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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.

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.

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.

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 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.