PHYSICAL OCEANOGRAPHY

Purpose. Wind speed accuracy in diverse storm systems is crucial for weather prediction, climate studies and marine applications. This study aims to evaluate the performance of the European Centre for Medium-Range Weather Forecasts (ECMWF) fifth-generation atmospheric reanalysis (ERA5) for wind speeds in extratropical cyclones (ETCs), polar lows (PLs) and tropical cyclones (TCs), as well as to propose a correction function for potential biases.

Methods and Results. We compared the ERA5 wind speeds with the data from the Advanced Microwave Scanning Radiometer-2 (AMSR-2) satellite for various storm events. Statistical metrics, including bias, root mean squared error (RMSE) and correlation coefficient (R), were calculated to quantify discrepancies between the two datasets. Based on the observed biases, a simple exponential correction function was proposed to adjust the ERA5 wind speeds. The effectiveness of the correction function was evaluated through visual comparisons and quantitative analyses. The analysis revealed that the ERA5 systematically underestimated wind speeds across large areas within ETCs, PLs and TCs compared to the AMSR-2 observations. The proposed correction function successfully improved the agreement between ERA5 and AMSR-2 wind speeds in ETCs and PLs. However, applying the same function to TCs revealed significant structural discrepancies between the ERA5 and the AMSR-2 wind fields within these systems.

Conclusions. This study demonstrates effectiveness of the proposed correction function in enhancing wind speed accuracy for ETCs and PLs, bringing them closer to AMSR-2 observations. However, further research is necessary to develop approaches for addressing wind speed biases in TCs, considering the unique characteristics and limitations of existing reanalysis data. This research contributes to improving our understanding and representation of wind speeds in diverse storm systems, ultimately aiding in more accurate weather forecasting and climate monitoring.

Purpose. The work is aimed at developing and implementing a geographic information system (GIS) that provides an opportunity for online work with the Argo floats data in the Black Sea and for its application to assess the float drift velocities in different sea layers.

Methods and Results. The geoinformation system is developed based on a client-server architecture using PostgreSQL DBMS to store the Argo float data, the jQuery, Plotly and mapbox gl libraries and, therefore, to implement a user interface and a cartographic service. The floats drift velocities are calculated and analyzed using the information provided by the Argo project in the public domain. The information is received from the autonomous drifting profiling floats and includes data on their satellite positioning, drift depths and profiling. The velocities at the float drift horizon are calculated using the data on its trajectory, meanwhile GIS assumes the possibility to recalculate velocities swiftly when new observation data are received, adjust calculation methodology, expand the range of statistical characteristics as well as to add a number of additional options. The Argo data array (early 2005 – mid 2022) was included in the system current version to calculate and analyze velocities. Application of GIS made it possible to estimate floats drift velocities in the Black Sea, specify mean velocity values as compared to the previous studies and show its seasonal variability in different layers of the sea.

Conclusions. The online services of the Argo project are complemented by the developed GIS that simplifies processing and scientific analysis of the Black Sea oceanographic data significantly with no need to use additional scripts, data downloads and external visualization systems. The examples of applying the system for the assessment of floats drift velocities at different depths and in certain parts of the sea are shown. In the future, GIS can be supplemented with new modules, such as automatic downloading of Argo data, operating with similar data arrays obtained, for example, from drifters or ADCP current profilers. Besides, it can be applied to any other regions.

Purpose. The research is purposed at substantiation of the design of floating marine stations with a wave energy generator. The proposed construction is of higher efficiency as compared to the known designs due to the application of roll oscillations and resonance operating mode.

Methods and Results. The resonance method of converting wave energy into electrical energy, as well as the design of an autonomous station based on the resonance conversion of the station hull roll oscillations into the electrical energy are described. The method implies adjusting the frequency of natural angular oscillations of the floating station hull to the significant wave frequency. It has been theoretically proved that the conversion of roll oscillations energy is more effective than the conversion of vertical oscillations. This is due to the fact that the amount of damping during vertical vibrations of a cylindrical body in water exceeds the amount of damping during angular oscillations of such a body. Besides, the proposed method is shown to be effective for applications in the development of measurement systems and storage devices. The design of a floating station is proposed for implementing the resonance method for converting wave energy. It is shown that adjusting the frequency of natural onboard oscillations of the station hull to the significant wave frequency can be done using the additional filled tanks. The algorithm for adjusting the hull roll oscillations to the resonance with significant wave frequency is described. The kinematic scheme for a mechanical converter of roll energy into electrical one is proposed.

Conclusions. The results of theoretical studies were validated experimentally using the device test model in a wave experimental basin. They show that the hydrodynamic efficiency of the proposed wave converter increases as the wave heights decrease.

Purpose. The work is aimed at studying the statistics of width-to-length ratio of a breaking crest for an active phase of wind wave breakings and at assessing the possibility of approximating the geometric shape of a breaking with an ellipse.

Methods and Results. The experimental data including the video records of sea surface and the wind speed measurements were obtained at the stationary oceanographic platform in the Black Sea Hydrophysical Subsatellite Polygon of Marine Hydrophysical Institute (near the village of Katsiveli, the Southern Coast of Crimea) in 2015–2019. Processing the in situ data made it possible to form the arrays both of lengths and areas of the breakings and the synchronous measurements of wind speed.

Conclusions. The distributions of random variables proportional to the minor-to-major axes ratios of breakings are obtained. It is shown that these distributions are coincident for the whitecaps of different scales. This fact testifies to the similarity of breaking geometric shapes. The average value of breaking axis ratio obtained from all the data is 0.41. It is concluded that the average geometric shape of the breaking boundaries is satisfactorily described by an ellipse with eccentricity 0.91. The ratio of the speed of a breaking centroid to that of its anterior front is estimated to be 0.9.

Purpose. Development of regional satellite algorithms requires the information on bio-optical properties of a particular water area. The present study is aimed at comparative analysis of bio-optical properties of optically complex waters differing in their trophic status.

Methods and Results. The study combined the results of measuring the spectral bio-optical properties in the waters of the Black, Azov, Barents and Norwegian seas, the Arctic and Southern oceans (Atlantic sector) and Baikal and Teletskoye lakes. Spectral coefficients of light absorption by phytoplankton, non-algal particles and colored dissolved organic matter were measured in accordance with the International Ocean Colour Coordinating Group Protocols. The study areas included the waters with trophic levels from the oligotrophic to the eutrophic ones (the chlorophyll a concentrations in the surface layers varied from 0.066 to 24 mg⋅m−3) and with high heterogeneity in their bio-optical properties: the total non-water light absorption at the wavelength of 438 nm varied from 0.021 to 0.97 m−1.

Conclusions. In all the regions, a high (within an order of magnitude or higher) spatial variability in the values of light absorption coefficients by all the optically active components and their ratios was noted. This fact indicates the optical complexity of waters in each of the regions under study. The regional specificity of parameterization coefficients for light absorption by phytoplankton, non-algal particles and colored dissolved organic matter was shown. The revealed parameterization coefficients for light absorption by the optically active environment components can be used to develop regional satellite algorithms for assessing water quality and productivity indicators. Based on the empirically revealed dependencies, the following additional indicators of water quality were proposed: the euphotic zone depth and the spectral characteristics of downwelling irradiance which can be retrieved based on remote sensing data.

Purpose. The aim of the study is to analyze the features of mean long-term structure and intra-annual variability of the characteristics of temperature fronts on the ocean surface in the Patagonian shelf region.

Methods and Results. Mean daily values of the ocean surface temperature from the NOAA OI SST data and the geostrophic velocity components on the surface at the nodes of a 0.25° regular grid from the CMEMS reanalysis for 1993–2020 were used. It is shown that at the western periphery of a large-scale cyclonic meander formed by the currents in the Patagonian shelf region (south of 45°S), three branches of the Subantarctic Front are traced; they correspond to theWest Falkland Current and to two jets of the East Falkland Current. North of 45°S, where one Falkland Current jet is observed, one branch of the Subantarctic Front is identified. On the eastern periphery of the meander, the front corresponding to the common stream of the Brazil Current and the Falkland Return Current is revealed. Besides, south of 40°S, a separate branch of the Subantarctic Front corresponding to one more recirculation of the Falkland Current is observed. It is shown that at the meander western periphery, the branches of the Subantarctic Front are most intensified in February – March, at its eastern periphery – in March – April, and at the meander northern peak (in the zone of the Brazil – Falkland Confluence) – in April – May and November.

Conclusions. It is found that on the western periphery of the cyclonic meander, south of 45°S, the main branch of the Subantarctic Front approximately follows the 900–1000 m isobaths, north of 45°S – the 150–170 m isobaths, and closer to the Brazil – Falkland Confluence – the 50–60 m isobaths. At the meander eastern periphery, north of 40°S, the main branch of the front is very close to the 800–1000 m isobaths, south of 40°S – to the 1000–2500 m isobaths. It has been established that the differences between the seasonal cycles of intensity of the Subantarctic Front branches are related to the dissimilar warming and cooling rates of surface waters separated by these branches.

Purpose. Sea breeze circulation is a common mesoscale phenomenon near the coasts of water bodies. However, at the moment, a number of the published review papers on this topic remain small. Therefore, the purpose of the work is to complement the existing reviews on sea breezes by generalizing the available knowledge on the influence of air heating intensity near the land surface, atmosphere stratification, synoptic background wind and the Coriolis force upon the sea breeze circulation.

Methods and Results. An overview of the results of studies involving the theoretical research methods, namely linear theory and two-dimensional numerical simulation, is presented. At first, the sea breeze circulation is considered within the framework of linear theory. Further, a technical description of two-dimensional models and the breeze features obtained applying these models are presented. The published works having been reviewed made it possible to consider the influence of four main factors (heat flux, atmosphere stratification, background synoptic wind and the Coriolis force) upon the breeze circulation.

Conclusions. Within the framework of linear theory, the breeze circulation represents an internal inertial-gravity wave with a diurnal period. Depending on the uniformity of vertical profile of the background synoptic wind, its influence on the linear sea breeze circulation leads to the asymmetry of circulation relative to the coast and to limitation of the breeze height. In a nonlinear regime, the important feature of breeze circulation obtained by applying numerical simulation consists in formation of a gravity current propagating over the surface. The nonlinear regime implies a fairly clear dependence of the velocity of gravity current front propagation on its height. The main manifestation of the background wind influence upon the gravity current is the change in its height that results in formation of a stationary or rapidly spreading current. Due to the Coriolis force influence, both within the framework of linear theory and in the nonlinear regime, an along-coastal velocity component is formed that leads to a decrease of the velocity component perpendicular to the coastline.