Download Hydrography of the eastern part of the Aegean Sea during the

Journal of Marine Systems 88 (2011) 502–515
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Journal of Marine Systems
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j m a r s y s
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, 35340 Izmir, Turkey
a r t i c l e
i n f o
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
a b s t r a c t
Hydrographic features of the Aegean Sea were studied using CTDs collected in 4 cruises from 1991 to 1993. 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 1991. 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 1992. 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.
© 2011 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.: + 90 232 278 5565; fax: + 90 232 278 5082.
E-mail address: [email protected] (E. Sayın).
0924-7963/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmarsys.2011.06.005
for the new forming EMDW. Samuel et al. (1999) analysed individual
monthly wind stress fields over the Mediterranean for 1980–1993
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 1988. Roether et al. (2007), showed that
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).
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
503
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 1993.
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 1992. 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 1991.
Fig. 3. Temperature, salinity and density near surface field (5 m) for summer 1991. The
locations of CTD stations shown with dark points.
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E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
Fig. 4. Summer 1991 temperature, salinity and density in the vertical section from Saros to Levantine. The path of vertical section is shown on the small map (lower right part of the first panel).
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
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 1993. 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.
505
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 1991. 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-Diagram for summer 1991 with the temperature, salinity and density values of the
intermediate depths. The density maximum measured in the Region 2 (Saros) at 393 m and
in the Region 9 (Lesvos) at 420 m is 29.37 kg/m3. Other dense waters are detected with a
density 29.28 kg/m3 at 382 m and 29.22 kg/m3 at 296 m in the Regions 14 (Chios off-shore)
and 13 (Chios on-shore) respectively. The value of 29.32 kg/m3 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 1992. The
locations of CTD stations shown with dark points.
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E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
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).
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
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 50–200 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 (39.1–39.15 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
16.5–17 °C, salinity of more than 39.15 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.
507
Fig. 8. The depth of the density 29.1 kg/m3 (Spring 1992).
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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
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
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) is found in
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 1992. The inflow from
Marmara Sea takes place in the opposite direction to the current below
due to the sea level difference.
509
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.
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Kusadasi in fall 1992. 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 29.44 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 29.43 kg/m 3 and 29.38 kg/m 3
at the depths of 330 m and 276 m respectively.
3.4. Winter 1993
Fig. 12. Fall general circulation pattern (after Sayin et al., submitted for publication) on
the background of average SST field of fall 1992.
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
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
511
Fig. 13. Fall 1992, temperature, salinity and density values of vertical section extending from west to east. The path of vertical section is shown on the small map (lower left part of the figures).
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
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E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
Fig. 14. Fall 1992 T-S diagram, temperature and salinity values are chosen below the
subsurface to determine where the densest points are located. 29.44 kg/m3 density was
measured in the Candarli (Region 10) area in the depth of 278 m, 29.43 kg/m3 in the
Chios Basin (Region 14) in the depth of 330 m and the density of 29.4 kg/m3 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).
Fig. 15. Temperature, salinity and density near surface field (5 m) for winter 1993.
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
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 σθ 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
E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
513
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 1992. The isopycnals deepen in Fall 1992 for a
while and shoal again up to February 1993. 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
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E. Sayın et al. / Journal of Marine Systems 88 (2011) 502–515
Fig. 17. Winter 1993 T-S diagram, 29.9 kg/m3 density was measured in the depth of
63 m between the Chios Island and the Cesme Peninsula, 29.8 kg/m3 in the coastal area
of Candarli Bay (Region 11) in the depth of 131 m and the density of 29.7 kg/m3 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).
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.
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/m3 in the major deep dense water formation period
(Winter 1993, red color) and for the Lemnos Basin (below panel).
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.
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