Satellite remote sensing techniques offer a wealth of optical, infrared (IR), and radar images of the ocean surface, where we can observe numerous elongated vortex structures known as filaments. These filaments become readily visible in the imagery due to the presence of surfactant films and/or floating algae clusters on the sea’s surface. Given their elongated form, automated vortex identification methods do not readily distinguish filaments from vortices. Nevertheless, both filaments and vortices exhibit notable characteristics such as high relative vorticity and kinetic energy. The process by which vortices transform into filaments is a result of their interaction with spatially non-uniform background currents. In this study, we apply the theoretical principles regarding the stretching of mesoscale ocean vortices to real ocean conditions, inferred from altimeter data. The primary objective of this research is to assess the proportion of mesoscale ocean vortices that undergo stretching to become filaments, consequently facilitating the redistribution of energy from the mesoscale to the submesoscale. We provide a total assessment of the portion of the World Ocean’s surface where mesoscale vortices undergo significant stretching. We present maps that indicate the geographical distribution of regions where vortex stretching is not restricted and offer an interpretation of the findings. The reduction in the inherent energy of vortices due to the stretching induced by the background flow is explained as a potential mechanism for energy transfer from the vortex to the flow, possibly leading to the manifestation of the negative viscosity effect within this system.

Numerous generalizations based on field studies show that the spatiotemporal coverage of Lake Ladoga with observational data is insufficient for a reliable quantitative assessment of the interannual variability of the characteristics and biogeochemical fluxes in the lake ecosystem. This paper presents estimates of the interannual dynamics of the main ecosystem characteristics and biogeochemical fluxes in the lake for the period from 1980 to 2020, obtained using a verified three-dimensional eco-hydrodynamic model of Lake Ladoga. The features of the lake ecosystem response to a 39 % decrease in the external phosphorus load during the study period, which is accompanied by a decrease in phytoplankton biomass and primary production by only 30 %, are revealed. The main mechanism explaining this response of the reservoir ecosystem is an increase in the recycling rate due to an increase in water temperature in the photic layer during the growing season, caused by climate change. The phosphorus turnover time in the lake was estimated to be approximately 5.4 years for the considered period of 1980–2020.

This study presents the results of the analysis of a long-term dataset of observations on the spatial-temporal variability of submesoscale eddy characteristics in Lake Ladoga using radar imagery. The initial data consisted of more than 3500 high-resolution Sentinel-1A/B satellite images for the period from January 2016 to December 2022. Generalized maps of the occurrence of submesoscale structures on the lake’s waters for the year and by seasons are presented. Average annual and seasonal estimates of the variability of the mean diameters of eddies with different type of rotation were obtained. It was shown that submesoscale structures are a common phenomenon throughout the lake during the period of direct thermal stratification and the presence of a near-surface pycnocline. Cyclonic structures with sizes up to 3 km were most frequently registered, which does not exceed the estimates of the average Rossby deformation radius for Lake Ladoga. Eddies were most often observed north of Valaam Island. No significant interannual variability in their characteristics was found; they were close to the average multi-year values. A pronounced intrayear variability in the characteristics of submesoscale structures was revealed, both in frequency and locations, and to a lesser extent in their sizes. It was established that eddies were most frequently observed in areas with depths of 50–100 meters, near frontal zones of different genesis. In specific cases, the instability in frontal zones in Lake Ladoga appears to be a dominant factor in the formation of small eddy groups, especially in the absence of significant topographic effects or wind influence.

The article focuses on the study of vortex ring generation and evolution in aquatic environments resulting from the discharge of a water jet into a flooded volume. It presents computational data based on well-known relationships from the literature, as well as results from simulations using a newly developed methodology. The characteristics of the vortex ring generator within the experimental setup, created using a large-scale hydrophysical water tank, are justified. Experimental studies were conducted under conditions of thermal stratification of the medium, with a temperature difference between the water jet and the tank. The experimental results on vortex ring formation and motion show good agreement with the computational data. The influence of the thermal stratification in the water tank on the vortex ring characteristics was found to be negligible. A significant effect of the dimensionless jet length on the key characteristics of the vortex rings was observed, along with the temperature differences between the water layer at the formation horizon and the jet, impacting the rings’ trajectory.

To solve inverse problems, in particular, the recovery of optically active substances from hydro-optical measurements, it is necessary to clearly understand the nature of these dependencies. The work analyzes such relationships between the fluorescence of dissolved organic matter, chlorophyll-a fluorescence and the beam attenuation coefficient at a wavelength of 660 nm based on direct measurements performed in Lake Teletskoye in August 2023. It is shown that the waters of Lake Teletskoye belong to the so-called waters of the second type (CASE2) according to the Morel classification, i. e. optically complex waters. 

Tidal dynamics along the shelf break and continental slope of the Kamchatka Peninsula, adjacent to the Pacific Ocean, are a significant but underexplored factor influencing the hydrological variability. This variability affects the distribution of early life stages of the Eastern Kamchatka population of Walleye pollock, a key species for Russian fisheries. Its spawning occurs mainly in the deep-sea canyons of Avacha and Kronotsky Bays. This study aims to describe the methodology developed to investigate the impact of tidally driven hydrophysical processes on pollock egg distribution, with a focus on its application in the deep-sea canyons of Avacha Bay. Two experiments were conducted in the "Central" and "Northern" canyons during the peak of pollock spawning in April 2024, coinciding with the spring tide when tidal effect on the environment is maximized. The experimental methodology was based on frequent hydrological profiling and layer-by-layer sampling of ichthyoplankton, carried out over a day. The study identified a 50-meter amplitudes of vertical oscillation of the thermocline, located at 320–420 meter between warm and cold intermediate layers, with a distinct diurnal rhythm in the "Central" canyon and semidiurnal one in the "Northern" canyon. These results highlight the critical role of tidal dynamics in shaping hydrophysical variability, which in turn potentially affects pollock eggs vertical redistribution and development in the deep-sea canyons of Avacha Bay. 

Laboratory modeling of hydrophysical processes is one of the methods for solving scientific and practical ocean research tasks. A hydrophysical water tank has been created at the St. Petersburg Branch of the P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences. The work in the tank is supported by a digital model that optimizes experimental programs and methodologies. The design and technological characteristics of the water tank allow for modeling multilayer stratification. This paper describes the tank, including its geometric dimensions (7 m × 2 m × 2.2 m — length, width, depth), instrumentation and measurement system, metrological support, and the technology for creating thermal stratification. Typical profiles of two- and three-layer stratification are provided. Based on similarity theory, the permissible scales for reproducing natural hydrophysical processes are evaluated. It is shown that the created hydrophysical water tank occupies an intermediate position between salt-stratified tanks and the large thermally stratified tank at the Institute of Applied Physics, RAS. Together with the tank’s digital model, it makes it possible to reproduce hydrological conditions covering the main types of stratification in lakes, seas, and oceans, while optimizing the temporal and functional parameters of the experiments. 

This paper focuses on the development of a digital model for a large thermally stratified water tank intended for hydro-physical studies. The model construction incorporates modern advances in computational hydrodynamics and high-performance computing to optimize and partially replace costly physical experiments. The precise establishment and maintenance of thermal stratification within the tank are achieved through fine-tuning the operation of heating/cooling systems, based on the use of the developed digital model. The digital copy is primarily viewed as a supplementary tool aimed at optimizing serial experiments. Simultaneously, the refinement of the numerical model through physical experimental data enables the extrapolation of laboratory-verified relationships to describe regimes characteristic of natural oceanic processes that are challenging to replicate in large-scale physical modeling. The digital copy serves as a constructive complement to the thermally stratified water tank, as it allows for a more efficient experimental methodology, achieving desired results while reducing both time and material costs.

St. Petersburg Branch of Shirshov Institute of Оceanology of Russian Academy of Sciences has constructed and commissioned an acoustically isolated water tank, equipped with an automated system for securing and moving measuring emitters, receivers (hydrophones), and the objects under study. The tank’s equipment includes emitters, hydrophones, multifunctional echosounders with side-scan sonar capabilities, amplifiers for both emitting and receiving channels, analog-to-digital converters, and portable computers with software for generating emitting and recording receiving signals. Functional models of both receiving and emitting channels have been created. A mathematical model of the water tank has been developed based on calculations of hydroacoustic fields using the method of imaginary sources. The quality of acoustic isolation in the water tank has been assessed, yielding satisfactory results. Standard models of objects have been manufactured for experiments investigating their reflective properties. The acoustically isolated water tank allows for research into the characteristics of hydroacoustic systems, methods for signal formation and processing in both active and passive modes, as well as processes occurring in aquatic environments.