A new mechanism for the renewal of the Sea of Japan intermediate water mass due to the transport of water volumes by mesoscale anticyclonic eddies in the summer season has been established. The structure and dynamics of three eddies over the continental slope east of the Peter the Great Bay are studied in the Sea of Japan based on the data of an autonomous mooring with the Aqualog profiler in June–July 2015. It is shown that the cores of the eddies had an ellipsoidal shape with an elongated lower part. Anticyclones, as isolated dynamic formations, transported water enriched with dissolved oxygen in the western-southwestern direction downstream the Primorye Current along the continental slope. This water based on its thermohaline characteristics corresponded to an intermediate water mass of low salinity in the source of its formation south of the Peter the Great Bay.

—We discuss the results obtained when performing a test acoustic-hydrological experiment in August 2022 at a marine test site from the coast of Sakhalin Island to Kita-Yamato Bank in the Sea of Japan. The methodology of preliminary studies in the water area intended for studying climatic variability of temperature regimes of the aquatic environment based on numerical modeling using the RAY computational program and the NEMO ocean hydrodynamic circulation model is presented. One of the main results is the average temperature of the marine environment calculated with high accuracy on the axis of the underwater sound channel in the Sea of Japan on the 1000-kilometer acoustic trace at the crossing of the vortex system. The shape of the measuring system and the technical and computational means and methods described in the article can be used as a basis for the organization of high-precision operational monitoring of thermodynamic processes in extended sea areas.

Intra-annual sea level fluctuations and variability of mesoscale processes based on eddy kinetic energy (EKE) were studied in the northern (northward of 41 N) Japan/East Sea (JES) using data from satellite altimetry for 1993–2020. Decomposition to empirical orthogonal functions (EOF) was performed of the high-pass filtered, with the cut-off period of 250 days, sea level anomalies. The leading mode accounting for the major fraction of the variance yielded sea level fluctuations which were simultaneous in the entire sea and occurred in the range from 70 to 250 days without any preferable timescale. EKE in the northern sea was also expanded to EOF and yielded the leading mode capturing mesoscale variability within the Primorye (Liman) Current and the Tsushima Warm Current. The seasonal signal was found in the simultaneous intra-annual sea level fluctuations, which matches that of EKE, and, as found in the earlier studies, of the mean currents. The sea level rises, the mean currents intensify and EKE increases in summer and fall and the opposite changes occur in winter and spring, with the seasonal extremes in October/November and March/April, respectively. This is in line with the EKE generation by instability of the mean currents. The intra-annual sea level fluctuations and EKE manifest rich variability on quasi-biennial, interannual and decadal timescales. However, in contrast with the seasonal signal, the low-frequency variability does not match, implying different kinds of forcing, probably by local wind in the northern JES and by the transport variations in the Korea – Tsushima Strait (KTS) in the southern JES. Intra-annual simultaneous SLA reveal changing relationship with Pacific Decadal Oscillation (PDO): both were in-phase in 1993–1994 and from late 2007 to 2013 and out-of-phase from 1997 to 2002, while there was no specific relationship in other times. However, the relationship of these SLA with the interannual KTS transport variation seems inconclusive.

In early October 2011, two surface buoys were deployed off Peter the Great Bay in the Sea of Japan. They drifted in that area first, then crossed the sea and were retrieved at the coast of the Honshu Island in late November 2011. Using the Argos-tracked drift coordinates, satellite altimetry data, and satellite infrared imagery, an eastern current with a speed of 10–19 cm/s was detected on October 4–11 off Peter the Great Bay where the southwestward flowing Primorye (Liman) Current was expected. From October 12 through November 5, the drift was mostly controlled by the wind, with the 5°–20° shift on the right-hand side from the wind. Based on the linear regression model, it was found that the wind accounted for more than half of the low-frequency, with inertial oscillations removed, variance of the drift speed, which on average, was equal to 23–24 cm/s per 10 m/s wind. Inertial oscillations estimated from wavelet rotary spectra of the drift velocity vectors were statistically significant most of the time, and their energy increased by two orders of magnitude in several hours after the wind strengthening to 17–18 m/s. Mesoscale eddies had a substantial impact on the drift, especially in the Tsushima Warm Current area.

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.

We investigated the transport of Arctic shelf water and its impacts on the downstream hydrographic structures in the Amerasian Basin by combining data collected in the summer of 2016 from two international cruises in the Pacific sector. The East Siberian Sea (ESS), in which cold shelf water with salinity of 31.0-32.8 is produced, was identified to be a major source of the near-freezing temperature water for the Makarov Basin. This relatively low-salinity shelf water was transported into the Chukchi Abyssal Plain and occupied the subsurface layer (30-60 m), which led to the formation of a subsurface oceanic front stretching along the Chukchi Plateau and northern side of the Mendeleev Ridge. The offshore transport of waters from the ESS caused an intrusion of the Atlantic Water onto the shelf and promoted the formation of the diapycnally mixed Lower Halocline Water (D-LHW) at the bottom of the outer shelf. Our findings indicate that the source of the D-LHW in the Arctic Pacific sector might extend from the Chukchi Plateau to as far as the Makarov Basin. We hypothesize that advection of waters from the ESS into the Makarov Basin promotes development of the subsurface halocline in the Makarov Basin. The volume transport of surface waters (salinity less than 30) on the ESS shelf was estimated to be about 0.6 Sv (10(6) m(3) s(-1)) eastward to the Canada Basin in 2016, consistent with the cyclonic Arctic circulation regime in 2016.

The possibility of applying ocean circulation modeling data to forecast the effective velocities of pulse acoustic signal propagation from the continental slope to the deep ocean is studied. Prediction of these velocities is crucial for reliable operation of acoustic navigation and ranging systems, while typical lengths of paths and the prediction accuracy requirements almost completely exclude the use of direct measurements for this purpose. Analysis of experimental data obtained on a 200-km-long acoustic path shows that the NEMO ocean circulation model used for reconstruction of the sound speed distribution along this path allows one to calculate with sufficiently high accuracy the effective velocities of propagation of pulse acoustic signals transmitted by a source on the shelf and received in the deep-water part of the Sea of Japan. The proposed method for effective velocities calculation is based on the adiabatic mode theory of sound propagation on the shelf, as well as on the fact that in the deep-water part of the path, the group velocities of low-number modes are very close to each other.

The Japan/East Sea is well ventilated and is the most oxygen-rich region in the Pacific. However, quantitative estimates of the turbulent fluxes are missing due to a lack of observational data. To assess turbulent mixing, we employ data from the moored profiler Aqualog survey of April-October 2015 near the northwestern boundary of this region where the oxygen maximum is observed. The survey allowed observation of collocated depth profiles of conductivity, temperature, ocean current, and dissolved oxygen 8 times per day. The data were processed by using the Mixing (MX) Oceanographic Toolbox based on the fine-scale parameterization of turbulent dissipation. The dissipation rate, the eddy diffusivity and the diapycnal fluxes of heat, salt and oxygen are estimated in the depth range from 130 to 350 m throughout the profiler deployment period. The survey-averaged diffusivity increased with depth from 0.5 x 10(-5) to 4.0 x 10(-5) m(2) s(-1). The month-to-month variability in the mixing in the intermediate water is presented. Early in May 2015, a transition in mixing occurred from the winter regime with upward turbulent fluxes of both heat and salt to the summer regime with the downward mixing of heat. The turbulent mixing was elevated in June when large anticyclonic eddies passed the profiler mooring. The application of the eddy diffusivities to the profiler mooring oxygen data yields an average downward oxygen flux of roughly 8.6 mol m(-2) month(-1).

Разработка систем акустической навигации и акустической дальнометрии в настоящее время является одной из наиболее актуальных практических задач акустики океана. В работе исследуется вопрос о влиянии крупномасштабных неоднородностей поля скорости звука в океане на точность решения задачи акустической дальнометрии. В качестве примера такой неоднородности нами выбран устойчивый антициклонический вихрь, наблюдающийся в южной части Японского моря в летний период. В работе проведены вычислительные эксперименты по исследованию влияния этого вихря на структуру звукового поля, формируемого на акустической трассе, проходящей через его центр, источником навигационных сигналов (ИНС), расположенным на шельфе. В ходе этих экспериментов по гидрологическим данным, полученным с помощью моделей глобальной циркуляции океана NEMO и ИВМ РАН, для этой трассы построена модель нерегулярного волновода «шельф–глубокий океан», после чего с помощью метода широкоугольных параболических уравнений выполнено моделирование акустического поля, формируемого ИНС в таком волноводе. Далее в работе также выполнен анализ модовой структуры этого поля, определены интервалы локализации различных модальных компонент сигнала и рассчитаны эффективные скорости распространения сигналов от ИНС на различных горизонтах приема, после чего исследовано влияние синоптического вихря на данные характеристики волновода. На основе этого анализа выполнены оценки влияния вихря на времена прихода сигналов от ИНС в точку приема, а также дополнительная погрешность решения задачи акустической дальнометрии, обусловленная этим влиянием. Результаты исследования показывают, что в рамках рассматриваемой методики решения задачи акустической дальнометрии даже относительно крупный неучтенный синоптический вихрь, ядро которого находится непосредственно на трассе, оказывает относительно слабое влияние на точность определения дальности (около 30 м для трассы протяженностью 300 км, или 0,01%). The development of acoustic navigation and acoustic ranging systems is currently one of the most important practical problems of ocean acoustics. In this study, the influence of large-scale inhomogeneities on the sound speed field in the ocean on the accuracy of acoustic ranging problem solution is considered. As a representative example of an inhomogeneity of this kind, we chose a stable anticyclonic eddy that is observed in the southern part of the sea of Japan in summer. In this work, computational experiments are conducted in order to study the influence of this eddy on the structure of the sound field formed along an acoustic path passing through the eddy's center by a source of navigation signals (SNS) located on the shelf. In the course of these experiments, a model of a range-dependent "shallow-to-deep-sea" waveguide was constructed along this path using hydrological data obtained from NEMO and INM RAS global ocean circulation models. After that, the acoustic field produced by the SNS in this waveguide was simulated by the method of wide-angle parabolic equations. The mode structure of the field along the path is studied, localization intervals of various modal components of the signal are determined, and the effective propagation velocities of signals transmitted by SNS are calculated at various reception horizons. The influence of the synoptic eddy on these waveguide characteristics is also investigated. On the basis of this analysis, the effect of the eddy on arrival times of the signals propagating from the SNS to the reception point is estimated, as well as the additional error in the solution of acoustic ranging problem caused by the presence of the eddy. The results of the study show, that within the framework of the considered technique of acoustic ranging problem solution, even the presence of a large unaccounted synoptic eddy, with its core located directly on the acoustic path, has a relatively weak effect on the accuracy of range estimation (about 30 m for a path 300 km long, or 0,01%).

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.

We analyze the results of measurements of the Tareev equatorial undercurrent in the Indian Ocean in February 2017. Sections from 3° S to 3°45′ N along 68° and 65° E crossed the current with measurements of the temperature, salinity, and current velocity at oceanographic stations. The maximum velocity of this eastward flow was recorded precisely at the equator. The velocity at a depth of 50 m was approximately 60 cm/s. The transport of the Tareev Current was estimated at 9.8 Sv (1 Sv = 10 6 m 3 /s).

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.

We present the results of in situ measurements of 134Cs and 137Cs 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 134Cs and 137Cs up to 13.5±0.9 and 22.7±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 makes them responsible for spreading, accumulation and downward transport of cesium rich water to the intermediate depth in the frontal zone.