Spatial and temporal variability and monitoring of hydrophysical fields of the Black Sea

As part of the Russian-Ukrainian program “The Black Sea as a Simulation Model of the Ocean,” the monitoring of the marine environment is considered using modern measuring systems. On the basis of historical and contemporary observation data, we estimate the spatial and temporal scales of dominant processes in the Black Sea. We describe the main measuring systems used to monitor the structure and variability of the hydrophysical fields. Examples characterizing the specific features of the Black Sea processes are presented.     

Numerical modelling of the seasonal variability of hydrophysical fields in the Black Sea

The results of research into the seasonal variability of hydrothermodynamic characteristics of the Black Sea are presented. These have been obtained using field data on the fields of temperature, salinity, and wind, and by mathematical modelling. The seasonal variability of the current and heat content fields and of the cold intermediate layer and other hydrophysical characteristics is discussed.Translated by Mikhail M. Trufanov.     

On the long-term variability of normalized radiance of the Black Sea

Basin-wide distributions of normalized spectral radiances averaged over different time periods were plotted based on the estimates of the average radiance for 20 × 20 km squares in 482 digital images of the Black Sea obtained with a SeaWiFS color scanner in 1998–2004. Significant distinctions of the annual average distributions in the amplitude, localization, and configuration of basin-scale inhomogeneities allowed us to consider these distributions as yearly spectral-radiance portraits of the Black Sea that reflect specific features of the annual behavior of the hydrometeorological conditions over the Black Sea region. Seasonal features of the radiance distributions related to the wind field were discovered. Substantial interannual variability in the errors of the standard atmospheric correction algorithm resulting in an underestimation of the shortwave radiances were revealed. It is shown that the river discharge and the stratification of the layer sensed substantially influence the yearly spectral-radiance portraits of the Black Sea.     

Experimental research on the hydrophysical characteristics and dynamics of waters in the north-west Black Sea in the region of the main Black Sea current

The results of measurements of hydrophysical parameters in the upper layer of the sea at an observation site located south-west of the Crimea are reported. The current pattern, the distribution of the upwelling and downwelling zones at the observation site, and the location of the mesoscale fronts are discussed.Translated by Mikhail M. Trufanov.     

Spatio-temporal variability of the relative transparency of Black Sea waters

The Black Sea relative transparency database covering the period from 1922 to 1993 is described. Seasonal and interannual variabilities of the Secchi disc depthz _σ are studied, along with the meteorological, hydrophysical and biological parameters affecting it. Translated by Vladimir A. Puchkin.     

Estimation of the long-term variability of hydrophysical characteristics of the Black Sea based on the assimilation of the climatic hydrological fields L and altimetry data

To study the long-term variability of the thermohaline and dynamic characteristics of the Black Sea, we use three versions of climatic fields, namely, the fields reconstructed in the model according to the old (1903–1982) and new (1903–2003) hydrological climatic data arrays of temperature and salinity and according to the data of satellite altimetry. The analysis of the altimetry-based climatic fields confirms the distinctions (established earlier according to the old and new data arrays) in the seasonal variability of the integral characteristics of temperature and salinity and in the structures of hydrophysical fields in the sea. It is shown that, in the winter-spring season, the thermohaline fields reconstructed according to the new and altimetry data arrays are characterized by a small elevation of the halocline (pycnocline) and the upper boundary of the cold intermediate layer. In all seasons, the altimetry-based surface geostrophic currents contain numerous mesoscale eddies with different signs of rotation. Moreover, in all seasons, the Rim Current reconstructed according to the altimetry data is characterized by a narrower jet almost along the entire its length. This jet is especially intense near the coasts of West Anatolia. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 4, pp. 3–17, July–August, 2006.     

Climatic and hydrophysical conditions of the development of hypoxia in waters of the northwest shelf of the Black Sea

The data of meteorological and oceanographic observations on the northwest shelf of the Black Sea for 1973–2000 are used to compute the characteristics of the entire area in the presence of hypoxia of waters under the pycnocline in the summer–autumn period and the area of surface waters with a level of salinity lower than 17.5‰ in May. The time of onset of the spring warming of air (stable transition through a temperature of 5°) is determined. A statistically significant positive trend of the air temperature (0.8° per 100 yr) is revealed in Odessa. The process of warming was observed mainly for the winter (1.5° per 100 yr) and spring (0.8° per 100 yr) periods and became especially intense since the beginning of the 1990s. On the basis of the data of correlation analyses, we establish a statistically significant relationship between the large-scale atmospheric processes [the index of North Atlantic Oscillation (NAO) and the wind conditions], the area of surface waters whose salinity is lower than 17.5‰, and the total area with hypoxia in the summer–autumn periods. For positive mean values of the NAO index (in January–March), we most often observe early spring with elevated repetition of the south and west winds with subsequent development of hypoxia in large areas of the northwest shelf. We propose an empirical regression model for the prediction of the total area of summer–autumn hypoxia of waters with predictors: the onset of the spring warming of air and the area of propagation of waters whose salinity is lower than 17.5‰ in May. The maximum error of prediction of the area with hypoxia does not exceed 5.5 ⋅ 10³ km², i.e., less than 2% of the total area of the northwest shelf in the Black Sea (to the north of 45°N).     

Influence of the dynamics of currents on the hydrophysical structure of the waters and the vertical exchange in the active layer of the Black Sea

An analysis of the data of measurements of the fine structure and microstructure fluctuations of hydrophysical fields in the upper 200-m layer of the Black Sea carried out using CTD profilers and a Baklan free falling microstructure and turbulence profiler revealed the existence of a positive correlation between the intensity of the fine structure and microstructure fluctuations and the dynamics of the currents. On the other hand, the level of the fine structure and microstructure fluctuations reflects the rate of the vertical turbulent exchange. It was shown that, in the case of the absence of the Black Sea Rim Current (BSRC) jet or clearly manifested mesoscale eddy structures, the vertical turbulent exchange in the pycnocline is weak, while, in the opposite case, it is stronger. The results obtained support the supposition that the interbasin dynamics play an important role in the maintenance of the rate of small-scale mixing in the pycnocline and halocline and provide the vertical transport of dissolved oxygen from the cold intermediate layer into the deeper layers of the sea.     

Thermohaline structure of abyssal and near-bottom waters of the Black Sea

The paper considers the thermohaline structure of Black Sea abyssal and near-bottom waters on the basis of exact measurements recorded in recent years under the HYDROBLACK and COMSBLACK international projects. The surface water mass (along with the cold mixed layer), the intermediate, abyssal, and near-bottom masses are identified. The upper boundary of deep waters is defined as a depth level where the density ratio decreases down to 0.1 This depth roughly corresponds to the 900–1100 m depth levels. Furthermore, the paper provides potential temperature and salinily values in the near-bottom layer, with an error of, respectively, 0.002°C and 0.002‰, at most. Translated by Vladimir A. Puchkin.     

Operational system for diagnosis and forecast of hydrophysical characteristics of the Black Sea

The automatic system of operational forecasting of the Black Sea state which functions at the Marine Hydrophysical Institute is presented. Principles of the system construction are considered; the marine environment models used for forecasting, the data streams required for the system functioning, and tools for validating and visualizing the results of sea-state calculations are described. Some examples of investigating a number of processes and phenomena in the Black Sea are given.     

Some statistical properties of the inter-annual variability of the relative transparency of Black Sea waters

The Secchi disk data-base compiled in the Black Sea from 1992 to 1993 has been investigated. The space-time variability of statistical properties (RMS deviations and trends) of the series of inter-annual anomalies of the relative water transparency is described. Translated by Vladimir A. Puchkin.     

Numerical analysis of hydrophysical fields on the northwestern shelf of the Black Sea

A numerical experiment with assimilation of hydrological observational data from a survey in October 2007 on the northwestern shelf of the Black Sea was carried out using the hydrodynamic model with nonlinear equations of motion, equations of heat and salt advection, and data assimilation. The results of this calculation are compared with thermohydrodynamic fields obtained without taking into account temperature and salinity measurements. It is shown that allowance for the observation data leads to qualitative and quantitative differences in the structure of the hydrophysical fields. Mesoscale eddies and intense jet streams that agree with satellite observations were found in the field of currents and were investigated. These eddies are not resolved in low-resolution field experiments.     

Short-range forecast of hydrophysical fields in the eastern part of the Black Sea

On the basis of an analysis of the results of modeling and a forecast of the basic hydrophysical fields in the easternmost part of the Black Sea for 2010–2012, the features of annual variability of regional circulating processes in this part of the sea basin are investigated. A forecast of a hydrological mode is made on the basis of the regional forecasting system developed at the Institute of Geophysics at Javakhishvili State University in cooperation with the oceanographic centers of the Black Sea riparian countries within the frame-work of the ARENA and ECOOP EU international scientific and technical projects. The regional system is one of the components of the Black Sea basin-scale Nowcasting/Forecasting System. The analysis of the material cumulated for the registered period shows that the easternmost water area of the Black Sea is a dynamically active zone where there is a continuous formation of different circulating processes considerably distinguished from each other.     

Modelling of the Black Sea seasonal variability

The paper discusses the results of numerical experiments on response of the Black Sea to the ‘real’ (mean monthly) atmospheric forcing. A new version of the multi-layer quasi-isopycnic model is applied which does not use the rigid-lid approximation and allows for a salt flux through the sea surface. Ways of obtaining quantitative agreement between the calculated data due only to the external forcing, without invokingin situ temperature/salinity observations are suggested. Translated by Vladimir A. Puchkin.     

Statistical structure of the large-scale fields of temperature and salinity in the Black Sea

In order to reconstruct the large-scale temperature and salinity fields by the method of optimal interpolation of the archival data, we compute the correlation functions and analyze the space and time variations of the statistical structure of the fields. On the sea surface, the thermohaline fields are spatially inhomogeneous. Thus, the correlation functions are anisotropic in the region of the northwest shelf and close to isotropic in the inner parts of the sea. The values of correlation length vary from season to season. In the layer of pycnocline, the temperature and salinity fields are anisotropic. In the zonal direction, the correlation length is 2–3 times greater than in the meridional direction. The indicated anisotropy becomes stronger in the winter season and weaker in the summer season as a consequence of the seasonal variability of large-scale circulation. We study the dependence of the error of reconstruction of the fields by the method of optimal interpolation on the form of approximation of the correlation functions with regard for anisotropy. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 51–65, January–February, 2008.     

Vertical mesostructure of deep waters in the Black Sea

Studies of mesoscaleT, S characteristics of deep waters in the Black Sea carried out during 1978–1988 are generalized. Transformed lower Bosphorus waters are spread in the layer from 400 m to the bottom following density isosurfaces corresponding to this water. This is the way that interlayers ofT, S inversions are formed; their vertical thickness reaches 100–200 m in the Anatolian flow. The streamwise decrease of theT, S maxima and the location of isotherms in the cross-sections of these tongues permit the estimation of the order of the values of the coefficients of the vertical and horizontal turbulent diffusion.Translated by Mikhail M. Trufanov.     

The effect of seasonal and inter-annual variability of river run-off on the salinity of coastal waters in the north-western Black Sea

The paper considers the correlation between the variability of river discharges in the northwestern Black Sea and water salinity (from 1962 to 1994). As the most complete series, we have chosen salinity data collected at the port of Odessa. The dominant oscillations in the intra-annual variability of river discharges (Danube, Dniester, Dnieper, Yuzhny Bug) and salinity attain about 2–3 months and one year, respectively. In the inter-annual variability of river discharges and water salinity, cyclicities equal to about 4–6 months and 9–12 years, respectively, have been identified. Through analysing the inter-annual variations, an inversely proportional dependence has been found between the overall river discharges and salinity (between discharges in April and salinities in May, the correlation coefficient has amounted to −0.61). For each river, we have obtained the following dependences: for Odessa-Dniester, −0.72; and for Odessa-Danube, −0.65. Translated by Vladimir A. Puchkin.     

On the large-scale and mesoscale variability of thermohaline characteristics in the north-eastern part of the Black Sea

The data collected during an 18-day station and nine hydrologic surveys have been analysed. Mesoscale and large-scale temperature and salinity oscillations were revealed. Mesoscale oscillations in the sea surface layer are induced by the diurnal course of solar radiation, and in the seasonal thermocline layer by internal waves with a predominant 6–10 h periodicity. Large-scale fluctuations are related to the passage of clockwise (cold) and anticlockwise (warm) meanders and vortices. It has been determined that the contribution of large-scale temperature and salinity oscillations to the total variability is 1·5 to 3-fold larger than that of the mesoscale ones.Translated by Vladimir A. Puchkin.     

Study of long-term variability of Black Sea dynamics on the basis of circulation model assimilation of remote measurements

The Black Sea dynamics for two decades (1993–2012) is analyzed. The study is carried out in numerical simulation with the use of a circulation model and assimilation of satellite measurements of free surface elevations and sea-surface temperatures (SSTs). The Black Sea circulation model has a spatial resolution of 4.8 km and 35 vertical levels, which thicken toward the surface. Arrays of ERA-Interim data are used for simulating the atmospheric forcing. A set of 3D arrays of the Black Sea hydrophysical fields is calculated on a regular grid with a time resolution of 24 h. The results are analyzed and compared with available contact measurements of temperature and salinity. This comparison shows a quite good quality of the resulted hydrophysical fields of the Black Sea.     

Numerical modeling of hydrophysical fields of the Black Sea under the conditions of alternation of atmospheric circulation processes

The hydrological regime of the Black Sea in the conditions of permanent alternation of atmospheric circulation processes was investigated on the basis of a baroclinic prognostic model of the sea dynamics. In the model, variations in the wind action were expressed as permanent alternation of 24 wind types characteristic of the Black Sea basin throughout the year. Thermohaline impact of the atmosphere was taken into account by specifying the annual trends of temperature and salinity at the sea surface, which was established from multiyear means of these parameters. The problem was solved numerically on the basis of the method of two-cycle splitting with the use of the grid with a horizontal spacing of 5 km. Results of the numerical experiment showed that, under the influence of a strong nonstationarity of atmospheric processes, the water circulation in the upper layer of the Black Sea changes qualitatively and quantitatively. The upper 20–30-m layer of the sea is particularly sensitive to atmospheric circulation variations. For any character of atmospheric circulation, the Black Sea circulation below this layer is nearly always cyclonic with internal cyclonic rotations.