Deep-sea anticyclones

Deep-sea anticyclones Jets Rim Current Wind forcing... 

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. 

The above considerations allow us to conclude that mesoscale areas of anticyclonic vorticity in the open eastern part of the Black Sea are not so rare and the lifetime of individual deep-sea anticyclones may reach approximately eight months. The reason for their long-term existence seems to he in their replenishment owing to the merging of adjacent anticyclones or to the energy transfer from the RC implemented by coherent vortical features (for example, to anticyclone A1 via the neighboring cyclone Cl interacting with the RC, Fig. 4A). 

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 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]. 

An important result of the comparative analysis of the water circulation in different years and the relevant information on the wind field consists in the establishment of the possibility of appearance and, sometimes, long-term (up to 8 months) existence of anticyclonic eddies in the open sea (beyond the continental slope) and in revealing the factors that favor this phenomenon. Deep-sea anticyclones are characteristic only of the eastern basin (Fig. 3), where their appearance results from the separation of the anticyclones formed due to the RC instability at the sites with a narrow continental slope (Caucasian and Anatolian coasts and the southeastern coast of the Crimea) from the coast. The wide and gentle northwestern slope imposes a stabilizing effect on the RC. The anticyclonic eddies that found themselves over this slope owing to their formation off the southwestern coast of the Crimea or transfer from the eastern basin propagate to the southwest never entering the deep-water part of the sea. 

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... 

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. 

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]. 

Ginzburg AI, Zatsepin AG, Kostianoy AG, Krivosheya VG, SkirtaAY, Soloviev DM, Stanichny SV, Yakubenko VG (2002) Separation of near-shore anticyclonic eddies from the Caucasian shore and their transformation into deep-sea eddies. In Zatsepin AG, Flint MV (eds) Multidisciplinary Investigations of the Northeast Part of the Black Sea. Nauka, Moscow, p 82 (in Russian)... 

The vertical structure of the zonal velocity components in the meridional sections across the eastern and western SBCGs is shown in Fig. 10. In March, the current structure is simpler than in September. In the eastern part of the sea, surface currents penetrate deeper than in the west. Locally, one can observe weak deep countercurrents beneath the surface streams however, there are no reasons to suggest a full change in the BSGC in the deep and, the more so, in the intermediate layers. Note that the climatic anticyclonic vorticity of the currents at the center of the Eastern Black Sea in September (Fig. lOd) is... 

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. 

Local synoptic wind impacts also favor the intensification of the horizontal water exchange in the Black Sea. For example, they cause wind effected phenomena and formation of filaments of coastal upwellings and provide the separation of coastal anticyclones from the eastern coast (under northerly winds) or removal of the shelf waters to the deep-water basin by anticyclones over the northwestern continental slope (under westerly winds). 

The surface circulation of the western basin apparently remains anticyclonic, and the deep circulation cyclonic, under the predominant winds. As far as the relatively fast surface currents are concerned, the transversal spatial scale of today s sea is too small for the Coriollis force to be significant, so that the direct wind drag matters rather than the Ekman transport. On the other hand, the bottom layer circulation seems to immediately follow the sea surface slopes in the classic barotropic manner, so the Coriollis force is still effective for the slower, near-bottom currents. 

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