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deep-basin water exchange processes in the Black Sea surface layer
Satellites, Oceanography and Society edited by David Halpern 9 2000 Elsevier Science B.V. All rights reserved.
273
Chapter 15 Remotely sensed coastal/deep-basin water exchange processes in the Black Sea surface layer A n n a I. G i n z b u r g a n d A n d r e y G. K o s t i a n o y E E Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia D m i t r y M . S o l o v i e v a n d S e r g e i V. S t a n i c h n y Marine Hydrophysical Institute, Sevastopol, Ukraine Abstract. The role of mesoscale structures (eddies, dipoles, jets) in horizontal mixing and coastal/deep-basin water exchange in the Black Sea was investigated with National Oceanic and Atmospheric Administration (NOAA) Advanced Very-High Resolution Radiometer (AVHRR) imagery during 1993 and 1996-1998, together with relevant meandaily meteorological data from seaports and available hydrographic data of different years. In summer 1993 two anticyclones with diameters of about 90 and 55 km co-existed without coalescence over the northwestern continental slope.
Cyclones at the eddies'
peripheries, entrained and ejected jets, filaments, and a pinched-off cyclone near Cape Hersones (44~
33~
associated with wind-driven coastal upwelling contributed
to the water exchange in the region. Four anticyclones about 50 km in diameter and associated cyclones at their peripheries were observed in the southeastern region in November 1996.
Surface circulation in the region was considerably changed over several days
because of the anticyclones' movements, and formations and disruptions of short-lived dipoles of anticyclones and associated cyclones at their peripheries. Near-shore anticyclonic eddies with diameters of 40-80 km and lifetimes up to one month, which form along the Caucasian coast and propagate with velocities up to 17 cm s-1 in the general direction of the Rim Current, can evolve into deep-sea eddies southwest of Novorossiisk. Offshore jets, up to about 200 km in length and associated with the anticyclones, are an effective mechanism of coastal/deep-basin water exchange in the northeastern region.
1.
Introduction The known schemes of the general circulation of the Black Sea (Neumann 1942;
Bogatko et al. 1979; Ovchinnikov and Titov 1990; Oguz et al. 1993) are based on hydrographic measurements of different years and include a basin-scale cyclonic boundary cur-
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274
rent over the continental slope, cyclonic gyres in the basin interior, and quasi-stationary or recurrent anticyclonic eddies along the basin periphery. The schemes do not suggest coastal/deep-basin water exchange across the boundary current, named Rim Current by Oguz et al. (1992) or Main Black Sea Current in the Russian literature. However, early hydrobiological measurements of Mediterranean Sea plankton in Sevastopol Bay (Bogdanova and Shmeleva 1967) and polychaete larvae observed from Cape Sarych to the Anatolian coast (Kiseleva 1953) were indicative of the water exchange. Investigations of mechanisms of the exchange are of utmost importance for the Black Sea because of the poor ecological situation in this semi-enclosed basin. Satellite imagery, which has been used for investigation of the Black Sea surface circulation since 1981 (Kazmin and Sklyarov 1982), has detected various nonstationary mesoscale features contributing to coastal/deep-basin water exchange. For example, in summer, Danube River water propagates along the western coast to the Bosphorus Strait with cyclonic vortices generated at the front between freshened coastal and saline offshore waters (Kazmin and Sklyarov 1982; Grishin 1993; Sur and Ilyin 1997). Few observations of complicated vortical structures, in particular dipoles, and jets were reported over the continental slope (Kazmin and Sklyarov 1982; Grishin 1993; Ginzburg 1994; Sur et al. 1994, 1996; Sur and Ilyin 1997). Mushroom-shaped flow extended about 160 km offshore from the coastal zone close to the Bosphorus Strait (Ginzburg 1995). Coastal eddies and filaments of upwelled coastal water occur along the Anatolian coast (Grishin et al. 1990; Oguz et al. 1992; Ginzburg 1994; Sur et al. 1994, 1996; Sur and Ilyin 1997). In summer, the meanders of the Rim Current are intense north of the Anatolian coast (Ginzburg 1994; Sur et al. 1994; Sur and Ilyin 1997) and south of the Kerch Strait (Sur and Ilyin 1997). Spatial (less than 30 km) and temporal (several days to a month) scales of the nonstationary features (Figure 1) make it practically impossible to investigate their generation and evolution by hydrographic surveys. Satellite observations with high space-time resolutions are necessary. Ocean dynamics can vary considerably even in the same region during several days, and joint analysis of satellite data with in-situ hydrometeorological data is desirable, where possible, to understand the reasons for the variability. We present new results of analysis of visible and infrared (IR) images from the National Oceanic and Atmospheric Administration (NOAA) satellites with hydrometeorological data to describe the role of eddies and associated nonstationary flow patterns in horizontal mixing and coastal/deep-basin water exchange in the northwest, southeast, and northeast Black Sea.
2.
Data NOAA satellite images in 1993 and 1996-1998 were received in high-resolution pic-
ture transmission (HRPT) format by the Russian State Committee for Hydrometeorology in Moscow and by the Marine Hydrophysical Institute (MHI) in Sevastopol, respectively.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
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Figure 1. Schematic of coastal/deep-basin water exchanges in the Black Sea, superimposed on bathymetry with locations of sites mentioned in text.
The dataset included 27 IR images of the northwestern Black Sea (26 for April-August 1993 and 1 for June 1996), 12 visible-band images for June 1993, and 25 IR images of the eastern area (November 1996, September-November 1997, June 1998). A visibleband image was constructed from a linear combination of AVHRR channels 1 and 2, and represented the information about light diffusion associated with concentration of suspended matter. Atmospheric corrections were applied. Data were mapped on a Mercator projection with l'-latitudinal and 1.5'-longitudinal resolutions. Sea surface temperature retrievals were computed with AVHRR channels 4 and 5, with a resolution of about 0.1~ Conductivity-temperature-depth (CTD) measurements were made over the northwestern continental slope on 16-17 June 1993 with Research Vessel Akvanavt of the P. P. Shirshov Institute of Oceanology. Also, mean-daily meteorological data were available from seaports in the Ukraine and Russia.
3.
Mesoscale Structures in the Northwestern Region The northwestern region is characterized by a very wide shelf, low salinity, high bio-
logical productivity, and pollutants due to discharges of large rivers (Danube, Dnieper, Dniester). Vortices and mushroom-shaped currents with horizontal scales of 15-50 km are frequently observed (Grishin and Subbotin 1992).
However, the most interesting
Ginzburg, Kostianoy, Soloviev, and Stanichny
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mesoscale features of the region are anticyclonic eddies up to 100 km in diameter, which occur over the continental slope to precondition water exchanges south of 45~
(Ginz-
burg 1994; Ginzburg et al. 1996). The anticyclonic eddy west of Sevastopol, which has been repeatedly detected by hydrographic surveys (Neumann 1942; Bogatko et al. 1979), was embodied in the Oguz et al. (1993) surface circulation pattern as the "Sevastopol Eddy."
However, satellite
imagery shows that two closely spaced mesoscale anticyclones are frequently observed in the region (Figures 2a and 3). Anticyclone A1 (mean diameter of about 90 km) was observed in NOAA images on 19 and 27 April 1993 and 2 June-23 August 1993; although images for May were absent, the eddy observed in April and June-August was likely the same A1 eddy. Anticyclone A2 was seen from 9 June to 19 July 1993 (Figures 2 and 3); see also visible images for 10, 12, and 13 June and 18 July 1993 from NOAA- 11 and Russian satellite Kosmos-1939 in Ginzburg (1994) and Ginzburg et al. (1996). From 9 June to 19 July, anticyclones A1 and A2 co-existed without coalescence within an area formed by an underwater ridge on the southwest and the 200-m isobath on the north and east. The largest displacements of the A 1 and A2 centers were 90 km and 60 km, respectively (Figure 4), and the A2 eddy was always oriented to the northeast of A1 (Figure 4). Displacement velocities were not constant.
In Julian days (JD) 160-161, both eddies
remained practically immobile; however, during JD 162-166 both moved southwestward at about 16 cm s-l. Directions of displacements of A1 and A2 also changed with time, with the same directions on JD 162-167 and 193-200, and opposite directions on JD 169-174 and 183-193. Therefore, the distance between A1 and A2 centers varied from a minimum 100 km on JD 168 (previous day to that shown in Figure 2a) to a maximum 140 km on JD 174 (Figure 3). Interaction of A1 and A2, even when they were far apart, is indicated in Figure 3 by the narrow jet. Directions of A1 and A2 displacements were not always associated with the southwestward direction of the Rim Current. The eddy diameter decreased as it approached the 200-m isobath. Absence of A2 on 2 August 1993 (Figure 5b) suggests collapse of the eddy or its coalescence with A1.
In hydrographic data recorded during September-
October 1993, Georgiev et al. (1994) found a 100-km diameter anticyclone located 80 km southwest from the A1 position on 23 August (JD 235 in Figure 4). It is quite possible that this anticyclone was A 1 shifted in the direction of the Rim Current. One to three short-lived (up to a week), small-scale (about 10-km diameter) cyclones sporadically formed at the A1 and A2 peripheries. Generation of the small cyclones was not correlated with wind direction or wind speed, which rarely exceeded 5 m s-1. However, on 30 July 1993 the northwesterly wind strengthened to 10 m s-1, and we speculate that it created an intense jet and 90-km diameter cyclone (B 1 in Figure 5c) at the A1 eastern periphery. (Ginzburg 1994).
The horizontal scale of the A1-B1 dipole was about 200 km
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
Figure 2. (a) NOAA visible image on 18 June 1993 with superimposed positions of CTD stations made during R/V Akvanavt survey on 16-17 June 1993. (b) Dynamic topography map (mm) at 5 dbar relative to 500 dbar. A1 and A2 mark centers of anticyclones.
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Figure 3. NOAA visible image on 23 June 1993.
The A1 eddy influenced the area 43~176
29~176
which was much larger than
the A1 area because of the generation of associated cyclones, entrainment of water, and ejection of jets. An example of water entrainment by A1 and A2 is seen in Figure 3, which shows turbid water from the Danube delta region along the northern peripheries of A1 and A2 and almost reaching to the Crimean shore (Ginzburg 1994). The added contribution to coastal/deep-basin water exchange was associated with anticyclones A1 and A2 and a cyclone detected by the hydrographic survey on 16-17 June 1993 (Figures 2a and 2b). The A1-A2 system and the short-lived (several days) cyclone were traced to 300-m depth. The current in the upper 150 m between A1 and the cyclone was southeastward with a speed more than 30 cm s-1, and reversed flow at the northern periphery of the cyclone had a core at 150 m and a speed of about 10 cm s-l (Ginzburg et al. 1998a). Coastal upwelling occurred with favorable winds, irrespective of shoreline or bottom topography orientation (Figures 5 and 6). Wind speed in summer 1993 was rarely above 5 m s-1. Upwelling occurred most often along Tendrovskaya Spit (Figure 5c) because of
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
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Figure 4. Positions of centers of anticyclones A1 (solid dot) and A2 (open circle) during April-August 1993. Numbers are dates in Julian day.
northwesterly winds. Offshore transport of upwelled water beyond the upwelling front was by short-lived (one to a few days) transversal filaments about 40 km long (longer near capes) and 4-14 km wide, separated by distances from 6 to 38 km. Sometimes the offshore end of the filaments terminated in vortices or vortex dipoles. The filament length considerably exceeded the 12-km baroclinic Rossby radius of deformation near Tendrovskaya Spit (Ginzburg et al. 1997). On 23 June 1993, cold water was carried in a filament more than 150 km from Cape Hersones (Figure 6), and the filament moved along the southeastern peripheries of A1 and A2. Maximum speed of filaments estimated from correlation of successive IR images on 2 and 3 August 1993 was 35 cm s-1. Filament formation time did not exceed 24 hours, and temperature contrast relative to surrounding water was 1.6-2. I~
Minimum temperature of upwelled water was 4-7~
colder than
that beyond the upwelling zone and corresponded to depths less than 20-30 m, which was within or just below the seasonal thermocline (Ginzburg et al. 1997). Shallowness of seasonal thermocline and pycnocline aided the occurrence of upwelling under light winds. The NOAA IR images in Figure 5 describe the generation and evolution of another nonstationary element of circulation associated with coastal upwelling--a pinched-off cyclone near Cape Hersones.
On 30 July 1993 (Figure 5a), upwelling between Cape
Hersones and Cape Sarych was created by steady 5 m s-1 northwesterly winds during the previous two days. Width of the upwelling zone was about 10 km, and offshore transport
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Figure 5. NOAA IR images in 1993 on (a) 30 July, (b-f) 2, 3, 4, 5, and 6 August, respectively. Darker (lighter) tone corresponds to colder (warmer) water.
of cold water by southward filaments was traced offshore for about 20 km. Surface water temperature changed from 11.3~ near shore on 30 July (Figure 7a) to 16~ at the terminal of the filament from Cape Hersones. Three days later, after wind speed increased on 30 July to 10 m s-l and decreased to 2-3 m s-1 on succeeding days, upwelling was absent, and the cold-water patch separated from the coast (Figure 5b). The 10-km wide patch expanded in its head part to 17 km. increased from 17.3~
Temperature within the patch gradually
at the center of the head part to 18.5~ in the tail part. One day
later the patch evolved into cyclonic eddy B2 (Figure 5c), with 18-km diameter (Figure 7b) and uniform temperature. In succeeding days the temperature gradually increased (Figure 7a) and the eddy diameter decreased to 7 km on 6 August (Figure 7b). The B2 cyclone moved southwestward across isobaths with a mean speed of about 9 cm s-l. By
281
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
Figure 6. NOAA IR image on 23 June 1993 with darker (lighter) tone corresponding to colder (warmer) water.
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Figure 7. Time variations of (a) sea surface temperature and (b) diameter of pinched-off cyclone.
6 August 1993 it was about 50 km from Cape Hersones (Figure 5f). In addition, B2 water combined with water along the periphery of cyclone B 1 to influence the water properties as far as 80 km from Cape Hersones.
282
4.
Ginzburg, Kostianoy, Soloviev, and Stanichny Mesoscale Dynamics in the Southeastern Region
A large anticyclonic eddy has repeatedly been detected with hydrographic data in the southeastern Black Sea (Bibik 1964; Bogatko et al. 1979; Oguz et al. 1993); this persistent circulation feature was named "Batumi Eddy" by Oguz et al. (1993). However, satellite imagery in the region revealed considerable mesoscale variability associated with one or several eddies existing at the same time (Ginzburg et al. 1998c). Figure 8 shows four anticyclonic eddies with diameters of 45-50 km: A1, in the open sea, forms a quasisymmetrical dipole with cyclone C1 at its southeastern periphery; A2, also located in the deep basin; and two coastal anticyclones, A3 and A4, which have cyclones on their peripheries. In particular, A3 had three associated cyclones. A similar system of three dipoles with one common anticyclone and three cyclones placed at 120 ~ from each other was observed for the first time in a laboratory experiment by Fedorov et al. (1989). Quasi-symmetrical dipoles of the A1-C1 type were previously observed on a satellite image of the southeastern basin (Kazmin and Sklyarov 1982; Fedorov and Ginsburg 1992), and appear to be typical of the regional surface circulation. During 13-18 November 1996, A4 practically stayed at the same place, whereas A2 and A3 moved southward and northwestward, respectively, with a mean speed of 8.5 cm s-l. A1, after displacement southwestward at 18 cm s-1 during 13-15 November,
Figure 8. Surface circulation in the southeastern Black Sea on 14 November 1996 with (a) NOAA IR image with darker (lighter) tone corresponding to colder (warmer) water, and (b) corresponding scheme.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
283
moved at 27 cm s-1 along the periphery of A2, supposedly under the influence of A2, and disrupted dipole A1-C1.
Nonstationary dipoles of A4 and its periphery cyclones were
continuously formed. Absence of satellite data after 18 November 1996 did not allow further monitoring.
It is evident that local surface circulation can be considerably
changed over several days because of anticyclones' movements, disruptions of dipoles, and formations of new ones. Anticyclone diameters in the southeastern region vary from 45-50 km (Figure 8) to about 100 km. For example, in June 1998 (not shown), an anticyclone completed a chain of anticyclonic eddies formed off capes of the Anatolian coast, with diameters increasing to the east (shown schematically in Figure 1). Eddy size was not related to the seasonal cycle. Dipoles, even short-lived ones, are a very effective mechanism of horizontal mixing in the surface layer. Entrainment of flow by a pair of eddies of opposite sign (e.g., dipole A1-C1 in Figure 8) transported warm water from Gudauta Bank to the open sea (Figure 8a). A narrow jet between A4 and a cyclone on its eastern periphery transported cold water from the deep basin to the southern coastal zone near 40~
(Figure 8). Fedorov
and Ginsburg (1992) observed offshore transport of coastal water rich in suspended matter and onshore flow of clear, open-sea water that was associated with three dipoles in the southeastern region.
5.
Eddies and Jets in the Northeastern Region A characteristic feature of the regional circulation is near-shore anticyclonic eddies,
which are formed within anticyclonic meanders of the Rim Current and move with speeds of 4-6 cm s-! in the same direction as the Rim Current (Krivosheya et al. 1998a, 1998b). A near-shore anticyclonic eddy with 40-km diameter (named NAE-1) formed on 7 8 September
1997 west of Novorossiisk, in the region of shelf/slope widening
(Figure 9a). NAE-1 progressively moved away from the coast, approximately southward during 7-18 September (Figures 9a and 9b), northwestward on 18-23 September, and then southwestward for 23 September-8 October. Its translation speed was uneven: about 8.4 cm s-1 between 12 and 17 September, 15 cm s -1 in the following six days, 4.5 cm s-1 from 24 September to 5 October, and 8.5 cm s-1 between 5 and 8 October.
Total
displacement of NAE-1 was about 115 km southwestward, with mean speed of about 4.5 cm s-1. The diameter increased from 40 km to about 75 km. The NAE-1 lifetime was greater than one month, but absence of cloudless images did not allow further monitoring of NAE-1.
No near-shore anticyclone eddy other than NAE-1 formed in the region
between 7 September and 23 September 1997, whereas after 24 September anticyclonic eddies began to appear in rapid succession. Figure 9c shows a chain of three near-shore anticyclonic eddies, each about 40 km in diameter, between NAE-1 and Gelendzhik, and a meander of the Rim Current near Tuapse that manifested itself as a new NAE one day
284
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Figure 9. NOAA IR images on (a) 7 September, (b) 13 September, and (c) 8 October 1997 with darker (lighter) tone corresponding to colder (warmer) water.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
285
later. All near-shore anticyclonic eddies moved northwestward along isobaths with speeds of 9-17 cm s-1. However, not all near-shore anticyclonic eddies had non-zero translation speed. For example, an 80-km diameter near-shore anticyclonic eddy remained at the same place south of Gelendzhik (Figure 3) for at least 11 days (Ginzburg 1994). Offshore jets are frequently observed with near-shore anticyclonic eddies. For example, a southwestward warm water jet about 85-km long occurred at the NAE-1 periphery (Figures 9a and 9b). A similar offshore jet, about 200 km in length, is seen in Figure 3. Evidently, the jets are an effective mechanism of coastal/deep-basin water exchange in the area.
6.
Conclusions
Analysis of satellite data revealed that one-to-several anticyclonic mesoscale eddies exist simultaneously in the northwestern, southeastern, and northeastern Black Sea. In particular, two closely spaced mesoscale anticyclones co-existed without coalescence over the northwestern continental slope for about 1.5 months in summer 1993 (Figures 2-4). Different dynamical situations were observed in different years in the same season, e.g., four anticyclonic eddies in the southeastern region in November 1996 (Figure 8) and only one in November 1997 (Figure 9). Translation speeds of anticyclones in all three regions reached 16 cm s-1. High speed of movement and close proximity of anticyclones, as in the southeastern region in November 1996, can produce an aliasing error in the interpretation of hydrographic data recorded over several days with inadequate spatial resolution. Water exchange between the coastal zone and deep basin in the Black Sea is determined, in large part, by processes associated with evolution of anticyclonic eddies, such as entrainment of coastal and open sea waters and generation of additional cyclones and jets at the peripheries. An added contribution to water exchange in the northeastern region is the separation of near-shore anticyclones near Novorossiisk and their evolution into deep-sea eddies. In addition, in the northwestern region, filaments of wind-driven coastal upwelling produce horizontal mixing, transport of cold water for offshore distances of several tens of kilometers and considerably greater distance near capes, and due to entrainment by mesoscale eddies beyond the upwelling zone. Characteristics of filaments in the northwestern region (temperature contrast relative to surrounding water, formation time, velocity, marked excess of length compared to Rossby radius of deformation) agree with observations of transversal filaments in other coastal zones of the ocean (Fedorov and Ginsburg 1992; Kostianoy and Zatsepin 1996; Kostianoy 1996). Further analysis of satellite and in-situ data is desirable to understand regularities of generation of pinched-off cyclones near Cape Hersones during coastal upwelling (Figure 5).
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Acknowledgments. This study was supported by the Russian Foundation for Basic Research (grants NN 98-05-64715 and 99-05-65528) and by the European Community grant INCO-Copernicus NIC 15-CT96-0111.
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Grishin, G. A., I. G. Makeev, and S. V. Motyzhev, Observations of circulation in the westem Black Sea by remote sensing, Morskoy Gidrofizicheskiy Zhurnal, 54-62 (2), 1990 (in Russian). Grishin, G. A., and A. A. Subbotin, Study of the vortex dipole in the Black Sea using spacebome data and shipboard observations, lssledovanie Zemli iz Kosmosa, 56-64 (5), 1992 (in Russian). Kazmin A. S., and V. E. Sklyarov, Some peculiarities of the Black Sea circulation inferred from the Meteor satellite, Issledovanie Zemli iz Kosmosa, 42-49 (6), 1982 (in Russian). Kiseleva, M. I., Polychaets larvae of the Black Sea, Avtoreferat dissertatzii (candidat biologicheskih nauk), Zoological Institute, Acad. Sci. USSR, Leningrad, 12 pp, 1953 (in Russian). Kostianoy, A. G., Investigation of the Sicilian upwelling on the base of satellite data, Tech. Rep., Stazione Oceanografica CNR, La Spezia, Italy, 99 pp, 1996. Kostianoy, A. G., and A. G. Zatsepin, The West African coastal upwelling filaments and cross-frontal water exchange conditioned by them, J. Mar. Sys., 7, 2-4, 349-359, 1996. Krivosheya, V. G., F. Nyffeler, V. G. Yakubenko, I. M. Ovchinnikov, R. D. Kos'yan, and E. A. Kontar, Experimental studies of eddy structures within the Rim Current zone in the northeastern part of the Black Sea, In Ecosystem Modeling as a Management Toolfor the Black Sea, 2, edited by L. I. Ivanov and T. Oguz, Kluwer Academic Publishers, Dordrecht, The Netherlands, 131-144, 1998a. Krivosheya, V. G., I. M. Ovchinnikov, V. B. Titov, V. G. Yakubenko, A. Yu. Skirta, Meandering of the Main Black Sea Current and eddies formation in the northeastern part of the Black Sea in summer 1994, Okeanologiya, 38, 546-553, 1998b (in Russian). Neumann, G., Die absolute topografie des physikalischen Meeresniveaus und die Oberflachenstromungen des Schwarzen Meeres, Ann. Hydrogr. Maritimen Meteorol., 70, 265-282, 1942. Oguz T., V. S. Latun, M. A. Latif, V. V. Vladimirov, H. I. Sur, A. A. Markov, E. Ozsoy, V. V. Kotovshchikov, V. V. Eremeev, and U. Unluata, Circulation in the surface and intermediate layers of the Black Sea, Deep-Sea Res., 40, 1597-1612, 1993. Oguz T., P. E. La Violette, and U. Unluata, The upper layer circulation of the Black Sea: Its variability as inferred from hydrographic and satellite observations, J. Geophys. Res., 97, 12569-12584, 1992. Ovchinnikov, I. M., and V. B. Titov, Anticyclonic vorticity of currents in the Black Sea coastal zone, Doklady Akad Sci., 314, 1236--1239, 1990 (in Russian). Sur, H. I., and Yu. P. Ilyin, Evolution of satellite derived mesoscale thermal patterns in the Black Sea, Prog. Oceanogr., 39, 109-151, 1997. Sur, H. I., E. Ozsoy, and U. Unluata, Boundary current instabilities, upwelling, shelf mixing and eutrophication processes in the Black Sea, Prog. Oceanogr., 33, 249-302, 1994. Sur, H. I., E. Ozsoy, Yu. P. Ilyin, and U. Unluata, Coastal/deep ocean interactions in the Black Sea and their ecological/environmental impacts, J. Mar. Sys., 7, 293-320, 1996. Anna Ginzburg, P. P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia. (email, [email protected]; fax, +7-095-124-5983)
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Chapter 15 Remotely sensed coastal/deep-basin water exchange processes in the Black Sea surface layer A n n a I. G i n z b u r g a n d A n d r e y G. K o s t i a n o y E E Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia D m i t r y M . S o l o v i e v a n d S e r g e i V. S t a n i c h n y Marine Hydrophysical Institute, Sevastopol, Ukraine Abstract. The role of mesoscale structures (eddies, dipoles, jets) in horizontal mixing and coastal/deep-basin water exchange in the Black Sea was investigated with National Oceanic and Atmospheric Administration (NOAA) Advanced Very-High Resolution Radiometer (AVHRR) imagery during 1993 and 1996-1998, together with relevant meandaily meteorological data from seaports and available hydrographic data of different years. In summer 1993 two anticyclones with diameters of about 90 and 55 km co-existed without coalescence over the northwestern continental slope.
Cyclones at the eddies'
peripheries, entrained and ejected jets, filaments, and a pinched-off cyclone near Cape Hersones (44~
33~
associated with wind-driven coastal upwelling contributed
to the water exchange in the region. Four anticyclones about 50 km in diameter and associated cyclones at their peripheries were observed in the southeastern region in November 1996.
Surface circulation in the region was considerably changed over several days
because of the anticyclones' movements, and formations and disruptions of short-lived dipoles of anticyclones and associated cyclones at their peripheries. Near-shore anticyclonic eddies with diameters of 40-80 km and lifetimes up to one month, which form along the Caucasian coast and propagate with velocities up to 17 cm s-1 in the general direction of the Rim Current, can evolve into deep-sea eddies southwest of Novorossiisk. Offshore jets, up to about 200 km in length and associated with the anticyclones, are an effective mechanism of coastal/deep-basin water exchange in the northeastern region.
1.
Introduction The known schemes of the general circulation of the Black Sea (Neumann 1942;
Bogatko et al. 1979; Ovchinnikov and Titov 1990; Oguz et al. 1993) are based on hydrographic measurements of different years and include a basin-scale cyclonic boundary cur-
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274
rent over the continental slope, cyclonic gyres in the basin interior, and quasi-stationary or recurrent anticyclonic eddies along the basin periphery. The schemes do not suggest coastal/deep-basin water exchange across the boundary current, named Rim Current by Oguz et al. (1992) or Main Black Sea Current in the Russian literature. However, early hydrobiological measurements of Mediterranean Sea plankton in Sevastopol Bay (Bogdanova and Shmeleva 1967) and polychaete larvae observed from Cape Sarych to the Anatolian coast (Kiseleva 1953) were indicative of the water exchange. Investigations of mechanisms of the exchange are of utmost importance for the Black Sea because of the poor ecological situation in this semi-enclosed basin. Satellite imagery, which has been used for investigation of the Black Sea surface circulation since 1981 (Kazmin and Sklyarov 1982), has detected various nonstationary mesoscale features contributing to coastal/deep-basin water exchange. For example, in summer, Danube River water propagates along the western coast to the Bosphorus Strait with cyclonic vortices generated at the front between freshened coastal and saline offshore waters (Kazmin and Sklyarov 1982; Grishin 1993; Sur and Ilyin 1997). Few observations of complicated vortical structures, in particular dipoles, and jets were reported over the continental slope (Kazmin and Sklyarov 1982; Grishin 1993; Ginzburg 1994; Sur et al. 1994, 1996; Sur and Ilyin 1997). Mushroom-shaped flow extended about 160 km offshore from the coastal zone close to the Bosphorus Strait (Ginzburg 1995). Coastal eddies and filaments of upwelled coastal water occur along the Anatolian coast (Grishin et al. 1990; Oguz et al. 1992; Ginzburg 1994; Sur et al. 1994, 1996; Sur and Ilyin 1997). In summer, the meanders of the Rim Current are intense north of the Anatolian coast (Ginzburg 1994; Sur et al. 1994; Sur and Ilyin 1997) and south of the Kerch Strait (Sur and Ilyin 1997). Spatial (less than 30 km) and temporal (several days to a month) scales of the nonstationary features (Figure 1) make it practically impossible to investigate their generation and evolution by hydrographic surveys. Satellite observations with high space-time resolutions are necessary. Ocean dynamics can vary considerably even in the same region during several days, and joint analysis of satellite data with in-situ hydrometeorological data is desirable, where possible, to understand the reasons for the variability. We present new results of analysis of visible and infrared (IR) images from the National Oceanic and Atmospheric Administration (NOAA) satellites with hydrometeorological data to describe the role of eddies and associated nonstationary flow patterns in horizontal mixing and coastal/deep-basin water exchange in the northwest, southeast, and northeast Black Sea.
2.
Data NOAA satellite images in 1993 and 1996-1998 were received in high-resolution pic-
ture transmission (HRPT) format by the Russian State Committee for Hydrometeorology in Moscow and by the Marine Hydrophysical Institute (MHI) in Sevastopol, respectively.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
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275
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Figure 1. Schematic of coastal/deep-basin water exchanges in the Black Sea, superimposed on bathymetry with locations of sites mentioned in text.
The dataset included 27 IR images of the northwestern Black Sea (26 for April-August 1993 and 1 for June 1996), 12 visible-band images for June 1993, and 25 IR images of the eastern area (November 1996, September-November 1997, June 1998). A visibleband image was constructed from a linear combination of AVHRR channels 1 and 2, and represented the information about light diffusion associated with concentration of suspended matter. Atmospheric corrections were applied. Data were mapped on a Mercator projection with l'-latitudinal and 1.5'-longitudinal resolutions. Sea surface temperature retrievals were computed with AVHRR channels 4 and 5, with a resolution of about 0.1~ Conductivity-temperature-depth (CTD) measurements were made over the northwestern continental slope on 16-17 June 1993 with Research Vessel Akvanavt of the P. P. Shirshov Institute of Oceanology. Also, mean-daily meteorological data were available from seaports in the Ukraine and Russia.
3.
Mesoscale Structures in the Northwestern Region The northwestern region is characterized by a very wide shelf, low salinity, high bio-
logical productivity, and pollutants due to discharges of large rivers (Danube, Dnieper, Dniester). Vortices and mushroom-shaped currents with horizontal scales of 15-50 km are frequently observed (Grishin and Subbotin 1992).
However, the most interesting
Ginzburg, Kostianoy, Soloviev, and Stanichny
276
mesoscale features of the region are anticyclonic eddies up to 100 km in diameter, which occur over the continental slope to precondition water exchanges south of 45~
(Ginz-
burg 1994; Ginzburg et al. 1996). The anticyclonic eddy west of Sevastopol, which has been repeatedly detected by hydrographic surveys (Neumann 1942; Bogatko et al. 1979), was embodied in the Oguz et al. (1993) surface circulation pattern as the "Sevastopol Eddy."
However, satellite
imagery shows that two closely spaced mesoscale anticyclones are frequently observed in the region (Figures 2a and 3). Anticyclone A1 (mean diameter of about 90 km) was observed in NOAA images on 19 and 27 April 1993 and 2 June-23 August 1993; although images for May were absent, the eddy observed in April and June-August was likely the same A1 eddy. Anticyclone A2 was seen from 9 June to 19 July 1993 (Figures 2 and 3); see also visible images for 10, 12, and 13 June and 18 July 1993 from NOAA- 11 and Russian satellite Kosmos-1939 in Ginzburg (1994) and Ginzburg et al. (1996). From 9 June to 19 July, anticyclones A1 and A2 co-existed without coalescence within an area formed by an underwater ridge on the southwest and the 200-m isobath on the north and east. The largest displacements of the A 1 and A2 centers were 90 km and 60 km, respectively (Figure 4), and the A2 eddy was always oriented to the northeast of A1 (Figure 4). Displacement velocities were not constant.
In Julian days (JD) 160-161, both eddies
remained practically immobile; however, during JD 162-166 both moved southwestward at about 16 cm s-l. Directions of displacements of A1 and A2 also changed with time, with the same directions on JD 162-167 and 193-200, and opposite directions on JD 169-174 and 183-193. Therefore, the distance between A1 and A2 centers varied from a minimum 100 km on JD 168 (previous day to that shown in Figure 2a) to a maximum 140 km on JD 174 (Figure 3). Interaction of A1 and A2, even when they were far apart, is indicated in Figure 3 by the narrow jet. Directions of A1 and A2 displacements were not always associated with the southwestward direction of the Rim Current. The eddy diameter decreased as it approached the 200-m isobath. Absence of A2 on 2 August 1993 (Figure 5b) suggests collapse of the eddy or its coalescence with A1.
In hydrographic data recorded during September-
October 1993, Georgiev et al. (1994) found a 100-km diameter anticyclone located 80 km southwest from the A1 position on 23 August (JD 235 in Figure 4). It is quite possible that this anticyclone was A 1 shifted in the direction of the Rim Current. One to three short-lived (up to a week), small-scale (about 10-km diameter) cyclones sporadically formed at the A1 and A2 peripheries. Generation of the small cyclones was not correlated with wind direction or wind speed, which rarely exceeded 5 m s-1. However, on 30 July 1993 the northwesterly wind strengthened to 10 m s-1, and we speculate that it created an intense jet and 90-km diameter cyclone (B 1 in Figure 5c) at the A1 eastern periphery. (Ginzburg 1994).
The horizontal scale of the A1-B1 dipole was about 200 km
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
Figure 2. (a) NOAA visible image on 18 June 1993 with superimposed positions of CTD stations made during R/V Akvanavt survey on 16-17 June 1993. (b) Dynamic topography map (mm) at 5 dbar relative to 500 dbar. A1 and A2 mark centers of anticyclones.
277
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Ginzburg, Kostianoy, Soloviev, and Stanichny
Figure 3. NOAA visible image on 23 June 1993.
The A1 eddy influenced the area 43~176
29~176
which was much larger than
the A1 area because of the generation of associated cyclones, entrainment of water, and ejection of jets. An example of water entrainment by A1 and A2 is seen in Figure 3, which shows turbid water from the Danube delta region along the northern peripheries of A1 and A2 and almost reaching to the Crimean shore (Ginzburg 1994). The added contribution to coastal/deep-basin water exchange was associated with anticyclones A1 and A2 and a cyclone detected by the hydrographic survey on 16-17 June 1993 (Figures 2a and 2b). The A1-A2 system and the short-lived (several days) cyclone were traced to 300-m depth. The current in the upper 150 m between A1 and the cyclone was southeastward with a speed more than 30 cm s-1, and reversed flow at the northern periphery of the cyclone had a core at 150 m and a speed of about 10 cm s-l (Ginzburg et al. 1998a). Coastal upwelling occurred with favorable winds, irrespective of shoreline or bottom topography orientation (Figures 5 and 6). Wind speed in summer 1993 was rarely above 5 m s-1. Upwelling occurred most often along Tendrovskaya Spit (Figure 5c) because of
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
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northwesterly winds. Offshore transport of upwelled water beyond the upwelling front was by short-lived (one to a few days) transversal filaments about 40 km long (longer near capes) and 4-14 km wide, separated by distances from 6 to 38 km. Sometimes the offshore end of the filaments terminated in vortices or vortex dipoles. The filament length considerably exceeded the 12-km baroclinic Rossby radius of deformation near Tendrovskaya Spit (Ginzburg et al. 1997). On 23 June 1993, cold water was carried in a filament more than 150 km from Cape Hersones (Figure 6), and the filament moved along the southeastern peripheries of A1 and A2. Maximum speed of filaments estimated from correlation of successive IR images on 2 and 3 August 1993 was 35 cm s-1. Filament formation time did not exceed 24 hours, and temperature contrast relative to surrounding water was 1.6-2. I~
Minimum temperature of upwelled water was 4-7~
colder than
that beyond the upwelling zone and corresponded to depths less than 20-30 m, which was within or just below the seasonal thermocline (Ginzburg et al. 1997). Shallowness of seasonal thermocline and pycnocline aided the occurrence of upwelling under light winds. The NOAA IR images in Figure 5 describe the generation and evolution of another nonstationary element of circulation associated with coastal upwelling--a pinched-off cyclone near Cape Hersones.
On 30 July 1993 (Figure 5a), upwelling between Cape
Hersones and Cape Sarych was created by steady 5 m s-1 northwesterly winds during the previous two days. Width of the upwelling zone was about 10 km, and offshore transport
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Ginzburg, Kostianoy, Soloviev, and Stanichny
Figure 5. NOAA IR images in 1993 on (a) 30 July, (b-f) 2, 3, 4, 5, and 6 August, respectively. Darker (lighter) tone corresponds to colder (warmer) water.
of cold water by southward filaments was traced offshore for about 20 km. Surface water temperature changed from 11.3~ near shore on 30 July (Figure 7a) to 16~ at the terminal of the filament from Cape Hersones. Three days later, after wind speed increased on 30 July to 10 m s-l and decreased to 2-3 m s-1 on succeeding days, upwelling was absent, and the cold-water patch separated from the coast (Figure 5b). The 10-km wide patch expanded in its head part to 17 km. increased from 17.3~
Temperature within the patch gradually
at the center of the head part to 18.5~ in the tail part. One day
later the patch evolved into cyclonic eddy B2 (Figure 5c), with 18-km diameter (Figure 7b) and uniform temperature. In succeeding days the temperature gradually increased (Figure 7a) and the eddy diameter decreased to 7 km on 6 August (Figure 7b). The B2 cyclone moved southwestward across isobaths with a mean speed of about 9 cm s-l. By
281
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
Figure 6. NOAA IR image on 23 June 1993 with darker (lighter) tone corresponding to colder (warmer) water.
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Figure 7. Time variations of (a) sea surface temperature and (b) diameter of pinched-off cyclone.
6 August 1993 it was about 50 km from Cape Hersones (Figure 5f). In addition, B2 water combined with water along the periphery of cyclone B 1 to influence the water properties as far as 80 km from Cape Hersones.
282
4.
Ginzburg, Kostianoy, Soloviev, and Stanichny Mesoscale Dynamics in the Southeastern Region
A large anticyclonic eddy has repeatedly been detected with hydrographic data in the southeastern Black Sea (Bibik 1964; Bogatko et al. 1979; Oguz et al. 1993); this persistent circulation feature was named "Batumi Eddy" by Oguz et al. (1993). However, satellite imagery in the region revealed considerable mesoscale variability associated with one or several eddies existing at the same time (Ginzburg et al. 1998c). Figure 8 shows four anticyclonic eddies with diameters of 45-50 km: A1, in the open sea, forms a quasisymmetrical dipole with cyclone C1 at its southeastern periphery; A2, also located in the deep basin; and two coastal anticyclones, A3 and A4, which have cyclones on their peripheries. In particular, A3 had three associated cyclones. A similar system of three dipoles with one common anticyclone and three cyclones placed at 120 ~ from each other was observed for the first time in a laboratory experiment by Fedorov et al. (1989). Quasi-symmetrical dipoles of the A1-C1 type were previously observed on a satellite image of the southeastern basin (Kazmin and Sklyarov 1982; Fedorov and Ginsburg 1992), and appear to be typical of the regional surface circulation. During 13-18 November 1996, A4 practically stayed at the same place, whereas A2 and A3 moved southward and northwestward, respectively, with a mean speed of 8.5 cm s-l. A1, after displacement southwestward at 18 cm s-1 during 13-15 November,
Figure 8. Surface circulation in the southeastern Black Sea on 14 November 1996 with (a) NOAA IR image with darker (lighter) tone corresponding to colder (warmer) water, and (b) corresponding scheme.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
283
moved at 27 cm s-1 along the periphery of A2, supposedly under the influence of A2, and disrupted dipole A1-C1.
Nonstationary dipoles of A4 and its periphery cyclones were
continuously formed. Absence of satellite data after 18 November 1996 did not allow further monitoring.
It is evident that local surface circulation can be considerably
changed over several days because of anticyclones' movements, disruptions of dipoles, and formations of new ones. Anticyclone diameters in the southeastern region vary from 45-50 km (Figure 8) to about 100 km. For example, in June 1998 (not shown), an anticyclone completed a chain of anticyclonic eddies formed off capes of the Anatolian coast, with diameters increasing to the east (shown schematically in Figure 1). Eddy size was not related to the seasonal cycle. Dipoles, even short-lived ones, are a very effective mechanism of horizontal mixing in the surface layer. Entrainment of flow by a pair of eddies of opposite sign (e.g., dipole A1-C1 in Figure 8) transported warm water from Gudauta Bank to the open sea (Figure 8a). A narrow jet between A4 and a cyclone on its eastern periphery transported cold water from the deep basin to the southern coastal zone near 40~
(Figure 8). Fedorov
and Ginsburg (1992) observed offshore transport of coastal water rich in suspended matter and onshore flow of clear, open-sea water that was associated with three dipoles in the southeastern region.
5.
Eddies and Jets in the Northeastern Region A characteristic feature of the regional circulation is near-shore anticyclonic eddies,
which are formed within anticyclonic meanders of the Rim Current and move with speeds of 4-6 cm s-! in the same direction as the Rim Current (Krivosheya et al. 1998a, 1998b). A near-shore anticyclonic eddy with 40-km diameter (named NAE-1) formed on 7 8 September
1997 west of Novorossiisk, in the region of shelf/slope widening
(Figure 9a). NAE-1 progressively moved away from the coast, approximately southward during 7-18 September (Figures 9a and 9b), northwestward on 18-23 September, and then southwestward for 23 September-8 October. Its translation speed was uneven: about 8.4 cm s-1 between 12 and 17 September, 15 cm s -1 in the following six days, 4.5 cm s-1 from 24 September to 5 October, and 8.5 cm s-1 between 5 and 8 October.
Total
displacement of NAE-1 was about 115 km southwestward, with mean speed of about 4.5 cm s-1. The diameter increased from 40 km to about 75 km. The NAE-1 lifetime was greater than one month, but absence of cloudless images did not allow further monitoring of NAE-1.
No near-shore anticyclone eddy other than NAE-1 formed in the region
between 7 September and 23 September 1997, whereas after 24 September anticyclonic eddies began to appear in rapid succession. Figure 9c shows a chain of three near-shore anticyclonic eddies, each about 40 km in diameter, between NAE-1 and Gelendzhik, and a meander of the Rim Current near Tuapse that manifested itself as a new NAE one day
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Ginzburg, Kostianoy, Soloviev, and Stanichny
Figure 9. NOAA IR images on (a) 7 September, (b) 13 September, and (c) 8 October 1997 with darker (lighter) tone corresponding to colder (warmer) water.
Remotely sensed coastal~deep-basin water exchange processes in the Black Sea
285
later. All near-shore anticyclonic eddies moved northwestward along isobaths with speeds of 9-17 cm s-1. However, not all near-shore anticyclonic eddies had non-zero translation speed. For example, an 80-km diameter near-shore anticyclonic eddy remained at the same place south of Gelendzhik (Figure 3) for at least 11 days (Ginzburg 1994). Offshore jets are frequently observed with near-shore anticyclonic eddies. For example, a southwestward warm water jet about 85-km long occurred at the NAE-1 periphery (Figures 9a and 9b). A similar offshore jet, about 200 km in length, is seen in Figure 3. Evidently, the jets are an effective mechanism of coastal/deep-basin water exchange in the area.
6.
Conclusions
Analysis of satellite data revealed that one-to-several anticyclonic mesoscale eddies exist simultaneously in the northwestern, southeastern, and northeastern Black Sea. In particular, two closely spaced mesoscale anticyclones co-existed without coalescence over the northwestern continental slope for about 1.5 months in summer 1993 (Figures 2-4). Different dynamical situations were observed in different years in the same season, e.g., four anticyclonic eddies in the southeastern region in November 1996 (Figure 8) and only one in November 1997 (Figure 9). Translation speeds of anticyclones in all three regions reached 16 cm s-1. High speed of movement and close proximity of anticyclones, as in the southeastern region in November 1996, can produce an aliasing error in the interpretation of hydrographic data recorded over several days with inadequate spatial resolution. Water exchange between the coastal zone and deep basin in the Black Sea is determined, in large part, by processes associated with evolution of anticyclonic eddies, such as entrainment of coastal and open sea waters and generation of additional cyclones and jets at the peripheries. An added contribution to water exchange in the northeastern region is the separation of near-shore anticyclones near Novorossiisk and their evolution into deep-sea eddies. In addition, in the northwestern region, filaments of wind-driven coastal upwelling produce horizontal mixing, transport of cold water for offshore distances of several tens of kilometers and considerably greater distance near capes, and due to entrainment by mesoscale eddies beyond the upwelling zone. Characteristics of filaments in the northwestern region (temperature contrast relative to surrounding water, formation time, velocity, marked excess of length compared to Rossby radius of deformation) agree with observations of transversal filaments in other coastal zones of the ocean (Fedorov and Ginsburg 1992; Kostianoy and Zatsepin 1996; Kostianoy 1996). Further analysis of satellite and in-situ data is desirable to understand regularities of generation of pinched-off cyclones near Cape Hersones during coastal upwelling (Figure 5).
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Ginzburg, Kostianoy, Soloviev, and Stanichny
Acknowledgments. This study was supported by the Russian Foundation for Basic Research (grants NN 98-05-64715 and 99-05-65528) and by the European Community grant INCO-Copernicus NIC 15-CT96-0111.
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