Eddies, anticyclonic

Another type of ordered structures related to the upwelling in the northwestern part of the sea is represented by cyclonic eddies with a diameter of 10-20 km that leave the coast off Cape Khersones and propagate across the depth contours beyond the shelf zone [22]. An additional contribution to the intra-shelf water exchange in this part of the sea is provided by the eddies (anticyclonic and cyclonic) with diameters about 20-50 km that are formed at the front of the freshened waters related to the Danube River runoff [16,21]. 

In the near-shore zone, selected drifters were repeatedly captured by NSAE and performed anticyclonic rotation with velocities up to 0.60-0.80 m s-1 (in the Batumi NSAE) nevertheless, no one of these eddies was noted in the averaged schematic. 

In [50], the mean annual wind field compiled according to the data of the Russian Climatic Reference Book was used. The mean wind speeds became two to threefold higher. The maximums of the velocity and cyclonic vorticity of the wind were confined to the eastern part of the Black Sea. The almost twofold decrease in the horizontal grid step (11 km) as compared to [48] allowed one to reproduce in [50] a system of subbasin cyclonic and anticyclonic eddies quasiperiodic over the longitude it clearly dominated over the large-scale BSGC. The latter is represented in [50] only in the weaker mean annual current fields. 

The eddies were formed off the eastern coast of the Black Sea and moved westward showing a decrease in the phase velocity in the narrowest area of the sea south of the Crimea. In the model version with a flat abyssal floor at a depth of 1540 m, the wavelength comprised 250 km in the east and 190 km in the west with a period of 160 days and a phase velocity of - 2.0-2.5kmday-1. The orbital velocities in the eddies in the surface layer reached 0.45 ms 1 and deeper decreased down to 0.25 ms-1 a depth of 70m and to 0.05 - 0.10 m s 1 at a depth of 1100 m. The wave regime was more intensive in the eastern part of the Black Sea in its western part, eddies dissipated above the continental slope and partially reflected from the western coast. In the study [50], the introduction of the abyssal bottom topography increased (reduced) the sizes and intensities of cyclonic (anticyclonic) eddies by a factor of 1.5-2. The cyclonic (anticyclonic) eddies became more alike the SBCGs (NSAEs) in Fig. 1. The period of the eddies grew up to almost two years, while their phase velocity decreased down to 0.4-0.5 km day-1. 

Below 500 m, the BSGC is poorly studied it significantly differs from the surface pattern by low mean velocities (not higher than 0.01-0.03 ms-1) and a prevalence of mesoscale eddies with anticyclonic vorticity instantaneous velocities here may reach 0.30-0.40ms-1 only owing to short-period (inertial, etc.) motions. 

Keywords Black Sea Coastal anticyclonic eddies Coastal upwelling ... 

The mesoscale variability in the Black Sea is mostly related to the meandering of the RC, to the formation of anticyclonic eddies between the RC and the coast, to their transformation into the deep-sea eddies, and to the interaction of the latter with the neighboring anticyclonic and cyclonic circulation elements and with the RC. The meandering jet of the RC together with the... 

Fig. 3 Scheme of the areas of most frequent observations of anticyclonic eddies in the Black Sea (circles) and trajectories of their motion (lines with arrows). The numerals show local values of nondimensional width of the continental slope according to [32]. Isobates 100 m and 1500 m are shown... 

A comparative analysis of the hydrodynamical situations that occurred in different years has made it evident that anticydonic eddies may represent a typical element of the circulation in the eastern deep basin at least during the warm season (April-December). This fact contradicts the traditionally accepted concept (see [1-3]). The appearance and existence of these kinds of anticydonic eddies in the deep basin is related to the separation of coastal anticyclones from the coast. Some events of this kind together with the subsequent movement of the deep-sea anticyclones were registered with the use of satellite information of high spatio-temporal resolution and derived from the hydrographic surveys of different years. 

A large long-living anticydonic eddy centered at 43°N and 34°E, in the area between the western and eastern cyclonic gyres (approximately abeam the southern extreme of the Crimea), was detected by the hydrographic survey of 1984 [6]. It was formed in September 1984 as a result of coalescence of two other anticyclones formed owing to baroclinic instability of the RC and to detachment of its meanders in the north (from the Crimean coast) and in the south (from the Turkish coast near Sinop). Its diameter exceeded 100 km, the maximum of the orbital geostrophic velocity was 25-45 cm/s, and the rate of the westward displacement was about 1 cm/s. Density and salinity anomalies related to this eddy were traced down to a depth of 1000 m and temperature anomalies were followed down to 300 m. 

One more deep-sea eddy transformed from a coastal anticyclone was observed over a month in the fall of 1997 [23]. This eddy was formed on September 6-8, 1997 west of Novorossiisk in the zone of the shelf/slope widening. Later on, it gradually moved southwestward away from the coast (Fig. 5) at a mean rate of 4.3 cm/s. During this time, its diameter increased from 40 to 75 km due to the entrainment of the adjacent waters. Then, most probably, it merged with an anticyclone off the southeastern coast of the Crimea. 

An analysis of the conditions that accompany the formation of the above-considered deep-sea anticyclonic eddies in the eastern part of the sea observed in 1984,1993,1997,1998, and 1999 allows us to suggest that the factors favoring their generation are the wind forcing and the particular features of the coastline/bottom topography. Correspondingly, the seasonal and interannual differences in intensity of the formation of these kinds of mesoscale anticyclones in the Black Sea are related to the variability in the RC intensity, which, in its turn, is controlled by the intensity of the wind forcing. 

The movement of anticyclonic eddies over the sea area including their separation from the coast and subsequent evolution in the situations considered is shown in Fig. 3. Frequent events of separation of anticyclones from the coast take place in the eastern part of the sea in the region of Novorossiisk-... 

The anticyclonic eddies over the Danube Fan supply waters rich in nutrients and chlorophyll to the deep sea and also influence the biological productivity in the western part of the Anatolian coastal zone. When the entrainment of the shelf waters by anticyclones is intensive, the southward alongshore flow is weak and the nutrient supply to the south is restricted, while at the absence of eddies it increases [25]. 

The contribution of the anticyclones separated from the coast to the water exchange between the coastal zone and the deep-water basin is caused by the entrainment of the coastal waters and their transfer to the open sea as well as by the formation of associated eddies and jets at the peripheries of the anticyclones. For example, waters with relatively low salinity (18.15 psu, at a salinity of the surrounding waters of 18.25 psu) of an evidently coastal origin were observed at the center of an anticyclonic eddy in November 1993, three months after its separation from the coast [23]. The reduced salinity of the waters of an evidently coastal origin observed in separating anticyclonic meanders and eddies was also noted in [5,11]. The advection of the surrounding waters at the periphery of the eddies at distances of about 150 km also represents an important mechanism for the water exchange between the shelf and the open sea. 

Short-living (from a few days to about two weeks) cyclones and jets often accompany the evolution of the anticyclonic eddies separating from the coast. For example, a jet 10 km wide terminated with a vortical pair, which moved southwestward from the northwestern periphery of an anticyclone separated... 

The influence of the anticyclones separating from the coast on the structure of the RC was also well traced in October-November 1993, when the offshore location of a deep-sea anticyclone centered approximately at 44°N, 36.8°E resulted in a departure of the RC from the coast and formation of a large meander [8,23], and in September 1997 (Fig. 5). Due to the movement of the anticyclonic eddies separated from the coast on the one hand and to the position of the RC changing with respect to the current location of the eddies and the wind direction on the other hand, situations are possible when the same deep-sea anticyclone finds itself now on the right-hand side of the RC mainstream or its branches and now on their left-hand side. In addition, the movement of the anticyclones from the eastern basin to the western... 

In addition to deep-sea anticyclones and large cyclonic eddies interacting with them (such as vortical pair Al-Cl in Fig. 4A), an important contribution to the water exchange between the coastal zone and the open sea is made by small cyclones with a diameter of 30-40 km and jets. Two cyclones of this type probably formed as a result of a velocity shear at the seaward boundary of the RC may be seen in Fig. 5. Moving to the northwest at a rate of approximately 10 cm/s and interacting with the RC and the anticyclone separating from the coast, they provided the transport of warm coastal waters over a distance more than 100 km away from the coast [23]. Small cyclonic eddies are also formed in the cyclonic meanders of the RC [8]. 

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