Seasonal variability and the types of currents in the upper layer of the Black Sea

Spectral analysis of the climatic circulation in the upper 10 m layer of the Black Sea has been carried out. The current field was calculated from a three-dimensional baroclinic non-linear model. The mode composition of the circulation was derived. It is shown that the surface climatic circulation can be represented as the superposition of a finite number of elementary currents of two types:n- andm-modes. Only 4n-modes are shown to be energy-dominant throughout the year. A relation is established between the different sources of energy supply of currents and the modes which assimilate this energy. Problems concerning the nature of the classical Black Sea currents such as the main Black Sea current (MBSC) and the Knipovich spectacles are discussed. A scenario of the field large-scale experiment is suggested which allows the reconstruction of the amplitudes of the first 10n- and the first 10m-modes from mooring data.Translated by Mikhail M. Trufanov.     

Variations of the seasonal behavior of geostrophic circulation in the Black Sea

On the basis of the results of processing of the archival hydrological data, we analyze the seasonal behavior of geostrophic circulation in the Black Sea and its long-term variations. It is shown that the variations of currents on the decadal time scales with different manifestations in different seasons lead to changes in the characteristics of the seasonal course of geostrophic circulation in the second half of the last century. The intensification of winter circulation and weakening of summer circulation observed since the mid-1970s result in the increase in the amplitude of the annual course of current velocity on the sea surface. We also discuss possible causes of variations in the intensities of geostrophic currents in the Black Sea.     

Assimilation of the Data of Satellite Altimetry in an Eddy-Resolving Model of Circulation of the Black Sea

The data of satellite altimetry are used to simulate the Black-Sea circulation. The altimetry data of the TOPEX/Poseidon and ERS satellites are prepared within the framework of the NASA Ocean Altimeter Pathfinder project. The additional data processing is performed to compute the dynamic level reflecting the circulation of the Black Sea. The altimetry sea-level is assimilated in an eddy-resolving model of circulation of the Black Sea based on primitive equations. The accuracy of the obtained fields of temperature and salinity is estimated by comparing with the data of large-scale hydrographic surveys according to the ComSBlack program. The prognostic capabilities of the proposed model are estimated by comparing the obtained results with the fields computed with the help of assimilation of the altimetry data.     

Numerical Analysis of the Effect of Active Wind Speed and Direction on Circulation of Sea of Azov Water with and without Allowance for the Water Exchange through the Kerch Strait

The effect of seawater movement through the Kerch Strait for extreme deviations in the level and speed of currents in the Sea of Azov caused by the action of climate wind fields has been studied using the Princeton ocean model (POM), a general three-dimensional nonlinear model of ocean circulation. Formation of the water flow through the strait is caused by the long-term action of the same type of atmospheric processes. The features of the water dynamics under conditions of changing intensity and active wind direction have been studied. Numerical experiments were carried out for two versions of model Sea of Azov basins: closed (without the Kerch Strait) and with a fluid boundary located in the Black Sea. The simulation results have shown that allowance for the strait leads to a significant change in the velocities of steady currents and level deviations at wind speeds greater than 5 m/s. The most significant effect on the parameters of steady-state movements is exerted by the speed of the wind that generates them; allowance for water exchange through the strait is less important. Analysis of the directions of atmospheric circulation has revealed that the response generated by the movement of water through the strait is most pronounced when a southeast wind is acting.     

Measurement and evaluation of the hydrodynamics and secondary currents in and near a strait connecting large water bodies—A field study

The straits connect two large water bodies show highly strong and stratified currents related to meteorological, morphological and hydrodynamic conditions. In some cases, spatial and temporal changes of the stratified currents and their thickness, direction and magnitude are so complex. This complexity directly affects the circulation pattern in the region, water exchange between both ends of the straits and migration of fish species. In order to understand general characteristics of this kind of straits and identifying the complexity of the hydrodynamics of the region and evaluate the secondary currents and recirculation need long term, intensive, field work and measurement studies. As an example of this kind of hydrodynamically complex straits, Bosphorus strait is selected for a field study. The Bosphorus strait has a strongly stratified two-layer system and a unique case of the maximal exchange regime typical of strait flows, which is largely determined by conditions at the Black Sea. Although the Bosphorus strait has distinct two-layer stratification with an associated two-layer system exchange, no continuous current measurements have been made so far, previous measurements all having been random sampling.In this paper, a detailed measurement program has been applied to Bosphorus strait. In the measurement program, a short-term current profile measurement at selected locations at southern part of the strait has been conveyed. Additionally a long-term measurement of current profile has been performed at a selected critical location (in front of the Dolmabahçe Palace) where a recirculation flow exists in the strait. The scope of this paper is to present the techniques and the results of analysis of measurement data. In the measurements the current profile (magnitude and direction) has been determined at every 1 m depth intervals from the surface to the sea bottom at 3 min duration at every hour. Measurements provide that lower-layer flows in northward direction from the Sea of Marmara towards the Black Sea, whereas the upper-layer flow comes from the Black Sea and flows towards the Sea of Marmara in the opposite direction of lower layer. The Bosphorus strait exhibits distinctive features associated with variations in its width and depth. The meandering features of Bosphorus also cause recirculation flows. These results of measurements are presented, discussed and compared with previous studies.     

On numerical modelling of currents over the north-west shelf of the Black Sea

Water circulation in the north-west part of the Black Sea during the summer period is calculated by means of a complete non-linear thermohydrodynamic model. We have also studied the contribution of wind forcing, the thermohaline regime, the coastline configuration, and the bottom topography to the generation and transformation of a flow field. It has been shown that the thermohaline regime and wind forcing account for 30 and 70% of the total circulation, respectively. Variation in the nature of circulation at weak winds is due to thermohaline factors. Vertical motion plays a crucial role in theT, S- andH ₂ S-field formation.Translated by V. Puchkin.     

Features of the hydrological structure and circulation in the north-western Black Sea: shipboard and satellite observations in 1989–1990

The results of shipboard and satellite observations of the synoptic variability in the region of the continental slope south of the Tarkhankut Cape are considered. Ship observations revealed a two-layer circulation system of currents. An anticyclonic meander is localized within the upper mixed layer. A cyclonic disturbance, related to the main Black Sea current, occupies the water column below the seasonal thermocline. The interaction between the thermohaline fields is presumably induced by the anticyclonic atmospheric circulation. The satellite-derived images revealed a complex pattern of the large-scale dynamics of seawater over the entire north-western Black Sea.Translated by Mikhail M. Trufanov.     

Modeling of Long-Term Evolution of Hydrophysical Fields of the Black Sea

The long-term evolution of the Black Sea dynamics (1980–2020) is reconstructed by numerical simulation. The model of the Black Sea circulation has 4.8 km horizontal spatial resolution and 40 levels in z-coordinates. The mixing processes in the upper layer are parameterized by Mellor-Yamada turbulent model. For the sea surface boundary conditions, atmospheric forcing functions were used, provided for the Black Sea region by the Euro mediterranean Center on Climate Change (CMCC) from the COSMO-CLM regional climate model. These data have a spatial resolution of 14 km and a daily temporal resolution. To evaluate the quality of the hydrodynamic fields derived from the simulation, they were compared with in-situ hydrological measurements and similar results from physical reanalysis of the Black Sea.     

Dramatic weakening of the ear-shaped thermal front in the Yellow Sea during 1950s–1990s

The ear-shaped thermal front(ESTF), formed by the convergence of the Yellow Sea Warm Current(YSWC) and the Shandong Coastal Current(SCC), is a very important oceanic phenomenon in the Yellow Sea(YS) in winter. In situ measurements and reanalysis datasets all demonstrate that the ESTF has been weakening during1950s–1990s, and a similar weakening trend is also found in winter monsoon over the YS. Numerical experiments show that the weakening of winter monsoon can induce an anomalous circulation in the YS on multi-decadal timescale with northward anomalous currents along China's coast and southward anomalous currents in the central YS—generally opposite to seasonal mean circulation. The anomalous circulation causes slowdown of the YSWC and the SCC, and thus weakens the ESTF. Since the ESTF plays important roles in regional ocean dynamics and air-sea interactions, its weakening has important implications for regional climate in the YS in winter.     

Modeling Black Sea circulation with high resolution in the coastal zone

We present a numerical model of Black Sea circulation based on primitive equations with improved spatial resolution in the coastal zone. The model equations are formulated in a two-pole orthogonal coordinate system with arbitrary locations of the poles and a vertical σ coordinate. Increased horizontal resolution is gained by displacing the pole into the vicinity of the separated subdomain. The problem is solved over a grid with a variable step. The northern coordinate pole is displaced to the vicinity of Gelendzhik; the grid step varies from 150 m in the coastal zone to 4.6 km in the main basin. We simulated the fields of currents, sea level, temperature, and salinity under the given atmospheric forcing in 2007. The model is capable of reproducing the large-scale Black Sea circulation and submesoscale variations in the coastal currents.     

南海环流的三维数值模拟

本文采用Backhaus的三维海流模式,运用半隐式及C-网格方法求解基本方程,对南海各季的平均海流流场进行了数值模拟。将模拟结果与已有的研究结果进行比较,其主要流系基本上是相符的,夏季表层基本上为一反气旋型环流;冬季则转变成气旋型;在冬季,从50m层起开始显露出“南海暖流”的存在。这些表明南海海流的某些主要特征基本上已经被此模型成功地再现出来,同时此模型又给出了垂直方向各层的海流情况,在目前尚缺乏深层实测海流资料的情况下,上述深层海流的模拟结果有一定参考意义。     

A near-inertial current event in the homogeneous deep layer of the northern Sea of Japan during winter

Under strong surface wind forcing during winter, direct current observations in the northern Sea of Japan show the existence of strong near-inertial currents in the deep water that is characterized by the extremely homogeneous vertical structures of temperature and salinity. However, the mechanism generating internal waves in the deep water of the northern Sea of Japan has not been well understood. In this study, to clarify the dynamical link between the surface wind forcing and near-inertial currents in the deep water of the northern Sea of Japan, we drive a general circulation model taking into account realistic wind stress, ocean bottom and land topography. In the northern Sea of Japan, the numerical results show that vertically coherent horizontal currents with a speed of ~ 0.05 m s^?1 are excited throughout the homogeneous deep water. A two-layer model successfully reproduces the pattern of the horizontal current velocities shown by the general circulation model, indicating that internal waves emanate westward from the northwestern coast of Japan through coastal adjustment to the strong wind forcing event and, while propagating into the ocean interior, they excite evanescent near-inertial response throughout the lower layer below the interface.     

Numerical experiments on the four-dimensional assimilation of the observation data derived in the Black Sea in June 1984 on the basis of a numerical energy-balanced model

A procedure for the four-dimensional (4D) analysis of the hydrophysical fields in the Black Sea with an assimilation of the temperature and salinity (T, S) data was realized on the basis of a numerical model which involves the primitive equations of motion, and the heat and salt advection equations. Two experiments were carried out which differed by the observation data assimilation procedure. Analysis has shown that the observation data assimilation procedure realized using the energy-balanced model allows the reproduction of some synoptic features of the circulation in the Black Sea. A comparison of two computations demonstrates the efficiency of assimilating the measurement data on the basis of the 4D analysis as compared with the sequential objective analysis.Translated by Mikhail M. Trufanov.     

Monitoring of the Black-Sea Dynamics Based on Satellite Technologies

We present a survey of the principles and methods of satellite monitoring of the ocean, analyze the prospects of application of the data of satellite measurements to the operative diagnostics and prediction of circulation in the Black Sea, and discuss the physical foundations of remote sensing, the possibilities of contemporary equipment aimed at the observation of the sea surface from satellites, and the applicability of satellites as retransmitters of the data of measurements from free-drifting buoys. The contemporary oceanographic investigations carried out from satellites enable one to use the accumulated data for the numerical analysis of currents in seas and oceans in the model of general circulation as well as for the creation of a system of operative diagnostics and prediction of the state of marine environment. Examples of experimental realization of elements of the system of operative diagnostics of the state of the Black Sea based on the use of the satellite data are presented and the prospects of prediction of currents in the basin are analyzed.     

Modeling processes of the protrusion of near-coastal anticyclonic eddies through the Rim Current in the Black Sea

The eddy-resolving (1/30)° version of the low-dissipative DieCAST [7] ocean circulation model is used for modeling processes of the protrusion of near-coastal anticyclonic eddies (NAEs) through the Rim Current (RC). Under mean climatic forcing, the model realistically reproduces the evolution of the Caucasian NAE (CNAE) from its generation, formation of an attached anticyclonic meander, protrusion through the RC, and, finally, to the formation of an isolated anticyclonic eddy and its dissipation within the Eastern Cyclonic Gyre of the Black Sea. The process of double protrusion of the CNAE and the Kizil-Yirmak NAE into the RC, their passages through the RC, and merging in the eastern part of the Black Sea is also considered. The modeled space-time parameters of NAE evolution agree well with satellite observations [15, 23].     

Vertical structure of geostrophic currents in the fields of density of water and hydrostatic pressure

We analyze geostrophic currents with nonmonotonic vertical profiles, of velocity and present the distributions of the density of water and hydrostatic pressure corresponding to possible types of the vertical kinematic structure. To prove the existence of the indicated complex layered structures, we use, the data of instrumentalin situ measurements of currents in the Black Sea and in the west part of the Tropical Atlantic. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Spatial structure and variability of fields of currents, attenuation factor of light, and temperature in the surface layer of the northwest part of the Black Sea

On the basis of the results of simultaneous observations of parameters of currents, attenuation factor of light, and temperature performed aboard a moving vessel, we established the main characteristics and features of the circulation of waters and the structure of transparency and temperature fields in the surface layer of the northwest part of the Black Sea. We investigate the correlation between the variability of fluid dynamics and redistribution of transparency and temperature fields. The measured currents are compared with those calculated using the actual field of atmospheric pressure during the time of observation. It is shown that the results obtained in the areas of steady currents in the west and central regions are in good agreement. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

On the influence of salinity exchange on the circulation of a fluid in an enclosed basin

The mechanism responsible for fluid circulation in an internal basin of the Black Sea type is studied using a two-dimensional model for convective currents generated by the inhomogeneous horizontal gradient of salinity exchange. Analytical investigations and calculations have shown that the generation of cyclonic motions occurs due to an imbalance of salt fluxes at the fluid lateral boundaries of the basin. Under the influence of salt fluxes from the straits and the fresh water from rivers, the circulation tends to become streamlike. At the site of reduced salinity, the stream narrows and becomes more intensive. Translated by V. Puchkin.     

A numerical experiment on modelling synoptic eddies in the Black Sea during the summer season

An energetically-balanced model has been used to perform an eddy-resolving experiment on the calculation of the baroclinic circulation in the Black Sea. As the initial and boundary conditions, historical data on the density and tangential wind stress for the summer season have been used. Following the integration of a set of model equations, an array of anticyclonic eddies formed on days 50–60 on the margins of the Black Sea rim current. Off the Caucasus, Crimean, and Bulgarian coast, anticyclonic eddies are shown to be quasi-stationary. The north-western Black Sea and Anatolia coastal zone are areas of Black Sea rim current instability, where synoptic eddies — both cyclonic and anticyclonic — are generated.Translated by Vladimir A. Puchkin.     

Lagrangian Transport by Currents Variable in Time in the Model of Wind-Induced Circulation of the Black Sea

We construct a simple model of seasonal variability of circulation in the Black Sea, which satisfactorily explains the data of altimetric satellite observations. The model is used to describe the nonstationary wind-induced currents in a round basin and study the trajectories of water particles in the surface layer of the sea. It is shown that, for nonstationary currents, these trajectories can be quite complicated. It is also assumed that Lagrangian mixing can be caused by the complex trajectories of water particles.