Journal of Marine Systems

Transcription

1 Journal of Marine Systems 88 (2011) Contents lists available at ScienceDirect Journal of Marine Systems journal homepage: Hydrography of the eastern part of the Aegean Sea during the Eastern Mediterranean Transient (EMT) Erdem Sayın, Canan Eronat, Şeniz Uçkaç, Şükrü T. Beşiktepe DEU Institute of Marine Sciences and Technology, Baku Bulvarı, No 100, Inciralti, Izmir, Turkey article info abstract Article history: Received 9 December 2010 Received in revised form 23 May 2011 Accepted 22 June 2011 Available online 4 July 2011 Keywords: Hydrography Aegean Sea Water Mass Eastern Mediterranean Transient Deep Water Formation Cascading Hydrographic features of the Aegean Sea were studied using CTDs collected in 4 cruises from 1991 to This period covers an interesting large-scale change in the thermohaline circulation in the Eastern Mediterranean and the Aegean Sea known as the Eastern Mediterranean Transient (EMT) and was first described during early 1990s. The cruise data is analyzed to depict the spatial variability of the water masses of the Aegean Sea in this period. We found Levantine Waters to be particularly prominent during the spring of 1992, more than in the other seasons. In the Central Aegean Sea the circulated cold water mixes with the upwelling water and very dense water remains behind in summer The Levantine Surface Water (LSW) is blocked and does not penetrate further to the north because of the existing upwelling water seen near Saros (Buyukkemikli Cape) and off-shore side of Baba Cape (product of northerly wind) especially in summer 1991 and in fall The water masses are nearly homogeneous vertically with the influence of strong wind and convective mixing in winter time. In this study, we underline the contribution of the eastern part of the Aegean to the rising isopycnal levels during the EMT. In turn, i) the dense water cascades from eastern shelf of the Aegean Sea, ii) the upwelling and downwelling processes in the northern and central basins, iii) the cooling effect of Black Sea Water (BSW) on upwelled water at the south of the Lemnos Island (open sea convection) and iv) local wind forcing Elsevier B.V. All rights reserved. 1. Introduction The Aegean Sea is connected with the Marmara Sea through the Dardanelles Strait, with the Ionian Sea through the Kithira, Antikithira and Elafonisos Straits and with the Levantine through the Rhodes, Kassos and Karpathos Straits (Fig. 1). It has more than 2000 islands forming small basins and narrow passages, and a very irregular coastline and bathymetry. The North Aegean Trough is the deepest basin in the North Aegean Sea. This deep basin is connected to the Chios basin with a sill of 350 m. In the south, the Chios basin communicates with the Cretan basin mainly through the passage between the shallow Kiklades Plateau and the west coast of Turkish Mainland. The Eastern Mediterranean Transient (EMT) which is the important period starting from 1987 and ending 1996 caused drastic changes in the water dynamics of the Mediterranean Sea (Theocharis et al. 1999). The source of Eastern Mediterranean Deep Water (EMDW) had shifted from the Adriatic to the Aegean Sea. The process has been called as the Eastern Mediterranean Transient (EMT). The EMT is discussed in relation to different factors and reasons in Zervakis et al. (2004). They reviewed the response of the Aegean Sea to climatic variability and mentioned the role of the North Atlantic Oscillation on EMT. According to Demirov and Pinardi (2002), wind stress has played an important role Corresponding author. Tel.: ; fax: address: erdem.sayin@deu.edu.tr (E. Sayın). for the new forming EMDW. Samuel et al. (1999) analysed individual monthly wind stress fields over the Mediterranean for showing an intensification of the winter mean wind stress over the Aegean Sea occurred in the latter half of this period. Velaoras and Lascaratos (2005) found abrupt increase in density in the intermediate depths during 1987 and 1988 in the Aegean Sea due to the decrease in the temperature. In 1993, an even more intense density increase was observed, characterized this time by an abrupt salinity increase, as well as a temperature drop. Gertman et al. (2006) reported that EMDW already began overflowing the sills of the Kassos and Antikithira Straits as early as during the winter of Roether et al. (2007),showedthat the outflow of Aegean dense water (Cretan Deep Water, hereafter CDW) in 1993 of nearly 3 Sv and that the total outflow of CDW over the sills of Kassos and Antikithira Strait during the EMT amounted to about twice the total volume of the Aegean Sea. Levantine surface waters flowing into the Aegean Sea before 90's, could be a main triggering mechanism for EMT (Klein et al., 1999 and Roether et al., 1996). The enhanced presence of these highly saline water masses in the early 90s, presumably signifies the intrusion of high salinity water of Levantine origin into the North Central Aegean following the massive outflow of dense Aegean water into the Eastern Mediterranean. This salinity intrusion was one of the preconditioning factors that enhanced dense water formation in the Aegean during the last major deepwater formation phase of the EMT (Velaoras and Lascaratos, 2005, 2010). Temporal evolution of the water masses of the Aegean Sea before during and after the EMT studied by Sayin and Besiktepe (2010) /$ see front matter 2011 Elsevier B.V. All rights reserved. doi: /j.jmarsys

2 E. Sayın et al. / Journal of Marine Systems 88 (2011) time for several Aegean shelves by Ivanov et al. (2004) and Durrieu de Madron et al. (2005). iii) The upwelling and downwelling processes in certain regions obtained from the model and in-situ measurements for the time summer 1991 (Sayin et al., submitted for publication). iv) The cooling effect of Black Sea Water (BSW) on the upwelled water at the south of the Lemnos Island (open sea convection) in all seasons from 1991 to 1993 (Nittis et al., 2003) and v) Local wind force enhancing the penetration of dense water formed near the coastal area towards the Central Aegean Sea during all seasons in 1992 and In-situ measurements and related modelling studies combination with wind data enable us to identify BSW influence in the North Aegean and seasonal developing eddies in the Central Aegean which have been discussed to find out the role of the water masses forming and the processes taking place in the eastern Aegean Sea related to the increase of densities at intermediate depths. This density change has been also influenced by cascading dense water from coastal area, upwelling in Baba and Buyukkemikli Cape, occurrence of the Cyclonic Central Gyre and open sea convection between Lesvos and Lemnos Plateau. Fig. 1. The Aegean Sea geography and bathymetry. The box in the map of the Mediterranean provided below shows the position of the Aegean Sea. The main objective of present study is to give additional explanations for the density increase at intermediate layers in the Aegean Sea during the major dense water formation episode (1993) and before, which can be summarized within 5 items; i) The air temperature and therefore water temperature in winters 1992 and 1993 the lowest since 1980 (Velaoras and Lascaratos, 2005). ii) The dense water cascades from the shelf of eastern Aegean Sea during spring A similar temperaturecontrolled dense water cascading was reported in spring and winter Fig. 2. Summer general circulation pattern (after Sayin et al., submitted for publication) on the background of average SST field of summer Fig. 3. Temperature, salinity and density near surface field (5 m) for summer The locations of CTD stations shown with dark points.

3 504 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 4. Summer 1991 temperature, salinity anddensity in the vertical section from Saros to Levantine. The path of vertical section is shown on thesmall map (lower right part of thefirst panel).

4 E. Sayın et al. / Journal of Marine Systems 88 (2011) The paper is divided into five parts. Following this introduction, the methodology is explained. Third part deals with the analysis of cruise data. The next section discusses some results of the study. The last section is conclusion. 2. Material and Methods Monitoring studies in the Aegean Sea have been carried out by the Institute of Marine Sciences and Technology of Dokuz Eylul University with R/V K. Piri Reis. The seasonal CTD data were collected during 4 cruises from summer 1991 to winter Pressure, temperature and conductivity were measured in situ by using a Sea Bird CTD (911plus). Sea Bird CTD sensors are calibrated by the Northwest Regional Calibration Center (operating under contract to NOAA) once a year. Wind was analyzed using the data from the Gokceada (in the vicinity of Saros Bay), Dikili, Izmir and Kusadasi meteorological stations. T-S diagrams are used to explore the inter-seasonal variability of the local hydrography in order to determine existing water masses. Vertical sections, horizontal contour maps, vertical profiles are drawn to analyze the data in three-dimensional. are dominant during cruise time over Izmir area. The wind intensity decreases further towards south. SE and NW directions are general features for the Kusadasi region. But N direction is frequently seen with rather high occurrence of 15% during cruise time. The longest duration of wind coming from N direction is 22 h. There is no other wind direction in considerable duration affecting the surface current in Kusadasi Bay during summer 1991 cruise. The general circulation pattern of the Aegean Sea for the summer period was obtained from the model studies of Sayin et al. (submitted for publication). Fig. 2 shows the main characteristics of summer circulation pattern. One cyclonic eddy CAgE (Central Aegean Eddy) is formed in the western side of Lesvos Island and the second one is evolved in the Chios Basin (Chios Eddy) western off-shore side of Chios Island. Occurring cyclonic Central Aegean Eddy (CAgE) contains the densest water mass in the core. It is the site of the formation of Aegean Intermediate Water. In the southern basin, warm saline LSW (Levantine Surface Water) and LIW (Levantine Intermediate Water) penetrate into the Aegean through the eastern straits of the Cretan Arc. LSW cannot be observed 3. Hydrography of the Eastern Aegean Sea 3.1. Summer 91 Summer cruise covers the area from Marmara Sea to Levantine and has 208 casts from July 4 to August 27. We focus mainly the Eastern Aegean area. The weather condition is variable during the cruise time in summer Although the expected general wind direction is NNE obtained from the long term analysis for summer months over the Saros area, the prevailing wind direction is 70% from ENE during cruise time. The longest duration of wind coming from ENE is 22 h. The intensity of wind decreases to the southwards. Generally the prevailing wind is from ESE in all times over the Dikili region. But the wind is variable (no lasting wind) over the Dikili area during cruise time. Two directions W and NE Fig. 5. TS-Diagramforsummer1991withthetemperature, salinityanddensityvalues of the intermediate depths. Thedensity maximummeasured in theregion 2 (Saros) at 393 m and in the Region 9 (Lesvos) at 420 m is kg/m 3. Other dense waters are detected with a density kg/m 3 at 382 m and kg/m 3 at 296 m in the Regions 14 (Chios off-shore) and 13 (Chios on-shore) respectively. The value of kg/m 3 is measured at 342 m in the Region 7 (Lemnos south). Region 1 (Marmara) and Region 17 (South Aegean Sea). Fig. 6. Temperature, salinity and density near surface field (5 m) for spring The locations of CTD stations shown with dark points.

5 506 E. Sayın et al. / Journal of Marine Systems 88 (2011) due to prevailing northerly and northwesterly winds influencing especially the eastern coastal area of the North Aegean Sea. It makes a strong front due to occurring upwelling water and spreads up to the Lesvos Island (Fig. 3). LSW starts to transform extensively in the Central Aegean Sea due to cooling on the way to the north and mixing with the saline and cold upwelling water. LIW influences the Central Aegean Sea. It can be recognized even in the west of Samothraki Island. The eastern part of North Aegean Sea is influenced by; 1. Flowing Levantine (surface and intermediate) waters to the north along the eastern coast of the Aegean Sea up to the vicinity of Gokceada Island. 2. Forming cyclonic Central Aegean Eddy (CAgE) and Chios Eddy (Fig. 2, after Sayin et al., submitted for publication). 3. Upwelling processes taking place near the Buyukkemikli, Baba Cape, in the west of Lesvos and Chios islands (The last two are well captured by the SST field, Fig. 2). 4. Dynamics of Canakkale (Dardanelles) two layer exchange. BSW is warmer than the water near the Baba Cape. The upwelling influences the area from (Saros) eastern coast up to Chios Island in the north-south direction seen from the monthly average SST fields (Fig. 2). Occurring upwelling in the eastern Aegean Sea is studied first by Unluata (1986). Southeasterly wind dominates over the area of Kusadasi. It explains why the upwelling area is limited in the south direction before the Chios Island and upwelled water turns to its right hand side to the west. The Etesian (dry north wind of the Aegean Sea, which blows mainly in summer period) influences the waters in Saros Bay and off- Fig. 7. Spring 1992 temperature, salinity and density in the vertical section from North Aegean Sea to Levantine. The path of vertical section is shown on the small map (lower right part of the figures).

6 E. Sayın et al. / Journal of Marine Systems 88 (2011) shore of Baba Cape, causing upwelling. Southeasterly winds, which are prevalent through the year in the vicinity of Edremit Bay, trigger the local upwelling processes and force the cold-water masses coming to the surface and to flow outwards through Musellim Strait to the west (Fig. 3). These two flows carrying different upwelling cold and dense water combine each other and flow from Baba Cape towards the central Aegean Sea. This water can be also seen at the surface especially in summer having a salinity of 39.0 psu and temperature less than 19 C. The upwelled water flows towards south influencing a big area from west side of the Lesvos Island up to the Chios Basin forming two cyclonic eddies. The cold cores can be identified even from the SST field (Fig. 2). These frequently seen cyclonic eddies bring the cold and dense water from depth to the surface layers leaving the whole water column homogeneous. Gertman et al. (2006) mentioned about the existing of CAgE with homogeneous Aegean Intermediate Water (AgIW) water in the core. The saline subsurface LIW affects the upwelling area with a thickness of 150 m between the depths of m. It is easily traceable from the salinity field (N39.0 psu) of vertical section extending north south direction (Fig. 4). The vertical section passes through some regions are given in inset. LIW isolates the cold upwelling water from deeper part in the north. It is found just under the Ekman layer. The water is relatively warmer in summer in the eastern shallow area of the Aegean Sea. It warms up faster due to its shallowness (small volume). The complex land and sea distribution of the shallow coastal area of middle Aegean Sea is not suitable for an upwelling. Downwelling process occurs northern side of the North Aegean Trough and upwelling south of it (Zodiatis, 1994). While weak downwelling takes place in the region of northwest of Samothraki Island, upwelling is seen in the Central Aegean Sea. The shoaling isopycnals in these upwelling and deepening isopycnals in downwelling regions are clear from the density vertical section (Fig. 4). The densest water is observed in the west part of Lesvos Island under the LIW (29.37 kg/m 3 ) at intermediate depths (~420 m) (Fig. 5). The regions that LIW penetrates, is especially near eastern coast in the North Aegean Sea. The density of LIW is lighter compared to Saros and Athos basins containing denser waters at the intermediate depths (Fig. 4). LIW reaches even up to the vicinity of Saros Bay with its typical temperature (near 16 C) and salinity ( psu). It is under the depth of 300 m in the Levantine basin and approximately in the depth of 100 m in the west of Gokceada Island. The upwelling was the main structure in summer 1991 and preconditioning for strong deep-water formation in wintertime. Upwelling processes take place near the Buyukkemikli and Baba Cape because of the blowing moderate wind from north direction. The cold and relative salty upwelling water (saltier than BSW) occupies the area from north to south near the coast of Buyukkemikli Cape, near the mouth of the Dardanelles strait and the west coast of Lesvos and Chios islands in summer 1991 (Fig. 3). Instead of this cold water, warm LSW occupies the same area from south to north in spring 1992 (Fig. 6). Dardanelles inflow carries BSW near the Lemnos Island and fills the surface of the North Aegean Sea with less dense water coming from the Black Sea. In winter and early spring coastal water is relatively colder than the off-shore water while temperature values are expected to be higher in general along the coastal area in late spring and in summer. The LSW and LIW flow towards north between Chios Island and mainly near the west of Cesme Peninsula. LSW extends to the vicinity of Gokceada, to the south of Somothraki Island. LIW reaches to the Baba Cape. Because of existing Levantine Waters, temperature difference reaches 1.5 C between the eastern coastal water and the offshore water. The Levantine light waters are observed obviously with temperatures of C, salinity of more than psu and density of approximately 29.0 kg/m 3 in the vertical section along west east direction in the Central Aegean Sea (Fig. 9). Therefore the Levantine Waters can be detected from temperature field easily rather than salinity due to slight salinity difference between the waters of Levantine origin and the Central Aegean Sea in spring-time (Figs. 6, 7 and 9). The relatively colder and saltier water in the Central Aegean Sea and offshore side of the Candarli Bay (Figs. 7 and 9) probably is the mixture of several water masses; the Levantine origin water, the water mass formed due to winter convection, the old upwelling water from previous fall and summer and new upwelling water mainly observed around Lesvos Island. The upwelling water at the surface in the north of the Lesvos Island is pushed towards to west. The cooled air temperature over Dardenelles area is carried by northerly wind to the west of Lesvos (another upwelling area) and its effect results in decreasing temperatures at off-shore. Therefore it is another reason that the 3.2. Spring 1992 The monitoring study covers area from Marmara to Levantine Sea. The survey is carried out in April/May 1992 (57 days). The sampling is consisted of 147 CTD casts. Again the data from the CTD stations shown in Fig. 6 is considered to analyse the Eastern Aegean Sea. The wind blowing from NNE during spring cruise has an agreement with the prevailing direction of wind. NNE is seen 30% in all times over the Gokceada Island and its occurrence is even more 50% during cruise time. The longest duration of the wind from NNE direction is 42 h. It influences the surface current in the North Aegean Sea. The wind intensity decreases towards south. In Dikili region the longest duration is not more than 9 h even from the prevailing direction ESE. Over Izmir Bay the longest duration of wind coming from WNW direction (14 h) in the cruise time is also the prevailing wind in all time. The prevailing winds in regions take place according to the distribution land and sea masses. The wind from west is the prevailing wind in Izmir. In Dikili region, the wind from west direction has no long fetch because of the Lesvos Island that hinders the wind from west. It is thought that the prevailing wind blows parallel to the coast locally depending on the location of the meteorological center. The intensity of wind decreases towards south further and it is not easy to determine a certain wind direction. Fig. 8. The depth of the density 29.1 kg/m 3 (Spring 1992).

7 508 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 9. Temperature, salinity and density values of vertical section extending from west to east. The station which is shown by an arrow is shown also in Fig. 10. The path of vertical section is shown on the small map (lower left part of the figures). heaviest surface water observed in the western part of Lesvos Island. This mixture water is denser than its environment can be noticed from the depth of the 29.1 kg/m 3 isopycnal contour (Fig. 8). The probable water movements around the Lesvos Island could be as a result of the local prevailing winds. The wind from ESE in Edremit and from NNE in Saros bay environments can cause an upwelling near the

8 E. Sayın et al. / Journal of Marine Systems 88 (2011) northeastern coast of Lesvos and downwelling near the northern coast of Edremit Bay. It means the water coming from Candarli Bay area through Dikili strait influences the surface water of Edremit Bay. LSW and LIW separate the eastern shallow coastal area from the Aegean Sea water. Coastal water that is under the influence of cold easterly wind from Turkish mainland is appropriate for the dense water formation. The high dense water from Edremit Bay has a connection with the water near Baba Cape. The dense water from the deeper part of Edremit Bay is contrary to surface water flows towards the south direction combining with the water of Candarli intermediate layer. Not only the excess evaporation in summer but also the frequently taking place upwelling process is responsible to find the very high density values in Edremit Bay. The prevailing wind from ESE direction is suitable to trigger the upwelling process near the east coast of the Lesvos Island in all seasons. The highest density value (29.84 kg/m 3 )isfoundin the deep part of the Edremit Bay (Fig. 10). The North Aegean intermediate water is relatively colder and fresher than the intermediate water observed in the Marmara Sea. Flow through the Dardanelles Strait is driven by the density differences between the Marmara Sea and the Aegean, and the sea level difference between these seas. Waters, which has contrasting properties and originates from the Black Sea and the Aegean, supply the two layers stratified flows in the Dardanelles. The North Aegean communicates with the Marmara Sea under 20 m from the surface due to the density difference in spring The inflow from Marmara Sea takes place in the opposite direction to the current below due to the sea level difference. concern. The CAgE and Chios Eddy are resolved as in summer circulation (Fig. 12, after Sayin et al., submitted for publication). The wind directions SSW and NNE direction dominate over the Saros area during autumn 1992 cruise with occurrence of 53% and 19% respectively. The wind from both directions reaching time to time up to 10 m/s intensity can be effective for the surface current. Namely the longest duration of wind coming from SSW and NNE direction is more than 72 h and 53 h over the area respectively. The wind from south and southeast is dominant from the region Dikili (SE 27%), Izmir-Cigli (SE 22% and S 18%) up to Kusadasi (SE 22%). The longest duration of wind coming from SE and SSE are 18 and 13 h over the Dikili area and from south is 11 h over the Izmir-Cigli area. Besides the south wind the duration of the winds from other directions are not long enough to change the surface current. Further south the dominance of wind from south direction continues. The longest duration of wind coming from S and SSW are 15 and 13 h over the Kusadasi area. Temperature is warmer near coastal area and LSW is not seen in the North Aegean Sea (Fig. 11). LIW is blocked in the vicinity of 3.3. Fall 1992 The fall survey is carried out in mainly September/October 1992 (45 days). 151 CTD stations are visited. The general area coverage is from Marmara to Levantine Sea. The CTD stations that belong to the Eastern Aegean Sea are shown in Fig. 11. The circulation pattern in fall 1992 is not different much than the summer circulation from the point of view of eastern Aegean Sea Fig. 10. TS-Diagram for spring 1992, temperature, salinity and density values of the intermediate depths. Region 1 (Marmara), Region 3 (Dardanelles), Region 4 (Samothraki), Region 5 (Baba Cape), Region 8 (Lesvos East), Region 9 (Lesvos West), Region 13 (Chios on-shore), Region 16 (Gokova), Region 17 (South Aegean). Fig. 11. Temperature, salinity and density near surface field (5 m) for fall 1992.

9 510 E. Sayın et al. / Journal of Marine Systems 88 (2011) Kusadasi in fall The coastal salty water formed as a result of strong evaporation is cascaded into the central Aegean Sea, to the west of Lesvos Island. This is another mechanism (precondition) for the forming densest water in the central Aegean Sea. It is interesting to know which extension of Levantine Waters penetrates into the Aegean Sea and how intense the dense water cascade from the coastal area. The answer can be found by analyzing the data of the vertical section perpendicular to the coast (Fig. 13). The LSW cannot penetrate up to the Central Aegean as observed in summer period. On the other hand the LIW is even seen in the vicinity of Canakkale Strait at the depth about 100 m. The LSW that reaches the Chios area can be detectable from the vertical section at the surface with temperature of 20.5 C and salinity of 39.0 psu (Fig. 13). Any cascade from coastal areas is not obvious, unlike in spring. The coastal area is warmer and saltier than spring period. The contrary influences of temperature and salinity cause no change in density. The T-S Diagram of water masses greater than 100 m deep shows relatively denser waters in different regions (Fig. 14). Isopycnals rise to the offshore side of Candarli with kg/m 3 density is observed at the depth of 278 m. The other dense waters are found in North Sikiros and Chios with the densities of kg/m 3 and kg/m 3 at the depths of 330 m and 276 m respectively Winter 1993 Fig. 12. Fall general circulation pattern (after Sayin et al., submitted for publication) on the background of average SST field of fall Kusadasi Bay. Mixed water with BSW influences the area from the Baba Cape up to the Kusadasi area making a big cyclonic movement. It is remarkable that Saros Bay have a very distinguish water mass. It is colder and denser than its environment. The reason for such cold water is the blowing wind first from NNE and then from SSW direction. Therefore upwelling water covers whole Saros bay. Northerly wind influences the Baba Cape environment as well and this region consists of two types of water, one is the Cape upwelling water and the other one is the cold BSW. The possible mechanism in the vicinity of the Central Aegean Sea is that the southeast wind maintains that the cold coastal water forming eastern coast of the Lesvos Island is pushed through Musellim Strait and is combined with Saros upwelling water and mixed water from Baba Cape. The new mixed water flows into the core of CAgE and south of it. This mechanism is partly seen from Figs. 11 and 12. CAgE has the densest water mass in the core because; - The cyclonic behavior of CAgE is suitable for upwelling. - An occurring chimney type shape makes the air-sea interaction easier. - The cold upwelling water from the adjacent basins and west of the Lesvos Island is carried with the local prevailing northerly wind system into the Central Aegean. - Although the coming relatively cold Black Sea water does not reach the Middle Aegean area, the cold air over the BSW can be carried with the help of the northerly wind frequently blowing in to the North Aegean (Open Sea Convection). - The possible salty dense water cascade is from the shallow coastal area. The evaporation has a stronger effect on salinity in shallow water. - The salt content of Levantine Waters is slightly high compared to the salinity of the Aegean Sea in the same depth of water column. The distribution of such salty water influences the area to form denser water in the core of CAgE. In the southern part (Edremit and Lesvos) the wind from SE direction does not allow the mixed water influences the coastal area. SE is also prevailing wind direction over the area Izmir, Cesme and The number of CTD stations visited is 57 during winter cruise. They are implemented in January/February 1993 from Jan 18 to Feb 21 (40 days). The survey area coverage is from Marmara Sea to Chios basin. The general wind conditions do not change during winter 1993 cruise time over the Saros and Dikili area. NNE and SSE directions are most frequently seen wind directions over the Saros and Dikili area respectively. The NNE wind over Saros is stronger than the SSE wind (~ 9 m/s) over the Dikili (2 3 m/s) in average. The longest duration of wind coming from the direction NE is 28 h over the Saros region. The second and third longest duration of wind coming from the directions NNE and ENE are 22 and 13 h respectively. Over the Dikili area the occurrence of SSE wind is 30% and wind from other directions is rare. The longest duration of wind coming from the direction SSE is 32 h during cruise time. The wind conditions in Izmir and Kusadasi region are different than the general picture. The longest duration of wind coming from NE and NNE are 14 and 16 h over the Izmir area and from N is 34 h over Kusadasi area. The northerly winds over both areas are not strong in intensity. The intensity of wind is 2 3 m/s for Izmir and 1 2 m/s for Kusadasi environment. The strong upwelling in summer period continues until early fall because of the blowing moderate wind from NNE direction all the time. Increasing BSW volume in the vicinity of Dardanelles strait together with the upwelling water does not allow Levantine waters, especially LSW, penetrating into the North Aegean Sea in summer and fall seasons. On the other hand the Levantine Waters penetrate even to the north of the Lemnos Island mainly in spring time. The decreasing volume of BSW and the lack of upwelling water in Saros Bay and near the Baba Cape play important role for the being Levantine Waters in the North Aegean Sea. The water column is homogeneous in the central area and in Chios region in wintertime. The Central Aegean has the coldest and the saltiest water at the surface compare to other periods because of the occurring strong winter convection process. The winter convection is rather effective in the Central Aegean as result of doming isopycnals particularly in the core of CAgE and Chios Eddy during summer and autumn (preconditioning). Therefore the densest water forms in the Central Aegean in winter. The data is limited up to Cesme Peninsula. Even the surface water is very cold due to very cold air temperature occurs in winter 1993 (Fig. 15). The warm surface water of Levantine that exists during the

10 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 13. Fall1992, temperature, salinityanddensityvaluesofverticalsectionextendingfrom west toeast. Thepathof verticalsection is shownonthesmallmap (lowerleft partofthefigures). last spring time loses its temperature. But it makes strong front with colder BSW as well. It is saltier and denser than environment. The temperature value of 14 C seen at the surface is the temperature of intermediate or bottom water in the Central Aegean Sea seen in other seasons. The salinity values are also very high even 39.5 psu detected near the west coast of Lesvos Island. It explains that the upwelling

11 512 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 14. Fall 1992 T-S diagram, temperature and salinity values are chosen below the subsurface to determine where the densest points are located kg/m 3 density was measured in the Candarli (Region 10) area in the depth of 278 m, kg/m 3 in the Chios Basin (Region 14) in the depth of 330 m and the density of 29.4 kg/m 3 in the southwest of Lemnos Island (Region 6) in the depth of 276 m. Region 16 (Gokova), Region 17 (South Aegean). process in the cores of two eddies forming in the Central Aegean Sea brings cold and salty water to the near surface layers (preconditioning). The domed isopycnals can be outcropped to the surface during the winter convection process. Central Aegean Sea homogeneous water along the water column can be observed in the vertical section extending from the Marmara Sea to Candarli area (Fig. 16). The density of water reaches even 29.5 kg / m 3 near the coastal area of the Baba Cape at the depth of 100 m. Winter 1993 T-S diagram shows very high densities that were not measured before and written in any study. The density of 29.9 kg/m 3 was measured in the depth of 63 m between the Chios Island and the Cesme Peninsula, 29.8 kg/m 3 in the coastal area of Candarli Bay in the depth of 131 m and the density of 29.7 kg/m 3 near the coastal area of Baba Cape in the depth of 82 m (Fig. 17). 4. Discussion The data of current study is convenient for the comparison with the data of Velaoras and Lascaratos (2005) selecting approximately the same regions by analyzing the time evolution. Unfortunately our measurements do not cover deep basins in order to make one to one comparison. After comparing our study with the studies of Sayin and Besiktepe, 2010 and Velaoras and Lascaratos, 2005, we find out that the main mechanism of EMT is the increasing of density in the Aegean Sea before and especially during the major deep dense water formation period (1993). The shoaling isopycnals in the Aegean Sea especially in the Central Aegean (Fig. 18a) related to the occurring strong winter convection processes in the cold winters (1992 and 1993) and very effective in 1993 (major deep dense water formation period) with outcropping high isopycnals at the surface. The density levels start to increase from summer 1991 to winter 1993 not only due to severe winter conditions also as a result of salt intrusion from Levantine Sea to the Aegean Sea (Velaoras and Fig. 15. Temperature, salinity and density near surface field (5 m) for winter Lascaratos, 2005), dense water cascade from colder coastal area (Eastern Aegean Sea) during cold period and dense salt-water cascade as a production of excess evaporation from coastal area in summer time (Sayin and Besiktepe, 2010). The dense-water cascade is very apparent towards the Central Aegean Sea from coastal area for all seasons (Figs. 3, 6, 11 and 15). It is found that the Central Aegean Basin is the site of the formation of AgIW (Gertman et al., 2006). AgIW depending on its density renews either the intermediate or the deep water of the Cretan Sea. Therefore the density levels in the Central Aegean Sea play crucial role for the EMT event. The 29.2 and 29.3 kg/m3 σ θ isopycnals shoal from summer 1991 up to spring 1992 and then outcropping of the high isopycnals take place in winter 1993 (Fig. 18). The increasing isopycnal levels in the water column during spring 1992 reach the maximum during the major deep dense water formation time (winter 1993) and deepen after, because of the maximum water exchange takes place between the Aegean Sea and the Mediterranean. This deepening rate of the isopycnals (after Velaoras and Lascaratos, 2005, their

12 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 16. Temperature, salinity and density in the vertical section from North Aegean Sea to offshore of Candarli area. The path of vertical section is shown on the small map (lower right part of the figures). Fig. 14) is approximately 5.7 m/month in the intermediate water level. The shoaling rate is about 22.2 m/month (Fig. 18b). This bigger rate shows that the two severe cold winters 1992 and 1993 play very important role for EMT event. The forming cyclonic gyres should be taken into account for having homogeneous dens water in its core coinciding with the 1992 and 1993 cold winter conditions. The intrusion of highly saline surface and intermediate waters masses of Levantine origin into the Central Aegean Sea acts as a preconditioning factor together with the dense water cascade from the coast for the forming dense water. The shoaling high isopycnals were not observed only in the Central Aegean Sea, but as well in all basins in the Aegean Sea. Fig. 18b shows the shoaling isopycnals in the Lemnos basin before the major deep dense water formation time (winter 1993) with an exception in fall The isopycnals deepen in Fall 1992 for a while and shoal again up to February There is a continuous deepening of the isopycnals from 1993 onwards in the Lemnos basin. 5. Conclusions After a drastic change in the thermohaline circulation in the Mediterranean Sea, the source of Eastern Mediterranean Deep Water had shifted from the Adriatic to the Aegean Sea. This process has been called as the Eastern Mediterranean Transient (EMT). The Central Aegean Sea played an important role on EMT event. It is the

13 514 E. Sayın et al. / Journal of Marine Systems 88 (2011) Fig. 17. Winter 1993 T-S diagram, 29.9 kg/m 3 density was measured in the depth of 63 m between the Chios Island and the Cesme Peninsula, 29.8 kg/m 3 in the coastal area of Candarli Bay (Region 11) in the depth of 131 m and the density of 29.7 kg/m 3 near the coastal area of Baba Cape (Region 5) in the depth of 82 m. Marmara (Region 1), Saros (Region 2), Dardanelles (Region 3), Lemnos south (Region 6), Candarli off-shore (Region 10). Fig. 18. The depth of isopycnals obtained from cruise data of current study and from the study of Velaoras and Lascaratos (2005) for the Central Aegean (upper panel, there is no density level less than 29.4 kg/m 3 in the major deep dense water formation period (Winter 1993, red color) and for the Lemnos Basin (below panel). main site of water mass formation and the main contributor to EMT event. The observed high densities near the surface have not been seen in the Aegean Sea before winter 1993 strongly related to the severe winter condition. The water column in the core of the Central Aegean Gyre was homogenized by the strong winter convection processes increasing the air sea interaction. When the cyclonic gyre was enhanced during the EMT, isopycnals rose closer to the surface. This coincided with the 1993 cold winter condition resulting in surface cooling and mixing sufficient to cause deep convection. The shoaling high isopycnals were not observed only in the Central Aegean Sea, but as well in all basins in the Aegean Sea. There is a continuous deepening of the isopycnals from 1993 onwards in all basins. The intrusion of highly saline surface and intermediate water masses of Levantine origin into the Central Aegean Sea, acts as a preconditioning factor together with the dense water cascade from the coast. The Levantine Waters can be detected even in the North Aegean Sea with its warm and saline water characteristics. These waters change also the water characteristics of the Central Aegean Sea. The warm and more saline surface waters mean the more evaporation in summer seasons. We found Levantine Waters to be particularly prominent during the spring of 1992, more than in the other seasons. In the gyre area the cold circulated water mixes with the upwelling water coming from north in summer time. The LSW is blocked and does not penetrate further to the north because of the existing upwelling water seen near Saros (Buyukkemikli Cape) and off-shore side of Baba Cape (a product of northerly wind) especially in summer and in fall time. The water masses are rather homogeneous vertically with the influence of strong wind mixing and convective mixing in winter time. After analyzing the Saros data, it can be concluded that the Saros region is not the site where very dense water formation occurs. But occasionally (in case of NNE wind) observed dense upwelling water in the limited area affects the North Aegean Sea. The relatively cold and saline upwelling water from the adjacent basins (Saros and Baba Cape Area) are carried with the local prevalent northerly wind system into the Central Aegean Sea. Besides the upwelling water, Open Sea Convection related to the cold air over the BSW can influence the Central Aegean Sea. CAgE (Central Aegean Eddy) is the place that AgIW (Aegean Intermediate Water) forms extensively. It means that this place is the source of the main water body of the Aegean Sea and it gives information about the possible occurrence of EMT through the shoaling of the isopycnals. A continuous monitoring in that region is necessary for prior determination of next possible EMT. References Demirov, E., Pinardi, N., Simulation of the Mediterranean Sea circulation from 1979 to 1993: Part I. The interannual variability. Journal of Marine Systems 33 34, Durrieu de Madron, X., Zervakis, V., Theocharis, A., Georgopoulos, D., Comments on Cascades of dense water around the world ocean. Progress in Oceanography 64, Gertman, I.F., Pinardi, N., Popov, Y., Hecht, A., Aegean Sea water masses during the early stages of the Eastern Mediterranean Climatic Transient ( ). Journal of Physical Oceanography 36 (9), Ivanov, V.V., Shapiro, G.I., Huthnance, J.M., Aleynik, D.L., Golovin, P.N., Cascades of dense water around the world ocean. Progress in Oceanography 60, Klein, B., Roether, W., Manca, B.B., Bregant, D., Beitzel, V., Kovacˇevic, V., Lucchetta, A., The large deep water transient in the Eastern Mediterranean. Deep-Sea Research I 46, Nittis, K., Lascaratos, A., Theocharis, A., Dense water formation in the Aegean Sea: numerical simulations during the Eastern Mediterranean Transient. Journal of Geophysical Research 108 (C9), Roether, W., Manca, B.B., Klein, B., Bregant, D., Georgopoulos, D., Beitzel, V., Kovacevic, V., Luchetta, A., Recent changes in the eastern Mediterranean deep waters. Science 271, Roether, W., Klein, B., Manca, B.B., Theocharis, A., Kioroglou, S., Transient Eastern Mediterranean deep waters in response to the massive dense-water output of the Aegean Sea in the 1990's. Progress in Oceanography 74, Samuel, S., Haines, K., Josey, S., Myers, P.G., Response of the Mediterranean Sea thermohaline circulation to observed changes in the winter wind stress field in the period Journal of Geophysical Research 104 (C4), doi:10. 99/1999JC

14 E. Sayın et al. / Journal of Marine Systems 88 (2011) Sayin, E., Besiktepe, S., Temporal evolution of the water mass properties during the Eastern Mediterranean Transient (EMT) in the Aegean Sea. Journal of Geophysical Research 115, C doi: /2009jc Sayin, E., Besiktepe, Ş., Uçkaç, Ş., and Eronat, C., (submitted for publication): General circulation of the Aegean Sea. Theocharis, A., Nittis, K., Kontoyiannis, H., Papageorgiou, E., Balopoulos, E., Climatic changes in the Aegean Sea influence the Eastern Mediterranean thermohaline circulation. Geophysical Research Letters 26 (11), Unluata, U., A review of the physical oceanography of the Levantine and the Aegean basins of the Eastern Mediterranean, in relation to monitoring and control of pollution. Institute of Marine Sciences, METU Technical Report, 55. Velaoras, D., Lascaratos, A., Deep water mass characteristics and interannual variability in the North and Central Aegean Sea. Journal of Marine Systems 53, Velaoras, D., Lascaratos, A., North Central Aegean Sea surface and intermediate water masses and their role in triggering the Eastern Mediterranean Transient. Journal of Marine Systems 83, Zervakis, V., Georgopoulos, D., Karageorgisa, A.A.P., Theocharis, A., On the response of the Aegean Sea to climatic variability: A Review. International Journal of Climatology 24, Zodiatis, G., Advection of the Black Sea water in the North Aegean Sea. The Global Atmosphere and Ocean System 2,