Vol 31, No 1 (2024)
Analysis of observations and methods of calculating hydrophysical fields in the ocean
Spectral reflectance coefficient, color characteristics and relative transparency of the Black Sea waters in spring, 2019 and 2021: comparative variability and empirical relationships
Abstract
Purpose. The work is purposed at studying spatial variability of the sea reflectance coefficient using the field data, as well as at obtaining empirical relationships between the colorimetric and bio-optical characteristics and comparing them with the already published data.
Methods and Results. The measurement data on spectral reflectance coefficient of the water column and the Secchi disk depth obtained in the expeditions of the R/V Professor Vodyanitsky in the northern and northeastern parts of the Black Sea in April 18 – May 13, 2019 and April 22 – May 8, 2021 were used. Based on the reflectance spectra, the following color characteristics were calculated: dominant wavelength, water color purity, hue angle, and the inherent optical characteristics (absorption of dissolved organic matter and backscattering by suspended particles). Variability of the sea reflectance coefficient and its color characteristics was analyzed for similar periods in spring 2019 and 2021. For the combined data for 2019 and 2021, the relationships between the Secchi disk depth and the reflectance coefficient at maximum, as well as the dominant wavelength were obtained and compared to the known data. For the first time, the empirical relationships connecting the hue angle with the dissolved organic matter absorption and the backscattering by suspended particles were obtained to calculate the inherent optical characteristics of the Black Sea waters.
Conclusions. The distribution of the reflectance coefficient observed in 2019 is more typical of late spring, whereas the similar distribution observed in 2021 rather indicates the continuing winter-spring development of phytoplankton communities that is typical of the deep part of the Black Sea waters in the years with cold winters. Significant correlations between the colorimetric and bio-optical parameters of seawater were established. They can be used as a part of the empirical and semi-analytical algorithms for comprehensive assessing (including application of remote sensing data) the hydrooptical characteristics of the Black Sea waters.
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Interdecadal variability of large-scale atmospheric circulation in the Atlantic-European sector conditioning surface temperature anomalies in the Black, Barents and Norwegian seas
Abstract
Purpose. The study is aimed at analyzing and assessing the interdecadal variability of winter hydrometeorological fields in the Atlantic-European sector during different phases of the Arctic and North Atlantic Oscillations indices.
Methods and results. The structure of positive (negative) anomalies of the geopotential height was analyzed based on the composite maps of geopotential height anomalies at levels H1000 and H50, the North Atlantic Oscillation index was scrutinized using the data from the Climate Prediction Center archives, the sea surface temperature anomalies were surveyed applying the information from the Japan Oceanographic Data Center. The researches covered two periods: a decade of negative and a decade of positive values of the Arctic and North Atlantic Oscillations indices. During a decade of positive values of these indices, the Azores anticyclone and the Icelandic cyclone are intensified, while the Siberian anticyclone weakens. And, on the contrary, during a decade of negative values, the Siberian anticyclone strengthens, while the Azores anticyclone and the Icelandic cyclone wane. Atmospheric circulation in the Atlantic-European sector (the Western Europe subregion) is formed being affected by the Atlantic air masses, and in the Eastern Europe subregion – by the Azores anticyclone and the Siberian anticyclone spur. During a decade of positive phase of the Arctic and North Atlantic Oscillations, the Black Sea surface temperature decreases and becomes lower than the climatic, whereas that of the Barents and Norwegian seas – higher. During a decade of negative phase of the Arctic and North Atlantic Oscillations, the surface temperature of the Black Sea becomes higher, and that of the Barents and Norwegian seas – lower.
Conclusions. During different phases of the Arctic Oscillation, interdecadal variability in the polar vortex intensity affects the redistribution of atmospheric mass between the center of a polar vortex and its boundaries. The consequence of this phenomenon consists in strengthening (weakening) of the Azores, Siberian and Icelandic centers of atmospheric action as well as formation of the interdecadal variability of atmospheric circulation in the Atlantic-European sector. As a result, the pressure structures conditioning the anomalies in surface air and sea surface temperatures with opposite signs are formed in the subregions of the Atlantic-European sector.
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Formation of large anomalies in the thermal conditions of waters on the western and eastern shelf of the Sakhalin Island
Abstract
Purpose. The study is aimed at identifying the mechanisms forming large anomalies in the water thermal conditions on the western and eastern shelf of Sakhalin Island being impacted by the atmospheric processes in spring-summer periods.
Methods and results. The data of coastal observation stations performed by the Hydrometeorological Centre of Russia in 1980–2021 permitted to study and assess the multi-year variability of water thermal regime in the fishery regions on the western and eastern Sakhalin shelf from May to August. The extreme fluctuations of monthly average thermal conditions of the water areas were revealed. The years known for formation of large negative and positive anomalies in the water thermal conditions were determined using the criterion analysis method. The fact that frequency of arising of large negative anomalies exceeds that of large positive ones was found. It was established that the mechanisms forming large anomalies were conditioned by the regional features of atmosphere circulation, i.e. by the abnormal changes in development and spreading of the atmosphere action centers (summer Far East depression, Okhotsk anticyclone and Hawaiian maximum). The cause-effect relations were determined.
Conclusions. In Sakhalin Island coastal regions, formation of large anomalies in the water thermal regime is conditioned by the abnormal changes of the baric fields structures in the regional atmosphere action centers, as well as by the changes in their local impact.
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Variability of decadal horizontal thermohaline gradients on the surface of the Barents Sea during summer season in 1993–2022
Abstract
Purpose. The paper is aimed at comparative analysis of the decadal horizontal gradients of thermohaline fields in the Barents Sea during the summer periods in 1993–2022 derived from the reanalysis and satellite measurements with the aim to select the most suitable data array for studying the surface manifestations of frontal zones.
Methods and Results. The fields of decadal and background thermohaline gradients on the Barents Sea surface were calculated for the summer periods in 1993–2022 based on the monthly mean temperature data from GHRSST OSTIA, MODIS/Aqua, and VIIRS/Suomi NPP, as well as on the monthly mean temperature and salinity data from CMEMS GLORYS12V1 and MERCATOR PSY4QV3R1. The quantitative estimates of temperature and salinity gradients were obtained for certain decades using different data arrays, and a comparative analysis of these estimates was performed along with a description of the physical and geographical characteristics of frontal zones. Maximum thermohaline gradients on the surface were observed in July. Based on the data from all the sources, the background horizontal thermal gradient has been increasing over three decades. During a summer period, the Polar Frontal Zone was identified on the surface of the Barents Sea in all the data arrays, whereas the Coastal and Arctic Frontal Zones were observed in the salinity field based on the CMEMS GLORYS12V1 and MERCATOR PSY4QV3R1 data.
Conclusions. The difference between the calculated estimates of horizontal temperature gradient can exceed 0.01°C/km that is comparable to the magnitude of the average climate gradient in the Barents Sea. The thermal gradient values obtained from the CMEMS GLORYS12V1 and MERCATOR PSY4QV3R1 reanalysis data are the closest to this estimate. This fact makes it possible to classify these data arrays as the most preferable ones for the analysis of the surface manifestations of frontal zones in the Barents Sea.
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Variance of short-period sea level oscillations in the Black Sea: seasonal and interannual variations
Abstract
Purpose. The study is aimed at investigating the peculiarities of seasonal and interannual variations of the variance of short-period sea level oscillations in the Black Sea.
Methods and Results. The peculiarities of changes in the variance (energy) of synoptic (2–30 days) and mesoscale (2 h – 2 days) sea level oscillations in the Black Sea were studied based on the analyses of long-term sea level observation series. The results of spectral analysis made it possible to find out that on the eastern coast of the Black Sea, the spectral density of sea level oscillations increases from summer to winter and decreases from winter to summer within the frequency range of 0.1–0.8 cycles/day. As for the northwestern and Crimean coasts, the spectral density is practically the same in autumn and winter, further it decreases in spring and summer. The interannual changes of the variance of synoptic oscillations on the eastern sea coast are characterized by a negative trend achieving – 0.25 cm2/year in Batumi and – 0.41 cm2/year in Poti. The variance of mesoscale sea level oscillations has negative trends with the rates from – 0.21…– 0.24 cm2/year in Odessa and Nikolaev to – 0.13 cm2/year in Gelendzhik.
Conclusions. The variance of short-period sea level oscillations in the Black Sea increases from summer to winter and decreases from winter to summer that is related to the intensification of cyclonic activity in the atmosphere during autumn and winter. A local seasonal decrease in the variance of sea level oscillations is observed in the estuaries of large rivers in winter due to the developed ice cover preventing the formation of wind surges and seiches. At that, the higher the oscillation frequency, the stronger the ice cover influence.
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Experimental and field research
Ratio between trough and crest of surface waves in the coastal zone of the Black Sea
Abstract
Purpose. The work is aimed at analyzing variability of the ratio between trough and crest of the sea surface waves, as well as the relationship of this ratio with the skewness of sea surface elevations.
Methods and results. The analysis is based on the wave measurements performed from the stationary oceanographic platform located near the Southern Coast of Crimea in the Black Sea. The depth at the place where the platform is installed is about 30 m. The analyzed data array totals 17,083 twenty-minute measurement sessions. The freak waves were identified by the abnormality index AI equal to the ratio between the maximum wave height per session and the significant wave height. The freak waves with index AI > 2 were observed in 562 measurement sessions. This corresponds to a probability of their occurrence equal to 3.3%. The AI values range from 1.16 to 2.79. The ratio between the trough Th of the highest wave and its crest Cr is in the range 0.37 < Th/Cr < 1.47, at that the average value is 0.79.
Conclusions. Statistical characteristics of the waves revealed in the presence of freak waves differ noticeably from those obtained at AI < 2. In the situations when AI < 2, the probability of an event when the trough Th of the highest wave exceeds its crest Cr is 10.9%. The event with Th/Cr > 1 does not occur if AI < 1.4. When there are waves satisfying condition AI > 2, the probability of an event Th/Cr > 1 is 19.4%. It is shown that condition Th/Cr > 1 is not necessary for arising of a negative skewness of sea surface elevations. The probability of skewness large deviations from a zero value both towards positive values and towards negative ones, is higher at AI > 2 than at AI < 2. The statistical relationship between the skewness and the Th/Cr ratio is observed only for freak waves.
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Major ion composition of waters in the Kerch Strait and the adjacent areas
Abstract
Purpose. The work is purposed at studying the influence of water exchange processes between the Black and Azov seas upon the characteristics of major ion composition (MIC) and other hydrochemical indicators of the Kerch Strait waters, as well as the impact of changes in the relative content of major ions of water salt composition upon the accuracy in determining salinity values. The MIC transformation during mixing of the sea surface waters and the Taman Bay ones in the Kerch Strait is investigated. The errors in calculating salinity by the standard methods are assessed for the Kerch Strait, the northeastern Black Sea and the Taman Bay waters.
Methods and Results. The concentrations of major ions determining MIC in the Kerch Strait, Black Sea and Taman Bay surface waters in 2019–2023 were defined by the potentiometric titration method. The water salinity values were obtained in four different ways.
Conclusions. It was established that the salinity value ~18.66 calculated by a sum of the major ions corresponds to the surface waters in the northeastern part of the Black Sea, that conforms to the practical salinity value ~18.10 calculated using the CTD probe data. On the average, MIC of these waters is characterized by the following relative content of major ions: Clˉ = 54.1%,
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Mathematical modelling of marine systems
Energy flows between the mean currents and the mesoscale eddies in the eastern and western parts of the Black Sea
Abstract
Purpose. The study consists in analyzing the energy flows between the currents of different scales in the eastern and western parts of the Black Sea.
Methods and Results. The energy balance components were determined based on the results of numerical calculation of current fields in the Black Sea performed using a two-layer eddy-resolving model subjected to a forcing of a wind with cyclonic vorticity. A complete non-stationary large-scale sea circulation was represented conventionally as a sum of mean currents and mesoscale eddies. Conversions between the kinetic and available potential energy, as well as the energy flows between the mean currents and the mesoscale eddies were calculated separately for the eastern and western parts of the Black Sea. Besides, the advective energy flows across the meridional boundary between two parts of the sea were also calculated.
Conclusions. The main contribution to the potential energy advective transfer from the eastern part of the Black Sea to its western part is made by the mesoscale eddies. This advective energy transfer results in arising of differences in the energy flow directions in the currents of different scales in the eastern and western parts of the sea. In the Black Sea eastern part, energy is transferred from the mean currents to the mesoscale eddies, whereas in its western part, a reverse energy flow is observed. It is shown that the kinetic energy exchange between the movements of different scales is provided by the Coriolis force work, which for the initial currents is equal to zero, but turns out to be non-zero separately for the mean flows and the mesoscale eddies.
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Numerical modeling of the Black Sea response to the intrusion of abnormally cold air in January 23–25, 2010
Abstract
Purpose. The work is aimed at studying the response of the Black Sea upper layer to the intrusion of cold air in January 23–25, 2010.
Methods and Results. The NOW coupled mesoscale sea – atmosphere model with the 1 km resolution was used to study the sea fields numerically. The change in sea surface temperature in January 23–25, 2010 resulted from the cold intrusion was reproduced. The basic factors which had influenced the change in the upper layer temperature, namely horizontal advection, cooling of the sea surface due to the sensible and latent heat fluxes and the impact of vertical turbulent mixing were considered and quantitatively assessed. The main changes that took place in the cold intermediate layer were investigated.
Conclusions. The change in vertical distribution of the monthly average temperature, salinity and density is considered based on the Copernicus reanalysis data for 2009–2010. The presence of a cold intermediate layer at the average depth 60 m in all the months except for the transitional winter-spring period is shown. The results of NOW modeling reveal the fact that decrease in the surface temperature over the most sea area occurred as a result of heat and mass exchange with the atmosphere. The influence of horizontal advection and mixing through the lower boundary of the cold intermediate layer was manifested only in certain small areas, in other words, it produced a local effect. Convective cooling spanned the upper mixed layer up to the depths about 40–45 m and amounted to ~ 1°С. Besides, it is shown that during the cold air intrusion, the depth of cold intermediate layer increased. The notion that the local cold waters in the shallow northwestern part of the sea are secondary as a source of formation of the cold intermediate layer has been confirmed. The density of colder, but less saline coastal water prevents its sinking to the upper boundary of cold intermediate layer.
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Mathematical model of a flooding process in the Don delta during extreme surges
Abstract
Purpose. The work consists in developing a mathematical model to study the flooding processes in the Don delta during the extreme surges.
Methods and Results. The level fluctuations in the Taganrog Bay are calculated based on solving a system of equations for long waves in a homogeneous incompressible fluid in the Coriolis force field. The problem is solved by the finite-difference methods on high-performance computing systems. The algorithm for determining the area of the Don delta flooding in the process of a surge is given. It is based on comparing the heights of the delta area to the water level and on taking a decision whether to flood or drain a computational cell. The calculation results are compared to the water level values observed at the gauging station, and are also displayed as a map diagram of the flooded area.
Conclusions. The proposed model should be applied in case of the extreme surges when a significant delta area is flooded. The model makes it possible to calculate accurately the hydrodynamic parameters of a flow including the magnitude of water level difference. The proposed algorithm determining whether flooding or draining is required for a computational domain, permits to reveal the areas in the Don delta where flooding depends on the wind conditions.
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