An expedition on cruise 62 of the R/V Akademik Oparin was organized by the V.I. Il'ichev Pacific Oceanological Institute, Far Eastern Branch, Russian Academy of Sciences, and lasted from December 14–29, 2020. Its tasks included oceanographic measurements and water sampling for hydrochemical and zooplankton analyses along climate monitoring transects of the international programs CREAMS (132°20′ E) and NEAR-GOOS (134°00′ E); a survey of mesoscale eddies near the southern coast of Primorye Region; the recovering of two autonomous bottom stations (moorings); and a study of the mercury, methane, and tritium contents in water (and also in the atmospheric layer for mercury) in the northern part of the Japan Sea (Fig. 1a).

Fig. 1.
figure 1

(a) Sketch map of operations of POI FEB RAS expedition on R/V Akademik Oparin (cruise 62) in northwestern part of the Japan Sea, December 14–29, 2020. Points show position of oceanographic stations; triangles, position of two moorings; isobaths in meters; (b) interannual variability of water temperature (°C) in bottom layer, 2900–3500 m, in Central Basin of the Japan Sea; linear trend of potential temperature of 0.0018°C/year for 1995–2020 is shown; (c) vertical salinity distribution (PSU) on transect across an anticyclonic eddy at Primorye shelf margin; vertical lines—CTD soundings; areas of extreme salinity values and secondary intrahalocline eddy core (LS) are shown.

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Due to the pandemic, the cruise was postponed to an unfavorable period in terms of weather conditions. The vessel often had to depart the operations area and take refuge in the coastal zone. Nevertheless, the majority of the expedition program was completed successfully. In a short time, a vast amount of operations were performed: 37 stations with CTD sounding were made, 23 of which with water sampling from surface to bottom, 12 stations with CTD sounding down to 1200 m on a transect across an eddy, and 6 samples for an assay of the radioactive hydrogen isotope tritium were taken. Thirty-five zooplankton samples were collected at 20 stations. Mercury in the atmospheric water layer was measured along the entire route of the vessel, and its flux between the sea and the atmosphere was measured out at two stations in the open sea and three at coastal stations. Water sampling to determine the methane content was also carried out along the entire route of the vessel with an interval of 3 h; a total of 102 samples were taken. Hydrochemical analyses included onboard determination of dissolved oxygen, nutrients, pH and alkalinity (334 samples each), as well as collection and preservation of samples to determine salinity, chlorophyll-a, and humic substances at the laboratories of the institute (162 samples each).

The main preliminary results of the expedition are the following:

(1) The monitoring transects show a continuing increase in the temperature of bottom waters of the deepwater Central Basin of the Japan Sea, associated with weakening of convection. A decrease in the growth rate from 0.0018°С/year for the entire observation period from 1995 to 2020, up to 0.0016°С/year over the last 10 years has been noted (Fig. 1b).

(2) It was confirmed that the development of upwelling in the area of cyclonic circulation in the northwestern part of the sea in autumn–winter results in more intensive cooling of the surface layer. The influence of the topographic effect is shown: the upwelling has been linked to the Pervenets and Vasilkovsky seamounts, above which the ascent of water has been established, extending to the surface layer and resulting from cyclonic vorticity.

(3) A detailed survey of the anticyclonic eddy off the southeastern coast of Primorye showed a double-core structure (Fig. 1c). A subsurface homogeneous layer formed by warm waters with low salinity (33.67–33.75 PSU), a secondary intrahalocline core of waters with low salinity (34.03–34.04 PSU) was found in the 120–190 m layer, indicating subduction of Primorsky Current waters during the evolution of the eddy. Anomalies in the anticyclonic zone can be traced down to the bottom layer (450 m).

(4) Based on the measurement results at two moorings equipped with acoustic Doppler current profilers and oceanographic parameter sensors and moored on the shelf margin at depths of ~100 m in the vicinity of Nakhodka and the village of Valentin (Fig.1 a), at the beginning of September 2020, for the first time a pattern of changes in water circulation and temperature in autumn–winter period was obtained: transition of the structure of the Primorye Current from baroclinic at the end of summer to barotropic in autumn; development of upwelling; its relaxation; and periodic anomalies caused by mesoscale eddies.

(5) Zooplankton is represented mainly by wintering groups; therefore, the data obtained make it possible to characterize the life cycles of species that are the main source of food for commercial planktivorous fish: large copepods of the genera Calanus, Eucalanus, Neocalanus, Metridia, and Pareuchaeta. The greatest species diversity and maximum abundance in plankton are observed near the Yamato Rise. The uneven distribution of individual groups can serve as current markers.

(6) Mercury measurements performed for the first time in winter showed that, at this time, transfer of air masses from polluted regions of China has a significant effect on its content in the surface atmospheric layer over the Japan Sea. In general, winter concentrations were higher than autumn (October 2019 and 2020), probably due to the burning of more fuel. The maximum mercury flux values from sea to atmosphere were in warmer and more saline waters of the central Japan Sea; the minimum fluxes, in the colder and less saline coastal waters of Primorye Region.

(7) Based on the methane measurements on this and previous expeditions, three areas with increased contents have been distinguished: above the Yamato Rise, off the coast of Primorye near the villages of Preobrazhenie and Olga, probably related to peculiarities in the tectonic structure of the seafloor.