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This paper presents a statistical analysis of time series obtained from the Aqualog profiler moored at the upper continental slope off the southeast Russian coast in the northwestern Sea of Japan from mid-April through mid–October 2015. The vertical conductivity, temperature and depth (CTD) profiles with a high resolution (6 hours in time and 1 m in depth) were measured between 64 and 300 m, and the buoyancy frequency was estimated from these data. Buoyancy frequency and temperature data were decomposed to empirical orthogonal functions (EOFs) in the depth-time domain. Fluctuations in the vertical stratification revealed by the buoyancy frequency were assessed and linked to the passage of dynamic structures near the mooring. In the anticyclonic structures, the stratification weakened above 120 m due to isopycnal bending down and strengthened below 165 m. Stratification changes occurred on an 8–13-day timescale due to the passage of mesoscale eddies near the mooring. Energetic fluctuations were also found over longer 20–30- and 80–130-day time scales. These processes were accompanied by temperature anomalies associated with horizontal advection above 150 m and with vertical pycnocline fluctuations in the deeper layer. Temperature anomalies moving in the vertical direction with an average speed of 1.7 × 10–2 cm/s (14.7 m/day) on timescales from 4 to 13 days were detected at the border between these layers which exhibited different kinds of stratification changes.

For the first time, the long-term measurements of meteorological and oceanographic characteristics were measured using the anchored autonomous complex of the WaveScan buoy on the Southwestern shelf of the Peter the Great Bay (Japan Sea) from 21 April to 23 December 2016. The buoy anchor was set at 50 m depth. During 10 minutes of each hour, meteorological characteristics of the near surface layer of the atmosphere, sea water temperature at the horizon of 1.5 m, and vertical profiles of the current velocity vector from the near-surface layer to the near-bottom one were measured with resolution 4 m. During 20 minutes of each hour, the characteristics of waves on the sea surface were measured. In General, most of the meteorological characteristics measured on the buoy are in good agreement with the data of the NCEP-DOE AMIP-II, ERA-Interim and ERA5 reanalysis. The significant wave height for the whole frequency belt, mean wave period and mean spectral direction from the WaveScan Buoy measurements have the best statistical relationship, confidence level is 99%, with the correspondent significant height of the waves, mean wave period and mean spectral wave direction from reanalysis ERA5. The features of variability of vertical profiles of the current velocity vector on the synoptic and seasonal time scales that depend on wind speed and vertical stratification of density are determined. In the warm season the a significant left turn of the vector of the measured current velocity with depth is observed in the seasonal pycnocline below the surface layer of friction. At wind speed, not exceeding 5 m/s, the angle of the current velocity vector left turn reaches 170° in the bottom layer, where countercurrent is formed. During the passage of the Lionrock tropical cyclone, when and the daily mean wind velocity increase to 9 m/s, the angle of left turn of the current velocity vector in the pycnocline decreases to 20°. In the cold season a classic right turn of the current velocity vector is observed in the upper boundary layer, and the vertically average velocity vector within the 50 m layer, as well as the total drift flow in the Ekman friction layer, deviate 90° to the right from the surface wind velocity vector. The left turn of the current velocity vector at the buoy installation point was not observed in the cold season.

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.

During the the 73th cruise of the RV Professor Gagarinskiy and 53th cruise of the RV Academic Oparin oceanological studies of the north-western Pacific, Japan and Okhotsk Seas were conducted. New data on the structure sea bottom, water composition as well as slope convection of the Japan Sea.

The study is based on unique data from the autonomous Aqualog profiler installed in the northwestern Japan Sea at the continental slope off the southeastern Primorye coast from April 18 through October 14, 2015. Data set for the 64–300 m depths, with the depth step of 1 m and time step of 6 hours, and the time series averaged for the 320–420 m depths, with the time step of 6 days, are used. Variability in the upper and lower profiled layers is found to be of different nature. Above 100–150 m short-lived (2–4 days) temperature and salinity anomalies are mostly of the same sign. Based on our earlier analysis of satellite information it is possible to link these anomalies with water intrusions of different origins. Conversely, these anomalies are usually of the opposite signs and are longer-lived (up to 10–15 days) in the lower layer which can be explained by vertical displacements of isopycnals, probably due to passing mesoscale and submesoscale disturbances. Based on wavelet transform it is found that the longest-period fluctuations (with the periods of 80–110 days) were the most intense in the entire profiled layer throughout the entire record. Isopycnal displacements with the periods of 80–110 days were also found, being in-phase with the temperature fluctuations. The wavelet transform also revealed that variability strengthened in the upper layer and weakened in the lower layer from mid-August onwards.

The study analyzes data of ship-based observations on 45 stations performed aboard the R/V Akademik M.A. Lavrentyev on cruise 33 May 7–18, 2004, in the northwestern part of the Sea of Japan (35–44° N, 130–137° E). The following in situ data were used: CTD-data, assimilation number, concentrations of nutrients (nitrogen, phosphorus, and silicon compounds) and chlorophyll a (Сhl a). Satellite data on Сhl a concentration, diffuse attenuation coefficient at a wavelength of 490 nm, primary production (PP), and Photosynthetically Active Radiation (PAR) available from the Climate Change Initiative Ocean Colour (CCI-OC) and Ocean Productivity databases were also used for the same stations. The Chl a concentration in the first optical layer estimated from the results of ship-based measurements, was on average 0.55 ± 0.58 mg/m3, but the satellite-derived estimates were almost twice as high (0.95 ± 0.36 mg/m3). Ship assessments of PP were 1870 ± 900 mgC m–2 day–1; the value obtained using satellite data was 1.5 times smaller: 1226 ± 432 mgC m–2 day–1. Vertical Chl a profiles showed that the largest amount of Chl a was concentrated in the 20–45 m layer. Measurements of the assimilation number showed that most production occurs within the 0–55 m layer in the south of the study area and within the 0–30 m layer in the north. The weak correlation between ship- and satellite-derived Chl a and PP values can be explained by low accuracy of satellite-derived estimates.

We studied the migrations of young spotted seals during their annual cycle. In May 2017, we attached satellite tags (SPOT-293A) to three individuals (two underyearlings and one yearling) captured at their breeding ground in Peter the Great Bay, western Sea of Japan/East Sea. The operational time of the installed tags ranged from 207 to 333 days; a total of 27195 locations were uploaded. All three seals migrated east and further north along the coast of the mainland. The average daily migration speed of the seals ranged between 70 and 135 km/day. The yearling moved faster than the underyearlings. During early August, they arrived at their summer habitats, which were located in the northern part of the Tatar Strait (Sea of Japan/East Sea) for the underyearling seals and in Aniva Bay (Sea of Okhotsk) for the yearling seal. While moving from the place of tagging to the summer feeding grounds, the seals covered a distance of 2300 to 3100 km. From August to October, each seal permanently stayed within the same isolated area. The reverse migration of all three seals began in November. When the seals traveled south, they used the same routes by which they had moved north in the spring, but they moved at a faster speed. By December, two seals returned to their natal islands, where both stayed until their transmitters stopped sending signals (in March 2018).

Cold-core (T 27.24 kg/m3 were observed in the near-bottom layer at the shelf break in Peter the Great Bay (PGB) of the Japan/East Sea in March 2010. The anomalously cold water was 0.4 ml/l richer in oxygen than was the surrounding water, and it contained more suspended particles. The observations were carried out by using a moored automatic mobile Aqualog profiler. Profiling occurred as frequent as every hour, allowing us to obtain data with high temporal resolution. The Aqualog profiler delivered vertical profiles of the ocean current velocity, acoustic backscatter at 2 MHz, temperature, and salinity between the depths of 20 and 105 m. Other oceanographic instruments were mounted at fixed depths on the mooring line to measure current velocity, temperature, conductivity, dissolved oxygen, chlorophyll-a fluorescence, and turbidity. Complementary data included ship-borne CTD casts and satellite-borne imagery and scatterometry as well as coastal weather station records. The Regional Ocean Modeling System (ROMS) was employed to study the origin and evolution of the cold water. The model simulated the ocean dynamics at a 600-m horizontal resolution in PGB from 2009 to 2010. The model was forced by the surface momentum, heat and fresh water fluxes of the NCEP-DOE Reanalysis 2. The lateral boundary condition of the model was obtained from nesting into a Japan Coastal Ocean Predictability Experiment model data set. According to the ROMS simulation, the circulation in Ussuri Bay in the northeastern bay of PGB was anticyclonic in February–March 2010. The submesoscale cyclonic vortices generated around the anticyclonic gyre. The submesoscale cyclones tended to move southward out of the bay, and they transported the anomalously cold water towards the outer shelf. As a result, the cold water anomalies were often observed to persist for as long as 2 days near 42.5°N, 132°E. Lagrangian analysis confirmed that this cold water observed by the Aqualog profiler originated in Ussuri Bay. The model simulation showed that the submesoscale cyclonic eddies played a specific role in supplying the densest water from the northern part of PGB to the outer shelf, where the dense water was then entrained by the mesoscale eddies in the Primorye Current zone and could cascade down the continental slope into the deep northern basin of the sea. This transport of the densest water by the submesoscale eddies was estimated to be 5–10·10−3 Sv in February–March 2010.

Herein, we report field studies of the seasonal and daily dynamics of the mixing zone in the micro-tidal mouth of the Partizanskaya river. The position of the mixing zone was determined according to the volume of river run-off. The upper boundary of the mixing zone was observed at 5 (at a river run-off of 73.67 m3/s) to 12.5 km (at 7.7 m3/s) from the mouth bar. The estuary is characterized by a two-layer circulation of water, which results in a halocline with a salinity gradient of up to 30% per 1 m. Variations of salinity in the rifts caused by tides reaches 27% over 3 h.

Field studies of the seasonal and daily dynamics of the mixing zone were carried out in the microtidal estuary of the Partizanskaya River. The position of the mixing zone was determined by river runoff volume. The upstream boundary of the mixing zone was located at a distance from 5 (at river discharge of 73.67 m3/s) to 12.5 km (at 7.7 m3/s) from the mouth bar. The estuary features a two-layer water circulation, resulting in the formation of halocline with a gradient of water salinity of up to 30‰ per 1 m. Salinity variations at rifts, caused by tides, reach 27‰ over a 3-hour period.

According to the Plan of Complex Scientific Research of the World Ocean for 2017–2022, the V.I. Il’ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences, continues to study the northern part of the Sea of Japan and the Tatar Strait, carrying out a three-year plan of expeditionary research of this region. The expedition aboard the R/V Akademik Oparin (cruise no. 55) spanned from October 2 to 19, 2018. Geophysical and gas-geological study of the water area in the central and southern parts of the Tatar Strait was continued. On the rookery of Tyuleny Island, in the Sea of Okhotsk, information on sea lions and larghas were recorded, three individuals of northern fur seal were tagged with satellite trackers to study their migration routes. CTD observations of the upper layer (down to 600 m) were made on transects across mesoscale eddies in the northern part of the Sea of Japan. Expeditionary research was supported by the Council of the Earth’s Hydorsphere of the Ministry of Science and Higher Education of the Russian Federation.

The hydrological and hydrochemical parameters of the Tumen River estuary were collected at 13 stations in May and October 2015. Vertical temperature, conductivity, dissolved oxygen, chlorophyll fluorescence, and turbidity profiles were obtained. Water was sampled from the surface and bottom layer. The water samples were analyzed for major ions, pH, salinity, concentrations of dissolved oxygen, major nutrients, dissolved organic carbon, humic matter, and δ18О and δD isotopes. This estuary is attributed to microtidal type with a flushing time of about 10 h. A phytoplakton bloom occurred in the top layer of the estuary. For surface horizons, the hydrochemical parameters show a linear correlation with salinity. In the bottom horizons, all these parameters, except for major ions and δ18О and δD isotopes, reveal substantial nonconservative behavior. The nonconservative behavior of the hydrochemical parameters in the bottom waters was mainly caused by degradation of the phytoplankton biomass at the water/sediment interface. Hypoxic conditions were established in the bottom waters of the estuary in May.

The pioneering long-term survey of the NW Japan Sea thermohaline structure was implemented for almost half a year from April 18, 2015 through October 15, 2015. The measurements were carried out by using the SBE 52-MP CTD probe at the moored profiler Aqualog. The profiler mooring was deployed at the continental slope in the area east of Peter the Great Bay. A novel approach to the primary processing of the profiler CTD data is discussed. It features a new data processing software CTD data_cor_SBE_52-МР. The profiler data is verified based on ship measurements at a cross-slope transect on May 30, 2015. The processing and verification of the SBE 52-MP CTD observations allows for the data quality improvement up to the WOCE standards. The profiler mooring data and the ship measurements reveal variations in the ocean temperature, salinity, and density stratification due to ocean eddy passage nearby the mooring site. The NOAA/AVHRR infrared satellite imagery is used for detection and analysis of this eddy and its movement southwestward along the Primorye Current zone. Overall, the study demonstrates a powerful potential of combined analysis of long-term time series of the in situ data at fixed geographical location, the ship borne oceanographic section, and the multispectral satellite information in compliance with the 4D oceanography requirements.

The paper discusses the results of an experiment conducted in the Sea of Japan in March 2016 on an acoustic track 194 km long in winter hydrological conditions. The most complex case of propagation of pseudorandom pulse signals from the shelf to deep water in the presence of gyre formation on the acoustic track. An analysis of the experimentally obtained pulse characteristics show that at all points, a maximum, in terms of amplitude, first arrival of acoustic energy is recorded. This is evidence that at a given depth horizon, pulses that have passed the shortest distance through a near-surface sound channel at small angles close to zero are received first. The calculation method of mean sound velocity on the track, based on the satellite data of surface temperature monitoring, is proposed. We expect that the results obtained with this method can be successfully used for the purposes of acoustic range finding and navigation.

Through the Cooperative Study of the Kuroshio and Adjacent Regions (CSK) program during 1965–1979, the capacities of current member states (MSs) of the Sub-Commission for the Western Pacific (WESTPAC) of the Intergovernmental Oceanographic Commission (IOC) were enhanced with regard to regional ocean science and data management. Following the termination of the CSK in 1979, each MS continued the work to advance ocean science. The results of scientific studies of the Kuroshio and its adjacent regions have been published by various experts including many from the MSs of the WESTPAC; however, to-date, there has been no systematic approach to the research of the Kuroshio and its adjacent regions. This review considered the Kuroshio from the regional perspective of experts of the MSs, that is, from the perspectives of MSs, science, and the future prospects. Experts from each MS reviewed past activities and contributions and reviewed the knowledge gaps in the fields of physical, biological, and biogeochemical science. Many scientific questions remain regarding the path of the Kuroshio from south to north, as well as associated phenomena, including mesoscale eddies and fronts, the important roles of ocean variations in adjacent regions, and the different roles and mechanisms of air–sea interactions in low- and mid-latitude areas. Despite considerable effort by many biologists, substantial gaps remain in our biological knowledge of the region. The Kuroshio and its adjacent regions comprise one of the areas of the world with high biodiversity; however, there has been insufficient research into what is the cause of this high biodiversity. From a biogeochemical aspect, high-resolution spatiotemporal observations will be required to understand interactions with physical processes both in the Kuroshio region and in the marginal seas. It has been highlighted that long-term fixed-location observations will be needed to understand the key mechanisms of biogeochemical processes, particularly in relation to climate change. Finally, the report summarized the future perspectives. Based on recognition of the current circumstances and with acknowledgment of the potential short-term future capabilities of MSs, the possible uses of new technologies and frameworks were discussed. Since the implementation of the United Nations Convention on the Law of the Sea, which came into force in 1994, it has been difficult to conduct observations in the exclusive economic zone (EEZ) of other regional states. Thus, new frameworks and/or technologies will be needed to ensure the success of future studies of the Kuroshio.

For more than three months in 2011, in situ monitoring of temperature (T), salinity (S) and dissolved oxygen concentrations (DO) was carried out using a Water Quality Monitor (WQM) station deployed on the seafloor of Amursky Bay (Japan/East Sea). During this period, hypoxia in the bottom waters persisted for 93 days. In the summers of 2012 and 2013, the spatial distribution of DO was measured during ship surveys. Using these time series of DO, the biological oxygen demand (BOD) and ventilation rates in bottom waters were estimated from May 10 to August 7. The seasonal change in the dominant direction of the wind, which occurs twice a year (spring and autumn), was an important natural factor in development and termination of seasonal hypoxia in the bay. Dominant southern winds in the summer induced downwelling circulation on the northwestern part of the Japan/East Sea shelf. Under this circulation, hypoxia developed in the bottom waters of Amursky Bay. In autumn, dominant northern winds induced upwelling, causing the advection of cold, oxygenated seawater into the bay, ending the period of hypoxia. Short-term fluctuations in wind direction in the summertime influenced spatial and vertical distribution of T, S and DO. At the end of the summer, the oscillation of the downwelling/upwelling circulations revealed complicated temporal-space distributions of hydrological parameters in Amursky Bay.

We studied in detail a mesoscale anticylonic eddy that has been sampled in the R/V Professor Gagarinskiy cruise (June–July 2012) in the area east of the Kuril Islands in the northwestern subarctic Pacific. Lagrangian approach was applied to study formation, structure and evolution of this feature called the eddy A and of its parent eddy B using a simulation with synthetic tracers advected by the AVISO velocity field. We used different Lagrangian methods and techniques to identify those eddies and their boundaries, to know their structure and to document their deformation, metamorphoses and splitting. It has been found that the eddy A was born as a result of splitting of the eddy B with the core water to be borrowed from the eddy B which, in turn, was influenced by the Okhotsk Sea water flowing into the ocean through the Kuril straits. The periphery of the eddy A was formed mainly by East Kamchatka Current water in the process of its winding onto the eddy A core by portions. All these processes have been documented in detail with the help of drift and tracking Lagrangian maps computed forward and backward in time with a large number of synthetic tracers distributed over the studied area. We have found a Lagrangian structure of those eddies and the ways how they have gained and released water. Simulated and measured locations of the center of the eddy A and its boundary have been be estimated to coincide with the accuracy of ≈ 7 –10 and ≈ 15 –20 km, respectively. Our simulations were validated in part by tracks of available surface drifters and Argo floats. We presented CTD hydrographic observations of the Kuril eddy A from the surface to deep waters and compared observed and simulated results in order to establish origin and properties of water masses constituting that eddy.

It has been established that the waters of the transboundary Razdolnaya (Suifen) and Tumannaya (Tumen) Rivers are highly polluted by nutrients. Loads of nutrients supplied by the Razdolnaya and Tumannaya Rivers cause eutrophication and induce seasonal hypoxia in areas off the river mouths in the Amursky Bay (northwestern part of Peter the Great bay (PGB)) and the Far Eastern Marine Biosphere Reserve (FEMBR) area (southwestern part of PGB). In both cases, hypoxia developed in the bottom waters in topographic depressions in the basins after the peak nutrient fluxes that occur in summer due to the monsoon climate. In 2013, anoxia occurred in the FEMBR area as a result of unusually heavy atmospheric precipitation at the end of July. The hypoxic areas expanded over approximately 60 km2 and 17 km2 in Amursky Bay and the FEMBR area, respectively. The hypoxia durations were approximately three months in 2011 in the semi-enclosed Amursky Bay and three weeks in 2014 in the more open FEMBR area. The observed hydrochemical anomalies (oxygen, phosphorus, silica, pH, DIC, ammonium) suggest that the main cause of hypoxia is degradation of “excess” diatoms on the water/sediment interface under dynamic water limitation. Mooring observations by a water quality monitoring station registered very high chlorophyll concentrations (approximately 50 mg/m3) in the bottom waters of the FEMBR area. These high chlorophyll concentrations may be attributed to the topographic depression, which works as a sediment trap and accumulates organic matter. Sharply increasing of transparency of water column in the FEMBR area caused by replacement of coastal waters on the waters of open sea can to develop periphyton on seafloor. There is synchronization in deoxygenation of regional water basins (in our case it is the shelf and open part of the Sea of Japan) and global basins (oceans) which may be caused by variations in the chemical composition of atmosphere.

Surface water samples were collected at 15 sampling sites in the southeastern Japan Sea along the Japanese Archipelago for analysis of polycyclic aromatic hydrocarbons (PAHs). Water samples were fractionated by filtration through a glass fiber membrane (pore size 0.5 µm) and analyzed by high-performance liquid chromatography with fluorescence detection. Thirteen PAHs having 3 to 6 rings were found in the dissolved phase (DP) and 12 were found in the particulate phase (PP). The total (DP+PP) PAH concentration ranged from 6.83 to 13.81 ng/L with the mean±standard deviation (S.D.) concentration of 9.36±1.92 ng/L. The mean±S.D. PAH concentration in the DP and PP was 5.99±1.80 and 3.38±0.65 ng/L, respectively. Three-ring PAHs predominated in the DP, while the proportion of 4-ring PAHs was higher in the PP. The mean total PAH concentration in the southeastern Japan Sea was higher than the concentration in the northwestern Japan Sea (8.5 ng/L). The Tsushima Current, which originates from the East China Sea with higher PAH concentration, is considered to be responsible for this higher concentration.

The East Sea (Japan Sea), a small marginal sea in the northwestern Pacific, is ventilated deeply down to the bottom and sensitive to changing surface conditions. Addressing the response of this marginal sea to the hydrological cycle and atmospheric forcing would be helpful for better understanding present and future environmental changes in oceans at the global and regional scales. Here, we present an analysis of observations revealing a slowdown of the long-term deepening in water boundaries associated with changes of water formation rate. Our results indicate that bottom (central) water formation has been enhanced (reduced) with more (less) oxygen supply to the bottom (central) layer since the 2000s. This paper presents a new projection that allows a three-layered deep structure, which retains bottom water, at least until 2040, contrasting previous results. This projection considers recent increase of slope convections mainly due to the salt supply via air-sea freshwater exchange and sea ice formation and decrease of open-ocean convections evidenced by reduced mixed layer depth in the northern East Sea, resulting in more bottom water and less central water formations. Such vigorous changes in water formation and ventilation provide certain implications on future climate changes.

Submesoscale cold-core (T < 0°C) eddies were observed near the shelf break in Peter the Great Bay of the East/Japan Sea in March 2010. The observations were carried out at the moored automatic mobile profiler Aqualog station. The profiling was as frequent as every hour allowing us to obtain data with high temporal resolution. Aqualog delivered vertical profiles of ocean current velocity, acoustic backscatter at 2 MHz, temperature, and salinity between the depths of 20 m and 105 m. Below the profiling range, the InterOcean S4 current meter with temperature and salinity sensors was mounted on the mooring line. Above the profiling range, the RBR XR420 CTD logger with dissolved oxygen, chlorophyll-a fluorescence, and turbidity sensors was also mounted on the mooring line. Additionally we used the data of ship-born CTD casts, the satellite-born imagery and scatterometery, and the coastal weather station records. The data analysis suggests that the cold-core submesoscale eddies play an important role in supplying the densest water (σθ > 27.24 kg/m3) from the northern part of Peter the Great Bay to the shelf break zone where it cascades downward into the deep northern basin of the Sea. The volume of the densest water in an eddy, according to our rough estimate, is 0.5-1 km³. At the sea shelf, the densest water is richer in oxygen by 0.4 ml/l than surrounding water. It also contains much more suspended particles brought by the rivers.

The results of the temperature profiles and heat flux field studies in the upper layer of the bottom sediments of Amurskiy Bay in the Sea of Japan are presented. These studies were carried out from November 22 to March 25, 2010. The obtained values of the heat flux from the sediments to the water layer can explain the formation of the warm water layer with high salinity near the bottom in Amurskiy Bay during the winter season.

We have studied the characteristics of mesoscale eddies in the Peter the Great Bay and the adjacent area of the northwestern Sea of Japan. The characteristics were determined using infrared images (NOAA/AVHRR satellite data) for July–October periods of 2000–2004. It is established that anticyclonic eddies are always present in the indicated area, since they have been identified on 85% of the available cloudfree images. Characteristics of 43 anticyclonic eddies have been determined. Their diameters vary from 20 to 110 km, with the most probable sizes within 20–60 km. The typical eddy lifetime varies from 3 to 30 days. However, some medium-sized eddies (40–60 km) have been observed for a longer time (up to 40–60 days) and large eddies (60–110 km in diameter) remained for up to 1–2 months. Eddies drift southwestward and southward along the continental slope at a typical speed of 3–6 cm/s. There are interannual variations in the vortex structure of water flows in the area studied, which are manifested by the presence of a single large eddy occupying a large part of the bay in some years (2000, 2002, and 2003) or by the formation of several smaller eddies (2001 and 2004). The eddies contain warmer and less saline shelf waters (compared to the surrounding water) and transport it in their cores over quite a long distance. Thus, eddies are an essential element of the process of the intermediate low-salinity water-mass formation in the Sea of Japan. In addition, eddies play important role in the cross-shelf water exchange, providing rapid ventilation of the coastal zone and determining the variability of the biological processes.

The results of nitrate in situ measurements by the In Situ Ultraviolet Spectrophotometer (ISUS MBARI) in the northern part of the Japan Sea are reported. The observations were implemented in three cruises of R/V Akademik M.A. Lavrentyev during 2009–2010 including SoJaBio expedition. The instrument was attached to CTD/water sampling system and thus allowed to measure high resolution vertical profiles of nitrate concentration as well as profiles of water temperature, salinity, dissolved oxygen and chlorophyll-a content down to 1000 m depth. The ISUS data were calibrated using results of chemical analysis of nitrate content in water samples taken at standard depths. Comparison of discrete samples and vertical profiles showed a good correlation of R =0.93 for 240 pairs of data. On a background of general increasing of nitrate concentration with depth the ISUS profiles showed at many stations an existence of local nitrate extremes which form inversions or step-like structure of the profiles. They had a typical vertical scale from a few tens to hundred meters and were observed in various layers below seasonal pycnocline (40–50 m) and down to 1000 m. These anomalies coincide well with the anomalies in dissolved oxygen, temperature and salinity profiles and thus could be explained by mesoscale advections of low nutrient/high oxygen water associated with mixing processes in the shelf/slope area as well as at the frontal zone and mesoscale eddies. High nutrient/low oxygen intrusions were also detected at depth 40–60 m and were associated with high salinity subsurface water transported from the southern part of the sea. However, a process of organic matter remineralization just below pycnocline could be also responsible for increasing nitrate concentration there as well for the formation of local maximum or step-like structure in the subsurface layer (40–150 m) observed at many stations located close to the shelf area. Anomalies of larger vertical extent (up to 200–300 m) were associated with mesoscale anticyclonic eddies which carry water of lower nutrient and higher oxygen content. All these peculiarities are important for understanding the biodiversity features observed during the SoJaBio expedition.

During the hydrological survey from November 4 to 9, 2009, the underway measurements of pH, temperature, and salinity of the surface water in the Peter the Great Bay were carried out using the cell without the liquid junction in the running system. Using the measurement data, the partial pressure of carbon dioxide (pCO2) was computed which was below its atmospheric value and varied from 290 to 360μatm. Thus, during the period under study, the surface water of the bay forms the runoff for the atmospheric carbon dioxide. The minimum values of pCO2 are associated with the warm stream entering the bay from the southwestern part of the open sea. The maximum values of pCO2 are associated with the local upwelling of bottom waters. It is suggested that the horizontal convection caused by the nonuniform cooling of coastal water in the autumn forms the revealed mesoscale dynamic structures.

To evaluate the structure and photosynthetic activity of the phytoplankton of the northwestern part of the Sea of Japan, the results of the surveys conducted from February 26 to March 9, 2003, during the 30th cruise of the R/V Akademik Lavrentyev were used. The species composition was considered taking into account the influences of the hydrological conditions. In particular, the studied water area was divided into four zones with various sizes of the mixed layer depth (MLD). An inverse relationship between the MLD and the content of chlorophyll a (Chl a) in various zones was established. The diatom algae were the basic dominants of the composite structure of the phytoplankton everywhere. It is revealed that, at a MLD increase, smaller forms of microalgae receive development. The intrapopulation relations are caused by the adaptation of various species of phytoplankton to the influence of external factors, among which the size of MLD is fundamental.

In July 2007, integrated studies of the Amur Estuary and the adjacent aquatic areas were performed on board R/V Professor Gagarinskii within the project of the Amur River basin exploration. On the basis of the data obtained during the cruise, the carbonate system of the Amur Estuary in the summer period was considered. It was shown that the distribution of the carbonate parameters in the Amur Estuary and the adjacent aquatic areas points to the high intensity of the bio-geochemical processes of production and mineralization of organic matter. It was found that the organic matter destruction is prevailing over the photosynthesis in the riverine part of the estuary. This aquatic area is a source of carbon dioxide for the atmosphere and rates as a heterotrophic basin. On the contrary, the surface waters at the outer boundaries of the estuary (the Gulf of Sakhalin and the Tatar Strait) act as a sink of the atmospheric carbon dioxide, which is caused by the intense photosynthesis in this area. This part of the estuary is treated as an autotrophic basin.

A response of the circulation in the Japan/East Sea (JES) to different kinds of wind forcing is studied, with the emphasis on the warm season, using a primitive equation oceanic model. Wind forcing is based on typical patterns obtained from complex empirical orthogonal functions of 1°-gridded NCEP/NCAR 6 h winds for 1998–2005. These patterns are distinguished by a prevailing wind direction. Northwestern wind and strong cyclonic (C) curl prevail in winter, while a variety of patterns occur in the warm season, differing in the wind direction and curl. Three model runs are performed to examine the circulation in response to a prevailing C wind stress curl or an alternating C and anticyclonic (AC) curl or a strong C curl in the warm season. The simulated features are consistent with the observational evidence, in particular with thermal fronts and frequent eddy locations derived from multi-year infrared satellite imagery. The simulated C circulation intensifies and the subarctic region extends southward with the strengthening of a summer C wind stress curl over the JES. Variability of Subarctic Front (SF) in the western JES (between 130°E and 133°E) is strongly affected by summer wind stress curl. Forcing by an AC curl tends to shift SF northward, while SF shifts to the south under the forcing by a C curl, reaching the southern Ulleung Basin in the case of the strong C curl. In the northwestern JES (off Peter the Great Bay, Russia, and North Korea), the SF northwestern branch (NWSF) is simulated. It is a known feature in autumn and early winter and can also occur in the warm season. The simulation results suggest an AC wind stress curl as the forcing of the formation of the NWSF in the warm season. The Siberia Seamount and sharply bending coastline near Peter the Great Bay facilitate partial separation of the Primorye (Liman) Current from the coast. The wind stress curl can be an additional forcing of the Tsushima Warm Current (TWC) branching off the Korea Strait to the East Korea Warm Current (EKWC) and the offshore branch (OB). In the warm season, the simulated TWC bifurcation occurs farther north, the EKWC is strong, and the OB is weak under the forcing of the AC wind stress curl. The EKWC is weak and the OB is strong under the forcing of the strong C wind stress curl.

Variability of sea surface temperature (SST) in the Japan/East Sea (JES) was studied using complex empirical orthogonal function (CEOF) analysis. Two daily data sets were analyzed: (1) New Generation 0.05˚-gridded SST from Tohoku University, Japan (July 2002-July 2006), and (2) 0.25˚-gridded SST from the Japan Meteorological Agency (October 1993-November 2006). Linkages with wind stress curl were revealed using 6-h 1˚-gridded surface zonal and meridional winds from ancillary data of the Sea-WiFS Project, a special National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) product (1998-2005). SST anomalies (SSTA) were obtained by removing the seasonal signal, estimated as the leading mode of the CEOF decomposition of the original SST. Leading CEOF modes of residual SSTA obtained from both data sets were consistent with each other and were characterized by annual, semiannual, and quasi-biennial time scales estimated with 95% statistical significance. The Semiannual Mode lagged 2 months behind the increased occurrence of the anticyclonic (AC) wind stress curl over the JES. Links to dynamic processes were investigated by numerical simulations using an oceanic model. The suggested dynamic forcings of SSTA are the inflow of subtropical water into the JES through the Korea Strait, divergence in the surface layer induced by Ekman suction, meridional shifts of the Subarctic Front in the western JES, AC eddy formation, and wind-driven strengthening/weakening of large-scale currents. Events of west-east SSTA movement were identified in July-September. The SSTA moved from the northeastern JES towards the continental coast along the path of the westward branch of the Tsushima Current at a speed consistent with the advective scale.

The variability of sea surface temperature (SST) in the Japan Sea is investigated using the complex EOF analysis of daily data produced at Tohoku University, Japan (New Generation SST; 2002–2006). The relationship with the wind field is investigated from the daily NCEP/NCAR reanalysis data with a 1° spatial resolution. Anomalies in the SST (SSTAs) are calculated by subtracting the basin-average annual variation estimated as a leading mode of temperature. The leading mode of an SSTA represents a adjustment to the annual mean variation, most significant in December in the zone of subtropical waters entering the sea through the Korean Strait and in the northwestern sea, over which a cyclonic wind curl develops in the cold period. The semiannual variability mode is identified, which is characterized by the largest temperature increase (decrease) in the western branch of the subarctic front (in the Tatar Strait), which lags by two months behind the semiannual changes in wind curl over the sea. An episodic SSTA movement is detected in the northern part of the sea, which moves from east to west along the western branch of the Tsushima Warm Current with a speed corresponding in magnitude to an advective scale.

According to the results of the international expedition aboard the R/Vs Roger Revelle and Professor Khromov in the summer 1999, areas with low oxygen contents (below 210 μM/kg) and those with increased contents of dissolved inorganic carbon and phosphates were found that roughly coincided with one another. These areas are located near the bottom on the southwestern slope of the Tsushima Basin in the region of the Korea Strait and on the continental slope in the region of the Tatar Strait in the northern part of the sea at about 46° N. The set of hydrochemical data points to a high geochemical activity in the near-bottom layer of the areas noted. This activity is confirmed by direct observations of the composition of the interstitial water in the sediments collected in the northern part of the sea during the expedition of R/V Akademik M.A. Lavrent’ev in 2003. It was supposed that the main cause of the increased geochemical activity is the runoff of suspended and dissolved matter from the Korea and Tatar straits. In the areas mentioned, the near-bottom waters are characterized by low values of the nitrogen-phosphorus ratio (below 10), which is geochemical proof of the denitrification process occurring under the conditions of high oxygen concentrations characteristic of the Sea of Japan. Based on the value of the annual production in the Sea of Japan, a rate of denitrification equal to 3.4 × 1012 gN/year was calculated. Hence, it is confirmed that the geochemical processes in the near-bottom layer have a direct influence on the spatiotemporal characteristics of the hydrochemical properties of the waters of the Sea of Japan.

The content of chlorophyll (Chl), the intensity of delayed fluorescence (DF), the primary production (PP), and the concentrations of macronutrients were measured in the northwestern part of the Sea of Japan during cruise 30 of R/V Professor Gagarinskii in the autumn of 2000. Sixty-three stations were executed in two steps: October 12–17 (stations 2–34) and October 29–November 3 (stations 35–63). At all the stations, CTD probes were performed. Three cross sections were executed twice within a 2-to 3-week interval, during which structural changes in the hydrological fields took place due to the development of an upwelling. During this time, the length of a day and the daily photosynthetically active radiation (PAR) decreased notably. The vertical profiles of Chl and DF showed pronounced maxima. The waters of the shallow-water stations were characterized by an increased Chl content as compared to the deep-water stations. The content of Chl in the photosynthetic layer amounted from 11 to 30 mg/m2; the PP values were from 120 to 520 mg C/m2/day. At the shallow-water stations, during the second step, the Chl concentration increased by roughly 60% (on average, from 0.47 ± 0.22 to 0.77 ± 0.42 mg/m3); the PP value grew by about 40% (from 268 ± 67 to 370 ± 72 mg C/m2/day). At the deep-water stations, this increase was small (the Chl concentration and PP value increased from 0.37 ± 0.20 to 0.41 ± 0.18 mg/m3 and from 356 ± 72 to 377 ± 74 mg C/m2/day, respectively). The changes noted proceeded due to the increase in the nutrient concentrations during the time between the two surveys, despite the decrease in the length of the day and of the daily PAR. The observations performed showed that, during the autumn time, the horizontal water advection across the shelf, caused by the development of the upwelling, plays the key role in the variations of the production characteristics of the Primor’e coastal zone.

Data from the R/V Mirai cruise (May‐June 2000) have been examined to discover how mesoscale processes associated with eddy dynamics directly affect the water masses, the distributions and the vertical fluxes of the dissolved oxygen, nutrients and dissolved inorganic carbon in the western subarctic Pacific. Using maps of the temperature, salinity, dissolved oxygen, nutrients, chlorophyll and sea-air pCO 2 difference we show that the boundaries of the anticyclone eddies in the study region were composed of high productivity coastal Oyashio water. The coastal waters were wrapped around the anticyclone eddies (thus creating a high productivity belt) and intruded inside of them. Using SeaWifs data we demonstrate that temporal variations in the position and the strength of anticyclone eddies advected the Kuril island coastal high productivity waters to the pelagic part, resulting in temporal variations of the chlorophyll in the Oyashio region. Computed vertical fluxes of the dissolved oxygen (DO), inorganic carbon (DIC) and silicate show that the anticyclonic eddies in the Kuroshio-Oyashio Zone are characterized by enhanced vertical fluxes of the DO and DIC between the upper (σ θ = 26.7‐27.0) and lower (σ θ = 27.1‐27.5) intermediate layer, probably due to the intrusions of the Oyashio waters into the eddies.

We present the results of in-situ measurements of $^{134}$Cs and $^{137}$Cs released from the Fukushima Nuclear Power Plant (FNPP) collected at surface and different depths in the western North Pacific in June and July 2012. It was found that 15 month after the incident concentrations of radiocesium in the Japan and Okhotsk seas were at background or slightly increased level, while they had increased values in the subarctic front area east of Japan. The highest concentrations of $^{134}$Cs and $^{137}$Cs up to 13.5 ${\pm}$ 0.9 and 22.7 ${\pm}$ 1.5 Bq m$^{-3}$ have been found to exceed ten times the background levels before the accident. Maximal content of radiocesium was observed within subsurface and intermediate water layers inside the cores of anticyclonic eddies (100 - 500 m). Even slightly increased content of radiocesium was found at some eddies at depth of 1000 m. It is expected that convergence and subduction of surface water inside eddies are main mechanisms of downward transport of radionuclides. In situ observations are compared with the results of simulated advection of these radioisotopes by the AVISO altimetric velocity field. Different Lagrangian diagnostics are used to reconstruct the history and origin of synthetic tracers imitating measured seawater samples collected in each of those eddies. The results of observations are consistent with the simulated results. It is shown that the tracers, simulating water samples with increased radioactivity to be measured in the cruise, really visited the areas with presumably high level of contamination. Fast water advection between anticyclonic eddies and convergence of surface water inside eddies make them responsible for spreading, accumulation and downward transport of cesium rich water to the intermediate depth in the frontal zone.

Received 15 October 2002; accepted 13 January 2003; published 19 February 2003. [1] Direct water mass renewal through convection deeper than 1000 m and the independent process of dense water production through brine rejection during sea ice formation occur at only a limited number of sites globally. Our late winter observations in 2000 and 2001 show that the Japan (East) Sea is a part of both exclusive groups. Japan Sea deep convection apparently occurs every winter, but massive renewal of bottom waters through brine rejection had not occurred for many decades prior to the extremely cold winter of 2001. The sites for both renewal mechanisms are south of Vladivostok, in the path of cold continental air outbreaks. INDEX TERMS: 4283 Oceanography: General: Water masses; 4243 Marginal and semienclosed seas; 4223; 4215 Climate and interannual variability (3309). Citation: Talley, L. D., V. Lobanov, V. Ponomarev, A. Salyuk, P. Tishchenko, I. Zhabin, and S. Riser, Deep convection and brine rejection in the Japan Sea, Geophys. Res. Lett., 30(4), 1159, doi:10.1029/ 2002GL016451, 2003.

Based on detailed hydrological and hydrochemical surveys carried out in each of the four seasons of 2008, Amursky Bay in the north west quadrant of the Japan Sea was found to experience seasonal hypoxia. The primary process of hypoxia formation is a microbiological degradation of the ¡§excess¡¨ amount of diatoms under rather low photosynthetic active radiation in bottom layer and weak water dynamics. The microbiological decay of dead diatoms under light deficient conditions intensively consumes dissolved oxygen and produces phosphates, ammonium, silicates, and dissolved inorganic carbon. Existence of a phytoplankton ¡§excess¡¨ is caused by phytoplankton bloom resulting from nutrient pulses into Amursky Bay. There are two main sources of these nutrients: the waste waters of Vladivostok city and discharge from Razdolnaya River. The river delivers more than two times the amount of nutrients than the waste waters of Vladivostok. It is suggested that the phytoplankton ¡§excess¡¨ might be caused by an enhanced supply of nutrients delivered into the surface layer resulting from the increased discharge of the river on a short time scale. Our data suggest that hypoxia is seasonal, with a peak at the end of summer. The upwelling of the Japan Sea water in the beginning of the fall season and its advection across the shelf is the primary process by which the hypoxia is destroyed. During the winter, strong vertical mixing due to termohaline convection makes the water column uniform and brings more oxygen into the water along with high primary production under the ice. Thus, during the winter season, the ecosystem of Amursky Bay recovers completely.

[1] We observed a sudden initiation of bottom-water formation in the East/Japan Sea associated with a severely cold winter in 2000–2001. An increase in dissolved oxygen concentration as well as decreases in temperature and nutrient concentrations for the bottom waters provides unequivocal evidence that cold, oxygen-rich and nutrient-poor surface waters were injected directly to the bottom. Since the conveyor-belt in the East Sea has been undergoing dramatic change with a complete halt to bottom-water formation since the mid-1980s, this sudden episode of bottom-water formation could easily be detected. Though the amount of bottom water formed was rather small, being only about 0.03% of the volume in the past time, the observation clearly demonstrates that the conveyor-belt is directly connected to the weather system.