Influence of the water exchange
through the Dardanelles on the
thermohaline structure of the Aegean Sea
by
V.l. VLASENK0 1, N.M. STASHCHUK 1, V.A. IVANOV 1,
E.G. NIKOLAENK0 1, 0. USLU 2 and H. BENLl2
1
Marine Hydrophysical Institute, Ukrainian Academy of Sciences,
2, Kapitanskaya St., Sevastopol 335000, Ukraine.
2
Institute of Marine Sciences and Technology, 1884/8, Sokak NO10,
35340 Jnciralti-Izmir, Turkey.
ABSTRACT
On the basis of experimental data and of a high resolution thermohydrodynamic model the plume front off the Dardanelles is studied. It is
generated by an inflow of Black Sea surface brackish waters to the highlysaline Aegean Sea and represents a quasi-stable climatic formation. The
frontal zone has a complicated horizontal and vertical structure, incorporating secondary non-stationary frontal interfaces. The latter occur and develop under the influence of severe winds of various directions. When winds
of appropriate directions occur with speeds of 10 m/s and more, the front
may be driven back towards the strait to a distance of several miles away
from it. During the intensification of wind, the main frontal interface is
likely to become discontinuous, and secondary fronts may occur at both
sides of the front. Under such conditions an inversion and intrusion interlayers can take place within the frontal zone.
With the help of mathematical simulations the processes of formation
and evolution of the plume front were reconstructed. Numerical estimates
of the thermo-hydrodynamic characteristics of the Dardanelles plume front
were close to the real ones. This concerns the extent of the frontal zone,
the location, width and orientation of the frontal interface as well as the
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values of the temperature and salinity gradients both at the front proper
and within the frontal zone. The model transfrontal circulation fields show
that the latter circulation has the form of two cells with horizontal axes
parallel to the front.
RESUME
Nous avons etudie le front des Dardanelles a partir de donnees experimentales et d'un modele thermo-hydrodynamique a haute resolution. Ce
front, cree par la penetration des eaux de surface saumatres de la mer Noire
dans les eaux salees de la mer Egee, constitue une formation climatique
pratiquement stable. La zone frontale presente une structure verticale et
horizontale complexe avec des interfaces frontales secondaires non stationnaires. Ces dernieres se developpent sous I' inf! uence de vents forts de
directions diverses. Lorsque ceux-ci soufflent a 10 m/s et plus, le front peut
etre renvoye vers le detroit sur une distance de plusieurs kilometres. Quand
le vent forcit, la principale interface frontale est susceptible de devenir discontinue et des fronts secondaires peuvent naitre aux deux extremites du
front. Dans de telles conditions, des couches intermediaires peuvent prendre
place, par inversion et intrusion, al' interieur de la zone frontale.
A !' aide de simulations mathematiques, nous avons reconstitue Jes processus de formation et d'evolution du front. Les estimations numeriques des
caracteristiques thermo-hydrodynamiques du front des Dardanelles se soot
revelees proches des valeurs reelles. Ceci est vrai pour l'etendue de la zone
frontale, la position, la largeur et !' orientation de !'interface frontale ainsi
que pour les valeurs des gradients de temperature et de salinite sur le front
proprement dit comme a l'interieur de la zone frontale. Les champs de circulation transfrontale du modele revelent la presence de deux cellules avec
des axes horizontaux paralleles au front.
INTRODUCTION
The Dardanelles play an essential role in the water exchange between
the Aegean and Black seas. After their passage across the Marmara Sea,
Black Sea water masses enter the Aegean Sea through the Dardanelles. The
interaction of Black Sea surface brackish waters with the highly-saline Afon
basin leads to the formation of the Dardanelles plume front. This region of
the Aegean Sea is located between areas with markedly different physical
and geographical conditions, where an intense interaction and transformation of several water masses within a relatively small area take place. Here,
the investigation of fronts becomes particularly important.
This investigation is based on a vast number of in situ observations and
relies on a mathematical simulation to describe the structure and evolution
of the frontal zone occuring in the Aegean Sea in the Dardanelles area. The
data were compiled in the course of small-scale hydrological surveys in the
Aegean Sea from 1984 till 1993 during cruises of the Ukrainian R/V-s
Professor Kolesnikov, Mikhail Lomonosov, Mgla and the Turkish vessel
Piri Reis in the immediate vicinity of the strait and at some distance from it.
The location of the stations is represented in Fig. 1.
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Fig. 1- Location of stations in the Aegean Sea during various cruises.
THERMOHALINE STRUCTURE
OF WATER IN THE DARDANELLES AREA
Black Sea waters enter the Aegean Sea through the Dardanelles. The
strait is 120 km in length, ranging in width from 1 296 m to 18 520 m, and
with a maximum depth of 106 m. According to BOGDANOVA (1969) the
interface between the two water masses in the strait rises from 20 m at the
northern entry to 10 m at the southern one.
As the surface water from the Afon basin is basically inflowing from the
Black Sea, a unique natural phenomenon takes place in the Dardanelles
area: there, two seawater masses with markedly different characteristics
converge. As a result, a quasi-stationary front emerges in the Aegean Sea,
often observed in the triangular area defined by Lemnos Island, Lesbos
Island and Ios-Eustratios Island in the area of contact of Marmara SeaBlack Sea brackish waters running from the Dardanelles and highly-saline
waters from the Afon basin (Fig. 1). It is similar in structure to the fronts
occurring on the margins of the lenses resulting from the discharges of large
rivers, such as the Amazon or Siberian rivers. In accordance with the classification of FEDOROV (1983), the frontal interface studied here is related to
the category of ageostrophic climatic discharge fronts.
Another remarkable feature of this front is that, owing to a relatively
constant inflow of brackish waters from the Marmara Sea to the mixing
zone, the frontogenesis here is not markedly seasonal, as is the case in the
majority of rivers. However, seasonal fluctuations of water exchanges
through the Bosphorus and Dardanelles exist and they are connected with
the changes in the inflow of river waters to the Black Sea. For this reason,
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the Bosphorus current becomes more vigorous in spring to reach a maximum
in June-July; then it decays, reaching a minimum in September-October.
The vertical structure of currents in the Dardanelles is double layered.
The maximum speed of the upper current is observed at the surface, where
it exceeds 100 emfs, and swiftly decreases with depth. Below 20 m the
remaining part of the strait is filled with highly-saline Aegean Sea waters,
which are being transported by the countercurrent to the Marmara Sea and
then through the Bosphorus to the Black Sea. The speed of their transport is
not high, staying slightly superior to 10 cm/s in the countercurrent core.
In summer Black Sea waters quitting the Dardanelles have a salinity of
about 26-28 %0 (OYCHINNIKOV et al., 1976). As they propagate further, their
salinity increases up to 36-37%0. The waters outflowing from the strait at
some distance from the latter start to propagate in the form of separate flows
rather than one flow. Owing to that, low salinity values are observed in the
surface salinity field along the northern and western shores of the sea (33-34
and 36-38%o respectively). In winter the reduction of the inflow of Black
Sea waters and an intensive convective mixing caused by significant
cooling of the air lead to the general increase of surface waters salinity (up
to 35.0-38.9%0) and their strong cooling (down to 12-17°C).
Shipboard and satellite data (GRISHIN et al., 1994) show that the front is
readily traceable not only in the surface salinity field, but also in the field of
sea surface temperature (SST). Multiple IR images of the Dardanelles area
distinctly show a high-gradient SST zone (GARVINE, 1977). The difference
in the temperatures of thoroughly-transformed Black Sea waters and the
surface water from the Afon basin amounts to about 2°C in the frontal zone.
In the surface salinity field the contrast is much more pronounced, being larger than 10%0.
The space thermohaline variability in that region is characterized by an
intermittent T,S-correlation (GRISHIN et al. , 1994). Negative T,S-correlation
for the front implies that temperature and salinity differences contribute to
the density drop at the front. This should result, in its turn, in the intensification of dynamic effects and fronts. In other words, thermal fronts with the
additional contribution of salinity to the density drop and salinity fronts with
the additional thermal contribution to the density drop in their performance
are better pronounced than purely thermal fronts. In any case, the ultimate
result of frontogenesis, irrespective of the nature of a local deformation
field, apparently depends in large measure on the nature of the background
spatial variability of temperature and salinity, with the salinity variability
likely to play a more important role.
FORMATION OF THE FRONT
Although part of the Mediterranean basin, the Black Sea has a very distinct hydrological regime. The water balance of the Black Sea is characterized
by a large excedent of river discharge and precipitations over evaporation,
while in the case of the Mediterranean the annual rate of evaporation by far
exceeds the total precipitations and river runoffs. Therefore, throughout the
year, the Black Sea level remains above that in the Mediterranean.
As a rule, brackish Black Sea waters outflowing from the Dardanelles
form a well-pronounced plume traceable over a large range in the adjacent
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zone. As the density of Black Sea waters is much lower than that of Aegean
Sea waters, and as Black Sea fluids are continuously inflowing from the
strait to the mixing zone, the plume's free surface rises above the level of
ambient waters. This causes the plume to spread over the Aegean Sea
waters. Horizontal turbulent diffusion and turbulent mixing through the
lower interface between brackish waters and salty waters hamper the propagation of Black Sea waters in the Afon basin and contribute to the generation of sharp frontal boundaries along the plume's perimeter.
The fro ntogenesis of this plume front is driven by horizontal pressure
gradients, resulting basically from the inclination of the surface of brackish
water layer and the opposite inclination of the front between the plume
waters and ambient ones. As Jong as the two water masses interact, the front
and the frontal zone will exist. The front may change its position due to
fluctuations of the intensity of brackish water source. The front's location
can be also impacted by tides and drift currents.
The limits of plume distribution depend primarily on the volume of
brackish waters running into the zone of mixing. Precise data for water
transport through the Dardanelles in various hydrological seasons are not
available. According to OVCHINNIKOV et al. (1976), the volume of the overall Black Sea waters transported to the Mediterranean Sea amounts to about
183 km3 per year. It seems hardly possible to draw any conclusion here
about the typical size of the brackish plume by comparing this quantity with
the volume of the Danube discharge. The point is that all characteristics of
the frontal zone (position of the sharpest front, average and minimum salinity of brackish water in the Jens, values of horizontal and vertical gradients,
etc.) turn out to be different for the runoffs through the delta or the estuary.
This is shown in Fig. 2 which depicts seasonal vertical distributions of temperature and salinity for stations El near the Dardanelles and E2 off the
strait near the Lemnos Island. Summer profiles indicate that the salinity at
point E2 is 3.5%o lower than that recorded at El. Such unexpected behaviour of the salinity field supports the view that frequently isolated lenses
are likely to detach from the front (or else, the front may change its orientation). In other words, it is likely that measurements were made at a time
when the front position was not typical. The influence of various factors on
the structure and location of the front will be discussed below.
The front proper limiting the plume of brackish waters is shown in
Fig. 3. The section made in cruise 27 of the RN Professor Kolesnikov is
shown by a dashed line in Fig. 1. It is seen from Fig. 3 that the front is located in the upper 15 m layer. The salinity front bounds the plume along its
outer side, facing the open sea. Right beneath it a sharp vertical pycnocline
with the prevalent contribution of halocline is formed. The values of vertical
salinity gradients there attain significance 0.66 %o/m. As in the case of
plume river lenses, the ratio between the average thickness of the brackish
waters plume and its cross section has an order of 104 •
MATHEMATICAL MODELING
To describe frontal dynamics, a complete system of Reynolds hydrodynamic equations including non-linear members as well as turbulent exchange
terms was used. Following FOFONOFF (1962) density was calculated from
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~aetnity C"I- >
JD
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Fig. 2 - Vertical temperature/salinity distribution at stations El and E2 for winter, spring and
summer (from the data obtained by RN K. Piri Reis).
temperature and salinity, using the state equation. The infl uence of the
Coriolis force at the first stage of front formation was ignored.
On the sea surface the "rigid-lid" condition and shear of velocity induced
by the tangential wind stress were used. At the bottom the kinematic nonflow and no-slip conditions were imposed. At the open boundary free-flow
conditions were prescribed. The temperature and salinity distribution for
inflowing Marmara Sea and Aegean Sea waters corresponded to the month
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~
-....;,
-10
~
~-20
'G
-30
0
""'
'i::. -10
'-""
39.2
-t:'
....,,
~
~ -20
-30
Fig. 3 - Temperature/salinity transects across the plume front in the Aegean Sea (from the data
collected during cruise 27 of the RN Professor Koles11ikov).
of September. The vertical current velocity profile for the Dardanelles Strait
was taken from OVCHINNIKOV et al. (1976). The formulated task was solved
numerically. Computational grid steps ranged from 100 to 200 m horizontal ly and equalled 2 m in the vertical. The time step was set at 3 minutes, in
conformity with the Courant-Friedrichs stability requirement.
With the difference approximation of the equations used, an implicit difference scheme was applied, as described in VLASENKO (1993). The method
of variable directions on a rectangular grid with the 2nd-order monotonous
approximation at the upper temporal half-layer and with the approximation
performed using central differences at lower layer was used.
COMPARATIVE ANALYSIS
OF THEORETICAL AND IN SITU DATA
The major external factor responsible for the front formation and for the
support of transverse circulation at all stages of front existence and evolution is an inflow of brackish fluid into the denser stationary fluid. First,
consider the process of front formation in the absence of wind (Fig. 4). The
movement of fluid begins from the state of rest. The colder and Jess salty
Marmara Sea water travels from the Dardanelles Strait in the Aegean Sea
with a speed of 100 cm/s. There is a subsurface countercurrent near bottom
whose maximal speed is about 10 emfs.
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56
0
A,1<11l
-20
tL(SITl/S)
- 40
-&o
a
0
- 20
w(sm/s)
- 40
-60
0
S(%o)
- &
c
0
-20
- 40
'f'(oC)
- 60
Fig. 4- Vertical cross-sections of (a) cross-frontal velocity, (b) vertical velocity, (c) salinity and
(d) temperature 4 days after the beginning offrontogenesis (model data).
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From analysis of these pictures it follows that brackish waters moving
from the strait outcrop near the exit of the strait. At the edge of the front the
moving water spreading over the water at rest subducts at the site of front
formation. The existence of such a zone of descending water masses before
the front was indicated by numerical simulation, reflecting adequately the
real picture of water circulation. A similar phenomenon was indicated in the
data collected during a cruise of the RIV K. Piri Reis (see Fig. 5) in
September in the vicinity of the Dardanelles Strait at stations El, E2, E3.
As seen from Fig. 4, a frontal zone of about 4 km in width had evolved
by that time at a range of 45 km away from the straits, with the horizontal
salinity gradient at the sea surface within the frontal area attaining
1.6 %0/km, and with a temperature gradient of l.0°C/km. The front has a
thermohaline nature, that is, it manifests itself simultaneously in the temperature and salinity fields. Due to the peculiar profiles of the initial temperature and salinity distributions, the frontal interface is more pronounced in
the salinity field than in the temperature field. Changes in water density
within the front are basically governed by salinity fluctuations. Therefore, in
accordance with the velocity profile of the inflowing water, the lower border
of the plume is centered at a depth of 10-15 m. This is in a good agreement
with experimental data (compare Fig. 3 and Fig. 4).
Now let us focus on the horizontal gradients values, by which the front
is primarily identified. The gradient is calculated as a ratio between the drop
of a hydrophysical characteristic to the width of site of measurements. Since
parameter variation is non-linear, the gradient value changes across the
front. For various areas within frontal zones and even for different sections
of the front, gradient values also will be different.
0
:.attmtv
femper.. {u.rf' (°C)
E2
E1
E3
E7
ES
0
10
10
20
20
;;i
ES
0
( 0
/oo)
t.ittmitv
E3
0
E8
E7
E3
E'.2
10
;
~
20
30
30
30
40
40
40
50
50
50
60
60
60
70
10
70
80
80
80
90
go
Z,"1
Fig. 5 - Temperature, salinity and conveationaJ density at the transect for statioas "El-E2-E3E7-E8" (from the data compiled by RIV K. Piri Reis).
Bulletin de /'/11stitlll oceanographique, Monaco, n° special 17 (1996)
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Within the frontal zones multifrontal structures are likely to occur.
Therefore, when calculating gradient values from survey data compiled in
test areas with a small spatial step (1 mile, at best) we obtain values describing only this frontal zone rather that the front proper. Clearly, these values
will be lower than the respective values for fronts. The question of how
large are these discrepancies remains open. For the frontal section presented
in Fig. 3, the values of temperature and salinity gradients (derived as a result
of a survey with a 2 mile step) are 0.27°C/km and 0.16%0/km respectively.
These values are large in comparison with the average climatic ones. At the
same time, according to the high-resolution measurements (a flow system
with registration made every 100 m), gradient values in the inflow frontal
interface close to the Dardanelles Strait amounted to more than 0.60°C/km
and l.82%0/km (Fig. 6).
0
15 ]
0.
ffl.
'"'
c:...>
0
Q,
~
.....
~
t3
'-
32 0~
~
0
~ 12
~
~
~
ff
~
(0
0
t6op
2040x,km
Fig. 6 - Sea surface temperature and salinity across the Dardanelles front (from the data collected in cruise 31 of the RN Professor Koles11ikov).
The gradients presented above, derived through simulations of the front
formation, appear at first sight to be somewhat overestimated (l.0°C/km and
l.6 %0/km) but they are consistent with the field data. This implies that numerical simulation of the physical phenomenon conducted here is realistic.
Now let us turn from the analysis of integrated quantitative front characteristics to the qualitative examination of the interaction of two water
masses in the area of front's fore boundary. As indicated above, the vertical
distribution of current velocities in the Aegean Sea in the vicinity of the
Dardanelles is characterized by the occurence of a velocity shear, with the
countercurrent propagating in deeper layers below 10-15 m. As brackish
water masses descend in the fore front area to a larger depth, they are entrained by the oncoming countercurrent under the water mass spreading over
the sea surface. In the fore front area a specific subsurface "tongue" of brackish waters forms at depths ranging from 20 to 40 m, and is elongated
towards the strait. Its shape is readily traceable by the isotherms 18-19°C
shown in Fig. 4d and the isohalines 36.5-38.0%0 in Fig. 4c. Obviously, such
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conditions should result in the formation of inverse structures both in the
temperature field and in the salinity field , that eventually may lead to an
unstable fluid stratification . At first sight, this "non-physical" result derived
from the numerical calculations has been experimentally confirmed. Fig.7
shows temperature, salinity and density profiles obtained at station E2 close
to the strait. These profiles indicate that the pre-Dardanelles area is characterized not on ly by an " inverse thermocline" situation , when cold water
underlies the warm one, but also by the existence of interlayers of heavy
water overl ying a lighter one. This is caused by the interaction of the two
water masses described above.
lempe.:alJLze
f.!./P
/q.fir7
fatl'l
f<l.tl'l
2tUfJ
:n.o°so&-nil, 35.txl
Mt»
" " 22/lll
10 'I'"~'
"28-""
I " ' J ll./lll
I
24<1lll
26.llll
21!.llll
deasdy<4J
Fig. 7 - Inverse fealu res in the vertical distributions of temperature, salinity, and conventional
density at slation E2 on 14 May 1991 (from the data compiled by RN K. Piri Reis).
Another illustration of the mechanism responsible for the "sinking" of
brackish waters is a transect s hown in Fig. 8, which was made in cruise 6 of
the RN Professor Kolesnikov. The picture displays the salinity field in the
cross-front direction, where a tongu e of brackish waters derived through
modeling (Fig. 4c) is readily visualized.
EVOLUTION AND TRANSFORMATION
OF THE MODEL FRONT
One of the intrinsic properties of plume fronts is their ability to exist for
a long time, primarily by virtue of the continuing action of the source of
brackish waters. In th at case, th e frontal interface confining the brackish
plume or a lens assumes some quasi-stationary position. On the other hand,
frontal zones are known to be dynamical formations, within which high gradient fea tures become alternately marked and diffused and which, in turn,
are affected by many factors controlling their instability and evolution.
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157
-601
-80
-fOO
Fig. 8 - The salinity transect across the plume front in the Aegean Sea (from the data compile d
during cruise 6 or the R/V Professor Kolesnikov).
The newly-evolved plume frontal interface close to the surface is subjected primarily to wind forcing. When a complicated configuration of the
shoreline and bottom relief is involved, it is impacted, in addition, by tidal
circulation and upwelling circulation. These are the main factors affecting
the dynam ics of the thermohaline front in the area under study. As regards
the other factors, one should pay attention to the influence of the mesoscale
deformation field, connected, for example, with the system of frontal eddies.
Eddies of th is type a re known to eme rge owing to a baroclinic-barotropic
instability of currents within frontal zones. Those eddies, as well as th e
topogenic eddies a nd mushroom-like feat ures related to the frontal interfaces of the Black Sea and Mediterranean Sea, have been already described
by GRIS HI N et al. (1989, 1992). Whe n these are evolving or mov ing, th e
intensity of the mesoscale deformation field may become locally higher or
weaker, which, in turn, contributes to the sharpening, relaxation or displacement of the fron t (FEDOROV, 1983).
T he Aegean Sea, with its complex bottom relief, a jagged coastline and
many islands, appears to have the most complicated hydrom eteorologica l
regime among the Mediterranean Sea basins. The main factor governing tbe
bas ic climatic c haracteristics in this region is the interact io n of the main
baric centers of the atm osphere. Amongst these is the Azores anticyclone,
the w in ter-time anticyclo ne over Eurasia and summer-ti me cyclones over
Africa and Asia Minor. C hanges in the intensity and extent of these highand low-pressure areas, in turn, cause changes in the nature of atmospheric
circulation, in the position of atm ospheric fronts, in the sense of movement
and frequenc y of cyclone passing, i.e. all aspects determining air transport
conditions and weather in different seasons of the year.
Meteorological co ndi tio ns vary greatly from season to season in th e
Aegean Sea. For exa mple, north- and northeast-oriented winds prevail in
summer w hile winds of opposite directions prevail during winter. This difference is reflected in the seasonal temperature/salinity structure which is readily seen from the vertical profiles in Fig. 2.
Hence, the fronts structure and position essentially depends on the speed
and direction of wind. In the subsequent numerical experiments this
influence was simula ted by imposing wind fo rcing on the formed quasi158
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stable condition. The first numerical experiment simulated the effect of the
south-westerly wind blowing for one day with a speed of 7 mis in the
Dardanelles direction. Such meteo conditions are characteristic of the winter
season. Fig. 9 shows the temperature/salinity/horizontal velocity fields after
12 hours of wind forcing. Comparison of Fig. 9 with Fig. 4 distinctly reveals
the wind-induced displacement of the front towards the Dardanelles, as well
as the initial stage of the formation of a second front within the plume frontal
zone, whose area by that time had essentially reduced. As the calculations
have shown, such wind action during one day may displace the front to a distance of about 8 km from the strait, generating a complicated thermohaline
structure of waters within the subsurface layers. Against the background of
z.m
wind
24
0
36
4D
x,i<m
-20
. 4')
1(°C)
- 60
a
Q
- 20
- 60
11.(Sm/S)
c
Fig. 9 - Model data for the fields of temperature, salinity and horizontal velocity after 12 hours
of south-westerly wind forcing (with a wind speed of 7 rn/s).
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diffusion-intrusion processes occurring in the area of interaction of differently-oriented flows, a subsurface salinity front had evolved there.
The duration of wind action was one day. In the day following the cessation of wind forcing, the structure of the plume frontal zone did evolve. The
fields of temperature, salinity and horizontal flows resulting from such evolution are shown in Fig. 10. Notwithstanding the termination of the locking
wind, the sharpness of the front did not alter. Residual effects of the wind
forcing, in the form of a "two-cell" pattern of drift circulation, are readily
visualized in the vertical transect of the horizontal circulation component.
Because of the delay intrinsic to the system, currents directed towards one
another in the upper layer facilitate a collision of two different water masses.
z.m
wind
0
-2/J
-40
-60
U(SITI/$)
c
Fig . 10 - Model data for the fields of temperature, salinity and horizontal velocity 24 hours
after the termination of the south-westerly wind fo rcing.
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This results in the formation of a very narrow frontal interface, about 1 km
in width, with large gradients in both the salinity field and the temperature
field, with the layering of waters continuing. The latter fact is confirmed by
the occurrence of separate lenses in the lower strata, having closed
salinity/temperature isolines, however, at different ranges from the front.
A wind blowing from the Dardanelles during one day with a speed of
7 m/s was used in the next series of calculations. Here, under the impact
of a wind-induced drift current, one observed an increase of the frontal
zone width and the formation of an isolated lens in the salinity field, detached fro m the main front and transported by the residual drift current.
This process is illustrated in Fig. 11, which shows vertical sections of the
Ll.(sm/S)
c
Fig. 11 - Model results for fields of temperature, salinity and horizontal velocity after 12 hours
of north-easterly wind forcing with speed of 7 m/s.
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161
temperature and salinity fields, as well as the horizontal current velocity
component field, after one day of north-easterly wind forcing. Further evolution of the thermohaline fields within the frontal zone resulted in the formation of a new front 20-25 km away from the strait (Fig. 12). In the
temperature field, to the contrary, the formation of a detached lens was not
observed. However, the emergence of a secondary front within the extensive frontal zone is traceable here as well. Clearly the thermohaline structure of waters in the subsurface layers incorporating a system of inversions
and intrusions is very complex. This is supported by the observation that a
z,m
wind
X,l<nt
0
- '2J)
- 4D
S(%o)
0
U(sm/s)
c
Fig. 12 - Model results for fields of temperature, salinity and horizontal velocity 24 hours after
the termination of north-easterly wind forcing.
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large number of developed or slamming intrusions are practically always
present not only in the immediate vicinity of the frontal section proper but
also within the frontal zone. Thus, for example, during the oceanographic
research conducted in the Aegean Sea in December 1993, we observed a
similar picture under quiet weather on 26 December 1993, and three days
after the front had been driven nearly as far backwards as the Dardanelles
by the south-oriented severe wind with a speed up to 25 mis and of two
days' duration. Fig. 13 shows multiple different-size intrusions and inversions, which are the traces of the front in the studied area.
rtmpnawie
fS.1111
fS.f/J
11.10
IS.JI
Ttmpeiatuz~
1'.18
fJ'.14
0
JUIJ
11.!lJ
IUO
IMlJ
lf.41
15.50
p~.....-;,......~~~~~~~~~
II.DJ
J9~
J9.1$
JIN
JlKJ
Saluzl'f
H.llf
19 DJ
U.10
1$.0J
Dtnsil;t.
Fig. 13 - Inverse features in the vertical distributions of temperature, salinity and density at
stations 5734 (26 Dec. 1993) and 5740 (29 Dec. 1993 , Fig.l ) (from the data obtained in
c ruise 31 of the RN Professor Kolesnikov).
As indicated previously, these different scale features in the fields of
temperature and salinity are always observed during oceanographic studies
in this particular area of the Aegean Sea. Comparison of their characteristics
with the model data shows good agreement.
CONCLUSIONS
The thermohaline frontal zone in the Dardanelles area represents a
quasi-stable climatic formation, which is rather stable in terms of its geographical location. Despite the fact that the major mechanism responsible for
the occurrence and maintenance of steady frontal interfaces in this area is
the interaction of two water masses differing in salinity, the front and frontal
formations are readily identifiable in all seasons in the surface temperature
field, as well. The frontal zone has a complicated horizontal structure, incorporating secondary non-stationary frontal interfaces. The latter occur and
develop under the influence of severe winds of various directions, which are
a permanent natural phenomenon in this region.
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As the experimental investigations have shown, under such conditions
the position of the near-surface plume front depends to a great extent on
the wind action. When winds of appropriate directions occur with speeds
of 10 m/s and more, the front may be driven back towards the strait to a
distance of several miles away from it. During the intensification of wind,
the main frontal interface is likely to become discontinuous, and secondary
fronts may occur at both sides of the front which, in turn, may be rather
extensive and have gradients close in scale to the gradients across the main
front. Under the conditions of an intermittent T, S-correlation, a very
complicated thermohaline structure of waters in the subsurface layers takes
place within the frontal zone, incorporating a great number of inversions
and intrusion interlayers.
By using simulation, we succeeded in reconstructing processes of formation and evolution of the plume front and also in deriving numerical
estimates close to the real ones. This concerns the extent of the frontal
zone, the location of the frontal interface, the latter's width and orientation,
as well as the values of the temperature and salinity gradients both at the
front proper and within the frontal zone. The model transfrontal circulation
fields have shown that the latter circulation has the form of two cells with
horizontal axes parallel to the front. The acquired model pictures of vertical
motions in the frontal zones are consistent with the views and estimations
of other authors.
In the course of modeling the front, thermohaline intrusional interlayers
occured, developed and dissipated in the immediate vicinity of the frontal
interface. The numerical experiments conducted have shown the conditions
of their generation to be thoroughly dependent on the magnitude of the horizontal and vertical turbulent exchange coefficients.
Comparison of in situ observations with the model data is satisfactory,
which suggests that the model developed here for the generation of a plume
front is quite realistic.
REFERENCES
BOGDANOVA A.K., 1969. - Water exchange through the Bosphorus and its
influence on the hydrology and biology of the Black Sea. - Kiev,
Naukova dumka, in Russian, 295 p.
0VCHINNIKOV l.M., PLAHIN E.A., MOSKALENKO L.V., NEGLAD K.V. ,
0SADCHIN A.S., F'EDOSEEV A.F., KRIVOSHEYA V.G., VOITOVA K.V.,
1976. - Hydrology of the Mediterranean Sea. - V.A. Burkov ed.,
Leningrad, Hydrometeoizdat, in Russian, 376 p.
GRISHIN G.A., NIKOLAENKO E.G., 1994. - The structure and evolution of
the quasi-stationary thermohaline front in the Aegean Sea from the shipborne and satellite data. -Earth Res. from Space, 1: 79-85.
GRISHIN G.A., EREMEEV V.N., MOTYZHEV S .V., 1989. - On gravitational
instability of the Black Sea main current. - in Russian, Dok/. Akad.
Nauk USSR, 306, Suppl. 2: 466-471.
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Science Series n°2
GRISHJN G.A., SUBBOTIN A.A., 1992. - Study of the vortex dipole in the
Black Sea using spaceborne and shipboard observations. - Earth Res.
from Space, 5: 56-64.
FEDOROV K.N., 1983. - Physical nature and structure of oceanic fronts. Leningrad, Hydrometeoizdat, in Russian, 296 p.
FOFONOFF N.P. , 1962. - Physical properties of sea-water. - In: The Sea,
1, Wiley-Interscience: 3-30.
GARVINE R.W. , 1977. Observations of the motion field of the
Connecticut River plume. -J. Geophys. Res. , 82, Suppl. 3: 441-454.
VLASENKO V.I., 1993. - Nonlinear model for the generation of baroclinic
tides over extensive inhomogeneties of the seabed relief. - Sov. J. Phys.
Oceanogr., 4, Suppl. 4: 241-250.
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