This paper provides information on the integrated geophysical, sedimentological, and hydrophysical investigations, passing meteorological and biological observations in the Eastern Tropical Atlantic during the cruise 63 of the R/V Akademik Ioffe in October–December 2022. The preliminary scientific results are discussed.

In this study, we discuss the water structure in the Kane Gap, which is a poorly studied part of the Tropical Atlantic’s eastern basin. Our study is based on results of measurements carried out during the 63rd cruise of R/V Akademik Ioffe in 2022. Moreover, the variability of monthly mean parameters of these waters according to the GLORYS12v1 reanalysis is shown. Intermediate waters in this area are represented by a mixture of Antarctic Intermediate Water (AAIW) and Mediterranean water (the proportion of the latter reaches 26

The goal of the work : to determine the water structure in the Central part of East Atlantic and to identify the features of the distribution of hydrological, chemical and biological characteristics of the upper layer in the autumn period Materials and methods: the data presented in the work were obtained on the 63rd cruise of the R/V «Akademik Ioffe» (September 29, 2022–December 8, 2022). During the expedition, hydrological and chemical (temperature, salinity, pH, dissolved oxygen, phosphates, nitrites, nitrates and silicates) characteristics were measured from the surface to the bottom, as well as biological (number, biomass and species diversity of phytoplankton), taken with chemical samples on depths of 0, 10, 20 and 50 m. Chemical analysis was carried out in the ship’s laboratory, according to certified methods for sea and fresh waters. Phytoplankton samples were thickenedusing a reverse filtration funnel through track (nuclear) membranes and fixed with Lugol’s solution to a final concentration of 5%. To study the species diversity, abundance and biomass of plankton microalgae, samples were processed by direct microscopy of algae in a living and thickened drop (V = 0.01 ml), as well as in a counting chamber (V = 0.5 ml) using an Olympus CX43 light microscope (magnification 10×20, 10×40). Identification of taxonomic groups and species composition of microalgae, calculations of the abundance and biomass of phytoplankton, as well as the Sörensen-Czekanowski floristic similarity coefficient according to the adoption of the methods of the « IBSS RAS» were carried out. The practical significance of the research : assessment of the production stock in a poorly studied region of the Central Eastern Caucasus, identification of mechanisms for influencing production processes, assessment of the variability of the water structure against the background of climate change.

The structure and dynamics of different water masses in the Tropical Atlantic is investigated using the results of instrumental measurements. The main attention is focused on the Vema fracture zone and the northern part of the WOCE A15 section. In the Vema Fracture zone an intradiurnal variability of hydrological parameters was observed. We also assess the long-term variability of hydrological parameters along the WOCE A15 section.

Herein we provide information on the integrated geological, geophysical, sedimentological, and hydrophysical investigations and passing meteorological and biological observations, as well as on the surface hydrobiological sampling in the Tropical Atlantic during cruise 60 (leg 2) of the R/V Akademik Ioffe in December 2021 to February 2022. The preliminary scientific results are discussed.

In this study, we estimated the transport and potential temperature of the Antarctic Bottom Water (AABW) and its variability on intra-annual, interannual, and decadal scales based on eight reanalyses and direct measurements at 26° N in the subtropical North Atlantic. The GLORYS2v4 and GLORYS12v1 reanalyses were selected, which give the closest results of the estimates for average AABW transport, calculated below 5000 m, and their variability with RAPID. It was revealed that the seasonal cycle of AABW transport for 2004–2015 is described by superposition of the annual and semiannual harmonics of ~80% with an amplitude of ~0.4 Sv, with maxima in April and October, and minima in February and July. The intra-annual temperature cycle is described by the annual harmonic, the contribution of which is more than 70% of the total variability, with a maximum in spring and minimum in autumn, and an amplitude of ~0.003°C. The study highlights the characteristic periods of variability in the transport and potential temperature of AABW of 12 and 6 years, with maxima in 2009 and 2014 and minima in 2006 and 2011 based on the ORAS5 and GLORYS2v4 reanalyses and RAPID estimates. The amplitude of interannual fluctuations in the transport and potential temperature of AABW according to the ORAS5, GLORYS2v4, and GLORYS12v1 reanalyses is ~0.6 Sv and ~0.01°C, respectively. Analysis of the long-term trends in average annual AABW transport showed an increase of 0.2 Sv based on the reanalyses and RAPID data with a slight decrease in temperature (–0.04°C) for 2004–2015.

Приведены сведения о комплексе геолого-геофизических, литолого-палеоокеанологических, гидрофизических и биологических исследований в Центральной Атлантике в 45-м рейсе НИС “Академик Николай Страхов” в октябре–ноябре 2019 г. Обсуждаются предварительные результаты экспедиции.

This paper addresses changes of Antarctic Bottom Water (AABW) transport through the Vema Fracture Zone based on direct measurements and Glorys12v1 reanalysis. Instrumental measurements of currents and other principal hydrological characteristics were carried out during the 45th cruise of the R/V Akademik Nikolaj Strakhov in November 2019. According to the current velocity measurements in 2019, the transport in the Vema Fracture Zone is 0.9 Sv. Moreover, based on the distribution of dissolved oxygen concentration, it is more correct to consider the isotherm 1.7°С rather than 2°С as the upper boundary of the AABW. The reanalysis showed very good agreement with direct measurements. The average transport of bottom water during the reanalysis period is 0.66 Sv. In general, there is a tendency towards an increase in both transport and temperature over the 25-year period covered by reanalysis. The measurements of the bottom water temperature during twenty successive samplings showed changes from 1.36 to 1.41°С within a day. This interval almost completely covers the entire range of values observed in previous years.

Variability of oxygen and hydrogen sulphide concentrations in the Gdansk basins deep water of the Baltic Sea for the last 25 years was analyzed. It was shown that suboxic and anoxic conditions in deep water were mainly determined by Baltic inflow frequency and intensity. Oxygen concentration in the bottom layer of the Gdansk basins increased strongly in 13 months after Major Baltic Inflow events (MBI). There were three periods of strong increasing of dissolve oxygen concentration in deep water since 1992 till 2018 after MBI in 1993, in 2003 and 20142016. It was shown that at the bottom of the Gdansk basins hydrogen sulphide was detected during 79 months till 22.5 years after MBI. Quality evaluation of dissolved oxygen distribution reanalysis was also conducted in this work. It can be noted good reproduction of field data by the reanalysis in its quality part; however, in most cases the observable maximum concentrations in dissolved oxygen distribution are understated, sometimes even several times.

We executed investigations of Mediterranean lenses detection in the Atlantic using data recorded by buoys-profilers of the “Argo” project (more than 11200 profiles). We investigate the region from 28° to 47° N and from 7° to 27° W for the period from 2001 to 2010. From these data, we revealed “lenses profiles” with the salinity values exceeding 36 psu (2043 cases). We demonstrate the allocation of Argo buoys in the area investigated and indicate their working periods; we also show the periodicity of the “lenses profiles.” It was found out that the Argo buoys traced individual lenses and depicted correctly their movements, the vertical scale of the eddy, its location in the depth, and the hydrological peculiarities of the lenses. We estimated the number of lenses occurring in different periods and analyzed the interaction of the Argo buoy with the intrathermocline lenses. The importance of these investigations for studying the Mediterranean water dynamics was pointed out.

During cruise 54 of the R/V Akademik Mstislav Keldysh to the southwestern Kara Sea (September 6 to October 7, 2007), a large amount of hydrophysical data with unique spatial resolution was obtained on the basis of measurements using different instruments. The analysis of the data gave us the possibility to study the dynamics and hydrological structure of the southwestern Kara Sea basin. The main elements of the general circulation are the following: the Yamal Current, the Eastern Novaya Zemlya Current, and the St. Anna Trough Current. All these currents are topographically controlled; they flow over the bottom slopes along the isobaths. The Yamal Current begins at the Kara Gates Strait and turns to the east as part of the cyclonic circulation. Then, it turns to the north and propagates along the Yamal coast over the 100-m isobath. The Eastern Novaya Zemlya Current (its core is located over the eastern slope of the Novaya Zemlya Trough) flows to the northeast. Near the northern edge of Novaya Zemlya, it encounters the St. Anna Trough Current, separates from the coast, and flows practically to the east merging with the continuation of the Yamal Current. A strong frontal zone is formed in the region where the two currents merge above the threshold that separates the St. Anna Trough from the Novaya Zemlya Trough and divides the warm and saline Arctic waters from the cooler and fresher waters of the southwestern part of the Kara Sea. This threshold, whose depth does not exceed 100–150 m, is a barrier that prevents the spreading of the Barents Sea and Arctic waters to the southwestern part of the Kara Sea basin through the St. Anna Trough.

During the 27th cruise of the research vessel Akademik Ioffe in 2009, the measurements in the area of the main saddle of the Chain fracture zone were carried out. They included the CTD-sounding and determination of current speeds by lowered acoustic Doppler current profiler (LADCP). As a result, the structure of waters was determined and the transport of bottom waters was estimated as 0.11–0.17 Sv that is significantly less than the previous estimates. The variability of bottom potential temperature values as compared with the measurements carried out in 1991–1994 is registered. At the station situated on the main saddle, the potential temperature increased by 0.1°C and at the station located behind the saddle, it decreased by 0.034°C.