global warming. [ABSTRACT FROM AUTHOR]

The climate-induced decrease in fish catches in South Korea has been a big concern over the last decades. The increase in sea surface temperature (SST) due to climate change has led to not only a decline in fishery landings but also a shift in the fishing grounds of several fish species. The habitat suitability index (HSI), a reliable indicator of the capacity of a habitant to support selected species, has been widely used to detect and forecast fishing ground formation. In this study, the catch data of the chub mackerel and satellite-derived environmental factors were used to calculate the HSI for the chub mackerel in the South Sea, South Korea. More than 80% of the total catch was found in areas with an SST of 14.72–25.72 °C, chlorophyll-a of 0.30–0.92 mg m−3, and primary production of 523.7–806.46 mg C m−2 d−1. Based on these results, the estimated climatological monthly HSI from 2002 to 2016 clearly showed that the wintering ground of the chub mackerel generally formed in the South Sea of South Korea, coinciding with the catch distribution during the same period. This outcome implies that our estimated HSI can yield a reliable prediction of the fishing ground for the chub mackerel in the East/Japan Sea and South Sea of South Korea.

Data derived from altimetry shows that since 1993 the mean sea level over the East /Japan (EJS) Sea is increasing at a rate of ~3 mm/year, but tide gauge records indicate that a multidecadal reversal trend occurred prior to the early 1980s. We here characterize and quantify the multi-decadal trend of mean sea level in the EJS from the reconstructed sea levels and the in-situ ocean profiles over the past 60 years. Our analysis shows that sea level trends have undergone a shift, revealing a declining trend before the early 1980s, followed by a rising trend from the early 1980s onward with a near basin-wide sea level fluctuation. The trend reversal strongly corresponds to changes in the upper-ocean heat content over the EJS, revealing a negative correlation with the Pacific Decadal Oscillation (PDO) index that correlates negatively with wind stress curl (WSC) in the subtropical North Pacific. The PDO-related WSC, which changes the transport of the western boundary current in the subtropical gyre, may account for the observed trend reversal in the EJS sea level on a multi-decadal time scale. [ABSTRACT FROM AUTHOR]

Sea surface temperature (SST) retrieved from Advanced Very High Resolution Radiometer (AVHRR) onboard National Oceanic and Atmospheric Administration (NOAA) polar orbiting environmental satellites were validated in the East/Japan Sea (EJS) using surface drifter measurements as ground truths from 2005 to 2010. Overall, the root-mean-square (rms) errors of multichannel SSTs (MCSSTs) and non-linear SSTs (NLSSTs) using global SST coefficients were approximately 0.85°C and 0.80°C, respectively. An analysis of the SST errors (satellite – drifter) revealed a dependence on the amount of atmospheric moisture. In addition, satellite-derived SSTs tended to be related to wind speeds, particularly during the night. The SST errors also demonstrated diurnal variations with relatively higher rms from 0.80°C to 1.00°C during the night than the day, with a small rms of about 0.50°C. Bias also exhibited reasonable diurnal differences, showing small biases during the daytime. Although a satellite zenith angle has been considered in the global SST coefficients, its effect on the SST errors still remained in case of the EJS. Given the diverse use of SST data, the continuous validation and understanding of the characteristic errors of satellite SSTs should be conducted based on extensivein-situtemperature measurements in the global ocean as well as local seas. [ABSTRACT FROM AUTHOR]

In this study the structure and seasonal variations of deep mean circulation in the East/Japan Sea (EJS) were numerically simulated using a mid-resolution ocean general circulation model with two different parameterizations for the eddy-topography interaction (ETI). The strong deep mean circulations observed in the EJS are well reproduced when using the ETI parameterizations. The seasonal variability in the EJS deep layer is shown by using ETI parameterization based on the potential vorticity approach, while it is not shown in the statistical dynamical parameterization. The driving mechanism of the strong deep mean currents in the EJS are discussed by investigating the effects of model grids and parameterizations. The deep mean circulation is more closely related to the baroclinic process and potential vorticity than it is to the wind driven circulation. [ABSTRACT FROM AUTHOR]

Denitrification rates in the East/Japan Sea (EJS) were examined with extended Optimum Multi-Parameter (eOMP) analysis. The potential denitrification locations expected from the eOMP analysis occurred only in the Ulleung Basin (UB) and near the Tatar Strait (TtS) of the Eastern Japan Basin (EJB). Estimated denitrification rates were -0 .3 -3 and -4 -1 1 prnol N m 2 d 1 in the UB and in the EJB, respectively. These rates agree with previous published results. The EJS's rates were lower than reported for other marginal seas. However, considering the rapid EJS response to climate change, we predict that denitrification may be enhanced in the near future. [ABSTRACT FROM AUTHOR]

We address the trends in water temperature variability to understand the local global warming effects through different time scales using daily sea surface temperature (SST) measurements from 1966 - 2004. The abrupt SST changes off of the east coast of Korea are related to similar changes observed in the North Pacific. Seasonal water temperatures have increased by at least a factor of two or more in winter than in summer. While SSTs on annual timescales have increased significantly, SSTs on interannual timescales have decreased slightly at all stations. The anomalies in SST decadal time scales reveal maxima in temperature occurring in the mid-to-late 1970's, circa 1990 and circa 2000. These maxima coincide with major peaks in the North Pacific Gyre Oscillation (NPGO) with a lag of 4 - 7 months. The most significant warming appears in the long-term SST trends at each station where it approaches +0.05°C yr-1. Overall, the SST off the east coast of Korea has increased at rates that are far greater than what is reported in many other marginal seas. [ABSTRACT FROM AUTHOR]

Total dissolvable metals (Co, Ni, Cu, Cd, and Pb) in both surface waters and the water columns were acquired in the southern East/Japan Sea during a cruise around the Ulleung Basin in June 2001 to understand the spatial distributions of the metals. Concentrations in offshore surface waters were found to be Co 60 ± 12 pM, Ni 2.16 ± 0.25 nM, Cu 1.85 ± 0.55 nM, Cd 0.134 ± 0.018 nM, and Pb 155 ± 40 pM. Spatial distributions in surface waters showed that metal levels were generally enhanced at coastal sites in both Korea and Japan, where the metal distributions indicated complex patterns due to inputs, biogeochemical processes, and physical factors including upwelling. The Co distributions in the water columns seemed to be influenced predominantly by surface and bottom inputs, scavenged rather than regenerated at depth. For Cd, there was generally good agreement between the Cd and PO depth distributions, in agreement with the literature. The Cd/PO ratio from the water columns was found to be 0.133-0.203, lower than that in other marginal seas (e.g. the East/South China Seas and the Philippine Sea) of the western Pacific Ocean; this might be a result of the fast ventilation rate in this sea. The vertical Pb profile showed typical scavenged-type behavior with a surface maximum and deep minimum. From a comparison of inputs from the atmosphere and the Tsushima Warm Current, atmospheric deposition is substantial enough that it cannot be ignored, and its role in metal cycling is more significant in the offshore zone. [ABSTRACT FROM AUTHOR]