Estuarine, Coastal and Shelf Science (2002) 54, 601–620
doi:10.1006/ecss.2000.0668, available online at http://www.idealibrary.com on
Coastal–open Ocean Exchange in the Black Sea:
Observations and Modelling
E. V. Staneva,f, J. M. Beckersb,g, C. Lancelotc, J. V. Stanevad,h, P. Y. Le Traone,
E. L. Penevaa and M. Gregoireb,i
a
Department of Meteorology and Geophysics, University of Sofia, 5 James Bourchier Street, 1126 Sofia, Bulgaria
Université de Liège, GHER, Sart-Tilman B5, B-4000 Liege, Belgium
c
Universite Libre de Bruxelles, Ecologie des Systèmes Aquatiques, Campus de la Plaine, CP 221,
Boulevard du Triomphe, B-1050 Bruxelles, Belgium
d
National Institute of Meteorology and Hydrology, Sofia, Bulgaria
e
CLS–Space Oceanography Division, Toulouse, France
b
Received October 1998 and accepted in revised form February 2000
The interaction between physical and biological processes in the areas of continental margins governs the variability of
ecosystems. The complexity of processes in these areas requires detailed studies combining modelling and surveying
efforts. One promising step in this direction was undertaken in the framework of the EROS 21 project, focusing on the
shelf part of the north-western Black Sea. In the present paper, we focus on the results of physical studies aiming to
improve the understanding of the fundamental exchange processes in the ocean margins, as well as to quantify some of
them in the Black Sea. We illustrate the capabilities of circulation models to reproduce physical processes with different
time- and space-scales: coastal waves, internal waves, baroclinic Rossby and topographic waves. Another class of
important phenomena in the coastal zone is associated with convection. Sources at the sea surface and in the outflow
areas give rise to plume dynamics that play a crucial role in the vertical mixing and provide the mechanism for water-mass
formation. Most of the results are illustrated for the shelf part of the Black Sea. The verification of simulations is
performed by comparison with survey data, altimeter data from the Topex/Poseidon mission and radiotracer observations. The latter, in combination with simulations from circulation models, are used to trace the penetration of tracers
into the intermediate and deep layers. We show that although most 90Sr is introduced by river runoff, large amounts of
this signal penetrate the halocline in the Bosphorus Straits area and along the southern coast. Another important fraction
of the river water penetrates the intermediate layers at the shelf edge in the north-western Black Sea.
2002 Elsevier Science Ltd. All rights reserved.
Keywords: coastal waves; upwelling; circulation; internal mixing; water mass formation; ventilation of coastal zone
Introduction
The coastal zone covers only 8% of the entire ocean
surface, but its role is dominant for most processes,
particularly those related to the exchange between
land and ocean. This key area includes estuaries and
deltas and the entire region between the shoreline
and the beginning of the deep ocean. It provides a
boundary layer for physical, chemical and biological
processes. Growing interest in better quantifying the
export from/into the coastal zone results from its
f
Corresponding author. Present address: ICBM, University of
Oldenburg, Postfach 2503, D26111 Oldenburg, Germany.
Tel: +49-44-1798 4061; Fax: +49-44-1798 3404. E-mail:
[email protected]
g
Research Associate, National Fund for Scientific Research,
Belgium.
h
Present address: Alfred-Wegener-Institut for Polar and Marine
Research, PO Box 120101, 27515 Bremerhaven, Germany.
i
Researcher, National Fund for Scientific Research, Belgium.
0272–7714/02/030601+20 $35.00/0
importance for the functioning of ecological and sedimentological systems. As is well known, an important
part of nutrients available to coastal ecosystems is
supplied from the land. This, along with the
favourable physical conditions, maintains a very high
primary production (about one quarter of the primary
production in the ocean is due to the coastal ocean).
Part of this material settles on the bottom and another
part enters the deep ocean. The ratio between the two
is not well known, but it is accepted that it greatly
varies in different ocean margins. This motivates
interdisciplinary studies on the ventilation of the
coastal zone, that is the exchange between coastal
waters and open-ocean, and the fate of organic matter
produced and imported.
The Black Sea is a basin where such exchanges are
of prime importance and must be correctly understood. It is a deep basin (greatest depths of about
2002 Elsevier Science Ltd. All rights reserved.
602 E. V. Stanev et al.
2200 m), with a large shelf and continental slope
covering 30–40% of its surface. This land-locked
basin is located in the temperate and subtropic
climatic zone, having a negative freshwater balance at
its surface caused by excess evaporation (evaporation
minus precipitation yields about 50 km3 per year, with
} zsoy & U
} nluata,
precipitation of 300 km3 per year; O
1998. However, it is its wide drainage area, covering a
large part of Europe and Asia and providing a total of
fresh water supply of about 350 km3 per year that
makes it very different from most other seas in temperate and subtropical areas. The excess fresh water at
the surface of the Black Sea, the restricted exchange
with the Mediterranean Sea through the Straits of
Bosphorus and the basin shape and topography have
fundamental consequences for its physical system,
creating a unique chemical and biological environment. The river runoff affects most of the physical
characteristics, which makes them strongly dependent
on the hydrological cycle over large areas of Europe,
as well as on coastal processes. This dependency is
even stronger for the biological processes, since the
latter are affected by the pollution of river waters
originating from a vast area of Europe. This has
resulted in the well-known eutrophication, observed
over the past 30 years, and in the complete deterioration of the Black Sea ecosystem. Eutrophication is
known as a process of intensive algal bloom, which has
a strong local dependency. The cause, characteristics
and, in particular, the quantification of the processes
associated with the recent changes are not easily
addressed from the point of view of observations,
since the data are too sparse. Much can be done using
numerical simulations, and an illustration of this is
given by Lancelot et al. (2002) in this issue. However,
the physical and biological processes governing the
accumulation of biomass require us to address this
phenomenon as three-dimensional, and deal with
mass fluxes due to currents and turbulence. In
this paper, we will illustrate the progress made in
the understanding of physical issues that are directly
related to the functioning of the biological systems,
and we will compare some simulations with in situ and
satellite data from surveys and remote sensing. Since
the coastal–open ocean exchange is not a local process, we will analyse the circulation using basin-wide
data and numerical simulations. Further details on the
dynamics and biological transformations can be found
in the accompanying papers (in this issue) of Beckers
et al. (2002) and Lancelot et al. (2002). Instead, the
present paper details and validates the physical modelling results underlying these papers. We will show
that the output from numerical modelling presents an
important supplement to existing data.
Data and models description
Data
Several data sets have been used to compare model
simulations with observations. The climatic data set
describes monthly mean temperature and salinity
based on more than 25 000 stations over the last
70 years (Altman et al., 1987).
The temperature and salinity database of the
Co-operative Marine Science Black Sea (CoMSBlack)
programme (Oguz, et al., 1993, 1994; O
} zsoy &
U
} nluata, 1998) was used to describe synoptic features
in the Black Sea circulation. In particular, the data
collected during three quasi-synoptic surveys (2–29
September 1991, 4–26 July 1992 and 2–14 April
1993) were interpolated onto a regular grid with
resolution in the meridional and in the zonal
direction at 35 levels.
The temperature and salinity profiles measured on
board the RV Professor Vodyanitsky, during the cruise
of April–May 1997 (Lancelot & Egorov, 1997), were
used to study the shelf–open sea exchange in the
north-western Black Sea.
The recent US/French mission Topex/Poseidon
(T/P) provided the scientific community with high
quality altimeter data. Their errors are below 3 cm
rms, thus the accuracy of estimates on the variations
of Black Sea mean sea-level (MSL) is quite good.
T/P data from October 1992 to July 1997 of the
latest version of Topex/Poseidon (T/P) M-GDRs
distributed by AVISO (MGC-B, version 2) were
used (AVISO, 1996). Standard altimetric corrections
are applied, except for ocean tides and atmospheric
pressure, which are very small and are not corrected
for the Black Sea. Sea-level anomaly (SLA) relative
to a 4-year mean (1993–1996) is then obtained
using a conventional repeat-track analysis and a
suboptimal space/time interpolation method (Le
Traon et al., 1998) onto a 0.20.2 spatial grid,
with 10-day averages from the 1 to 3 days repeat
track data.
Atmospheric forcing was derived from the climatic
handbook edited by Sorkina (1974) and the meteorological data of the Hadley Centre, United Kingdom
Meteorological Office (UKMO). This data set
includes twice-daily temperature, relative humidity
and wind velocity for the period June 1993–May
1995. The resolution is 0.44 in latitude and longitude. The climatic data set of Sorkina (1974) originates from coastal and ship measurements (67 000
measurements in total). The procedure for the calculation of wind stress is described by Staneva and
Stanev (1998).
1
12
1
9
Coastal–open ocean exchange in the Black Sea 603
Numerical models of the Black Sea circulation
We will here briefly review two basin-wide models: the
DMG and GHER model. They are described in detail
in the references cited below. The GHER model is
presented in this issue by Beckers et al. (2000), and
will, therefore, only be described schematically,
focusing on its major differences from the DMG
model. Both models use a set of primitive equations
for velocity, temperature and salinity in hydrostatic
approximation. The DMG model is based on the
Modular Ocean Model (MOM) code (Pacanowski
et al., 1991), widely used in ocean modelling, while
the second one is based on the GHER mathematical
model code (e.g. Nihoul et al., 1989; Beckers, 1991).
The application of these models to the Black Sea is
documented in the papers of Stanev et al. (1997)
and Stanev and Beckers (1999a, b). As can be seen
from these works, many differences exist between
the model formulations, numerical schemes and
parameterizations. However, initial conditions are
essentially the same and the forcing functions have
almost the same climatic characteristics. The forcing
functions used in the DMG-MOM model are based
on atmospheric analysis data (atmospheric temperature, humidity and wind). Aerodynamic bulk formulae are used to compute heat and momentum fluxes,
using stability-dependent exchange coefficients. The
realistic forcing of DMG-MOM ensures correct simulations of events associated with water-mass formation
and the ventilation of coastal regions. As shown in a
number of papers addressing parameterizations, sensitivity studies and intercomparisons with observations (Stanev et al., 1997, 1998; Staneva & Stanev,
1997, 1998; Staneva et al., 1999), the model is well
tuned to the Black Sea conditions and realistically
replicates the major circulation and thermohaline
properties of the Black Sea system. The forcing of the
GHER model is more simple and includes only the
seasonal variability of atmospheric forcing (the sea
surface temperature and salinity are relaxed to
monthly mean climatological data). River discharges
from the three main rivers in the Black Sea (i.e.
Danube, Dnepr and Dnestr) are also prescribed. In
addition, momentum fluxes are computed from the
climatological monthly data and are interpolated at
each time-step. The data used to force both models
are described in detail in the paper of Staneva and
Stanev (1998).
Bottom topography is taken from the UNESCO
bathymetric map and discretized with the model resolution. One important difference between the two
models is the vertical discretization. MOM uses geometric depth as vertical co-ordinates, with variable
thickness of model layers, and the Arakawa B-grid in
T 1. Models used in this study, and their resolution
Resolution
horizontal
(km)
vertical
(levels)
DMG-MOM
coarse
fine
28
9
24
24
GHER
coarse
fine
15
5
25
25
Models
the horizontal. To avoid possible artefacts associated
with the amplification of basin waves over the abyssal
plain (Stanev & Rachev, 1999), we increase the discretization there. Thus, it varies from 5 m in the
surface 20 m layer, 10 m until a depth of 90 m,
decreasing to 400 m in the deep homogeneous layers
increasing again to 60 m in the deepest levels. The
GHER model uses the double co-ordinate system
(which allows us to represent the abyssal plain precisely) in the vertical and the Arakawa C-grid in the
horizontal. An important physical difference between
the two models is related to the use of a 2D prognostic
variable: total stream function in MOM and free sea
surface in the GHER model. The subgrid parameterizations are also different, and are documented in the
above papers. Both models have been developed such
that they can be run with two different resolutions in
the horizontal (Table 1). This allows good efficiency
when studying large-scale circulation and relatively
slow processes of water-mass formation with coarse
resolution. Fine resolution is used to study the impact
of eddies on the circulation (e.g. Staneva & Stanev,
1997; Stanev & Staneva, 2000; Gregoire, 1998).
Since the Rossby radius of deformation in the Black
Sea is of the order of 20–30 km, the two fineresolution models (with horizontal resolutions of 9
and 5 km for the DMG-MOM and GHER models,
respectively) can be seen as eddy-resolving models. As
will be demonstrated when discussing the results of
the simulations, the eddy resolution is of utmost
importance if we want to adequately replicate the
exchange processes in the frontal area, as well as on
the shelf.
This paper addresses the simulation of the basinwide circulation, with a particular focus on the
description of the north-western shelf circulation
and on the estimation/quantification of the exchanges
between shelf waters and open-sea waters.
We will illustrate different results produced by the
two models, since they are complementary. It is worth
604 E. V. Stanev et al.
noting that the evolution of the circulation and
thermohaline fields simulated by the two models obey
the same type of behaviour. This proves that both
models are calibrated in such a way as to give close
results, in particular with respect to the seasonal
variability of circulation.
The Black Sea circulation: evidence from
observations
Black Sea surface elevation and currents
The circulation of the Black Sea has been widely
illustrated, and further details and references can be
found in Blatov et al. (1984), Stanev et al. (1988),
Simonov and Altman (1991) and O
} zsoy and U
} nluata
(1998). We restrict the analyses in this study to issues
directly linked to the coastal–open sea exchange,
presenting estimates based largely on new data and
modelling.
Currents in the Black Sea are mainly quasigeostrophic, but strong deviations from this balance
exist in the surface and bottom boundary layers, as
well as in the jet-like current (encompassing the entire
basin) where inertial force is substantial. Wind is
the main driving force, tending to create a cyclonic
general circulation (Stanev, 1990; Stanev & Beckers,
1999a). The buoyancy anomalies due to river runoff,
precipitation and evaporation enhance the cyclonic
circulation, since most of the fresh water enters the sea
in the coastal area (Stanev, 1990; Oguz et al., 1995;
Bulgakov et al., 1996). This forcing exerts an indirect,
but very strong, impact on the circulation, forming
(together with the exchange through the Straits of
Bosphorus) the unique vertical stratification.
It is accepted that the Black Sea can be divided into
two major circulation areas: the cyclonic (in the basin
interior) and the anticyclonic (between the jet current
and the coast). The anticyclonic area is narrow, since
the continental slope is very close to the coast over
most of the sea, and the circulation in this area is
dominated by a number of small coastal eddies. As
can be shown from the dynamical analyses of the
recent basin-wide quasi-synoptic surveys (Oguz et al.,
1993, 1994; Korotaev et al., 1998), the dynamic
height correlates with the general climatic pattern.
What is less well known, and is very impressively
illustrated by the recent T/P data, is the spatial
variability of the sea surface. In order to compare
it with the existing observations, we subtract the
climatic signal from the dynamic heights of three
ComsBlack surveys. The reference level is taken as
500 dbar, since the density anomalies below this
depth are very small, so that the results remain almost
unchanged if we consider a deeper reference level.
The results are shown in Figure 1(a–c). A number of
synoptic features dominates the anomaly pattern,
indicating strong mesoscale/sub-basinscale variability.
Horizontal scales vary between tents to 100 km. The
largest anomalies are observed in the areas of the
Batumi eddy and in the north-western Black Sea. As
can be seen from the comparisons between the
anomalies of dynamic heights obtained from hydrographic and T/P data, the agreement is satisfactory
[cf. Figure 1(c,d)]. This proves that the two types of
data are consistent and can be used as complementary
(a large number of observations of SLE in T/P data
and a complete description of the thermohaline fields
in the survey data). It is worth noting that the data
from the survey resolve eddies smaller than those from
the T/P, which is due to the relatively large distances
between tracks (Korotaev et al., 1998). This could
explain the higher slope in SLE estimated from the
survey data. We will give some further intercomparisons between our model estimates and both types of
data.
Seasonal variability
The sea-level elevation (SLE) oscillates with amplitudes of 10–20 cm. The maxima are associated with
large river runoff in spring and early summer. The
minima are observed in late autumn and are due to
small freshwater fluxes in summer and autumn. This
evidence, well known from measurements in coastal
locations, is nowadays supported by satellite data
(Figure 2). Along with the strong signature of seasonal
signal, these data give a well-resolved trend of about
3 cm yr 1 in the last 5 years (a similar trend in the
Caspian Sea is much higher), indicating possible longterm changes associated with the freshwater balance.
The altimeter data clearly allows us to identify
regions with higher or lower variability (e.g. energetic
Battumi and Sevastopol eddies, Figure 3).
The seasonal variability of the Black Sea circulation
is externally forced and carries a substantial part of the
spectral energy. In winter, not only does the wind
magnitude increase, but also its curl, which contributes to the intensification of circulation (Stanev,
1990; Staneva & Stanev, 1998). The decreasing
intensity of circulation in summer is also a direct
consequence of the change in the mechanical forcing.
The corresponding seasonal transitions are illustrated
by the shallowing of the halocline in the central
(cyclonic) part of the basin and its deepening along
the coast (anticyclonic part of the basin) in winter. As
a result, the slope of the halocline increases. The
Coastal–open ocean exchange in the Black Sea 605
47°N
(a)
Latitude
46°N
47°N
45°N
45°N
44°N
44°N
43°N
43°N
42°N
42°N
41°N
41°N
28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E
Longitude
–150 –100 –50
0
50
–120 –90 –60 –30
(c)
46°N
Latitude
28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E
Longitude
100 150 200
47°N
44°N
44°N
43°N
43°N
42°N
42°N
41°N
41°N
28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E
Longitude
50
100 150 200 250 300
30
60
90
120 150
(d)
46°N
45°N
0
0
47°N
45°N
–100 –50
(b)
46°N
28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E
Longitude
–180 –160 –140 –120 –100 –80 –60 –40 –20
F 1. (a, b, c) Anomalies of dynamic heights (mm) from hydrographic surveys. Dynamic heights calculated from annual
mean climatological data are subtracted in order to obtain the anomalies. The reference level is taken to be 500 m, which
explains why the plot does not cover the whole area. (a) September 1991; (b) July 1992; (c) April 1993. (d) Anomaly of
sea-surface elevation from the T/P data (April 1993).
inverse process takes place in summer, leading to a
decrease in the intensity of circulation.
Penetration of the signals from the sea surface into the
pycnocline
The Black Sea stratification (surface salinity of about
17·8 and salinity at 150 m of about 21) tends to shield
deep layers from the processes occurring in the surface
layer. The consequences of this ‘ decoupling ’ (studied
by Stanev, 1990) are impressively demonstrated by
the anoxic conditions below 150 m. The depth
reached by winter convection is governed by the
stability of the stratification and, unlike the ocean
basins at the same latitudes, is very small. Thus, the
upper layer is ventilated down to about 50–150 m.
The newly formed cold intermediate water (CIW) is
overlaid by the seasonal thermocline. The reduced
vertical exchange caused by the strong stability of
stratification shields the CIW from mixing with surface and deep waters (Stanev, 1990), and the cold
intermediate layer (CIL) is observed as a perennial
thermic, characteristic at depths ranging from 50 m
(the central basin) to 150 m (the easternmost Black
Sea).
Radiotracers give valuable information for estimating the speed of penetration of signals from the sea
surface into the deep ocean layers. In the case of the
Black Sea, the Chernobyl accident created such a
signal and made possible the evaluation of the rate of
mixing between the Mediterranean and Black Sea
waters, which contributes to internal mixing in the
Black Sea (Buesseler et al., 1991). Numerical models
can be used to test the contribution of different
mechanisms of mixing in the coastal–open sea
exchange. Since the limited amount of observations could not give reliable information about the
basin-wide exchange, we give some results of the
modelling, though a more detailed presentation of
the simulations will be given in the next section.
To study the exchange between surface and
deep waters, we add a new tracer—90Sr—to the
DMG-MOM model with horizontal resolution. The
parameterization of vertical mixing in the model is
stability-dependent and tuned against chemical data
1
4
606 E. V. Stanev et al.
180
90
Sr (Bq m–3)
(a)
5
0
10
15
20
25
120
SLA (mm)
60
100
Depth (m)
0
–60
200
–120
–180
1992
1993
1994
1995
Year
1996
1997
300
1998
F 2. Variability of the basin mean sea-level anomaly
(SLA, mm) from the T/P data.
(b)
Cl = 0.1
46°N
47°N
45°N
44°N
44°N
43°N
.4
Latitude
45°N
17
Latitude
46°N
42°N
43°N
41°N
42°N
Mean = 17.7
40°N
41°N
40°N
27°E 29°E 31°E 33°E 35°E 37°E 39°E 41°E
Longitude
20
25
30
35
40
45
50
55
60
65
70
75
F 3. Amplitude of the variations in sea-level elevation
(SLE, mm) from the T/P data.
analysed by Lewis and Landing (1991). The forcing
includes fluxes of 90Sr located in the river mouths in
the north-western Black Sea, which are calculated as a
product of river discharge times the measured concentrations. As seen from Figure 4(a), the simulated
distribution of 90Sr in the vertical correlates well
with the measurements. What is very important and
relevant to the present study is that it is not only
the entrainment of Black Sea water by the sinking
Mediterranean plume (Buesseler et al., 1991) that
governs the penetration of signals from the sea surface
into the deeper layers. One substantial part of the
diapycnal transport occurs along the jet stream, which
28°E 30°E 32°E 34°E 36°E 38°E 40°E 42°E
Longitude
F 4. The penetration of 90Sr into the Black Sea
pycnocline. The results have been simulated with the
DMG-MOM model with resolution. Detailed description
of the model setup is given in Stanev et al. (1998). (a) 90Sr
vs depth for 1992. Dashed line indicates 1 deviation of
simulated values from the basin mean. Squares correspond
to observations for the same period. (b) Horizontal mixing
pattern as seen in the distribution of 90S at t =14·4 (30 May
1991).
1
4
is associated with the time variability and synoptic
oscillations. In the context of the ventilation of the
shelf area, the Sevastopol eddies are of utmost importance. As seen from the analyses of observations by
Ivanov et al. (1997), and as shown in the theoretical
study of Staneva and Stanev (1997), the volume of
CIW expands in the area of anticyclones. This makes
them potentially important stock elements for substances transported from the shelf. The slope currents
propagate these pollutants rapidly along the basin
periphery, so that they can penetrate into the open sea
Coastal–open ocean exchange in the Black Sea 607
due to the diapycnal exchange. This is the case with
the Chernobyl 90Sr, which was discharged into the sea
by the rivers.
The strong stratification in the Black Sea and the
different depth of pycnocline in the cyclonic and
anticyclonic areas make the vertical profile of tracers
quite noisy when plotted against depth [Figure 4(a),
the squares correspond to observations in different
locations]. Plotting the data in t-co-ordinates usually
reduces the dispersion created by dynamical reasons
(e.g. Turgul et al., 1992) and makes possible the
appearance of some fundamental features associated
with the diapycnal mixing (Staneva et al., 1999).
However, the horizontal gradients of transient tracers
are usually small when plotted on t-surfaces and are
not easily detected from observations. In this case, the
simulations can give useful supplementary information about the mixing paths or the diapycnal penetration into the pycnocline. This is illustrated by the
simulated 90Sr at t =14·4 [Figure 4(b)]. It is clearly
seen that 90Sr penetrates the ispycnal surface from
its periphery, where the highest concentrations are
observed. Equally important, and at the same time
very peculiar, is the fact that the Chernobyl 90Sr
penetrates the pycnocline far from the region of its
origin (i.e. the rivers in the north-western part of the
sea) along the whole basin periphery. This demonstrates the importance of coastal circulation for the
vertical/diapycnic spreading of signals from the sea
surface into the interior.
Circulation on the north-western shelf
The Black Sea shelf consists of two distinct regions: a
very flat area in the north, lying approximately
between the Cape Tarhankut and the Danube delta,
with depths lower than 50 m, and a narrow belt of
about 50 km wide, extending from the Crimea
Peninsula to the coast of Bulgaria, with depths varying
between 50 and 100 m. In the first (very shallow)
region, the dynamics are strongly dominated by winds
and dissipation, whereas in the second one, they are
much more complex due to the interaction between
shelf and open-sea processes.
The recent quasi-synoptic measurements carried
out under the CoMSBlack and EROS 21 projects are
not analysed in detail for the shelf area. However,
salinity data [Figure 5(a)] demonstrate that the
dynamics close to the Danube delta are dominated by
the river plume. A well-defined front separates the
river water from the open-sea waters. Below the
pycnocline salinity reveals quite different patterns
[second column of Figure 5(b)], indicating that the
processes in the surface layers might be quite indepen-
dent from the ones in the deep layers. It is of particular
interest to show the vertical cross-sections in the
southern part of the plume, also indicating the
decorrelation between surface and deeper waters.
The circulation on the shelf has been addressed in
a number of experimental and model studies (e.g.
Blatov et al., 1984; Simonov & Altman, 1991;
Mikhailova & Shapiro, 1996). There is evidence to
suggest that the currents may rotate in a clockwise or
anticlockwise manner depending on the wind direction. Remote sensing data obtained from CZCS
(Barale & Murray, 1995) show that the plume originating from the Danube River often displaces to the
north or intrudes the shelf interior. This supports
the idea that the circulation is very changeable
(e.g. Stanev & Beckers, 1999a, b), which can be
easily explained by the small mechanical inertia of a
shallow-water column.
Physical processes: model–data
intercomparisons
Inventory of the physical processes affecting the coastal–
open ocean exchange and their representation by the
DMG-MOM and GHER model
The variability in the coastal ocean occurs over a wide
range of space- and time-scales that necessitate considering a wide range of phenomena, simultaneously.
This is exactly the case in the Black Sea, where the
scales of major processes in the coastal zone range
from regional to basin-wide. Direct atmospheric forcing and exchanges at the boundaries dominate the
dynamics in the coastal zone, and both free and forced
motions are important. The width of the continental
shelf and the characteristics of the slope area shape the
geometry of the processes.
An inventory of processes studied with the
DMG-MOM and GHER models (with a short
specification of them) is given in Table 2. We remind
the reader that some processes listed in Table 2 have
also been studied using other models. In the following, we will demonstrate the relevancy of some physical processes to the coastal–open ocean exchange,
using model results.
Waves
Coastal waves. Most of the modelling studies on
general ocean circulation focus on wind and thermohaline currents, neglecting the short, periodic seasurface variability by prescribing rigid lid boundary
conditions. Another large class of model studies
addresses tidal motions, neglecting the baroclinicity.
608 E. V. Stanev et al.
(a)
Salinity at 5 m
(b)
Salinity at 40 m
September 1991
(c) Cross section at 44.4°N
September 1991
September 1991
Vertical cross-section (m)
0
Latitude
45.5°N
45°N
44.5°N
44°N
10
20
30
40
50
60
July 1992
July 1992
July 1992
Vertical cross-section (m)
0
Latitude
45.5°N
45°N
44.5°N
44°N
10
20
30
40
50
60
April 1993
April 1993
April 1993
Vertical cross-section (m)
0
Latitude
45.5°N
45°N
44.5°N
44°N
10
20
30
40
50
60
April 1997
April 1997
April 1997
Vertical cross-section (m)
0
Latitude
45.5°N
45°N
44.5°N
44°N
29°E
30°E
Longitude
12.5 13 13.5 14 14.5 15 15.5 16 16.5 17 17.5 18
Salinity
29°E
31°E
0
3
6
30°E
Longitude
9
31°E
12 15 18 21 24 27 30 33 (S–18) × 100
Salinity
10
20
30
40
50
60
29°E
14.5
15
30°E
Longitude
15.5
16
16.5
17
31°E
17.5
18
Salinity
F 5. Salinity patterns on the north-western shelf from the CoMSBlack and EROS surveys. Horizontal plots at (a) 4 m
and (b) 40 m. (c) Vertical cross-section at 44·4N.
However, in deep baroclinic land-locked basins, both
the sea-surface oscillations and the baroclinicity are
important. Closed boundaries provide a wave-guide
for Kelvin waves (coastal-trapped waves, in the case of
a basin with realistic bottom), and their relationship
with other wave processes has been illustrated by
Coastal–open ocean exchange in the Black Sea 609
T 2. Inventory of the major physical processes and phenomena in the Black Sea studied using the DMG-MOM and
GHER models
Processes/phenomena
Black Sea reference
Waves
Surface
Stanev and Beckers (1999a)
Internal
Stanev and Beckers (1999a)
Basin oscillations
Rachev and Stanev (1997a)
Rachev and Stanev (1997b)
Stanev and Staneva (2000)
Stanev and Rachev (1999)
Topographic
Kelvin and Rossby
Coastal trapped
Front processes and eddies
Upwelling
Rim current
Baroclinic instabilities
Eddies (synoptic, quasi-permanent,
basin-scale)
Small-scale processes
Boundary layers and mixing
Ekman transport
Breaking waves and turbulence
Currents and water masses
Stanev and Beckers (1999a)
Stanev and Rachev (1999)
Stanev and Beckers (1999a)
Notes
subinertial oscillations with maximum
amplitudes at 1·2, 2·1, 4·4 and 6·3 h have
been studied
coupling between free surface and
pycnocline oscillations
western propagation caused by basin
oscillations
differentiation between basin and
topographic oscillation
analysis of characteristics of Kelvin waves in
a small basin
oscillations with periods exceeding inertial
period, with the coast on their right
Stanev and Beckers (1999a)
Stanev and Staneva (2000)
Staneva and Stanev (1997)
Stanev and Beckers (1999a, b)
Rachev and Stanev (1997)
Stanev and Staneva (1999)
Gregoire (1998)
Stanev and Staneva (2000)
generation of upwelling at Cape Kaliakra
characteristics of the rim current, transport,
vertical shear of the currents
Stanev et al. (1997)
parameterization in models and their impact
on simulations
surface and bottom plumes, gravity currents,
ocean–atmosphere exchange
mixing in intermediate layers caused by
breaking waves
wind- and buoyancy-driven currents, and
their relationships to water-mass formation
Simeonov et al. (1997)
Staneva et al. (1995, 1998)
Stanev and Beckers (1999a, b)
Staneva and Stanev (1997)
Stanev and Beckers (1999a, b)
Stanev et al. (1998)
Staneva et al. (1999)
Stanev and Beckers (1999a) in the case of the Black
Sea. The active free surface in the GHER model
makes it possible to simulate short, periodic barotropic oscillations. Their periods range between 1 and
10 h. Coupling between barotropic and baroclinic
oscillations results in baroclinic wave excitation and
wave shedding towards the open sea. This is shown
in time vs distance from the coast dependency
[Figure 6(a)]. It is clear that the coastal-trapped waves
are simulated in a narrow zone over the continental
slope. The wave speed can be measured by the slope
of the thick line in Figure 6(a) connecting equal
phases at different distances from the coast (giving
approximately 0·5–1 m s 1). The characteristics of
the oscillations change with time and, as indicated in
Figure 6(a) at location 14, high salinity water from
free and forced baroclinic oscillations
the impact of eddies on the circulation
the basin interior intrudes the coastal zone. Almost
simultaneously (after day 22–23), a decrease of the
amplitude of oscillations is observed.
The appearance of internal waves in the Black Sea is
very specific and they are well traced in the time vs
depth plot [Figure 6(b)]. Our simulations reveal two
layers (surface and deep) with large stratifications and
an intermediate homogeneous layer (the CIL). The
internal wave oscillations in the intermediate layer, as
seen in the temperature field, are insignificant. This
proves that the CIL acts as a thermic buffer, not only
at climatic time scales, but also at high frequencies.
However, the oscillations of the interface (permanent
halocline) interacting with the shelf/continental slope
might become a key element in the mixing process.
Breaking internal waves [or transformations of the
Temperature (°C)
(a)
8.18
68
8.1
7.6
0
77
7.
7.60
3
7.9
7.93
77
7.
7.43
0
8.1
8.18
8.0
93
8.10
1
0
7.43
8.1
0
8.18
18
8.
7.43
8.0
5
1
7.8
8.18
8.0
5
8.26
7.52
7.60
7.6
8
7.35
18
8.
8.10
7.52
7.
60
7.
68
7.
85
7.7
7
7.35
1
8.26
7.85
8.0
7.93
1
7.52
7.7
8.01
0
7.6
8.
18
7.43
7.68
7.8
7.35
8.
8.1
0
68
18
7.
7.52
7.
8.26
7.35
18
8.
7.85
68
7.6
Days from perturbation
8.10
7.43
8.10
7.
22
8
7.8
5
68
77
7.
23
8.1
6
8.2
7.
0
8.01
93
7.
8.26
7.52
7.
7.35
85
7.
8.01
7.35
7.43
77
7.
8.2
6
24
7
8.10
GHER
8.18
21
9
14
19
24
Point
Temperature at point 11 (°C)
(b)
7.
30
7.6
7
7.
43
7.80
2 7.80
2
7.9
8.0
5
9
7
8.1
8.29
6
4
8.5
8.54
8.66
6
9
9
9
8.7
8.7
9
8.7
1
8.9
9
8.7
8.7
8.
91
8.91
8.91
8.91
8.91
22
23
Days from perturbation
8.91
8.91
1
21
9
8.7
91
8.
8.9
91
8.
–250
20
9
8.7
9
8.7
9
8.7
9
8.7
8.7
9
6
8.6
8.6
6
8.6
8.66
8.6
4
8.5
4
8.42
8.54
8.5
4
8.5
8.54
8.4
2
8.42
2
2
8.4
8.29
2
8.4
2
2
2
2
2
8.4
8.4
9
8.2
9
8.2
8.2
9
8.2
9
8.2
7
8.1
8.17 8.05
7
8.1
5
8.0
7
8.1
7
7
7
7
7
8.1
8.17 8.05
5
8.05
2
2
7.9
7.9
2
0
0
7.9
2
7.9
7.9
7.6
7
7.6
7
7.8
0
7.8
7.8
8.4
8.0
7.9
2
5
7.5
5
7.5
7
7.6
55 .67
7.
7
8.1
8.4
0
8.1
8.4
7.8
0
8.1
0
7.8
0
7.8
7.8
5
8.0
5
8.4
0
0
8.0
8.1
7.8
7.8
Depth (m)
43
43
5
5
7.5
7.
7.
43
3
7.4
43
7.5
5
7.5
–150
7.4
43
7.
7.
3
7.43
3
7.4
7.
7.4
3
24
GHER
25
F 6. (a) Time vs distance from the coast plot of temperature simulated with the GHER model with 15-km horizontal
resolution at 150 m, 4327N. The abscise axis gives the distance from the coast (the number on the axis times 15 km).
(b) Time vs depth plot of temperature at location 11 simulated with the GHER model.
Coastal–open ocean exchange in the Black Sea 611
signal in the coastal region, as shown in Figure 6(a)]
might provide mixing in the halocline and affect the
thickness of the CIL. This leads to changes in the
stratification of the intermediate layer and depends on
the amount of energy provided by free-surface oscillation. These physical processes currently remain unexplored for the Black Sea, but might be responsible
for the control of the exchange between upper and
intermediate layers.
The oscillations of the halocline on different
time-scales could be of particular importance for the
physical control of the biological systems. We can
speculate that mixing (affecting different chemical and
biological compounds) will also be governed by wave
breaking. This might explain the importance of lateral
mixing in the Black Sea, providing the basin interior
with strong signals generated in the coastal areas.
Basin waves. While in the world ocean the Rossby
waves are responsible for the western propagation of
signals, the wave reflection in enclosed basins creates a
more specific regime, associated with the basin oscillations (Rachev & Stanev, 1997a; Demyshev et al.,
1996; Stanev & Rachev, 1999), that could exist in
barotropic as well as in baroclinic fluids. What makes
the Black Sea an interesting case is that the Rossby
radius of deformation (several tenth kilometres) is
comparable with some fundamental basin scales
(central narrow section, the narrow easternmost part
of the basin). The simulations with the DMG-MOM
demonstrate that the Rossby waves emerge from the
easternmost area and propagate to the west with a
phase speed of 2 cm s 1 [Figure 7(b)]. They dominate the dynamics when the model is forced with
stationary boundary conditions (Rachev & Stanev,
1997a), as well as when the forcing changes with
time (Stanev & Staneva, 2000), as is the case in
Figure 7(a). The meridional wind-stress component
and the simulated meridional velocity at the sea surface show quite different patterns. Wind maxima
occur in winter and are more pronounced in the
western Black Sea. The lack of slope in the wind
contours demonstrates that there is no substantial
signal propagating in the zonal direction (we exclude
from this analysis the synoptic processes in the atmosphere having very short time-scales). There is no
substantial correlation between the two types of data
(forcing and response), demonstrating that the westward propagation is rather an appearance of natural
oscillations. Thus, the atmospheric variability provides perturbations for the ocean system, but does not
shape the response. To give an idea of the consistency
of simulations with observations, Figure 7(c) shows
that the western propagation is also well pronounced
in the altimeter signal, although the phase speed is
slightly lower.
The analysis of model simulations demonstrates
that the basin oscillations induce strong changes in the
depth of pycnocline (Rachev & Stanev, 1997a). They
could also enhance the amplitude of the rim current
and affect the exchange between anticyclonic and
cyclonic areas. Since this process is extremely sensitive
to complicated bottom relief, we could speculate that
this type of wave could affect the anticyclonic eddies
between the main gyre and the coast. Some of the
most energetic eddies of this type originate in the area
of the Crimea Peninsula (Figure 1), and their interaction with the shelf could substantially affect the
mixing (including mixing of chemical and biological
matter) and the resulting water-mass formation.
Frontal processes and eddies
The Black Sea upwelling. In this section, we address
some processes that are crucially important for the
circulation and synoptic variability of any ocean
basin using simulation results produced by the
DMG-MOM and GHER models. In basins with
strong vertical stratification, the contrasts between
thermohaline characteristics in coastal and open-sea
regions are usually well pronounced, providing that
the mechanic forcing sufficiently maintains the slope
of the pycnocline steep. Under such conditions, the
instabilities associated with frontal oscillations and the
direct mechanic forcing from the atmosphere triggers
an intense upwelling. It has been found that there are
some areas (Cape Kaliakra, the southern coast and the
area west of the Crimea Peninsula, Figure 8) where
the upwelling is quasi-permanent (Stanev et al., 1988;
Sur et al., 1994; Blatov & Ivanov, 1992; O
} szoy &
U
} nluata, 1998; Gavarkievicz et al., 1999). In the
presence of strong changes of coastal line or topography (e.g. Cape Kaliakra), the oscillations of the
pycnocline may amplify the transport of CIW into the
surface layers. What has not been addressed in any
further detail in previous studies is the specific vertical
stratification in the Black Sea and the characteristics
times of processes acting in the horizontal and in the
vertical. While it takes about 1 year for the currents to
make one loop along the coast (Stanev et al., 1998),
the vertical penetration of the signals is much slower,
as shown by observations and modelling of the
penetration of radiotracers (see ‘ Penetration of the
signals from the sea surface into the pycnocline ’).
Since the horizontal–isopycnal mixing is much larger
than the vertical–diapycnal mixing, the water properties of each region tend to homogenize on isopycnals.
Thus, the properties of coastal waters dominate in the
612 E. V. Stanev et al.
(a)
JUL 1994
0.6
JAN 1994
0.5
0.4
JAN 1993
0.3
JUL 1992
0.2
JAN 1992
0.1
JUL 1991
JAN 1991
28°E 29°E 30°E 31°E 32°E 33°E 34°E 35°E 36°E 37°E38°E 39°E 40°E 41°E 42°E
Longitude
(b)
18
JUL 1994
15
9
JUL 1993
6
3
JAN 1993
0
–3
JUL 1992
–6
JAN 1992
–9
–12
JUL 1991
–15
28°E 29°E 30°E 31°E 32°E 33°E 34°E 35°E 36°E 37°E38°E 39°E 40°E 41°E 42°E
Longitude
(c)
JAN 1997
JUL 1996
JAN 1996
JUL 1995
JAN 1995
JUL 1994
JAN 1994
JUL 1993
27°E 28°E 29°E 30°E 31°E 32°E 33°E 34°E 35°E 36°E 37°E38°E 39°E 40°E 41°E
Longitude
–100
–80
–60
–40
–20
0
20
40
Anomaly of sea-level (mm)
60
80
100
120
Meridional velocity (cm s–1)
12
JAN 1994
JAN 1991
Wind stress
JUL 1993
Coastal–open ocean exchange in the Black Sea 613
46°N
Latitude
45°N
44°N
43°N
42°N
41°N
28°E
30°E
32°E
34°E 36°E
Longitude
–0.5 –0.4 –0.3 –0.2 –0.1 0
38°E
40°E
0.1 0.2 0.3 0.4 0.5 0.6
Vertical velocity (¥ 10–3 cm s–1)
F 8. Monthly mean vertical velocities in March 1984
at 7·5 m. Dark tones mark upwelling areas. Simulations are
carried out with the DMG-MOM model with resolution.
1
12
surface layers, while deep-water characteristics
dominate in the interior basin (Staneva et al., 1999;
Stanev & Staneva, 2000). Under such conditions, one
could represent the Black Sea as being composed of
two dynamically different sub-basins: coastal and
open sea. A similar division has already been suggested by Bulgakov and Korotaev (1984). However,
what has been realized in recent years is that the
circulation in the coastal (anticyclonic) area is
dominated by synoptic eddies, which requires the
addressing of the exchange between the two areas
from the viewpoint of non-stationary dynamics.
Accordingly, the physical and biological characteristics of the coastal/interior basin tend to upper/deep
sea characteristics. This ‘ regionalization ’ is consistent
with the general cyclonic circulation and the associated upwelling in the basin interior. The two branches
of the vertical circulation communicate by the
exchange in the slope area, the latter providing a
substantial part of the exchange in the Black Sea [see
Figure 4(b)].
The upwelling has not only important consequences for the vertical exchange of physical properties in the upper layer, it also affects the biological
productivity. The basin-wide upwelling in the Black
Sea interior supplies the intermediate layer with
hydrogen sulphide-rich waters, as shown from observations (Dobrujanskaya, 1967). Deep waters (rich in
ammonium) could also have a key significance for the
characteristics of the trophic chains in the upwelling
areas.
Systematic studies on the local aspects of coastal
upwelling are still sparse (e.g. Blatov & Ivanov, 1992;
Stanev et al., 1988; Oguz et al., 1992; Sur et al.,
1994; Kosnyrev et al., 1996; Vlasenko et al., 1996;
Gavarkievicz et al., 1999). The upward transport of
waters from the CIL into the surface layer opposes the
general trend of anticyclonic circulation in the coastal
zone bringing surface waters into the deeper layers.
Since the temperature of the CIW is lower than the
sea surface temperature in summer, this cold-water
mass clearly traces the upwelling region by giving
signals in the AVHRR data (mostly with synoptic and
mesoscale characteristics) in the warm part of the
year.
The generation of coastal upwelling in the
western Black Sea has been shown by the model of
Stanev and Beckers (1999a, b), but there are no
clear estimates about the contribution of this process
in the mixing of Black Sea waters. The localized
appearance of upwelling necessitates the very fine
resolution of numerical models (Demirov, 1994), and
we will show in the next subsection an example of
such simulations.
Basin-scale/mesoscale circulation. The rim current is one
of the most interesting physical phenomena in the
Black Sea dynamics. It is very narrow, which is due to
extremely strong density contrasts in the vertical, as
well as to very narrow continental slope. Realistic
modelling of this current requires a fine spatial resolution. This is shown in Figure 9, which compares
simulations of the basin-wide circulation carried out
with the GHER model with 15- and 5-km horizontal
resolution at the end of May. One can clearly see that
although the general pattern of circulation does not
drastically differ in the two simulations, the eddyresolving simulations are dominated by meanders,
eddies, filaments and dipole structures with scales that
are subgrid for the coarse model. The comparison of
the eddy scales with the ones in the observed data (see
Figure 9 and Figure 1, in addition to Oguz et al.,
1993, 1994; O
} zsoy & U
} nluata, 1998) demonstrates
that a 5-km resolution resolves most of the important
mesoscale features.
F 7. Time–longitude diagrams of (a) wind stress magnitude and (b) meridional velocity at the sea surface, 43.5N. The
slope of the contours gives a measure of the speed of westward propagation. The data used to plot this diagram are simulated
by the DMG-MOM model with resolution. The model is forced with atmospheric analysis data from UKMO. (c) Sea-level
anomalies (mm) from the T/P data at the same latitude.
1
12
614 E. V. Stanev et al.
Velocity scale:
0.07 m s–1
Depth: 10 m
(a)
45.75°
Latitude (N)
44.75°
43.75°
42.75°
41.75°
40.75°
27.334°
29.334°
31.334°
33.334°
35.334°
Longitude (E)
37.334°
39.334°
41.334°
46.705°
Velocity scale:
0.07 m s–1
Depth: 10 m
(b)
45.705°
Latitude (N)
44.705°
43.705°
42.705°
41.705°
40.705°
27.272°
29.272°
31.272°
33.272°
35.272°
Longitude (E)
37.272°
39.272°
41.272°
F 9. Horizontal currents at 10 m at the end of May simulated by the GHER model and forced with monthly climatic
data. (a) Simulations with horizontal resolution of 15 km. (b) Simulations with horizontal resolution of 5 km.
The baroclinic instabilities together with basin
oscillations and mesoscale eddies, give a very complicated picture of the exchange occurring between anticyclonic and cyclonic areas, depending on the
transition of the circulation between different dominating states (intense winter- and less intense
summer-state, Stanev & Staneva, 2000). The synoptic
eddies present a key element in the energy exchange.
The key point here is that the two states of circulation
are characterized by different slopes of the pycnocline
in the area of the rim current, and that the diapycnal
exchange between coastal and open waters might
be dependent on this slope, so that any mesoscale
features could affect this mixing.
Subgrid-scale processes
Small-scale processes are of the utmost importance
to the behaviour of geophysical fluids, and some of
them are listed in the third part of Table 2. Most of
these processes are not resolved by the basinwide numerical models, therefore they have to
be parameterized. For details on the different
parameterizations, and their impact on the model
performance, we recommend the paper by Stanev
et al. (1997). Here, however, we only mention that the
estimation of the impact of horizontal mixing is of
paramount significance when addressing the exchange
between coastal sea and open ocean. One could
expect that increasing the coefficient of horizontal
Coastal–open ocean exchange in the Black Sea 615
Currents and water masses
Currents. The simulations of the DMG-MOM surface
currents have magnitudes of about 20–40 cm s 1,
the total horizontal mass transport reaches several
sverdrups, with a larger part of it being located above
the pycnocline. This transport is approximately two-
(a)
Temperature (°C)
8.0
7.5
7.0
6.5
Jan
Mar
May
Jul
Sep
Time (months)
Nov
Mar
May
Jul
Sep
Time (months)
Nov
(b)
8.0
Temperature (°C)
mixing in the model would result in increasing the
exchange between the coastal and open-sea areas.
However, as our simulations demonstrate, this is not
always the case. Reducing the horizontal diffusion can
result in unrealistically large slopes of the halocline,
followed by an increase of eddy activity and enhanced
cross-gyre mixing (Stanev et al., 1997). Since the rim
current encompasses the entire basin, one could conclude that increasing the instability of the jet current
would increase the diapycnal mixing along the entire
slope area, partly compensated by a decrease in the
mixing (small diffusion coefficient) in the shelf break,
inhibiting the uptake of cold water from the coastal
zone. In such situations, slope currents take control of
cross-shelf mixing (Staneva & Stanev, 1997).
The interrelationship between horizontal and vertical exchange is demonstrated below, in the simulations with the DMG-MOM model; by analysing the
differences between mixing properties simulated by
coarse- and fine-resolution models (Table 1). The
atmospheric forcing is identical in both models,
except that the high-resolution model admits mesoscale structures in the sea surface temperature field,
and thus in the interactive heat fluxes. Since mesoscale heat fluxes generally enhance water formations,
we would expect a change in the CIL average temperature in a high-resolution model compared to one
of a coarse resolution. However, the more realistic
resolution of the physical processes in the fineresolution model did not result in any substantial
difference in the annually averaged temperatures in
the CIL, only in large amplitudes of the seasonal
signal (Figure 10). This comparison suggests that, by
increasing the eddy activity along the rim current
(which is also the usual situation in the real basin), the
rate of ventilation of the coastal zone increases. This
has far-reaching consequences. By exchanging waters
between the two areas (often diapycnally), the model
tends to change the vertical stratification. It is clearly
seen in Figure 10(a,b) that the vertical temperature
gradient is smaller at the end of winter and greater by
the end of autumn in the fine-resolution model. So,
the increase in the amplitude of seasonal temperatures
should affect the biology: directly, by influencing the
rate of biological processes, and indirectly, by acting
on the depth of the mixed layer.
7.5
7.0
6.5
Jan
F 10. Time-series of basin-averaged temperature
simulated by DMG-MOM model with resolutions (full
line) and (dashed line) at (a) 55 m and (b) 75 m.
1
12
1
4
fold larger under the fine-resolution model than under
the coarse one. One could ask whether this drastic
difference has a pronounced impact on the ventilation
of coastal regions and the intermediate layer, or
whether a compensation between eddy and mean
transport occurs, as in some ocean models (Cox,
1985; Bryan, 1986). Since the vertical circulation in
the Black Sea is much weaker than the horizontal one,
and large changes in the horizontal circulation are not
accompanied by large (in absolute values) changes in
616 E. V. Stanev et al.
Winter
Spring
20
40
60
80
100
120
140
200
20
40
60
80
100
120
140
200
400
400
600
600
800
800
1000
1000
1200
1200
1400
1400
1600
1600
1800
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Longitude (E)
1800
40.5° 41.5° 42.5° 43.5° 44.5° 45.5° 46.5°
Longitude (E)
–0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.5 0.6
–0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.5 0.6
5
3 –1
Contour interval (¥ 10 m s )
Contour interval (¥ 105 m3 s–1)
Summer
Autumn
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40
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100
120
140
200
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400
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40.5° 41.5° 42.5° 43.5° 44.5° 45.5° 46.5°
Longitude (E)
1800
40.5° 41.5° 42.5° 43.5° 44.5° 45.5° 46.5°
Longitude (E)
–0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.5 0.6
–0.6 –0.5 –0.4 –0.3 –0.2 –0.1 0.1 0.2 0.3 0.4 0.5 0.6
Contour interval (¥ 105 m3 s–1)
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F 11. Seasonal mean zonally-averaged vertical mass transport simulated by the DMG-MOM model with resolution,
forced with UKMO atmospheric analysis data.
1
12
the vertical circulation, the intensification of the
horizontal circulation does not automatically result in
pronounced vertical overturning (see Figure 11 and
also Stanev, 1990; Rachev & Stanev, 1997a) and does
not have a clear impact on the water–mass formation. The results of simulations are consistent with
observations and simple theoretical considerations
(Bulgakov et al., 1996). The simulated seasonal mean
horizontal mass transport ranges in values: 6, 8, 4
and 5106 m3 s 1 in winter, spring, summer
and autumn, respectively. The vertical overturning
is much smaller, amounting in the upper layer to
Coastal–open ocean exchange in the Black Sea 617
(a)
46.5°N
46°N
Latitude
45.5°N
45°N
44.5°N
44°N
43.5°N
43°N
28°E
29°E
30°E 31°E 32°E
Longitude
33°E
34°E
33°E
34°E
33°E
34°E
40
(b)
46.5°N
46°N
Latitude
45.5°N
45°N
44.5°N
44°N
43.5°N
43°N
28°E
29°E
30°E 31°E 32°E
Longitude
30
(c)
46.5°N
46°N
Latitude
45.5°N
45°N
44.5°N
44°N
43.5°N
43°N
28°E
29°E
30°E 31°E 32°E
Longitude
in the deep layer are 16104 m3 s 1 in winter,
9104 m3 s 1 in spring, 10104 m3 s 1 in summer
and 13104 m3 s 1 in autumn. Thus, we could
conclude that the vertical circulation is: (1) much
weaker than the horizontal one (about two orders of
magnitude, which is typical for stagnant basins;
Stanev, 1990), (2) about one order of magnitude
stronger than the river discharge or the transport
through the Straits of Bosphorus (104 m3 s 1,
O
} zsoy & U
} nluata, 1999, and (3) the patterns are more
irregular and variable compared to those of horizontal
transport. The above results support the idea that the
horizontal processes in the Black Sea are much more
active than the vertical ones, which is also due to the
stagnant conditions. The evidence that the vertical
circulation is about one order of magnitude larger
than the straits inflow supports the independent
results based on the analysis of radionuclide penetration into the pycnocline, and simulations on the
Mediterranean plume in the Black Sea, giving a value
of about 10 for the rate of entrainment of Black Sea
water by the Mediterranean plume (Buesseler et al.,
1991; Simeonov et al., 1997; Staneva et al., 1999).
The correlation of our estimates with this fundamental number, obtained independently, gives a credibility
for the simulated ventilation of intermediate and deep
layers.
In the context of the present study, the area west of
the Crimean Peninsula is very important as a key area
where the rim current attacks the continental slope.
The anticyclonic eddies simulated in this region
propagate westward (Figure 12) and shape the
exchange between the shelf and the open sea.
These eddies are also found in the survey data
[Figure 1(a,c)], as well as in the T/P data [Figure 1(d)
and Figure 3]. Their impact on the shelf–open ocean
exchange is associated with the large thermal capacity
of anticyclones caused by the deeper position of the
halocline. This has at least two important consequences for the physical and biological system related
to the deepening of the pycnocline in this area: (1) an
increase in the volume of the biologically active layer
and (2) mixing between coastal and open-sea waters
might become more efficient.
30
F 12. Snapshots of velocities at 2·5 m in the northwestern Black Sea. The plots are based on simulations with
the DMG-MOM model with resolution, forced with
UKMO atmospheric analysis data. (a) 20 January 1993;
(b) 29 June 1993; (c) 19 September 1993.
1
12
6104 m3 s 1 in winter, 4104 m3 s 1 in spring,
4104 m3 s 1 in summer and 5104 m3 s 1 in
autumn. The corresponding rates of vertical transport
Conclusions
We have demonstrated that the altimeter signal is
reliable for analysis of the annual variability of circulation. The large amount of such data enables a
precise mapping of basin-wide dynamics, as well as of
the variability in some dynamically important areas
(coastal anticyclones and the well-known areas of
the Batumi and Sevastopol eddies, Figure 3). The
618 E. V. Stanev et al.
multiple time-scales were demonstrated over a wide
range of frequencies (from several hours, governing
internal gravity oscillations and convection, to interannual). Such an illustration is the coupling between
barotropic and baroclinic oscillations. This process is
of significance for the dynamics in the coastal zone,
and one important fact has been given here associated
with the baroclinic wave excitation and wave shedding
towards the open sea with a phase speed of 0·5–
1 m s 1. A fundamental characteristic of the Black
Sea, intimately related to the coastal–open sea
exchange, is extreme vertical stratification. It prevents
vertical mixing, in which case the oscillations of the
interface (permanent halocline) interacting with the
shelf/continental slope are a key process. The transformations of the wave signal in the coastal region
(Figure 6) provide mixing in the halocline that might
affect the thickness of the CIL. This explains, at least
partially, the importance of lateral mixing in the Black
Sea, providing the basin interior with strong signals
generated in the coastal areas. Unfortunately, studies
in this field are almost non-existent for the Black Sea,
but what is already known from ocean studies is that
this is a potentially very important area for exploring
different scenarios regarding the transport and transformation of physical and biological matter on the
shelf break.
What has not yet been addressed in enough detail in
the context of horizontal mixing is the interaction
between the vertical stratification and the processes
acting in the horizontal. It takes about 1 year for the
currents to make a loop along the coast; however, the
vertical penetration of the signals is much slower.
Under such conditions, the horizontal–isopycnical
mixing is much larger than the vertical–diapycnal
mixing, and the water properties tend to align to the
isopycnals. The latter exhibit a large slope over the
narrow continental slope and split the Black Sea into
two dynamically distinct areas: coastal and open
ocean, where water properties are dominated by
surface- and deep-water characteristics, respectively.
What has not yet been analysed in the current models
is the extent to which the mixing parameterizations,
aligned along the model co-ordinates, are applicable
in areas of sharp slopes (even under very fine resolution). More elaborate parameterizations (Gent &
McWilliams, 1990; Griffies et al., 1998) have to be
further applied in order to reach a better consistency
between the models and the real mixing processes.
Developing new mixing parameterizations in extreme
areas such as the Black Sea is another challenging task
for the future. Since the vertical overturning (i.e. a
measure of internal mixing) appears to be about one
order of magnitude stronger than the river discharge
or the transport through the Straits of Bosphorus
(this factor is in agreement with the estimates from
observations of Buesseler et al., 1991), it is of fundamental interest to test the sensitivity of this estimate
to different parameterizations and water balance
scenarios.
Among the interesting dynamical features deserving
future interest, and in particular more profound
quantification, are the two branches of vertical circulation (i.e. upwelling in the interior and downwelling
in the coastal zone) communicating by exchanging
water and other properties in the slope area. The
position, slope and variability of the pycnocline in the
coastal area might have important consequences not
only for the physical system, but also, by increasing
the volume of the surface layer, the pelagic system.
What has now become clear is that the ventilation of
the coastal zone is controlled by the eddy activity
along the rim current, which has far-reaching consequences. By exchanging waters between the two areas
(often diapycnally), the model tends to change the
vertical stratification and the amplitude of the seasonal
signal in the intermediate layer. As mentioned
earlier, these exchange processes have to be further
investigated with models that have more elaborate
parameterizations of the mixing.
Acknowledgements
The authors thank M. H. Calvex for altimeter data
processing and V. Belokopytov for providing us with
gridded climatic data. The help of E. Cholakov, who
plotted some figures, is also acknowledged. Thanks
are also due to G. Korotaev for useful comments on
the manuscript. Data from the CoMSBlack surveys
have been prepared in the framework of the NATO
Black Sea project and been made available through
Black Sea Environment Internet Node (BSEIN). This
work has been supported by an EROS 21 research
contract with CEC grant IC20-CT96-0065. We
acknowledge the help of the UKMO for making
available meteorological analysis data under the
research contract with CEC EV5V-CT92 0121,
supplementary agreement CIPD CT93 0016. This is
publication No. 194 of the EU-ELOISE initiative.
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