Distribution of the thermohaline characteristics in the Aegean Sea related to water mass formation processes (2005-2006 winter surveys)

Abstract

Aiming at portraying the Aegean's water mass structure and identifying
Dense Water Formation processes, two winter cruises were conducted in
2005-2006, across the plateaus and depressions of the Aegean Sea. The
most prominent feature of the water mass distribution in the basin is a
distinct "X-shape" of the Θ-S characteristics, suggesting a
complicated coupling of the major Aegean sub-basins. The surface and
deep waters are relatively decoupled with diverse origin
characteristics, while the intermediate layers act as connectors of the
main thermohaline cells. The Central Aegean seems to play a key role due
to formation processes of water masses with densities equal and/or
higher than 29.2 kg/m3, that take place in the sub-basin and
disperses in the North Aegean. On the other hand, the South Aegean
appears greatly influenced by the Eastern Mediterranean circulation and
water mass distribution, especially under the Eastern Mediterranean
Transient status. The Transitional Mediterranean Water monitored in the
post-EMT period and characterized by low temperature at 14.2°C, low
salinity at 38.92 and low dissolved oxygen at 3.97 ml/l, with its core
around 750 m and above the saline (39.06) Cretan Deep Water, altered
significantly the South Aegean structure. The pre-EMT thermohaline
pattern of the Central and South Aegean deep layers were similar, while
the bottom density of the Central basin was higher than that in the
South Aegean. Thus, it is possible that the deep waters of the Central
Aegean acted as a dense water reserve supply for the deeper part of the
Southern basin.

Full text

Distribution of the thermohaline characteristics in the Aegean Sea
related to water mass formation processes
(2005–2006 winter surveys)
Vassilios D. Vervatis,
1
Sarantis S. Sofianos,
1
and Alexander Theocharis
2
Received 7 December 2010; revised 23 June 2011; accepted 7 July 2011; published 30 September 2011.
[1]Aiming at portraying the Aegean’s water mass structure and identifying Dense Water
Formation processes, two winter cruises were conducted in 2005–2006, across the plateaus
and depressions of the Aegean Sea. The most prominent feature of the water mass
distribution in the basin is a distinct “X‐shape”of the Q‐S characteristics, suggesting a
complicated coupling of the major Aegean sub‐basins. The surface and deep waters are
relatively decoupled with diverse origin characteristics, while the intermediate layers act as
connectors of the main thermohaline cells. The Central Aegean seems to play a key role
due to formation processes of water masses with densities equal and/or higher than
29.2 kg/m
3
, that take place in the sub‐basin and disperses in the North Aegean. On the other
hand, the South Aegean appears greatly influenced by the Eastern Mediterranean circulation
and water mass distribution, especially under the Eastern Mediterranean Transient status.
The Transitional Mediterranean Water monitored in the post‐EMT period and characterized
by low temperature at 14.2°C, low salinity at 38.92 and low dissolved oxygen at 3.97 ml/l,
with its core around 750 m and above the saline (39.06) Cretan Deep Water, altered
significantly the South Aegean structure. The pre‐EMT thermohaline pattern of the Central
and South Aegean deep layers were similar, while the bottom density of the Central basin
was higher than that in the South Aegean. Thus, it is possible that the deep waters of the
Central Aegean acted as a dense water reserve supply for the deeper part of the Southern
basin.
Citation: Vervatis, V. D., S. S. Sofianos, and A. Theocharis (2011), Distribution of the thermohaline characteristics in the
Aegean Sea related to water mass formation processes (2005–2006 winter surveys), J. Geophys. Res.,116, C09034,
doi:10.1029/2010JC006868.
1. Introduction
[2] In late 1980s and early 1990s the Aegean Sea drew the
attention of the oceanographic community, due to the dra-
matic shift of the Eastern Mediterranean thermohaline cir-
culation pattern. The abrupt changes in the Eastern
Mediterranean Deep Water (EMDW) posed a series of
questions on the stability of the regional thermohaline cir-
culation. Since then, several authors oriented their work in
the Eastern Mediterranean Transient (EMT) event, empha-
sizing the dynamical implication of the Aegean Sea as a
substantial source of regional deep waters [Roether et al.,
1996, 2007; Theocharis et al., 1999b, 2002; Klein et al.,
1999; Lascaratos et al., 1999; Samuel et al., 1999;
Malanotte‐Rizzolietal., 1999; Zervakis et al., 2000; Wu
et al., 2000; Boscolo and Bryden, 2001; Tsimplis and Josey,
2001; Stratford and Haines, 2002; Tsimplis and Rixen,
2002; Nittis et al., 2003].
[3] In the beginning of the 20th century oceanographic
studies focused in the dense waters masses of the Aegean. In
a process first proposed by Nielsen [1912] North Aegean
Dense Water (NAgDW), “a uniform cold and heavy layer
from surface to bottom north of Cyclades plateau,”could
flow toward the South Aegean and possibly contribute to the
EMDW. However, Schott [1915] re‐analyzing Nielsen’s
data confirmed his argument but disputed the fact that this is
a dominant process. Furthermore, Pollak [1951], Wüst
[1961] and later Hopkins [1978, 1985] minimized even
more the role of the Aegean Sea as a Dense Water Forma-
tion (DWF) area, as well as a contributor to the EMDW
compared with the Adriatic Sea. Under the same rationale
Plakhin [1971, 1972] did not comment of the Aegean being
a contributor to the EMDW, but made a step forward
describing a thermohaline cell triggered from DWF in Chios
and Cretan basins. Lacombe and Tchernia [1958] and Miller
[1974] did not change the suggested marginal character of
the Aegean Sea with respect to DWF. The only dissonance
comes from El‐Gindy and El‐Din [1986], using cruise data
from 1948 to 1972, suggesting that just outside the Cretan
1
Division of Environmental Physics, University of Athens, Athens,
Greece.
2
Hellenic Center for Marine Research, Institute of Oceanography,
Attiki, Greece.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JC006868
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, C09034, doi:10.1029/2010JC006868, 2011
C09034 1of16

arc straits 40% of the EMDW originated from the Cretan
Deep Water (CDW). The general consensus of the majority
of the authors in the pre‐EMT period, leads to the hypoth-
esis that the Aegean produces occasionally insignificant
amounts of dense waters. On the other hand, in the light of
new evidence, it becomes increasingly more likely that the
Aegean Sea’s location and atmospheric forcing make the
basin an advantageous DWF area.
[4] The Aegean Sea is a semi‐enclosed basin character-
ized by an alteration of shelves, sills and bottom depres-
sions. It is a semi‐enclosed basin located in the northeastern
Mediterranean (one of the four major basins of Eastern
Mediterranean Sea), covering an area of 180000 km
2
.Itis
connected to the Marmara and Black Seas through the
Dardanelles Strait in its northern part and connected to
the Levantine and Ionian Seas through several straits at the
southern end. The total volume of the Aegean Sea is about
75000 km
3
with an average depth of 415 m accounting for
the presence of extensive plateaus. Its topography is very
complicated, with over 3000 islands and islets and, in
consequence, introducing fragments of continental shelf
areas divided by numerous bottom depressions between
them. The deepest basins include the North Aegean trough,
encompassing the north Sporades, Athos, Lemnos and Saros
basins, with maximum depths up to 1500 m, the Skyros,
Chios and Ikaria basins in the Central Aegean with maxi-
mum depth of 1100 m, bounded to the south by the broad
Cyclades plateau and sills with depths less than 350 m, and
finally, the much bigger Cretan basin in South Aegean, with
maximum depth of 2500 m (Figure 1).
[5] The dominant winds are primarily from the north,
bringing cold and dry air through the Balkan Peninsula
[May, 1982], contributing to substantial heat fluxes for
surface buoyancy loss. The annual amount of evaporation
over the Aegean is around 1.3–1.5 m/yr [Jakovides et al.,
1989; da Silva et al., 1994; Drakopoulos et al., 1998] and
exceeds the sum of precipitation and river runoff 0.5 m/yr
and 0.11 m/yr, respectively [Poulos et al., 1997], while the
annual upward net heat flux is estimated at 26 W/m
2
[Poulos
et al., 1997]. The latter implies that the Aegean Sea, on an
average loses heat through its surface. A biased method to
classify a basin as a concentration or dilution is to examine
its density relative to the density of an adjacent basin
[Hopkins, 1978]. Zervakis et al. [2004] suggested that the
Aegean should be classified as a concentration basin, based
on the density distribution below the sills.
[6] The Aegean’s water masses and stratification are
subject to strong variability at various timescales [Zervakis
et al., 2004; Skliris et al., 2007], intense circulation pat-
terns [Korres et al., 2002; Olson et al., 2007], water mass
formation [Velaoras and Lascaratos, 2005, 2010; Gertman
et al., 2006; Vervatis et al., 2009] and strong forcing of both
wind and thermohaline character [Theocharis et al., 1999b;
Sofianos et al., 2005]. The combination of this forcing with
the complex sub‐basin circulation and mesoscale eddy field
render a complicated picture of the regional dynamics. All
observational studies [Gertman et al., 1990; Ovchinnikov
et al., 1990; Theocharis and Georgopoulos, 1993; Theocharis
et al., 1999b; Zervakis et al., 2000; Olson et al., 2007] and
numerical simulations [Wu et al., 2000; Korres et al., 2002;
Nittis et al., 2003; Sofianos et al., 2005], point out a general
cyclonic circulation in the Aegean Sea, while an anticyclonic
pattern dominates the northeastern corner of the basin
[Sofianos et al., 2005]. However, the most active dynamic
features are the mesoscale cyclonic and anticyclonic eddies,
which can extend to several Rossby radii of deformation
(around O(10 km)). Some of these features appear to be
permanent (i.e., cyclonic eddy in south Chios basin, anti-
cyclonic circulation in Lemnos basin), while others have a
transient character and yet the mechanisms responsible for
the formation and decay of these features are not clear
[Olson et al., 2007].
[7] The main water masses and their distribution can be
summarized as follows. The most characteristic feature of
the stratification pattern in the North Aegean is the surface
brackish Black Sea Water (BSW) outflowing from the
Dardanelles Strait. The BSW forms a front with the intru-
sion of the much saltier Levantine Surface Water (LSW)
entering the Aegean through the eastern Cretan arc straits.
The intermediate layers of the North‐Central Aegean are
occupied by Levantine Water (LW) of Levantine origin and/
or the locally formed Aegean Intermediate Water (AgIW)
[Gertman et al., 2006]. The Levantine Intermediate Water
(LIW) entering through the eastern Cretan arc straits follows
a northward path along the Turkish coast to be modified by
the aforementioned LW. Over the Lemnos‐Lesvos plateau
the LW and/or the AgIW get sub‐ducted below the BSW
thermohaline front. The vertical structure of the North
Aegean is comprised of a 20–70 m modified BSW surface
layer, of an intermediate LW and/or AgIW layer down to
400 m and of the locally formed NAgDW [Zervakis et al.,
2003, 2004; Velaoras and Lascaratos, 2005]. After expe-
riencing strong mixing in the North‐Central Aegean, and
being modulated by the presence of mesoscale eddies
[Zodiatis, 1994], the water masses appear at the western side
of the South Aegean, along the Evian coast and through the
Kafireas strait, with significantly lower salinities than the
ambient surface water masses. A less saline water mass
present in the sub‐surface layers of the South Aegean is the
Modified Atlantic Water (MAW). At intermediate depths,
the annually ventilated Cretan Intermediate Water (CIW), a
water mass more saline than the LIW, has often been
Figure 1. Aegean bathymetry and major features (basins,
plateaus and straits).
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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monitored above the core of Transitional Mediterranean
Water (TMW), a product formed by the mixture of LIW and
EMDW outside the Cretan arc. In the early 1980s, the deep
layers of the Cretan Sea were occupied by a homogenous
water mass similar to the TMW and there was no signal of
CDW type [Zervakis et al., 2000].
[8] Observations after the early 1990s revealed a dra-
matically different structure of the Cretan Seawater column.
During the EMT event, extremely dense and saline waters of
local origin, started filling the deep Cretan basin and over-
flowing through the sills of the Cretan arc straits [Roether
et al., 1996; Kontoyiannis et al., 1999; Theocharis et al.,
1999a, 1999b]. Due to its high density, the CDW dis-
placed water from the deepest parts of the Levantine and
Ionian basins of the Eastern Mediterranean, and as a con-
sequence the TMW masses intruded the Cretan basin.
During the EMT, the hydrography of the Cretan Sea
included a distinctive deep water mass close to the bottom of
the basin (800–2500 dbar), possibly formed by convection
in the open sea and/or in the surrounding shelf areas
[Theocharis et al., 1999b; Lykousis et al., 2002]. This
structure was not static but seemed to undergo considerable
changes during the same period. Following the EMT, the
CDW volume in the Cretan Sea has decreased, most probably
due to changes in the DWF processes [Theocharis et al.,
2006; Sofianos et al., 2007; Vervatis et al., 2009].
[9] Aiming at portraying the stratification of the Aegean’s
sub‐basins, as well as locating possible areas of DWF pro-
cesses, two cruises were conducted during the winters of
2005 and 2006 and the basic findings are described in this
paper. In section 2 we provide a brief description of the
cruises and the data collected. In section 3 the basic Q‐S
characteristics of the water masses during both winters are
reviewed and the hydrographic structure observed in the
major concavities of the Aegean is presented. Additionally,
in section 4 DWF structures are presented, under the scope
of potential sub‐basin coupling mechanisms. Finally, in
section 5 in order to quantify the variability of the water
mass structure, related to the EMT and its phases, an
intercomparison analysis is carried out.
2. Data and Methods
[10] Two cruises were carried out on board the R/V Aegaeo,
from 1 to 10 March 2005 and from 3 to 13 February 2006.
During the first cruise a total of 44 hydrographic CTD
stations were occupied while a total of 47 stations were
recorded in the second cruise (Figure 2). The majority of the
stations were concentrated in the deep concavities in order
to identify the dense water characteristics along the main
north‐south axis of the Aegean Sea and locate possible areas
of DWF. A small number of stations where occupied on this
axis across the Aegean’s plateaus, in order to identify pos-
sible shelf convection processes and to monitor exchanges
between sub‐basins.
[11] The profiles of temperature, salinity (conductivity),
and dissolved oxygen concentration were collected using a
Sea‐Bird CTD with a General Oceanics rosette, including
12 Niskin Bottles for the oxygen and salinity calibration.
The data set acquired was quality controlled, the dissolved
oxygen concentration was calculated with the Winkler
method [Carpenter, 1965] and the data set was filtered and
sub‐sampled at 1 dbar vertical resolution. For safety reasons
the maximum depth of each profile was approximately
10 dbar shallower than the stations sea bottom. All the tem-
perature and densities discussed in the present paper are
potential temperatures and potential densities, respectively.
In section 5, in order to compare the stratification of the
Aegean Sea in different phases of the EMT event, a data
set covering the winter of 1987 [Malanotte‐Rizzoli and
Robinson, 1988] has been analyzed together with the data
sets of the 2005 and 2006 winter cruises. From the
aforementioned data sets an averaged profile has been
computed for each Aegean sub‐basin. Finally, a data set
(MEDATLAS II) was used for analysis of the Aegean deep
Figure 2. Hydrographic CTD stations during the winter cruises of 2005 and 2006; magenta line denotes
cruise trajectory; first cruise station (start): square dot; last cruise station (end): diamond dot; North
Aegean (NAg): blue dots; Central Aegean (CAg): green dots; South Aegean (SAg): red dots; gray lines:
straits boundaries of the three major Aegean sub‐basins [Gertman et al., 2006].
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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water properties evolution, during the EMT [MEDAR Group,
2002].
3. Cruises Hydrographic Observations
3.1. Q‐S Characteristics of the Aegean Sea
[12] The temperature and salinity characteristics mea-
sured during the first cruise, in winter 2005, are depicted in
Figure 3. The projected data in a Q‐S diagram revealed a
diversity of the surface and deep layers between the North
(blue dots) and South (red dots) Aegean. The Q‐S char-
acteristics present a distinct “X‐shape.”At the surface, the
low salinity BSW layer and a warm LSW feature are
observed, whereas in the deep layers, locally formed dense
water masses of the NAgDW and CDW are found. The
same Q‐S pattern is also observed in the second cruise in
winter of 2006 (Figure 4). Differences between the two
years data sets are mainly due to surface variability of the
intruded BSW and LSW masses and different locations of
sampling (e.g., Lemnos/Athos basins in North Aegean,
Skyros/Chios basins in Central Aegean, central/eastern
Cretan basins in South Aegean). Spatial differences in the
deep layers across the Aegean sub‐basins, indicates DWF
variability and/or different Q‐S modification rates inside
the depressions [Zervakis et al., 2003, 2009].
[13] In Figures 3 and 4 the magenta triangle, following
Gertman et al. [2006], represents intermediate layers of the
Aegean, leaving outside the surface and deep waters. The
“X‐shape”of the Q‐S diagram implicitly suggests a tran-
sition in properties inside the magenta triangle where the
North and South Aegean Q‐S characteristics converge in the
Central Aegean, as clearly depicted in both cruises in
Figures 3 and 4 (green dots, upper panels). The Q‐S trace
indicates possible DWF, since high dissolved oxygen con-
centrations higher than 4.8 ml/L prevail from the surface
down to depths of 400 m (Figures 3 and 4, lower panels). In
general, relatively high dissolved oxygen concentrations
were recorded in intermediate depths across all three Aegean
sub‐basins.
3.2. Stratification and Circulation
[14] In the North Aegean, observations on a dense grid of
CTD stations from 2005 winter survey, illustrate the water
mass structure of Lemnos basin. A SW‐NE transect reveals
a strongly stratified water column in the region (Figure 5).
The most prominent water mass feature, characteristic of the
circulation pattern and exchange with the Dardanelles Strait
is the BSW intrusion, with surface temperature minimum
11.5°C, salinity 36.2 and density 27.5 kg/m
3
, in a wedge
like layer with depth changing from 100 m in the NE to 20 m
in the SW ends of section. This salinity minimum is combined
with the dissolved oxygen concentration maximum of the
intruding water. The BSW intrusion from the Dardanelles
Strait into Lemnos basin is divided in two basic branches
Figure 3. Q‐S diagram; winter 2005; North Aegean (NAg): blue dotes; Central Aegean (CAg): green
dots; South Aegean (SAg): red dots; Gray‐color bars: intermediate depths (db) and oxygen concentrations
(ml/lt). Q/S/s
Q
boundaries (magenta lines): NAg surface layers (S < 38.7), SAg surface layers (Q> 14.7°C),
Aegean dense waters (s
Q
> 29.2 kg/m
3
)[Gertman et al., 2006].
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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around the island of Lemnos, depicted also with surface
drifters [Olson et al., 2007]. The first is westward through
Saint Eustratius strait and the second is northward through
Lemnos strait. The second branch influences more the surface
layers of Lemnos basin since the BSW extends to greater
depths (Figure 5). Below the surface layer, a warmer and
more saline layer centered at intermediate depths, extends
down to 400 m in Lemnos basin (Figure 5, Q/S panels). This
denser water mass cannot be formed locally due to presence
of the BSW insulator layer at the surface [Zervakis et al.,
2003, 2004; Velaoras and Lascaratos, 2005; Gertman et al.,
2006] and is most probably advected from the Central
Aegean. This water mass (Figure 5, Qpanel) is a mixture of
LW and/or AgIW formed locally in the Central Aegean
(discussed in section 4), with varying values of temperature
13.9–14.4°C, salinity 38.7–39 and density 29–29.2 kg/m
3
.
In greater depths, below the main sill level, to the bottom of
Lemnos basin the dissolved oxygen concentration decreases
to 3.7 ml/l. The very low oxygen concentration of the deep
waters inside Lemnos basin indicates possible trapping of
“old”dense water from the early stages of the EMT. At the
time of the cruises in the deepest parts of Lemnos basin,
below 1000 m, the density was around 29.4 kg/m
3
gaining
buoyancy in comparison with the first stages of the EMT
[Zervakis et al., 2003, 2009].
[15] In the Central Aegean the locally formed water
masses, as well as the water masses of Black Sea and Le-
vantine origin, undergo strong mixing while being modulated
by the presence of mesoscale eddies [Zodiatis, 1994]. The
modified water masses follow the general cyclonic circula-
tion of the Aegean, exiting the Central Aegean via two main
paths to the South Aegean: a surface path through Kafireas
strait with Q‐S characteristics of 13°C, salinity at 38.1 and
28.8 kg/m
3
and a surface/intermediate path through Myco-
nos‐Ikaria strait. The latter exchange between the Central and
South Aegean is monitored from three stations across the
strait (Figure 6). Warm and saline waters of Levantine origin
(maximum values at 15.4°C and salinity at 39.1, Figure 6,
Q/S panels), enter the Central Aegean at the east end of
the strait with densities of 29 kg/m
3
(Figure 6, s
Q
/O
2
panels),
whereas cool and less saline AgIW egress from surface/
intermediate depths at the west side of the strait (minimum
values at 13.8°C and salinity at 38.7, Figure 6, Q/S panels)
with slightly increased densities of 29.1 kg/m
3
(Figure 6,
s
Q
/O
2
panels).
[16] The vertical water mass structure of the South
Aegean follows a post‐EMT typical three layer pattern
consisting of a surface, an intermediate and a deep layer,
but in contrast with the North Aegean the stratification is
much weaker (surface density 29 kg/m
3
; 1000 m density
29.18 kg/m
3
). However, none of the layers resemble the
Q‐S characteristics ofthe North Aegean. This can be explained
mainly by the very strong influence from the adjacent basins.
In Figure 7, a NW‐SE transect during the winter close to
Figure 4. Q‐S diagram; winter 2006; North Aegean (NAg): blue dotes; Central Aegean (CAg): green dots;
South Aegean (SAg): red dots; Gray‐color bars: intermediate depths (db) and oxygen concentrations (ml/lt).
Q/S/s
Q
boundaries (magenta lines): NAg surface layers (S < 38.7), SAg surface layers (Q> 14.7°C),
Aegean dense waters (s
Q
> 29.2 kg/m
3
)[Gertman et al., 2006].
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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Kassos strait, depicts the strong influence of Levantine region
to South Aegean. In contrast with Lemnos basin where the
surface stratification is dominated by the shallow (20–100 m
thickness) BSW layer, the South Aegean Mixed Layer Depth
(MLD) extends to 200 m (Figure 7). The horizontal tempera-
ture gradient, ascribed to a permanent cyclone in the eastern
Cretan Sea [Balopoulos et al.,1999;Theocharis et al.,1999b;
Kontoyiannis et al.,1999;Tsimplis et al.,1999;Georgopoulos
et al., 2000], is dominating the region (Figure 7, Q/S panels).
[17] The South Aegean intermediate layers consisted of
several intrusive and locally formed water masses. A mix-
ture of LIW and locally formed Cretan Intermediate Water
(CIW) lies between 200 and 500 m (Figure 7, Q/S panels,
intermittent core of 14.6°C and salinity at 38.98). A second
very distinct intermediate layer in the Cretan Sea is the
TMW, a mixture of LIW and old EMDW with its core
around 750 m, which enters from the adjacent Levantine
basin through Kassos strait at the sill depth. The TMW Q‐S
core characteristics are low temperature 14.2°C, relatively
low salinity 38.92 and low dissolved oxygen concentrations
3.97 ml/l (Figure 7) and had never been recorded inside the
Aegean in the pre‐EMT period. In the first stages of the
EMT, the TMW was present in the Cretan Sea in interme-
diate layers of about 300 m [Theocharis et al., 2006;
Sofianos et al., 2007; Vervatis et al., 2009]. Since 1995 the
EMT event started to decay confirming its transitional char-
acter. As a consequence, the TMW core was continuously
deepening, due to the deflation and outflow of the CDW from
the Cretan to Levantine basin through Kassos strait. At the
time of the cruises, the CDW horizon has dropped in depths
greater than the Kassos sill. The deep layers of Cretan basin
are filled by CDW, locally formed in the Aegean Sea, with
properties of 14°C, salinity of 39.06, 29.32 kg/m
3
and dis-
solved oxygen concentration of 4.2 ml/l, which is larger than
the overlying TMW.
4. DWF Processes
[18] Although the North Aegean is a dilution basin (winter
surface densities of 28 kg/m
3
influenced by the presence of
BSW), the deep layers are filled up to the sills with very
dense waters (29.4 kg/m
3
below a depth of 1000 m). Several
authors in recent studies [Zervakis et al., 2000, 2003;
Tragou et al., 2003; Nittis et al., 2003] suggested that the
northern Aegean can act in some exceptionally rare cases as
a concentration basin, under extremely cold and dry winter
Figure 5. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 6 CTD stations in the
winter of 2005 at Lemnos basin in the North Aegean (NAg); inside map blue large dot denote initial sta-
tion, following a SW‐NE direction of the magenta section (small blue dots); top axis indicating the code
names of the stations during the cruise; lower axis are in distance (km) from the initial station; major sea-
bed features: Lemnos basin.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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events such as the EMT. Under this scope two basic scientific
questions arise: Are the Aegean sub‐basins decoupled or is
there a connecting mechanism? Is the Aegean circulation
controlled by intrusions from adjacent basins or by local
processes?
[19] Investigating the role of the Central Aegean in DWF
processes, a number of stations were occupied during the
cruise of 2006 in Skyros, Chios and Ikaria deep basins,
aiming at capturing open ocean convection events and shelf
convection in Lemnos‐Lesvos plateau. In Figures 8 and 9
both eastward and westward transects are depicted, respec-
tively, introducing a winter snapshot of the North‐Central
Aegean. The most outstanding feature in both transects is
the surface isopycnal layers of 29.1 kg/m
3
and 29.2 kg/m
3
over almost the entire Central Aegean (Figures 8 and 9, s
Q
panel). The physical mechanism which makes this area
favorable for DWF is the combination of extreme saline
LSW, loss of buoyancy through intense winter cooling,
along its northward travel over the depressions and plateaus
of the Central Aegean and finally transformed to the
locally formed AgIW. This buoyancy loss mechanism is
enhanced by the trapping of water in the permanent and
semi‐permanent cyclonic features in the region (Chios and
Skyros cyclones) over the deep sub‐basins [Lykousis et al.,
2002; Olson et al., 2007]. Both deep water and shelf
convection processes across the Lemnos‐Lesvos plateau
are believed to contribute to DWF in the region. During the
2006 cruise the DWF processes extended to intermediate
depths around 400 m. The densest surface waters masses
observed above Lemnos‐Lesvos plateau at 29.3 kg/m
3
, while
in Chios and Skyros depressions the surface density was
measured at 29.2 kg/m
3
.
[20] Evidence of previous occurrences of DWF in the
region is revealed by the deep stagnant layers in Figures 8 and
9. In Skyros and Athos basins, a strong EMT signal seems to
be preserved since the late 1980s period, evidenced by
extreme dense waters trapped in the bottom depressions. In
Figure 9 the EMT signature is emphasized with low dissolved
oxygen at 3.4 ml/l, low temperatures at 13°C and salinities at
39.05, resulting to high densities of 29.4 kg/m
3
.Zervakis
et al. [2003, 2009] suggested a slow renewing of the
deep waters in both Skyros and Athos basins, due to an
internal wave braking mechanism.
[21] Intrusive water masses from adjacent basins, such as
the Levantine and Black Seas can play significant roles in
the Aegean Sea processes. The fact that two different water
Figure 6. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 3 CTD stations in the
winter of 2006 at the north edge of the Myconos‐Ikaria strait in the Central Aegean (CAg); inside map
green large dot denote initial station, following a W‐E direction of the magenta section (small green dots);
top axis indicating the code names of the stations during the cruise; lower axis are in distance (km) from
the initial station; major seabed features: Myconos‐Ikaria strait, Ikaria basin.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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masses (with very diverse temperature, salinity and density
characteristics) are intruding from the south and north ends
of the basin does not allow the development of a simple
thermohaline cell. The BSW, with very low density, is
covering big parts of the north region of the Aegean acting
as an insulator and preventing DWF processes [Zervakis
et al., 2004]. Coupling between the basins is achieved
through intermediate (and possibly deep) water mass for-
mation processes in the central part of the basin, creating a
rather complicated thermohaline cell. A replenishing mech-
anism of intermediate layers is introduced, through buoyancy
loss of the saline LSW. A mixture of LW and/or locally
formed AgIW recirculates at intermediate depths across the
Aegean. Convection was monitored in Chios basin during the
winter of 2006 (Figure 10), enhanced by the permanent Chios
cyclone [Sofianos et al., 2005; Olson et al., 2007]. Dense
water masses formed in Chios basin are observed to be
spreading isopycnally at 400 m. The Q‐S characteristics of
the dense water masses at intermediate depths of Chios basin
(Figure 10, Q/S panels) are also found in the North Aegean
(Figures 5, 8 and 9, Qpanel) and in the western side of the
Myconos‐Ikaria strait (Figure 6), flowing into the South
Aegean. The deep layers in different basins are decoupled
from each other due to very irregular seabed topography.
5. Pre/Post‐EMT State
[22] The stratification monitored during 2005 and 2006
cruises should be discussed in the framework of the EMT
climatic shift. The question is: how typical is this late‐post
EMT pattern? In order to answer the latter question, a pre‐
EMT reference year, namely data from a 1987 cruise
[Malanotte‐Rizzoli and Robinson, 1988], is compared to the
winter cruises of 2005 and 2006 (Figure 11, 1987–2005–
2006 map panels). Zervakis et al. [2000] presented a synoptic
log of the winter survey in the beginning of the EMT during
1987. A severe buoyancy loss took place in mid‐March
[Lagouvardos et al., 1998] a few days before the stations
depicted at Lemnos and Chios basin (Figure 11, 1987 map
panel, blue/green dots). Therefore, no significant EMT signal
was yet detected at that time in the deep layers. The stations in
the Cretan basin (Figure 11, 1987 map panel, red dots)
chronologically are placed in late‐March, but the EMT signal
monitored at least two months later in late‐April. Conse-
Figure 7. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 7 CTD stations in the
winter of 2005 at eastern Cretan basin in the South Aegean (SAg); inside map red large dot denote initial
station, following a NW‐SE direction of the magenta section (small red dots); top axis indicating the code
names of the stations during the cruise; lower axis are in distance (km) from the initial station; major sea-
bed features: eastern Cretan basin.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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quently, the 1987 mid/late‐winter profiles are considered to
be representative of the pre‐EMT state of the Aegean.
[23] In Figure 11 (Q/S/s
Q
panels for the North‐Central‐
South Aegean), an average winter profile is depicted, of the
thermohaline properties during the three years (1987, 2005
and 2006). The North Aegean profiles reveal a three‐layer
stratification (Figure 11, top row Q/S/s
Q
panels). Surface
BSW intrusion enhances the stratification, whereas LW and/
or AgIW replenish the intermediate layers. In the deep
layers NAgDW is trapped in the depressions. During 1987
the North Aegean surface waters appear to be more saline,
probably due to reduction of the BSW inflow [Zervakis et al.,
2000] and/or to intrusion of larger amounts of LSW/LIW
[Malanotte‐Rizzoli et al., 1999]. At the time of the cruises,
the typical North Aegean Q‐S characteristics significantly
changed, as the colder and less saline intermediate/deep
layers altered in more saline water masses (Figure 11).
Between the cruises, in late post‐EMT state, the deep layers
of Lemnos basin indicate a small buoyancy gain due to a
possible internal wave breaking [Zervakis et al., 2003, 2009].
This mechanism introduces a slow evolution of the North
Aegean deep layers in the post‐EMT period, in contrast to the
uplifting mechanism of the deep layers ventilation during
the EMT period.
[24] The South‐Central Aegean exhibits a completely
different structure during the pre/post‐EMT periods. A two‐
layer stratification in both areas during the pre‐EMT period
changed in a compound mode during the EMT phase. In
Central Aegean warm and saline surface water masses existed
on top of cold and less saline trapped deep layers (Figure 11,
middle row Q/S/s
Q
panels). During the winter cruises of
2005 and 2006 a much weaker stratification was observed,
resulting to an almost uniform dense water column in 2006 of
29.15–29.2 kg/m
3
(Figure 11, middle row s
Q
panel). The
DWF processes described in section 4, replenish the inter-
mediate layers of the Aegean sub‐basins. The evolution of the
deep layer characteristics during the EMT is presented in
Figure 12, where the average deep (below 600 m, extracted
from MEDATLAS II database) temperature, salinity and
density was computed form data obtained during cruises from
1986 to 2006 [MEDAR Group, 2002]. Because the pre‐EMT
temperature/salinity patterns of the deep layers of the Central
and South Aegean basins were similar (Figure 11, middle and
bottom rows Q/S/s
Q
panels, 1987; Figure 12), and the bottom
Figure 8. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 17 CTD stations in
the winter of 2006 at the eastern side of the North‐Central Aegean (NAg‐CAg); vertical dashed gray line
in seabed divides NAg‐CAg regions; inside map blue large dot denote initial station, following a N‐S
direction of the magenta section (small blue‐green dots for NAg‐CAg, respectively); top axis indicating
the code names of the stations during the cruise; lower axis are in distance (km) from the initial station;
major seabed features: Lemnos basin, Lemnos‐Lesvos plateau, Chios basin, Myconos‐Ikaria strait.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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density of the Central basin was higher than that in the South
Aegean at that time, it seems likely that the deep waters of the
Central basin acted as a reserve supply of dense water for the
deeper part of the Southern basin. In fact, while salinity
increased at deep layers in both basins, it was compensated by
a temperature increase that resulted in a decrease of deep
water density in the Central Aegean, while there was a
marked increase of density in the Southern Aegean deep
waters compared to pre‐EMT stratification (Figure 11, mid-
dle and bottom rows Q/S/s
Q
panels; Figure 12). Furthermore,
the coupling of the Central‐South Aegean deep layers is
complicated, due to entrainment phenomena, underlined
from the greater densities in Chios than in Cretan basin,
during the first stages of the EMT (Figure 12).
[25] The very complicated topography of the Aegean Sea
can also play a role in the evolution of deep water char-
acteristics. The topographic differences between the North
and Central Aegean, highlighted by the hypsographic dia-
grams in Figure 13, can explain the density differences in
the deep layers. Lemnos and Athos basins are very abrupt
with a wide, shallow shelf and contrasting deep depression,
favorable for replenishment by abrupt pulses of DWF on
shelf regions such as the Lemnos‐Lesvos plateau. The Chios
basin is more like a V‐shaped basin with gradual depth
change (Figure 13), and easier to replace deep water masses
by thermohaline circulation cell. However, in Central
Aegean in Skyros basin the seabed topography is similar with
the North Aegean Athos and Lemnos basins (Figure 13), and
the deep water thermohaline properties deviate from those in
Chios basin (Figures 4 and 9). This effect of topography on
deep water mass renewal can explain the density differences
between the depressions in the North and Central Aegean.
Although temperature, salinity and density increased during
the EMT in all northern deep basins, the density of the Central
Aegean appears lower in the post‐EMT, where renewal
processes are favored by the local topography.
[26] The EMT signal inside the Cretan Sea has recently
declined [Theocharis et al., 2006; Sofianos et al., 2007;
Vervatis et al., 2009] confirming its transitional character.
However, it is still remaining strong enough to obscure a
Figure 9. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 14 CTD stations in
the winter of 2006 at the western side of the North‐Central Aegean (NAg‐CAg); vertical dashed gray line
in seabed divides NAg‐CAg regions; inside map blue large dot denote initial station, following a N‐S
direction of the magenta section (small blue‐green dots for NAg‐CAg, respectively); top axis indicating
the code names of the stations during the cruise; lower axis are in distance (km) from the initial station;
major seabed features: Lemnos basin, Athos basin, Skyros basin, Kafireas strait, Myconos‐Ikaria strait.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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Figure 10. Transects of Q(°C), S, s
Q
(kg/m
3
) and oxygen concentration (ml/lt), of 6 CTD stations in
the winter of 2006 at Chios basin in the Central Aegean (CAg); inside map green large dot denote initial
station, following a SW‐NE direction of the magenta section (small green dots); top axis indicating the
code names of the stations during the cruise; lower axis are in distance (km) from the initial station; major
seabed features: Chios basin.
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Figure 11. Map panels: Aegean Sea winter hydrographic stations for the years 1987 (pre‐EMT), 2005
and 2006 (post‐EMT); North Aegean (NAg): blue dots at Lemnos basin; Central Aegean (CAg): green
dots at Chios basin; South Aegean (SAg): red dots at central‐eastern Cretan basin. Q/S/s
Q
panels: Aver-
aged profiles of Q(°C), S and s
Q
(kg/m
3
), for all three major sub‐basins.
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Figure 12. Q/S/s
Q
properties of the Aegean deep layers (averaged data below 600 m) during the pre/
post‐EMT period (1986–2006). Lemnos (North Aegean‐NAg, blue circles), Chios (Central Aegean‐CAg,
green squares) and eastern Cretan (South Aegean‐SAg, red diamonds) sub‐basins.
VERVATIS ET AL.: AEGEAN SEA THERMOHALINE CHARACTERISTICS C09034C09034
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clear understanding of the dynamics of the Aegean, since
the CDW remains extremely dense (29.32–29.34 kg/m
3
below 2000 m) and because the TMW evolution masks the
signals of the change in intermediate layers.
6. Summary and Discussion
[27] Two winter cruises in the late post‐EMT period were
conducted during the winter season of 2005 and 2006
aiming at depicting the distribution of the thermohaline
properties and possible DWF processes in the Aegean Sea.
Previous studies highlighted the dramatic changes that took
place in the region during the EMT [Gertman et al., 1990,
2006; Roether et al., 1996, 2007]. With respect to the last
decade observational studies [Balopoulos et al., 1999;
Kontoyiannis et al., 1999; Tsimplis et al., 1999; Theocharis
et al., 1999a, 1999b; Georgopoulos et al., 2000; Zervakis
et al., 2000, 2003, 2004; Lykousis et al., 2002], the cruises
designed to portray the water mass changes in the deep basins
of the North and South Aegean. The decoupled surface and
deep water masses, unveiled an intermediate conveyor belt
coupling the Aegean sub‐basins. A distinct “X‐shape”Q‐S
emphasizes that the intermediate layers connects the basins
through a main thermohaline cell. Occasional deep water
formation processes replenish the deep layers while the
intermediate waters serve in their preconditioning.
[28] Intrusive water masses from adjacent basins, such as
the Levantine and Black Seas influence the surface vari-
ability of the region, masking local effects. The deep layers
of the three major bottom depressions of the Aegean Sea are
decoupled from each other due to the very irregular seabed
topography. Intermediate layers are replenished through
buoyancy loss of the saline LSW. Both open ocean (Chios
and Skyros basins) and shelf convection (Lemnos‐Lesvos
plateau) DWF processes were monitored in the Central
Aegean. Intermediate dense waters were formed locally as a
mixture of LW and local waters, recirculating at interme-
diate depths across the North‐Central Aegean. On the other
hand, the South Aegean appears greatly influenced by the
Eastern Mediterranean general circulation and water mass
distribution, especially under the EMT status. The South
Aegean intermediate layers with a core at 750 m are influ-
enced by the TMW intrusion in the Cretan Sea, a mixture of
Figure 13. Hypsographic curves A(z) (km
2
) of cross‐sectional areas as a function of depth z (m). Merid-
ional cross‐sectional areas (magenta lines inside maps): Lemnos and Athos basins in North Aegean
(NAg), Skyros and Chios basins in Central Aegean (CAg), central Cretan and eastern Cretan basins in
South Aegean (SAg). Gray areas in transects and gray dashed line in hypsographic diagrams denote
the deepest layers of the basins (below 600 m).
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the LIW and the old EMDW entering at sill depth of the
Kassos strait from the adjacent Levantine basin.
[29] In order to quantify the variability of the water mass
properties related to the EMT and its phases, a pre‐EMT
reference year, 1987, was selected to be compared with data
from the winter cruises of 2005 and 2006. During the winter
of 1987, relatively fresh and less dense waters are monitored
in the deep layers. The BSW from the Dardanelles and the
LSW/LIW intrusion from the Cretan arc straits governed the
Aegean surface layers. However, this status changed dra-
matically during the EMT, due to the abrupt displacement of
the deep Aegean waters, together with the significant TMW
intrusion. In the post‐EMT period, the TMW layer has
similar characteristics with the dense water masses in pre‐
EMT period [Zervakis et al., 2000]. Furthermore, those
properties are very much alike to the Central Aegean DWF
properties monitored in the two recent winter cruises during
2005 and 2006. Therefore, the DWF processes in the Cen-
tral Aegean could not only trigger the intermediate conveyor
belt, but also act as an EMT relaxation mechanism inside the
Aegean.
[30]Acknowledgments. This work was supported by the Office of
Naval Research under contract N00014‐04‐1‐0524. The authors are deeply
indebted to the Captain and the crew of the HCMR R/V Aegaeo for con-
tinuous support during the whole measurement phase and to the Univer-
sity of Washington for the guidance and scientific suggestions/preparation
in both surveys. The thoughtful comments by the reviewers are gratefully
acknowledged.
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S. S. Sofianos and V. D. Vervatis, Division of Environmental Physics,
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(vervatis@oc.phys.uoa.gr)
A. Theocharis, Hellenic Center for Marine Research, Institute of
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