Eddy Statistics for the Black Sea by Visible and Infrared Remote Sensing

Abstract

Examination of vortical circulation features in the Black Sea surface waters is presented based on satellite optical and infrared images. The sequence of images used comprises the period from September 2004 to December 2008. The images were obtained by the Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectrometer (MODIS) sensors. Our analysis allowed us to gain new insights into Black Sea mesoscale circulation features. In particular, information on the spatial distributions of structures such as the Rim Current meanders, mushroom-like currents, near-shore anticyclonic eddies, and chains of shear eddies was obtained. As a result, a better understanding of Black Sea dynamics has been achieved.

Examination of vortical circulation features in the Black Sea surface waters is presented based on satellite optical and infrared images. The sequence of images used comprises the period from September 2004 to December 2008. The images were obtained by the Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectrometer (MODIS) sensors. Our analysis allowed us to gain new insights into Black Sea mesoscale circulation features. In particular, information on the spatial distributions of structures such as the Rim Current meanders, mushroom-like currents, near-shore anticyclonic eddies, and chains of shear eddies was obtained. As a result, a better understanding of Black Sea dynamics has been achieved.

1 Introduction

The Black Sea ranks among the most interesting water bodies of the global oceans. It has an extremely dynamical, mesoscale-dominated circulation and a highly eutrophic ecosystem. Another essential reason for studying the basin is its poor ecological conditions that are the result of its limited water exchange with the adjacent basins, weak vertical mixing, and a significant contamination from river discharges, waste from city and tourist resorts, oil and other discharges from shipping and oil terminals. Because most of the contamination comes from the shore and near-shore regions of the sea, the processes of horizontal mixing and cross-shelf water exchange are of great importance.

The commonly assumed scheme of the Black Sea general circulation based on decades of hydrographic surveys includes (i) a basin scale boundary current cyclonically flowing along the continental slope (the Rim Current), (ii) the Bosporus, Sakarya, Sinop, Kizilirmak, Batumi, Sukhumi, Caucasus, Kerch, Crimea, Sevastopol, Danube, Constantsa, and Kaliakra anticyclonic eddies in the coastal zone of the sea (Oguz et al., 2005), (iii) near-shore anticyclonic eddies (NAEs) between the Rim Current and the shore (Oguz et al., 1993). Traditionally, this scheme also includes a few semi-permanent cyclonic gyres in the deep part of the sea, but there are some disagreements on the number (2, 3 or more) of these sub-gyres. Moreover, those gyres were not confirmed during the drifter experiment carried out in 1999–2003 (Enriquez et al., 2005; Poulain et al., 2005).

Mesoscale circulation in the Black Sea is represented by meanders, anticyclonic and cyclonic vortices, pinched off eddies, vortex dipoles, filaments and jets. Numerous observations demonstrate that the mesoscale variability is very important in the transport of scalars especially in the coastal-deep basin water exchange across the Rim Current. Satellite imagery has made possible the detection of the various nonstationary mesoscale dynamical features that contribute to the exchange. Near-shore anticyclonic eddies are a characteristic circulation feature in this sea. Remote sensing contributed significantly to the studies of this form of mesoscale variability. Seasonal variability was clarified, and the role of eddies in the distribution of chlorophyll a concentration was described.

The study of small-scale circulation features in the near-coastal zone of the Black Sea was made possible recently with synthetic aperture radar (SAR) data (Lavrova and Bocharova, 2006; Lavrova et al., 2008).

As one can see the satellite observations provide a good resource for increasing our present level of knowledge on the mesoscale circulation in the Black Sea. However the satellite datasets were used in some rather limited way, e.g., for complementing some hydrographic observations (Zatsepin et al., 2003) or for analyzing short-term data sequences (Sur and Ilyin, 1997; Afanasyev et al., 2002). The present study attempts to characterize the Black Sea basin and mesoscale circulation from the Advanced Very High Resolution Radiometer (AVHRR) and Moderate Resolution Imaging Spectrometer (MODIS) images. As a result, a better understanding of the Black Sea dynamics has been achieved and a renewed comprehensive scheme of the general mesoscale circulation has been drawn up.

The remainder of this paper is structured as follows. After a short description of the dataset and methodology used we discuss the different types of vortical structures detected in the satellite-derived images (the Rim Current, its meanders and quasi-permanent eddies; NAEs; mushroom-like currents; eddy chains). Finally, conclusions are presented in Sect. 7.

2 Data and Methods

The work described is based mainly on the processing and analysis of satellite data (IR and sea color) with a spatial resolution of the order of 1 km and a temporal resolution of a few hours (up to six passes per day). The dataset includes the following satellite images entirely covering the Black Sea:

• AVHRR NOAA Sea Surface Temperature (SST) images obtained since September 2004–December 2008; total number of images is about 3,000;

• AVHRR MetOp-2 SST images obtained during January–December 2008; total number is about 100 images;

• MODIS Aqua SST, normalized water-leaving radiance (WLR) at 551 nm, and chlorophyll a concentration images obtained from April 2006 to December 2007; total number is about 250 images.

The data are provided by the Remote Sensing Department of the Marine Hydrophysical Institute (Sevastopol, Ukraine) (http://dvs.net.ua/).

Tracking ocean currents from space is possible in two different ways. The first – direct method – can be applied if there are some tracers in the water such as suspended organic or inorganic matter, ice floes, etc. Unlike infrared and visible data, SAR-data can reflect seawater circulation through surfactant – natural or anthropogenic – slicks. Because of their high surface resolution these data can represent small-scale eddies with diameter of a few kilometers or fragments of the mesoscale eddies.

Another way – indirect – consists in the estimation of the current velocity using satellite altimeter data. This method is beyond the scope of this paper (see e.g., Korotaev et al., 2003).

The most adequate products to track mesoscale circulation features are infrared thermal images derived from the AVHRR sensor, which is onboard NOAA polar orbiting satellites. Spatial and spectral resolutions of this sensor provide SST fields that allow monitoring vortical structures as small as 20 km in diameter.

Other product used is WLR at 551 nm derived from the MODIS sensor onboard the Aqua satellite. As WLR at 551 nm is affected by dissolved and particulate matter present within the water column, the contrasts of WLR fields are subjected to large spatial and seasonal variations. As a result the maximum value of WLR is usually observed in June when coccolithophore bloom takes place.

In Fig. 4.1 MODIS WLR at 551 nm and chlorophyll a concentration charts obtained during coccolithophore bloom (Fig. 4.1a and b) and in normal conditions (Fig. 4.1c and d). On June 20, 2006, during the maximum extent of coccolithophore bloom, the prevailing magnitude of WLR was within 1.5–2.5 mW·cm⁻²·μm⁻¹·sr⁻¹ (Fig. 4.1a). The typical for the Black Sea basin WLR magnitude can be accessed from Fig. 4.1c presenting the distribution of WLR on July 20, 2006, when the bloom was over. As we can see from Fig. 4.1c, maximum of WLR was hardly exceeding 1.2 mW·cm⁻²·μm⁻¹·sr⁻¹.

Fig. 4.1

Manifestation of the surface circulation features in MODIS WLR and chlorophyll a concentration images during a coccolithophore bloom (a and b respectively) and in the normal conditions (c and d). Two upper images were obtained on June 20, 2006 at 11:10 GMT, the lower ones – on July 20, 2006 at 11:25 GMT. Letters mark the most prominent vortical structures: A – an anticyclonic eddy, B – the Sevastopol quasi-permanent eddy, C – a mushroom-like current, D – the Batumi quasi-permanent eddy, E – the Sinop anticyclonic eddy

The third type of products used is chlorophyll a concentration. In general, these images are applicable for the purposes of circulation study only in the near-coastal zone where the pigment concentration is especially high. Two examples are given in Fig. 4.1 providing the possibility to compare the way in which circulation features were manifested in simultaneously obtained WLR and chlorophyll a concentration fields. Nevertheless, sometimes chlorophyll a concentration fields can provide very important information on the NAEs that cannot be retrieved by any other means (see some details and examples in Sect. 4).

The main obstacle associated with using optical imagery is the limited availability of clear sky scenes which does not allow the continuous monitoring of circulation features. In Fig. 4.1 and in all other MODIS images presented hereafter, areas covered by clouds are designated by white color used also as a land mask. In AVHRR SST images cloud cover is manifested in a different way: dense cover is white while in the edges it can be represented by the lowest temperature colors; we use purple and dark blue colors. Sometimes it is really difficult to define whether purple color means water temperature or just the edge of the clouds and this can be confusing while trying to retrieve precise SST data from the images. However, water circulation patterns are visible only in the areas unhindered by clouds, so the purple colors should not be confusing this aspect of the analysis.

Extraction of the vortical structures discussed below was performed manually. In thermal images the position of maximum temperature gradient with spiral or circular shape was used as an eddy border; in ocean color images such a border was coincident with flow, which had orbital velocity maximum characterized also by radiance maximum. Automated or semi-automated methods are hardly possible to use, because at times eddy identification is complicated even for a trained person familiar with the Black Sea hydrodynamics background.

3 Observations of the Rim Current and its Meanders. Quasi-Permanent Eddies

The most striking point about the Rim Current is that this basin-scale circulation feature confirmed by numerous hydrological observations is rarely evident in satellite images. It has been shown (see e.g., Gill, 1982) that the eddy kinetic energy would be much larger than that of the original gyre. So, this could be one of the reasons why eddies generated by the Rim Current are much easier to observe than the Rim Current itself.

Another reason concerns the spatial scale of the Black Sea basin. Due to comparatively small basin extent in latitude there are relatively homogeneous waters, so the Rim Current waters show very weak contrast and can hardly be detected (unlike e.g., the Gulf Stream).

Nevertheless, sometimes in the cold season there is enough thermal contrast between the Rim Current and adjacent waters, so satellite data can provide some opportunities to track the Rim Current especially in the western part of the sea (e.g., see Fig. 4.2a).

Fig. 4.2

Examples of the surface water circulation patterns typical for cold (a) and warm (b) seasons. (a) image captured by AVHRR NOAA-18 on February 25, 2008 at 11:08 GMT; (b) image captured by AVHRR NOAA-18 on September 7, 2006 at 23:33 GMT. Letters mark some mesoscale structures filling the zone attributed to the Rim Current: A – a mushroom-like structure, B – the Rim Current stream, C – the Rim Current meander, D and E – anticyclonic eddies, F – the Batumi quasi-permanent eddy

Due to the mentioned reasons, well-expressed manifestations of the Rim Current meanders in satellite images are rare. The region with the most frequent formation of an anticyclonic meander lie to the south-west of the Sevastopol zone of high hydrodynamic instability. One such meander is presented in Fig. 4.2b, marked with the letter C. In the generalized scheme (Fig. 4.4d) this eddy is mentioned as Western eddy (meander).

Both model data and observations manifest Black Sea circulation seasonal cycles (Korotaev et al., 2003; Poulain et al., 2005). Strong winds during the winter season cause an increase in the basin-scale Ekman transport. As a result, the circulation of the Rim Current is more clearly defined and intense during winter than in summer. The eddy activity is more pronounced at different scales in the summer season (Sur and Ilyin, 1997; Zatsepin et al., 2003; Shcherbak et al., 2008).

Satellite observations confirm this idea. The typical pattern of the Black Sea surface water circulation in cold season is well represented by this AVHRR NOAA-18 image acquired on February 25, 2008 at 11:08 GMT (Fig. 4.2a). In the image, the Rim Current is manifested as the warmest waters and shown by bright green color. One can see that it goes just along the coastline and very close to it. As has been mentioned above in Sect. 2, white, purple and dark blue fragments in the eastern part of the basin mean cloud cover.

In summer, closely packed vortical structures fill this region. Some of them are associated with the meandering Rim Current, while others are produced by atmospheric forcing and buoyancy fluxes (Sur and Ilyin, 1997). Some examples of such mesoscale features and the Rim Current trajectories are presented in Fig. 4.2b. This SST image was obtained by NOAA-18 on September 7, 2006 at 23:33 GMT. In the western part of the image one can observe a large mushroom-like structure (A), adjacent to the Rim Current stream (B) that forms an anticyclonic meander (C) at the Bulgarian coast. In the eastern part there are three anticyclonic eddies of different sizes: the smaller one at the Crimean coast (D), the bigger to the south from the Kerch Peninsula (E), and the Batumi eddy (F) in the south-easternmost part of the basin.

Another example of the warm season typical circulation is given in Fig. 4.1a and b. The most prominent circulation features are: an anticyclonic eddy in the vicinity of the Bosporus Strait (A), the Sevastopol quasi-permanent eddy (B), a mushroom-like structure (C, arrows show the centers of the antycyclonic and cyclonic parts), the Batumi quasi-permanent eddy (D), and the Sinop anticyclonic eddy (E).

Some interesting results were obtained on so called Black Sea quasi-permanent anticyclonic eddies that originated from the combined effect of hydrodynamic instability of the Rim Current and basin configuration. The Sevastopol, Batumi and Caucasus eddies are traditionally regarded as the greatest and most permanent of the quasi-permanent eddies.

Satellite observations have shown that the Sevastopol eddy is not just one distinct anticyclonic vortex as it has traditionally been thought to be (Sur and Ilyin, 1997; Afanasyev et al., 2002; Korotaev et al., 2003; Zatsepin et al., 2003; Oguz et al., 2005; Poulain et al., 2005; Stanev, 2005). Though sometimes this structure looks like a single eddy with 100–120 km diameter, more often it represents a whole system of closely packed eddies, mushroom currents and eddy dipoles. So, in Fig. 4.3 some examples of the Sevastopol eddy modifications are presented: (a) a “classical” one-core eddy – A; (b) two anticyclonic eddies designated by B and C; (c) an eddy (D) and a mushroom-like structure (E); (d) a chain of three anticyclonic eddies (F) and an associated mushroom-like structure (G); (e) two (three?) associated mushroom-like structures (H); (f) an anticyclonic eddy (I) with a number of associated smaller cyclonic ones (J).

Fig. 4.3

Different vortical structures generated at the place of the Sevastopol quasi-permanent eddy detected in AVHRR images obtained (a) May 29, 2008 at 18:46 GMT; (b) April 17, 2005 at 04:42 GMT; (c) June 2, 2006 at 03:48 GMT; (d) November 4, 2004 at 15:46 GMT; (e) June 3, 2008 at 03:01 GMT. A – a single anticyclonic eddy, B and C – a chain of two anticyclonic eddies, D – an anticyclonic eddy with an attached mushroom-like current (E), F – a chain of three anticyclonic eddies, in which one of the eddies is a part of a mushroom-like current (G), H – three closely packed mushroom-like structures, I – an anticyclonic eddy with a series of attached cyclonic eddies (I)

Fig. 4.4

Schemes of different vortical structures detected in AVHRR and MODIS images obtained from September 2004 to December 2008: (a) near-shore anticyclonic eddies; (b) mushroom-like currents; (c) chains of shear eddies. In these images the darker the color the more frequently vortical structures were observed. (d) – generalized scheme of quasi-permanent and most frequent non-stationary vortical structures: A – the Anatolian anticyclonic eddies, B – the Batumi quasi-permanent eddy, C – the Caucasus quasi-permanent eddy, CA – the regions of cyclonic vorticity along the Anatolian coast, K – the quasi-permanent shear eddy chain to the south from the Kerch Peninsula, N – near-shore anticyclonic eddies, S – the Sevastopol zone of high hydrodynamic instability, W – the Western quasi-permanent eddy (meander)

The Batumi quasi-permanent eddy lifetime is much greater than that of the Sevastopol eddy. Unlike the Sevastopol quasi-permanent eddy, the Batumi one more often has the form of a well-shaped anticyclonic structure (see Figs. 4.1a, D and 4.2b, F). Nevertheless, on the periphery of the Batumi eddy there are also multiple attached eddies and mushroom-like currents. In Fig. 4.4b and partly in Fig. 4.4c one can see how the vortical structures practically visualize the Batumi eddy boundary.

Some authors, on the basis of the numerical modeling and satellite altimeter observations, traditionally include in the number of the quasi-permanent anticyclonic eddies some other coastal eddies (e.g., Caucasus, Crimea, Bosphorus, Sakarya, Sinop, Kizilirmak, Kaliakra, Danube and Constantsa) (Korotaev et al., 2003; Oguz et al., 2005; Stanev, 2005). According to the observations presented some of them can hardly be referred to as quasi-permanent ones.

The anticyclonic structures observed in the Caucasus near-coastal zone differ greatly in size and location and generate irregularly. So, it is hardly possible to regard any of these eddies as quasi-permanent.

The Crimea quasi-permanent eddy is considered to be generated along the southern coast of the Crimean Peninsula (see e.g., Korotaev et al., 2003). Satellite observations revealed that anticyclonic eddies really generated in this region – both as elliptical NAEs and as spiral-like eddies e.g., that marked with D in Fig. 4.2b (see schemes presented in Fig. 4.4). However, most frequently this zone was occupied by the anticyclonic shear eddy chains due to baroclinic instability (Fig. 4.4c). For more details on eddy chains please refer to Sect. 6.

As for the eddies located along the southern coast (Bosporus, Sakarya, Sinop, Kizilirmak), it is also not quite true to recognize these four eddies as quasi-permanent because usually the Black Sea southern coast has a great number of the well-developed anticyclonic eddies. More detailed information on the eddy chains detected along the Anatolian coast is provided in Sect. 6.

The Danube, Constantsa, and Kaliakra anticyclonic eddies were not detected in the observations presented in this chapter.

4 Near-Shore Anticyclonic Eddies

Near-shore anticyclonic eddies (NAEs) form within a zone of coastal anticyclonic current vorticity (anticyclonic convergency zone) between the coast and the midstream of the Rim Current. They stretch along the coast due to lateral friction, so their most prominent feature is an elongated shape. Because of NAEs formation, the zone of coastal anticyclonic current vorticity has a bimodal current regime (i.e. back-and-forth motions along the shore when passing the NAE) (Titov, 2002).

NAEs growth gives rise to large meanders that could either detach and propagate in the open sea or stagnate for some time in coastal areas. Their separation from the coast and transformation into open sea eddies could provide horizontal mixing of the upper layer waters and result in deflection of the Rim Current offshore, formation of large meanders of the current around the eddies, and its branching when rounding such features (Zatsepin et al., 2003).

It is traditionally considered that NAEs are more pronounced and stable at the Caucasian and Anatolian coasts but they can form along the entire Black Sea coast, however, most frequently along the Caucasian one (Ovchinnikov et al., 1986).

The present study shows that NAEs arise only in the regions where (i) the width of the shelf is minimal and (ii) the Rim Current goes closely to the shoreline, i.e. along Caucasian and Bulgarian coasts. Anticyclonic eddies quasi-permanently generated along the Anatolian coast are of special origin, so they are examined in Sect. 6.

In Fig. 4.4a a generalized scheme of NAEs detected within the framework of the present study is given. In this scheme all the NAEs observed are marked with a grey ellipse and after that the ellipses are overlapped. As a result areas with frequent NAE formation look in the scheme as black patches. As we can conclude from the scheme, most frequently NAEs generate along the Caucasian coast and south-eastern coast of the Crimean Peninsula though sometimes they can arise along the Bulgarian coast.

Morphometric parameters of the NAEs detected are as follows: The longer axis of NAEs varies approximately between 30 and 150 km with an average of about 60 km; for the smaller axis, the Figures are 20, 75 and 50 km, respectively. Another discovered characteristic feature of NAEs is that along with a single NAE or chained medium size NAEs, we can often observe them combining in pairs as shown at Fig. 4.5. Letters A and B mark the position of the pair-forming NAEs centers. Usually such pairs include NAEs of about 90–120 km in length.

Fig. 4.5

Some examples of double NAEs detected in satellite images obtained (a) by AVHRR NOAA-18 on April 16, 2006 at 10:44 GMT; (b) by AVHRR NOAA-15 on April 10, 2008 at 14:11 GMT; (c) by MODIS Aqua (normalized water-leaving radiance, 550 nm) on November 24, 2006 at 10:40 GMT; (d) by AVHRR NOAA-15 on August 4, 2006 at 15:11 GMT. Letters A and B mark the center of the pair-forming eddies

5 Mushroom-Like Currents

Vortex pairs (dipoles) or mushroom-like currents (MLCs) are regularly observed on thermal and colour imagery of the coastal ocean. Many dipole observations have been reported by Fedorov and Ginzburg (1986). It was determined that the formation of a dipole at the sea surface is due to a uniform reaction of the upper layer to any spatially localized forcing from the atmosphere (wind-forcing) or due to the oceanic dynamics itself (e.g., instability of currents, pulsing exchange through straits). Dipoles represent the important effective mechanism of horizontal mixing and transport of heat and mass.

The horizontal dimensions of MLCs detected differ between 50 km and 250 km with a typical size of about 90 km. In most cases, the width of the “mushroom cap” exceeds the length of the “mushroom stem” by about 20–50%.

Coherent dipole structures are often observed along unstable boundary currents, such as the Black Sea Rim Current system, typically excited by density or wind impulses. The present examination of MLCs in the Black Sea basin showed the peculiar spatial distribution of these vortex structures. In Fig. 4.4b a scheme of the observed MLCs location is given. An MLC was approximated by two grey ellipses representing cyclonic and anticyclonic parts of MLC and a grey triangular marking the MLC’ jet position. When different MLCs overlap the colour in the scheme becomes darker.

As we can see from the scheme (Fig. 4.4b), most frequently MLCs can be detected in the Rim Current zone along the western coast of the sea with the maximal density in the region to the south-west of the Crimean Peninsula. Such pattern of MLCs spatial distribution confirms the fact that the region of the Sevastopol quasi-permanent eddy is a zone of high hydrodynamic instability as has been shown before.

Another region of frequent MLCs formation is located along the Caucasian coast. Such a peculiar spatial distribution suggests two different factors of the Black Sea MLC generation, namely: (i) the instability of the current that apparently dominates along the western seacoast and (ii) wind forcing that is great along the Caucasian coast especially affected by north-westerly winds, “bora” events.

6 Eddy Chains

In this study we define eddy chains in a wide sense; number of eddies in the chain can be as small as only two. Analysis of satellite images made it possible to subdivide eddy chains into different groups: (i) chains of shear eddies, (ii) eddy chain along the Anatolian coast, and (iii) small-scale eddy chains. The generalized scheme of the first group eddy chains is given in Fig. 4.4c. In this scheme all eddies in the detected chains were represented as circular grey-colour figures. As one can see from the scheme, practically all the chains are generated in the zone of the Rim Current.

Chains of shear eddies are considered as horizontal versions of the Kelvin-Helmholtz instability. These eddies have marked spiral forms and the centres of eddies are located along a straight line. Eddies forming a chain can be both cyclonic and anticyclonic. The chains of shear eddies can originate from different causes: when the currents separate from the shelf or shore, in the cases of a sharp change in the shoreline configuration, at fronts, in areas of local current shear, etc.

The region where chains of shear eddies were frequently detected is one to the south of the Kerch Strait. In this region ten eddy chains were detected in thermal images. The chains are situated above the continental slope between 44°N and 45°N and stretched from east to west. All eddies in these chains are anticyclonic; number of eddies in the chains varied from 2 to 4; diameter of eddies was within 30–100 km. The chains were generated in different seasons but most of them in April and May.

The second most frequent region of eddy chain formation is the near-coastal zone along the Caucasian coast. Within a stripe of near-coastal waters of approximately 100 km in width, nine chains of shear eddies were detected. All of them are cyclonic with an eddy diameter between 30 km and 90 km; number of eddies in the chains was 2 or 3. All the chains in this region were observed in the cold season (from September to March).

A characteristic feature of this type of eddy chains is their large total length compared to the diameter of eddies forming the chain. Figure 4.6a shows a typical chain of cyclonic shear eddies. This AVHRR NOAA-12 image was obtained on March 13, 2005 at 13:58 GMT. The chain is made up by 3 eddies with diameters not exceeding 60 km while the chain stretches out about 400 km.

Fig. 4.6

Chains of eddies different in origin: (a) chain of cyclonic shear eddies (A) detected in AVHRR NOAA-12 image obtained on March 13, 2005 at 13:58 GMT, (b) anticyclonic eddies of the Anatolian coast (B) and the adjacent cyclonic eddies (C). The AVHRR NOAA-18 image was obtained on December 4, 2007 at 10:26 GMT

Eddies of the Anatolian coast. The schemes shown in Fig. 4.4c and d reveal that all the southern near-coastal zone of the Black Sea is a region of quasi-permanent generation of eddies. The most numerous group among these eddies is made up of anticyclonic eddies with a diameter of 60–100 km. The generalized scheme of their location is presented in Fig. 4.4d where they are marked with the letter A. These eddies have spiral shape and form very closely to the coast; frequently well-formed spiral eddies are observed in the numerous semicircular bays along the eastern part of the coast as it is shown in Fig. 4.6b, letter B; arrows depict the eddy centres. This satellite image was obtained by AVHRR NOAA-18 on December 4, 2007 at 10:26 GMT. As anticyclonic eddies of the Anatolian coast originate under the current baroclinic instability and effects of the complicated shoreline configuration, they represent a special transitional type between NAEs and chains of shear eddies.

The sequence of AVHRR images obtained in February 2008 allowed us to follow the evolution of an anticyclonic eddy in the vicinity of the Bosporus Strait. Originally there was some cold water intrusion about 60 km length distinctively marked in the image obtained 20 February 20, 2008 at 08:11 GMT (Fig. 4.7a). During the following 24 h the linear intrusion was curling clock-wise until an enclosed ring was formed (Fig. 4.7b and f). During that period, the speed of the orbital motion was strikingly high and ranged between 1.2 km and 4.0 km per hour. At the same time the speed of movement downstream in the Rim Current was about 1 km per hour. On the third day of the observations it became clear that another anticyclonic eddy of the same size had formed downstream in the Rim Current and the deformation of the first eddy began (Fig. 4.7i). The last image obtained before clouds prevented further observations shows that the first eddy was almost destroyed and the second one continued its motion along the coast (Fig. 4.7p).

Fig. 4.7

Evolution of an anticyclonic eddy retraced by the sequence of AVHRR images obtained from February 20 to February 25, 2008

Another characteristic feature of this region is a cyclonic eddy chain generation that sometimes can be observed to the north from the row of the anticyclonic eddies. Chains of this type are schematically shown in Fig. 4.4c; in Fig. 4.4d they are marked with “CA”. In Fig. 4.6b such a chain of two cyclonic eddies marked with the letter C is shown. Despite all the differences in their shape and location these anticyclonic and cyclonic eddies are caused by the same mechanism – baroclinic instability. This mechanism of cyclonic vorticity transfer into the interior of the basin was successfully simulated in the laboratory experiments and described by Blokhina and Afanasyev (2003).

7 Conclusions

In this paper, we show the potential of MODIS and AVHRR imagery for studying circulation features with spatial scale exceeding 20 km. Our analysis allowed us to gain new results on the spatial distribution of the Black Sea mesoscale vortical structures. The most important of them are summarized below.

It was shown that the region to the south-west of the Crimean Peninsula is a quasi-permanent zone of high current instability that results in the formation of the Sevastopol anticyclonic eddy with numerous attached eddies along with mushroom-like currents, eddy dipoles, etc. The only other eddy that can be regarded as quasi-permanent is the Batumi one.

The region where NAEs are generated most frequently is along the Caucasian coast. Another possible place of NAE formation is the Bulgarian near-coastal waters (Fig. 4.4a). Near-coastal zone along the Anatolian coast gives rise to the quasi-permanent chains of anticyclonic spiral eddies that should be differentiated both from NAEs and from the chains of shear eddies.

Mushroom-like currents and eddy dipoles are most frequently observed in the Rim Current zone to the north of the western and eastern near-coastal waters of the Black Sea (Fig. 4.4b).

The region to the south of the Kerch Strait above the continental slope is a zone of quasi-permanent formation of chains of cyclonic shear eddies. Other areas with frequent chains of cyclonic shear eddies arising are near-coastal waters along the Caucasian and Anatolian coasts (Fig. 4.4c).

As a result a generalized scheme of quasi-permanent and most frequent non-stationary vortical structures is presented (Fig. 4.4d). In this scheme, there are three well-known quasi-permanent structures: the Batumi (marked with the letter B), Sevastopol (S) and Caucasus (C) eddies and one which has not yet been well studied – Western meander (W). Due to the multiple possible modifications of the Sevastopol eddy it was symbolized by two anticyclonic eddies. The Caucasus and Western eddies were drawn with a dashed line, because the former’s generation was not regular and the latter was a seasonal structure (usually it formed in autumn). Also there are provided the symbols for NAEs (N), Anatolian anticyclonic eddies (A) as well as the Kerch and Anatolian eddy chains (K and CA respectively). The Rim Current can be regarded as flowing around all the anticyclonic structures designated in the scheme from the inner side of the basin.