Vol 40, No 2 (2024)

Purpose. The study is purposed at deriving the discrete equations of absolute vorticity and potential vorticity for a three-dimensional stratified incompressible fluid as an exact consequence of the finite-difference equations of sea dynamics in the field of a potential mass force in the adiabatic approximation provided that viscosity and diffusion are absent. The purpose also consists in analyzing both the features of two-dimensional projections of the absolute vorticty equation onto the coordinate planes and the three-dimensional equation of potential vorticity.

Methods and Results. In order to determine the discrete analogues of absolute and potential vorticity, introduced is the additional grid on which the finite-difference equations for the components both of absolute vorticty and potential vorticity are written down. Two-dimensional analogues of the three-dimensional equation of absolute vorticity on the planes (x, y), (y, z) and (x, z) are obtained; they possess the feature of preserving vorticity, energy and enstrophy (square of vorticiy). A discrete equation for potential vorticity of a stratified incompressible fluid is derived from the finite-difference system of three-dimensional equations of sea dynamics in the adiabatic approximation at the absence of viscosity and diffusion.

Conclusions. In the case of a linear equation of state, the discrete equations of absolute vorticity and potential vorticity which are the exact consequence of finite-difference formulation are obtained. The equation of potential vorticity is of a divergent form, and two-dimensional analogues of the absolute vorticity equation on the planes (x, y), (y, z) and (x, z) have two quadratic invariants that provide preservation of the average wave number.

Purpose. The study is purposed at presenting and analyzing a new method for determining light absorption in the sea which for the first time made it possible to redirect almost all the scattered rays from the studied light beam to the photodetector along the path of its propagation in a weakly absorbing medium, as well as at showing that application of the new method providing such an efficient collecting of the scattered rays, permits not only to avoid significant errors from the strong influence of scattering upon the results of determining the light absorption, but also to give up the necessity in correcting the data by theoretical modeling.

Methods and Results. It is known that sea water is a weakly absorbing light-scattering medium in which the light propagation is accompanied by its attenuation that is many times stronger due to scattering than due to absorption. Therefore, the determination of light absorption by seawater at a receiving device requires collecting of not only the light that has traveled a certain distance in the absorption medium, but also all the light scattered along this path. Previously, proposed was the method for measuring the light absorption in a cylindrical mirror cuvette with a light source at the input and a collector with a photodetector at the output (reflective-tube absorption meter). Somewhat later, a similar method based on the phenomenon of total internal reflection was applied. Since these methods do not provide the sufficiently complete collection of scattered rays, the data are to be corrected by theoretical modeling. The authors propose a new method for determining the spectral absorption of light in a quartz glass conical cuvette with an external mirror cone. It is shown that the cone cuvette permits to collect most of the scattered rays in the beam passing through the water medium by means of more efficient redirection of these rays from the place of light scattering to the receiver. The rest part of the scattered rays that has left the cuvette reaches the receiver in the air space between the cuvette and the cone mirror due to multiple reflections from it. As a result, the new method makes it possible to redirect almost all of the scattered light to the receiver and thus to minimize the errors in determining the light absorption in a weakly absorbing medium. To quantify the advantages of the new method, the authors have calculated the geometric parameters of the scattered light propagation for a conical quartz cuvette in air and for the same cuvette placed inside the external cone mirror.

Conclusions. The combination of a conical quartz cuvette and an external mirror cone in the new method made it possible to collect all the rays scattered in a weakly absorbing medium in the receiver. Thus, it permitted not only to exclude their strong influence upon the determination of light absorption in the sea, but also to give up the necessity in correcting the data by theoretical modeling.

Purpose. The study is purposed at assessing the features of spatial-temporal variability of the characteristics of annual fluctuations of the Baltic Sea level based on satellite and contact measurements, their comparing with theoretical dispersion relations of the low-frequency waves of different types, as well as at investigating possible mechanisms of the amplitude modulation of annual fluctuations of the Baltic Sea level.

Methods and Results. The hypothesis on a wave nature of annual fluctuations of the Baltic Sea level is tested and the reasons for their amplitude modulation are investigated based on the harmonic analysis of satellite altimetry data and the 132-year series of tide gauge sea level measurements in Stockholm. It is shown that the wave-like annual perturbations in the sea level field propagate from the southwest to the northeast at the velocity from 0.06 to 0.36 m/s. Having been compared, the estimated characteristics of annual waves and the theoretical dispersion ratios of the low-frequency waves of different types has shown that they are identified as the internal Kelvin waves over the most of the sea area, and only in rare cases in the southwest of the sea, their characteristics agree with the theoretical dispersion relations of the baroclinic topographic Rossby waves. The perceptible interdecadal changes in the annual wave parameters in the sea level field were noted. As compared to the 1993–2021period, the decade 1993–2002 is characterized by a decrease of the Sa harmonic amplitude by 1.5–3 times, by later onset of the maximum of the sea level annual variation (about 1 month later), and also by a noticeable slowdown of the annual wave phase velocity in the sea southwest.

Conclusions. The reasons for the amplitude modulation of annual waves in the sea level field are related to the impact of the oscillations with periods 352, 374, and 379 days, which are identified in a form of small but significant amplitude maxima in the Fourier series spectra of sea level, wind speed and atmospheric pressure. One more mechanism of the amplitude modulation of annual waves is assumed to be related to the changes in frequency of the natural baroclinic oscillations of the Baltic Sea due to the interannual variations of its stratification.

Purpose. The study is purposed at obtaining the quantitative estimates of coastal upwelling influence on the distribution of chlorophyll a in the coastal zone of the southeastern Baltic Sea during the summer seasons in 2000–2019.

Methods and Results. Based on the data both on frequency and duration of upwelling events for June – August 2000–2019 and the chlorophyll a concentrations derived from multi-sensor satellite observations in the coastal zone of the southeastern Baltic Sea, the long-term and monthly average values of the studied parameters are obtained, and the influence of upwelling events on the chlorophyll a concentration in the sea surface layer is assessed. The spatial variability of chlorophyll a in the coastal areas is found to be related to the influence of upwelling events. On the average, the chlorophyll a concentration decreases by more than 1 mg/m3 after an upwelling of any duration and in all summer months. The concentration drop is most significant after the upwelling events lasting more than 6 days.

Conclusions. The reduced chlorophyll a concentrations (as compared to the pre-upwelling values) are observed in course of a week after a coastal upwelling event in the southeastern Baltic Sea.

Purpose. The purpose of the work is to analyze the features of vertical distribution of the dissolved oxygen and hydrogen sulfide in the deep part of the Black Sea in the modern period.

Methods and Results. The data obtained in 11 expeditions of Marine Hydrophysical Institute (MHI) RAS in the Black Sea within the economic zone of Russia in 2017–2019 were used. These surveys included more than 200 deep-sea stations at which by means of a cassette of 12 bathometers of the Sea-Bird 911 plus CTD Seabird-Electronics INC device the hydrochemical samples were taken at specific isopycnal surfaces; as a rule, it was σ_t = 16.30; 16.20; 16.10; 16.00; 15.95; 15.90; 15.80; 15.60; 15.40; 15.20; 15.00 and 14.60 kg/m³. Such a scheme made it possible to determine both the general vertical distribution of oxygen in the oxycline and the depth of hydrogen sulfide occurrence with the accuracy 0.05 kg/m³ in the conditional density scale.

Conclusions. In all the surveys, a decrease in oxygen content with depth (and occurrence of oxycline, respectively) began below the isopycnal surface σ_t = 14.5 kg/m³. The position of the sub-oxygen zone upper boundary defined by the isooxygen 10 µmol/l was not strictly isopycnal, but fell on the range of isopycns – σ_t = 15.7 – 15.85 kg/m³. However, it was not possible to identify a relation between the change in the position of the upper boundary and a certain hydrological season. For example, the deepest occurrence of the upper boundary below σ_t = 15.8 kg/m³ was observed both in November and December 2017, and August 2018. The lowering of isooxygen 10 µmol/l to σ_t = 15.9 kg/m³ in the Kerch shelf area is related to a more voluminous and colder intermediate layer over the shelf in December 2017. The position of hydrogen sulfide upper boundary determined by isosulfide 3 µmol/l was found raised almost to σ_t = 16.0 kg/m³ in only one of 11 surveys in April 2017. And in all other cases (including the one in August 2017, i. e. six months after it was raised) it was invariably within the range σ_t = 16.10–16.15 kg/m³. Over the past 25 years, the concentration of hydrogen sulfide at the depths 1750–2000 m has remained unchanged at the level 383 ± 2 µmol/l.

Purpose. The object of the work is to construct an automated system for calculating sea ice drift velocity fields using Sentinel-1A/B radar measurements based on the normalized maximum cross-correlation approach. The conditions and results of a numerical experiment aimed at evaluating the effectiveness of this technique for 63 pairs of radar images of the Fram Strait region for the summer-autumn periods in 2017 and 2018 are presented. Both the calculation algorithm and the qualitative and quantitative characteristics of the results are described in details. The effectiveness of the approach being applied to regular monitoring of ice drift is considered.

Methods and Results. The maximum cross-correlation (MCC) method was used for calculations. It is based on an automated finding of photographically similar fragments in the pairs of images, for which the sensing time interval is known. The Pearson correlation coefficient was applied as a proximity metric. As a result, 63 sea ice drift velocity fields were constructed for the Fram Strait region, each of which has a spatial scale of approximately several hundred thousand square kilometers. The method for filtering false correlations is proposed.

Conclusions. The approach applied in the study makes it possible to obtain automatically the sea ice drift velocity fields from the satellite data with high spatial resolution (40 m). The reconstructed velocity fields cover significant areas of the ocean surface. The method proposed for filtering false correlations permits to extract effectively the fragments with distortions resulting from the MCС algorithm limitations, from the calculation results.

Purpose. The purpose of the study is to describe comprehensively the extreme storm in the Black Sea in November, 2023 in the terms of characteristics of the wind and wave fields, based on model calculations, satellite data and field measurements.

Methods and Results. The atmospheric fields are calculated using the WRF model, and the wave fields – by the SWAN model. The wind and wave fields, as well as their development during the storm are described in details. The phenomenon of wave shading by the Crimean Peninsula is studied. The data available for the storm period were involved in comparing the calculation results to the data of satellite altimeters, the CFOSAT SWIM wave scatterometer, and the synthetic aperture radars. The data of contact measurements carried out during the storm by the standard equipment installed at the oceanographic platform at the Black Sea hydrophysical subsatellite polygon of Marine Hydrophysical Institute of RAS (the coastal zone of the southern coast of Crimea) are presented. The wave characteristics near the oceanographic platform were calculated using the nested grid method.

Conclusions. It is found that during the storm in the Black Sea in November 2023, the maximum wave heights and the maximum wave periods exceeded 9 m and 13 s, respectively. The calculation results are confirmed by a large amount of satellite data. The results of calculating the characteristics of waves near the oceanographic platform are consistent with the in situ measurements performed from the oceanographic platform. Since the applied configuration of models and their settings permitted to retrieve the fields of wave physical characteristics with a high degree of reliability, they can be used for an authentic forecast of extreme storms in the Black Sea. The shading of waves by the Crimean Peninsula has led to the two and more times decrease in the heights of extreme waves in the coastal waters from the southern tip of the peninsula to Cape Chauda (35.8 ^оE).