Gennady K. Korotaev
Marine Hydrophysical Institute
Kapitanskaya
Sevastopol
Ukraine

Extended abstract

1. INTRODUCTION

1.1. General characteristics of the Black Sea basin.

The Black Sea is a deep with a depth up to 2200 m elongated basin situated between 40056’ and 46033’N. Its maximum zonal length is 1148km. The Crimea peninsula and the Anatolian coast convexity divide the sea into two sub-basins. The minimum width of the sea is 258 km. The broad north-western shelf (NWS) occupies the northwestern part of the sea. Typical width of the shelf along the other coastlines is 2-12 km. Profiles of density show a well-pronounced permanent pycnocline situated at a depth of 150-300m. The Black Sea is connected with the ocean by the narrow Bosphorus Straits. Density stratification is determined mainly by salinity, which is near 22.5 ppt in the deep-sea against 18-18.5 ppt on the surface. Sea surface temperature (SST) varies seasonally from 8°C to 30°C, while the deep-sea temperature is about 8.5°C. Winter cooling on the NWS could reduce SST up to 6°C and produces a distinctive feature of the Black Sea thermal stratification, the so-called cold intermediate layer (CIL) situated at a depth of about 50-90m. A permanent feature of the upper layer circulation is the Rim Current, encircling the entire Black Sea and forming a large-scale cyclonic gyre. Direct observations of the current velocity from surface buoys suggest that the maximum speed of this stream is usually 40-50cm/s increasing sometimes up to 80-100cm/s. The Rim Current is concentrated above the shallow pycnocline and the volume transport of the current is estimated to be 3-4 Sv. There are also two smaller cyclonic gyres in the western and the eastern parts of the basin. Cyclonic circulation induces the rise of the sea level toward the coast. The amplitude of sea level variation in space depends on season and ranges from 25 to 40 cm.

1.2. New technology transfer to the basin
Most of the information about the Black Sea dynamics came for many years from hydrographic surveys and episodic current measurements from buoy stations. However the last decade gave the set of new and very efficient tools of marine observations. IR and sea colour imagery permits the description of major mesoscale features of the Black Sea circulation such as the meanderings of the Rim Current, quasi-stationary anticyclonic eddies, fronts. Modern altimeters measure the sea surface elevation with an accuracy of about four centimeters and along-track resolution about ten km providing a unique possibility to observe basin-scale and mesoscale circulation in the Black Sea continuously. The use of continuous sea level observations from Topex/Poseidon and ERS altimeters beginning from the spring of 1992 makes it possible to describe seasonal, interannual and mesoscale variability of the Black Sea circulation. A broad surface drifting buoys programme has been put in place in the Black Sea during the last five years providing the new insights into the surface circulation structure and transfrontal mixing in the basin. ADCP surveys have shown that the surface currents are almost uniform above the pycnocline. At last the profiling floats programme which took place in the basin beginning in September 2002, has produced information about seasonal and interannual variability of the basin stratification and intensity of the deep currents in the basin.

2. PHYSICS OF THE BLACK SEA CIRCULATION

New observations and numerical simulations with and without real data assimilation make it possible to improve significantly the physical understanding of the Black Sea circulation on a broad scale, providing a good basis for the following applications of theoretical results.

2.1. Black Sea stratification and circulation driven by buoyancy fluxes.
The intense fresh water supply by rivers reduces the basin salinity by comparison with what is typical for the Mediterranean Sea. Therefore the pressure gradient induces a deep flow of the saline water along the bottom of the Bosphorus Strait. The contrast between the fresh and salt-water produces a buoyancy flux, which forms the density stratification of the basin. Theoretical analysis, laboratory models and assimilation of the climatic hydrography show that the deep-water upwelling at the open sea should support the permanent pycnocline. The upwelling directly determines the age of the deep water and position of the hydrogen sulphide surface in the basin. Compensating downwelling induces the buoyancy-driven cyclonic circulation in the upper layer of the sea and modulates the mixing of saline water entering through the Bosphorus Strait with ambient waters.

2.2. Wind-driven circulation: seasonal cycle and interannual variability.
Maps of surface currents obtained from altimetry manifest an obvious annual cycle of the circulation. The Rim Current is the most intense in winter-spring seasons. Winter circulation shows two gyres but in spring the current jet belts the whole basin along the bottom slope. Summer circulation attenuates significantly and in the autumn season the Rim Current usually breaks on the set of eddies. Seasonal variability of currents is well presented by the temporal evolution of potential and kinetic energy and has a simple physical explanation. Increase of the cyclonic vorticity of wind in January produces intensification of the upwelling on the bottom of the Ekman layer and rise of the pycnocline. The most shallow pycnocline observes a quarter of period (i.e. three months) after the most intense wind stress curl. However the rise of pycnolcline in the central part of the basin should be compensated by its deepening near the coast due to the conservation of the fluid volume above the pycnocline. The deepening of pycnocline near the coast is significantly larger than its rise in the open sea as the volume of fluid replaced in the center of the basin should be preserved along the beach. Therefore the slope of pycnocline toward the coast increases significantly at the beginning of spring. The intensity of the Rim current, which is in geostrophic balance, is highest at the same time. The weakening of the wind stress curl vice versa is accompanied by the deepening of pycnocline in the open sea and its rise near the coast. Thus, the phase of current oscillation is determined by the seasonal cycle of the wind stress curl. The most intense currents are observed approximately three months later after intensification of the wind stress curl. Accordingly, the weakest currents are observed approximately three months after amplification of the wind stress curl. The decomposition of the observed sea level to the empirical orthogonal functions (EOF) is carried out to prove that the basin-averaged wind stress curl is responsible for the annual variability of the Black Sea currents.

The Black Sea circulation also manifests significant inter-annual variability. Inter-annual variability is obvious in both potential and kinetic energy evolution. Annual mean energy as well as the amplitude of its seasonal variation is changed a few times from one year to another. Inter-annual variability is also observed in the structure of eastern and western gyres. The energy budget equation is considered to understand the nature of inter-annual variability of the Black Sea circulation. This equation states that the rate of change of basin-integrated energy is equal to the difference between the work exerted by the wind stress over the basin and dissipation due to horizontal friction in the system. It is sufficient to have the altimeter and the wind stress data to estimate all terms of the energy budget equation. It is found that temporal variations of the left and right hand sides of the equation are in phase, implying that the basin integrated wind stress work is the main source of total energy variations over the basin.

2.3. Mesoscale circulation of the Black Sea.
Major features of mesoscale variability such as planetary waves, meandering of the strong jet and mesoscale eddies, which are well known from oceanic observations, could be found in the Black Sea. Continuous observations of the sea level by space altimetry provide a unique possibility to consider different types of mesoscale processes.
Evidence of Rossby waves.
It is well known that the seasonal cycle of the oceanic circulation is accompanied by the radiation of Rossby waves from the eastern coast of the basin. We have shown above that the Black Sea circulation manifests significant seasonal variability. Therefore we could await propagation of Rossby waves of annual period in the basin. Satellite altimetry gives direct evidence of the Rossby wave radiation as a part of annual cycle on the diagram of phase propagation of the sea level anomaly averaged between 42.95°N and 43.86°N. The western propagation, which is seen in the altimeter data results from the superposition of the sea level reaction to the direct wind forcing and Rossby waves radiated from the eastern coast. The superposition of two factors is consistent with the phase velocity of the observed signal. The western propagation of the signal is observed only in the North part of the Black Sea. Strong eastern jet compensates the western propagation of Rossby waves in the southern part of the basin.
Dynamics of mesoscale anticyclonic eddies.
Quasi-permanent anticyclonic features of the Black Sea circulation were pointed out a decade ago based on the hydrographic and space imagery data. Recent altimetry measurements make it possible to trace the continuous evolution of coastal anticyclones providing an independent validation of their schematic presentation and introducing a few additional new features. The revised scheme including all major quasi-persistent and recurrent features of the circulation pointed out earlier suggests persistency of the circulation system even at decadal time scales. Coastal anticyclonic eddies according to altimetry data are quasi-permanent but manifest significant inter-annual intermittency, which could not be defined in a deterministic way. However available long-term data set permits the evaluation of the coastal anticyclonic eddies statistics within the year. It is remarkable that in spite of the well-pronounced inter-annual intermittency of the life cycle of each eddy, the long-term statistics show seasonal evolution. Continuous mapping of the sea surface topography also allows the description of mechanisms of some anticyclone eddies formation and showing that the inherent property of the Black Sea dynamics is the anti-clock-wise transport of anticyclonic vorticity.
Transfrontal transport by mesoscale features.
Intense meandering of the Rim Current induces significant transfrontal exchange of coastal and open-sea waters. This process makes a significant input to the hydrological regime of the Black Sea upper layer and for the open sea blooming cycle. Altimeter data permit the identification of regions of the intense transfrontal exchange of coastal and open-sea waters and the specification of the most typical cases.

2.4. Deep circulation in the Black Sea basin.
The profiling float drift on the depth 1550m beginning from September 2002 provides a unique opportunity to understand the nature of the Black Sea deep-water circulation. Now there are two findings following from these observations. It was discovered that the deep flow in the Black Sea is very intense achieving a velocity up to 3-4 cm/s. Moreover the float being on the depth is transported along the isobaths. The topography control of the deep flow explains its intensity as the bowl-shaped bottom topography of the Black Sea is conditioned the intensification of deep currents.

3. INTERDISCIPLINARY SIGNIFICANCE OF THE CIRCULATION
Knowledge of the major principles governing the basin-scale circulation of the Black Sea allow for the consideration of interdisciplinary applications, which could have practical significance.

3.1. Predictability of the Black Sea circulation.
Shipping, estimation of the marine pollution transport or careful surface wave prediction all need accurate forecasting of currents. Three-dimensional numerical simulations with assimilation of altimeter measurements permit us to study the problem of predictability of the Black Sea circulation. The temperature, salinity and velocity fields from assimilation are taken as initial conditions for the prognostic run. Then prognostic simulations are compared with the same one obtained after assimilation of altimetry. The simulations show that the correlation of temperature, salinity and velocity fields obtained at the prognostic and assimilating modes is about 0.8 after 15 days of integration. Thus, our simulations indicate that the model is capable to forecast the sea state with the reasonable accuracy up to 15 days, if the wind stress field is known.

3.2. Regional climate.
Inter-annual variability of the Black Sea circulation produces some hints as to the importance of the local air-sea interaction. It is shown above that the annual cycle of the Black Sea circulation is induced by seasonal variation of the wind stress curl. The question arises what is the reason for winter intensification of the wind stress curl? Observations of SST and air temperature at the Black Sea show that the winter warming of the atmosphere by the sea surface creates a localized source of heat, which should induce cyclonic circulation in the regional atmosphere. In fact, the highest cyclonic vorticity of the wind above the sea is observed in January, when SST is higher than the air temperature. It decreases in May - August, when the air becomes warmer than the water. Air temperature maps above the sea and the land show positive air temperature contrast beginning from the end of August. However it is not clear how the local warming of the atmosphere could be efficient in interacting with larger scale processes. EOF decomposition of the climatic surface wind field over the Black Sea basin partly clarifies the question. It shows that the vorticity of the surface wind is described mainly by the third mode corresponding to the cyclonic-anticyclonic motion above the Black Sea while the first and the second mode describing the major part of energy produces almost uniform wind. Thus the seasonal cycle of the Black Sea circulation probably reflects the regional coupling of the sea and atmosphere, which could provide an important input to interannual changes of the local climate.

3.3. Ecosystem significance of the Black Sea circulation.
Analysis of the sea colour imagery obtained from SeaWiFS and other satellites shows a few examples of the blooming events modulation by the Black Sea circulation on the basin and regional scales. The major mechanism of the nutrient supplies to the open part of the sea is connected with propagation of the jet along the western coast of the basin. The jet transports nutrients until the mid of Anatolia and then turns to the open-sea waters providing the blooming event. Therefore the phase of the blooming depends on the dynamics of the coastal jet. However intense anticyclonic eddy sometimes occupies the whole NWS preventing the nutrients transport to the south. This abnormal circulation at the NWS, it seems, is responsible for the inter-annual variability of the basin-scale state of the ecosystem. The comparison of the altimeter-derived circulation with sea colour imagery definitely shows that mesoscale processes determine the regional variability of the ecosystem. Coastal blooming events are sometimes also induced by the synoptical upwellings appearing due to the special wind structure.

4. SUMMARY
New technologies transferred to the Black Sea basin have provided a significant increase in our understanding of the physical processes responsible for the formation and variability of the Black sea circulation. The achieved understanding creates a solid basis for the conversion of the knowledge to technologies and building of the Black Sea observing system according to the principles of GOOS.