Japan/East Sea Research Articles

Deep Convection Along the Continental Slope in the East/Japan Sea: A Large-Eddy Simulation Study

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.

Characteristics of Marine Heatwaves in the Japan/East Sea

Marine heatwaves (MHWs) are prolonged discrete events of anomalously warm seawater observed around the world. The occurrence of MHWs in the Japan/East Sea (JES), a middle latitude marginal sea with abundant aquatic resources, has severe impacts on marine ecosystems and human society. We used satellite remote sensing products spanning 1982–2020 to investigate the mean states and variations of JES MHWs. The results show that the JES MHWs occurred twice per year. More than 40% of the MHWs were shorter than the mean duration of 12.6 days and weaker than the mean intensity of 2.4 °C. Frequent MHWs were observed in August, while high mean intensities were observed in winter and spring in the western JES. In recent years, the area’s averaged yearly sum of MHW intensity in the JES has increased dramatically, with a positive trend of 29.62 °C days per decade—over twice the average global trend (12.37 °C days per decade), which is further confirmed by its first empirical orthogonal function mode. The strengthening trend in MHWs may be explained by oceanic dynamics rather than atmospheric forcing, and may result in increased surface latent heat flux from the ocean to the atmosphere.

Application of Non-stationary Extreme Value Analysis to Satellite-Observed Sea Surface Temperature Data for Past Decades

Extreme value analysis (EVA) has been extensively used to understand and predict long-term return extreme values. This study provides the first approach to EVA using satellite-observed sea surface temperature (SST) data over the past decades. Representative EVA methods were compared to select an appropriate method to derive SST extremes of the East/Japan Sea (EJS). As a result, the peaks-over-threshold (POT) method showed better performance than the other methods. The Optimum Interpolation Sea Surface Temperature (OISST) database was used to calculate the 100-year-return SST values in the EJS. The calculated SST extremes were 1.60–3.44°C higher than the average value of the upper 5th-percentile satellite-observed SSTs over the past decades (1982–2018). The monthly distribution of the SST extremes was similar to the known seasonal variation of SSTs in the EJS, but enhanced extreme SSTs exceeding 2°C appeared in early summer and late autumn. The calculated 100-year-return SSTs were compared with the simulation results of the Coupled Model Intercomparison Project 5 (CMIP5) climate model. As a result, the extreme SSTs were slightly smaller than the maximum SSTs of the model data with a negative bias of –0.36°C. This study suggests that the POT method can improve our understanding of future oceanic warming based on statistical approaches using SSTs observed by satellites over the past decades.

Mesoscale Dynamics and Eddy Heat Transport in the Japan/East Sea from 1990 to 2010: A Model-Based Analysis

The driving mechanisms of mesoscale processes and associated heat transport in the Japan/East Sea (JES) from 1990 to 2010 were examined using eddy-resolving ocean model simulations. The simulated circulation showed correctly reproduced JES major basin-scale currents and mesoscale dynamics features. We show that mesoscale eddies can deepen isotherms/isohalines up to several hundred meters and transport warm and low salinity waters along the western and eastern JES boundaries. The analysis of eddy kinetic energy (EKE) showed that the mesoscale dynamics reaches a maximum intensity in the upper 300 m layer. Throughout the year, the EKE maximum is observed in the southeastern JES, and a pronounced seasonal variability is observed in the southwestern and northwestern JES. The comparison of the EKE budget components confirmed that various mechanisms can be responsible for the generation of mesoscale dynamics during the year. From winter to spring, the baroclinic instability of basin-scale currents is the leading mechanism of the JES mesoscale dynamics’ generation. In summer, the leading role in the generation of the mesoscale dynamics is played by the barotropic instability of basin-scale currents, which are responsible for the emergence of mesoscale eddies, and in autumn, the leading role is played by instabilities and the eddy wind work. We show that the meridional heat transport (MHT) is mainly polewards. Furthermore, we reveal two paths of eddy heat transport across the Subpolar Front: along the western and eastern (along 138∘ E) JES boundaries. Near the Tsugaru Strait, we describe the detected intensive westward eddy heat transport reaching its maximum in the first half of the year and decreasing to the minimum by summer.

Local Sea-Level Rise Caused by Climate Change in the Northwest Pacific Marginal Seas Using Dynamical Downscaling

Global climate models (GCMs) have limited capacity in simulating spatially non-uniform sea-level rise owing to their coarse resolutions and absence of tides in the marginal seas. Here, regional ocean climate models (RCMs) that consider tides were used to address these limitations in the Northwest Pacific marginal seas through dynamical downscaling. Four GCMs that drive the RCMs were selected based on a performance evaluation along the RCM boundaries, and the latter were validated by comparing historical results with observations. High-resolution (1/20°) RCMs were used to project non-uniform changes in the sea-level under intermediate (RCP 4.5) and high-end emissions (RCP 8.5) scenarios from 2006 to 2100. The predicted local sea-level rise was higher in the East/Japan Sea (EJS), where the currents and eddy motions were active. The tidal amplitude changes in response to sea-level rise were significant in the shallow areas of the Yellow Sea (YS). Dynamically downscaled simulations enabled the determination of practical sea-level rise (PSLR), including changes in tidal amplitude and natural variability. Under RCP 8.5 scenario, the maximum PSLR was ∼85 cm in the YS and East China Sea (ECS), and ∼78 cm in the EJS. The contribution of natural sea-level variability changes in the EJS was greater than that in the YS and ECS, whereas changes in the tidal contribution were higher in the YS and ECS. Accordingly, high-resolution RCMs provided spatially different PSLR estimates, indicating the importance of improving model resolution for local sea-level projections in marginal seas.

First Concurrent Measurement of Primary Production in the Yellow Sea, the South Sea of Korea, and the East/Japan Sea, 2018

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.

Acidification and Deoxygenation of the Northwestern Japan/East Sea

Seasonal hypoxia in the bottom waters of the Peter the Great Bay (PGB) of the Japan/East Sea (JES) occurs in summer. Using the empirical relationship between dissolved oxygen (DO) and pH obtained for hypoxic conditions and available historical DO data, acidification rates were estimated. Carefully sampled time-series observations from the northwestern part of the JES, carried out from 1999 to 2014 along the 132°20′ E and 134°00′ E longitudes, were chosen to determine the interannual variability of the sea’s hydrochemical parameters (DO, pH, and TA—the total alkalinity phosphates, nitrate, and silicates). To limit the effects of seasonal and spatial variability, only data obtained in the warm period were used. Additionally, all data from depths shallower than 500 m were discarded because they are affected by high natural variability, mostly due to strong mesoscale dynamic structures. Our results demonstrated that the pH and DO concentrations measured in the Upper Japan Sea Proper Water (750 m), Lower Japan Sea Proper Water (1250, 1750, 2250 m), and Bottom Water (3000 m) have been decreasing in recent years. On the other hand, calculated normalized dissolved inorganic carbon (NDIC), CO2 partial pressure (pCO2), and measured nutrient concentrations have been increasing. Maximum rates of acidification and deoxygenation are occurring at around 750 m. The annual rate of increase of pCO2 in the water exceeds the atmospheric rate more than 2-fold at a depth of 750 m. The observed variability of the hydrochemical properties can be explained by the combination of the slowdown ventilation of the vertical water column and eutrophication. However, the results obtained here are valid for the subpolar region of the JES, not for the whole sea. The synchronization of the deoxygenation of the open part of the JES and PGB has been found.

Satellite-Derived Protein Concentration of Phytoplankton in the Southwestern East/Japan Sea

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.

Characteristics of Different Size Phytoplankton for Primary Production and Biochemical Compositions in the Western East/Japan Sea.

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.

Variability of primary production among basins in the East/Japan Sea: Role of water column stability in modulating nutrient and light availability

The primary productivity of the East/Japan Sea (EJS) displays strong spatio-temporal variability, largely driven by the oceanographic forcings that vary across the different regions of the sea. Consequently, the ecosystem responses to climate variability could be region-dependent. A model-based analysis from this study synthesized the complex biological-physical interactions across the EJS, and suggested an intricate role of water column stability in driving the observed seasonal and basin-scale variability. Compared to the northern EJS (e.g., the Japan Basin), the southern EJS (e.g., the Ulleung and Yamato Basins) has a more favorable mixing regime for higher productivity in both winter and summer, balancing the need for phytoplankton growth in terms of nutrient and light availability. The wintertime water column in the southern EJS is relatively stable compared with the northern EJS due to the influx of warm Tsushima Warm Current (TWC) water and also weaker surface atmospheric cooling, leading to decreased light limitation for primary production in the south than in the north. During the stratified summer season, the surface stirring and mixing, due to the energy dissipation of strong TWC inflow, promote a relatively less stable surface mixed layer in the south than that in the north, and thus alleviate the nutrient limitation at the surface. In addition, the current- and wind-induced upwelling along the east coast of Korea further enhances the productivity in the southwestern EJS. The general south-north ‘see-saw’ pattern of mixing regime, combined with upwelling-downwelling switching, could be the key driver for the observed production patterns in the EJS.

Temporal and Spatial Variability of Near‐Inertial Waves in the East/Japan Sea From a High‐Resolution Wind‐Forced Ocean Model

AbstractNear‐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.

Climatological descriptions on regional circulation around the Korean Peninsula

The coastal ocean climatology of the temperature and salinity around the Korean Peninsula [Yellow Sea (YS), South Sea of Korea (SS), and East/Japan Sea (EJS)] is derived from conductivity-temperature-depth (CTD) profiles, which are collected from historical hydrographic surveys conducted over the last 20 years (1995 to 2014), using a multivariate regression analysis having basis functions of the temporal mean, annual and semiannual cycles, and linear trend. Based on the temporal means of the temperature and salinity profiles, the regional mesoscale circulation is reviewed with regard to the boundary currents, geostrophic currents, and density compensation and layered density structures associated with distinct water properties. The temperature in the YS exhibits a two-layer system, with decreasing annual amplitudes onshore above 25 m and offshore below 25 m because of annually modulated tidal mixing. The annual and semiannual variability in the temperature in the EJS primarily appears in the surface (upper 50 m) and intermediate (between 50 m and 200 m) layers, respectively, because of annual surface heat fluxes and southward subsurface regional currents (e.g. the North Korea Cold Current). For the linear trend in the temperature, the onshore decreasing and offshore increasing tendencies in the EJS are described by the transport of long-term heat contents associated with the regional boundary currents. The multivariate regression analysis presented here explains approximately 90% of the variance in the temperature and salinity in the upper 100 m (corresponding to most of the depths in the YS and SS).

Estimation of significant wave period from wave spectrum

Although the significant wave period is one of the key parameters in the design of coastal structures, it is not calculated by the spectral wave model because it is obtained by the zero-crossing analysis. The present paper examines several formulas that relate the significant wave period to wave spectral parameters such as peak period, mean period, and peakedness parameter. The formulas are derived based on the wave measurements in two locations in the Japan/East Sea (JES). The derived formulas are then compared with the measured significant wave periods in other locations in JES. It is shown that the formula using the mean wave period, Tm−1,0, is the most accurate. This formula is further used in a spectral wave model, the significant wave periods from which are compared with the measurement. Additionally, the relationship between the significant wave height and period is obtained based on the wave hindcasting, from which the most probable significant wave period can be determined for a specific significant wave height. A comparison with the measurement shows that the relationship is relatively inaccurate in the locations where southerly swell waves are significant, which are not accurately taken into account in the numerical model.

Role of cold water and beta-effect in the formation of the East Korean Warm Current in the East/Japan Sea: a numerical experiment

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.

Annual New Production of Phytoplankton Estimated from MODIS-Derived Nitrate Concentration in the East/Japan Sea

Our main objective in this study was to determine the inter-annual variation of the annual new production in the East/Japan Sea (EJS), which was estimated from MODIS-aqua satellite-derived sea surface nitrate (SSN). The new production was extracted from northern (>40° N) and southern (>40° N) part of EJS based on Sub Polar Front (SPF). Based on the SSN concentrations derived from satellite data, we found that the annual new production (Mean ± S.D = 85.6 ± 10.1 g C m−2 year−1) in the northern part of the EJS was significantly higher (t-test, p < 0.01) than that of the southern part of the EJS (Mean ± S.D = 65.6 ± 3.9 g C m−2 year−1). Given the relationships between the new productions and sea surface temperature (SST) in this study, the new production could be more susceptible in the northern part than the southern part of the EJS under consistent SST warming. Since the new production estimated in this study is only based on the nitrate inputs into the euphotic depths during the winter, new productions from additional nitrate sources (e.g., the nitrate upward flux through the MLD and atmospheric deposition) should be considered for estimating the annual new production.

A Numerical Simulation of Long-Term Sea Level Change in the East Asian Marginal Seas

ABSTRACT Kim C.-H.; Jang C. J., and Kim M.-W., 2018. A Numerical Simulation of Long-Term Sea Level Change in the East Asian Marginal Seas. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 546–550. Coconut Creek (Florida), ISSN 0749-0208. To investigate the regional sea level rise (SLR) and its possible mechanisms in the East Asian Marginal Seas (Yellow Sea (YS), East China Sea (ECS) and Japan/East Sea (JES)) sea level variation was simulated using the global ocean-sea ice coupled model (GFDL MOM5-SIS) forced with European Centre for Medium-range Weather Forecasts reanalysis data for 58 years (1958–2015). Seasonal variation in the simulated sea level shows comparable amplitude and phase to those of the satellite altimetry for 1993–2014 in each sea region. Rates of the simulated SLR are compared with those of the reconstructed sea level (Hamlington et al., 2011) ...

Interannual Variation of Surface Circulation in the Japan/East Sea due to External Forcings and Intrinsic Variability

The interannual variation of surface ocean currents can be as large as seasonal variation in the Japan/East Sea (JES). To identify the major factors that cause such interannual variability of surface ocean circulation in the JES, surface circulation was simulated from 1998 to 2009 using a three-dimensional model. Contributions of atmospheric forcing (ATM), open boundary data (OBC), and intrinsic variability (ITV) of the surface flow in the JES on the interannual variability of surface ocean circulation were separately examined using numerical simulations. Variability in surface circulation was quantified in terms of variance in sea surface height, 100-m depth water temperature, and surface currents. ITV was found to be the dominant factor that induced interannual variabilities of surface circulation, the main path of the East Korea Warm Current (EKWC), and surface kinetic energy on a time scale of 2–4 years. OBC and ATM were secondary factors contributing to the interannual variation of surface circulation. Interannual variation of ATM changed the separation latitude of EKWC and increased the variability of surface circulation in the Ulleung Basin. Interannual variation of OBC enhanced low-frequency changes in surface circulation and eddies in the Yamato Basin. It also modulated basin-wide uniform oscillations of sea level. This study suggests that precise estimation of initial conditions using data assimilation is essential for long-term prediction of surface circulation in the JES.

δ13C and δ15N Values of Sediment-trap Particles in the Japan and Yamato Basins and Comparison with the Core-top Values in the East/Japan Sea

Investigation of sediment-trap deployments in the East/Japan Sea (EJS) showed that distinct seasonal variations in particulate organic carbon (POC) fluxes of intermediate-water sediment-traps clearly corresponded to changes in chlorophyll a concentrations estimated from SeaWiFS data. The prominent high POC flux periods (e.g., March) were strongly correlated with the enhanced surface-water phytoplankton blooms. Deep-water sedimenttraps exhibited similar variation patterns to intermediate-water sediment-traps. However, their total flux and POC flux were higher than those of intermediate-water sediment-traps during some months (e.g., April and May), indicating the lateral delivery of some particles to the deep-water sediment-traps. Distinct seasonal δ13C and δ15N variations in settling particles of the intermediate-water sediment-traps were observed, strongly supporting the notion of seasonal primary production. Seasonal variations in δ13C and δ15N values from the deep-water sediment-traps were similar to those of the intermediate-water sediment-traps. However, the difference in δ13C and δ15N values between the intermediate-water and the deepwater sediment-traps may be attributed to degradation of organic matter as it sank through the water column. Comparison of fluxweighted δ13C and δ15N mean values between the deep-water sediment-traps and the core-top sediments showed that strong selective loss of organic matter components (lipids) depleted in 13C and 15N occurred during sediment burial. Nonetheless, the results of our study indicate that particles in the deep-water sediment-trap deposited as surface sediments on the seafloor preserve the record of surface-water conditions, highlighting the usefulness of sedimentary δ13C and δ15N values as a paleoceanographic application in the EJS.

Hourly Observed Internal Waves by Geostationary Ocean Color Imagery in the East/Japan Sea

AbstractAs internal waves (IWs) are generated via internal tidal energy near the bottom shelf break of the Korea Strait, continuous evolutions of IWs are extremely difficult to observe using satellite observations. However, the Geostationary Ocean Color Imager (GOCI) has enabled the detection of IWs in the East/Japan Sea (EJS) on an hourly basis to investigate their propagation behavior. For an extended IW analysis, a total of seven packets were identified using GOCI chlorophyll-aconcentration (Chl-a) measurements around the Ulleung Basin and Dok Island in the EJS. The results demonstrate that the IW locations had lower Chl-avalues than non-IW-affected regions on 10, 11, 13, and 14 August 2013. The lower Chl-avalues are due to the significant wave heights of the IWs, which cause surface water masses to be dispersed. In addition, from the continuous trajectories of the waves in the hourly GOCI Chl-aimagery, quantitative physical parameters of the IWs, such as their pathway (northeasterly), phase speed (1.46–1.61 m s−1), amplitude (20.62–26.76 m), and period (12.29 h), could be obtained. Therefore, the advantage of using GOCI is the ability to detect and analyze the physical characteristics of IWs on an hourly time scale.

Distributions and fluxes of methylmercury in the East/Japan Sea

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 (190–200μgL−1 at 3000m water depth) than that found at the same depths of the Northwestern Pacific (30μgL−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 1.2pM, mean 0.59 ± 0.24pM) 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 ± 24fM) than in the Ulleung Basin (12fM), 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 (3.2nmolm−2yr−1) than in the Ulleung Basin (1.9nmolm−2 yr−1) due to the shallow thermocline depths in the Japan Basin. In contrast, the MeHg concentration in deep seawater (1000–3000m) was similar between the Japan Basin (530 ± 87fM) and the Ulleung Basin (610 ± 99fM) and significantly (p < 0.05) higher than in the North Pacific (24 ± 40fM) or North Atlantic (87 ± 96fM) 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.