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Journal of Marine Systems 47 (2004) 51–66
Influence of the Aral Sea negative water balance on its seasonal
circulation patterns: use of a 3D hydrodynamic model
D. Sirjacobs*, M. Gregoire, E. Delhez, J.C.J. Nihoul
Universite de Liege, GeoHydrodynamics and Environment Research (B5), B4000 Sart-Tilman, Belgium
Received 7 May 2003; accepted 31 December 2003
Available online 24 April 2004
Abstract
A 3D hydrodynamic model of the Aral Sea was successfully implemented to address the complex hydrodynamic changes
induced by the combined effect of hydrologic and climatic change in the Aral region. The first barotropic numerical
experiments allowed us to produce a comparative description of the mean general seasonal circulation patterns corresponding to
the original situation (1956–1960) and of the average situation for the period from 1981 to 1985, a very low river flow period.
The dominant anticyclonic circulation suggested by our seasonal simulation is in good agreement with previous investigations.
In addition, this main anticyclonic gyre was shown to be stable and clearly established from February to September, while
winter winds led to another circulation scenario. In winter, the main anticyclonic gyre was considerably limited, and cyclonic
circulations appeared in the deep western basin and in the northeast of the shallow basin. In contrast, stronger anticyclonic
circulation was observed in the Small Aral Sea during winter. As a consequence of the 10-m sea level drop observed between
the two periods considered, the 1981–1985 simulation suggests an intensification of seasonal variability. Total water transport
of the main gyre was reduced with sea level drop by a minimum of 30% in May and up to 54% in September. Before 1960, the
study of the net flows through Berg and Kokaral Straits allowed us to evaluate the component of water exchange between the
Small and the Large Seas linked with the general anticyclonic circulation around Kokaral Island. This exchange was lowest in
summer (with a mean anticyclonic exchange of 222 m3/s for July and August), highest in fall and winter (with a mean value of
1356 m3/s from September to February) and briefly reversed in the spring (mean cyclonic circulation of 316 m3/s for April and
May). In summer, the water exchange due to local circulation at the scale of each strait was comparatively more important
because net flows through the straits were low. After about 20 years of negative water balance, the western Kokaral Strait was
dried up and the depth of Berg Strait was reduced from 15 to 5 m. Simulation indicated a quasi-null net transport, except during
the seasonal modification of the circulation pattern, in February and October. A limited, but stable, water exchange of about 100
m3/s remained throughout the year, as a result of the permanent superposition of opposite currents.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Aral Sea; Inland water environment; Hydrodynamics; Mathematical model; Circulation; Water level; Anthropogenic period; Latitude
45jN, longitude 60jE
1. Introduction
0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmarsys.2003.12.008
* Corresponding author. Tel.: +32-4-36-6-36-47; fax: +32-4-36-
6-32-25.
E-mail address: D.Sirjacobs@ulg.ac.be (D. Sirjacobs).
Following the dramatic increases in societal water
withdrawals in the Aral Sea watershed, the annual
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6652
river discharge to the Aral Sea started to drop in 1960
(Bortnik, 1996). Since then, the global water balance
became persistently negative and led to a rapid
shrinkage of the Aral water body, compared to known
fluctuations at geological time scale (Boomer et al.,
2000). Consequent sharp decreases in level, volume
and areal extent of the Aral Sea triggered further
important environmental changes in the sea itself
(Aladin and Potts, 1992; Aladin et al., 1996; Letolle
and Chesterickoff, 1999; Small et al., 2001a) and in
the whole region (Micklin and Williams, 1996; Small
et al., 2001b). This significant human perturbation of
the natural hydrological cycle evolved into one of the
most severe human-induced ecological catastrophe of
the twentieth century.
The hydrodynamics of the Aral Sea is a funda-
mental aspect that cannot be neglected, if one aims to
understand the past and future evolution of the Aral
Sea and surrounding region. As a matter of fact, by
allowing one to go further than considering an homo-
geneous water body, a complete 3D hydrodynamic
model of the sea will help to solve many questions.
Water, salt and heat budgets are affected by complex
feedback conditioned by hydrodynamics. The non-
uniform distribution of salinity and temperature
throughout the water body (by stratification and by
limited exchanges between closed bays or shallow
zones and the main basins) affects evaporation rates,
biogeochemical reactions and winter ice cover forma-
tion (Tsytsazrin, 1995). Hydrobiology is also condi-
tioned by hydrodynamics: the distribution of nutrients
and pollutants (Tsytsazrin, 1995), the extension of
refuge zones for species less tolerant to saline con-
ditions and the connectivity of populations between
large basins as function of flow through the straits.
Studies that will combine 3D mathematical simula-
tions and in-situ hydrochemical measurements may
produce valuable information concerning the less
understood groundwater exchanges. Such 3D models
may also prove useful in testing the consequences of
various hydraulic engineering scenarios proposed for
improving water management in the remaining Aral
Sea water bodies (Aladin et al., 1995).
Although the Aral Sea catastrophe has been wide-
ly studied and numerous publications can be found
on various aspects of its causes and consequences
(Dement’ev, 1993; Ivanov et al., 1996; Zhamoida et
al., 1997; Aladin et al., 1998; Ferrari et al., 1999;
Lyatkher, 2000), comparatively few efforts were
devoted to understanding the general circulation of
the sea, as well as the complex history of Aral Sea
water masses, throughout the desiccation process. In
1956, a first qualitative chart of dominant surface
currents of the Aral Sea was established by Blinov (in
Barth, 2000). A two-dimensional model was used by
Barth (2000) to simulate, at different stages of the
dessication process, the Aral Sea depth-integrated
circulation resulting from typical cyclonic and anti-
cylonic wind fields. Yet, each of these simulations
was realized as isolated run and each assumed a
constant wind forcing.
Foreseeing the interest of using an integrated
mathematical tool to address the complex ecosystem
evolution following the Aral Sea’s shrinkage, a 3D
hydrodynamic model was adapted to the case of the
Aral Sea. This paper describes its implementation,
forcing and results of the first numerical experiments.
The consequences of the desiccation process on the
hydrodynamics of the Sea are studied by comparing a
simulation representing the original situation (1956–
1960) to a simulation corresponding to the average
situation for the period from 1981 to 1985, which was
a very low river flow period that occurred after a 10-m
water level drop. General seasonal circulation patterns
are compared for both periods, as well as seasonal
fluctuations of the transport of the main gyre and of
water mass exchange between the Small and Large
Aral Seas.
2. Mathematical model: description and forcing
2.1. Selection of the model
The general aim of this work is to implement a
mathematical tool allowing study of the effect of a
persistent negative water balance on the 3D seasonal
circulation, temperature, salinity and water-mixing
fields. For this, we used as a basis the numerical code
of a large-scale hydrodynamic model developed by
co-author Professor E. Delhez. This three-dimensional
model is baroclinic, has a robust turbulence closure
and was designed to address specifically the macro-
scale processes affecting the northwestern European
continental shelf. Detailed description of the model
and equations of state can be found in Delhez (1997).
Fig. 1. Bathymetry of the Aral Sea used for both simulated periods 1956–1960 and 1981–1985 (corresponding respectively to mean sea level
of (a) 1960 and (b) 1983).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 53
Fig. 2. Atmospheric parameters obtained from Aral Sea meteorological stations during both simulated periods: (a) location of stations, (b)
names, (c) number of stations in function regarding specific forcing; adapted from Bortnik and Chistyaeva (1990).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6654
This model is characterized by 10 horizontal layers,
uses vertical ‘sigma’ coordinates, and its code has a
parallel structure, running on eight processors for our
application. Originally designed for an environment
presenting strong tidal sea level fluctuation, some
subroutines of the model are devoted to the simulation
of local drying events of particular grid points, when
water depth falls below a particular threshold. Land
Fig. 3. Average seasonal variation of evaporation in particular stations of
Ostrov Lazareva); adapted from Bortnik and Chistyaeva (1990).
points can also be flooded when the sea level eleva-
tion of a neighbouring grid point rises above the local
land elevation. These particularities triggered the
selection of this model as the starting point for our
work, as they will allow the representation of a
changing Aral Sea coastline when launching long
term simulations (i.e., representing several successive
years). By then, particular attention should be devoted
the Aral Sea for the period 1961–1980 (Station 1: Uyaly, Station 2:
Table 1
Mean annual river discharge for the periods 1951–1960 and
1981–1985
(A) Mean annual salinity of river dischargea
g/l Amu Darya Syr Darya
1956–1960 0.55 0.81
1981–1985 1.75 1.40
(B) Mean annual river dischargeb
km3/year Amu Darya Syr Darya
1956–1960 37.94 10.04
1981–1985 2.24 0.22
a Calculated from Glazovsky (1995).b Calculated from Kostianoy (personal communication, in
Sirjacobs et al., 2001).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 55
to the potential bias introduced by these user-defined
water level thresholds, controlling the drying or flood-
ing of particular grid points, on the accuracy of the
global water and salt balance.
2.2. Forcing and validation data: compilation,
synthesis and choice of simulation periods
The forcings required are the seasonal fluctuations
of wind fields, precipitation, evaporation, water dis-
charge and salinity of the Amu Darya and Syr Darya
rivers, could cover, air temperature and humidity.
Extensive efforts were devoted to compile a large
data base of all potentially useful parameters in order
to cover the forcing and validation needs of the model
from 1950 to the present. Two periods (1956–1960
Fig. 4. Average seasonal variation of total river discharge in the Aral Sea
(1990).
and 1981–1985) were selected for studying the
changing hydrodynamics of the Aral Sea. This selec-
tion was made as a best compromise between data
availability and years of particular interest in the
drying process (from a river discharge and a hydro-
dynamic point of view). Most of the required forcing
data are available for these periods. In addition, the
mean seasonal salinity and temperature fields (surface
distributions and different cross sections) are provided
for both periods by Bortnik and Chistyaeva (1990), on
the basis of measurements at sea. Focusing our
simulations on these periods will allow us, on the
occasion of further work, to use these precious in-situ
data for model validation.
The original Aral Sea seasonal hydrodynamics is
simulated with the average forcing from 1956 to 1960.
This first investigation concerns a period of relative
stability of the water balance, before the beginning of
the drying process. However, wind, cloud cover and
evaporation data are missing for this period and are
assumed to be identical to the 1981–1985 period, as a
first approach. The consequences of the drying process
on the hydrodynamics of the Sea will be studied by
comparison of the mean 1956–1960 seasonal simula-
tion to one representing the average situation for the
years 1981 to 1985, a very low river flow period. As
suggested by previous work (Sirjacobs et al., 2001),
another feature of this 1981–1985 period is a change
in the mixing regime induced by winter cooling that
would have occurred around 1984, when the temper-
ature of maximum density became lower then the
freezing temperature, due to the increasing salinity.
for the period 1961–1980; adapted from Bortnik and Chistyaeva
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6656
The most important adaptations brought to the
selected model are listed in the following paragraphs.
2.2.1. Bathymetry
We used the bathymetry digitized by Barth (2000),
from a precise bathymetric map of the sea in its original
state (Russian military map, scale 1/450,000). The
whole Aral Sea is represented by a final grid of
100� 130 meshes (respectively in longitude and lati-
tude), each of them corresponding to about 2.92 km
side length. Fig. 1 illustrates the bathymetry used for
each simulated period, as well as the three sub-basins of
the Aral Sea (Small Aral Sea; Large Aral Sea, western
Fig. 5. Seasonal variations of the surface wind str
basin; Large Aral Sea, eastern basin) and the two straits
linking the Small and Large Aral Seas (Berg Strait and
Kokaral Strait). The morphometric relations between
the sea volume, area and level in the basin were
calculated from this digitized bathymetry and showed
satisfying agreement (Sirjacobs et al., 2001) with
similar relations given by several authors (Bortnik
and Chistyaeva, 1990; Perminov et al., 1993; Glazov-
sky, 1995; Ressl, 1996).
2.2.2. Atmospheric forcing
The meteorological forcing updating frequency
was adapted to the common optimum 1-month tem-
ess considered for both simulations periods.
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 57
poral resolution of the selected data sets. Wind and
total cloud cover data were extracted from the 24-
h ECMWF simulations (European Center for Medi-
um-Range Weather Forecasts, Re-Analysis Sample
Data 1979–1993), whereas the mean seasonal cycle
(monthly mean in situ data) of precipitation, surface
air temperature and humidity were obtained for both
periods from the very good synthesis made by Bortnik
and Chistyaeva (1990). This data set was compiled
from a varying number of meteorological stations
depending on the parameter and period (Fig. 2). The
resulting interpolated fields suggest that dominant
gradients are oriented north–south, with maximum
spatial ranges clearly occurring in winter for temper-
ature, and in summer for air saturation (relative
humidity). For each of the two simulation periods,
seasonal variability of evaporation was reconstructed
a) April
c) October
46.5
46
45.5
45
44.5
44
43.5
Lati
tud
e (d
ecim
al
deg
ree,
nort
h)
58.5 59 59.5 60 60.5 61 61.5Longitude (decimal degree, east)
46.5
46
45.5
45
44.5
44
43.5
Lati
tud
e (d
ecim
al
deg
ree,
nort
h)
58.5 59 59.5 60 60.5 61 61.5Longitude (decimal degree, east)
Fig. 6. Seasonal variations of the sea surface elevatio
by distributing total annual evaporation according to
the only seasonal information available about evapo-
ration: the mean seasonal cycle provided by Bortnik
and Chistyaeva (1990) for the period 1960–1980
(Fig. 3). The total annual evaporation considered for
the two periods are, respectively, set to conserve water
mass for 1951–1960 (i.e., equal to river discharge
plus precipitation over the sea surface) and set as 96
cm/year, the mean annual evaporation calculated from
Bortnik and Chistyaeva (1990) for the period 1981–
1985. The validity of these data was confirmed by a
previous water balance study (Sirjacobs et al., 2001).
2.2.3. River discharge
For both Amu Darya and Syr Darya and for each
simulated period, monthly water discharges were
calculated from data provided by Kostianoy (per-
b) August
d)December
46.5
46
45.5
45
44.5
44
43.5
Lati
tud
e (d
ecim
al
deg
ree,
nort
h)
58.5 59 59.5 60 60.5 61 61.5Longitude (decimal degree, east)
46.5
46
45.5
45
44.5
44
43.5
Lati
tud
e (d
ecim
al
deg
ree,
nort
h)
58.5 59 59.5 60 60.5 61 61.5Longitude (decimal degree, east)
n (m) corresponding to the period 1956–1960.
D. Sirjacobs et al. / Journal of Mar58
sonal communication), whereas average salinity of
the discharges were calculated from data published
by Glazovsky (1995) (Table 1). The mean annual
river discharges for the periods 1951–1960 and
1981–1985 were selected from previous water bal-
ance studies (Sirjacobs et al., 2001). Seasonal cycles
of river discharges were reconstructed with same
approach as described for the evaporation parameter,
on the basis of the mean seasonal cycle of river
discharge calculated for the period 1960–1980
(Bortnik and Chistyaeva, 1990) (Fig. 4). An identi-
cal seasonal cycle of the discharged water tempera-
ture was imposed for both rivers and both periods,
as a simple sinusoid defined by a minimum of 0 jCat the end of January and a maximum of 20 jC at
the end of July.
Fig. 7. Seasonal variations of the sea surface elevatio
2.2.4. Water budget
The water mass conservation subroutines were
adapted to take into consideration precipitation and
evaporation fluxes. They were considered as an
equivalent temporal variation of the elevation of the
sea surface in each grid point. No groundwater input
or infiltration were considered at this stage, as their
total contribution to the global water balance were
shown to be relatively low for the considered periods
(Sirjacobs et al., 2001) and as only limited data are
available for such an integrated simulation at the scale
of the whole Aral Sea.
2.2.5. Salt budget
The model takes into account the local effects of
river discharges and their salinity levels (Table 1a), as
ine Systems 47 (2004) 51–66
n (m) corresponding to the period 1981–1985.
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 59
well as the evaporation/precipitation fluxes, on the
salinity budget of the sea.
3. Aral Sea seasonal circulation: analysis of
simulation results
In this section, we present and discuss the first
results of monthly averaged currents obtained with the
previously described model functioning in barotropic
mode.
3.1. Changes in general circulation patterns
The typical seasonal variations of the surface wind
stress considered for both simulations are presented in
Fig. 5. Resulting hydrodynamic variations are sum-
Fig. 8. Seasonal variations of the vertically integrated circul
marized in Figs. 6–11, for both the earlier and later
simulated periods: sea surface elevation (Figs. 6 and
7), circulation (Figs. 8 and 9) and horizontal flow
across a longitudinal section centered on the main
gyre (Figs. 10 and 11). A general anticyclonic circu-
lation is observed for both simulated periods. As
illustrated in Fig. 12, this observation is in agreement
with results of previous studies (Barth, 2000). Our
seasonal simulations show that this main gyre is stable
and clearly established from February to September.
Its western current shows a separation in two branches
occupying the deep western basin and the west of the
shallow eastern basin, joining back at the approach of
Barsakelmes Island (e.g., Barsakelmes Current). Most
of the flow passes north from this island until October,
when it separates in two, with the northern flow
remaining dominant. The easterly wind begin to push
ation (m2/s) corresponding to the period 1956–1960.
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6660
the center of the main gyre southwards, and reduces
its transport (Fig. 13). Due to the southeasterly winter
winds (December), these changes continue and lead to
another circulation pattern: the main anticyclonic gyre
is considerably limited and almost separated in two
sub gyres, whereas we can see the appearance of
cyclonic circulations in the deep western basin and
in the northeast of the shallow basin. Results also
suggest the establishment of a winter anticyclonic
circulation in the Small Aral Sea. Our model confirms
also the more complex circulation of the Small Sea,
showing several cyclonic and anticyclonic gyres
strongly influenced by local bathymetry, as suggested
earlier by Barth (2000).
Although Barth (2000) used wind forcing cor-
responding to particular months of the years 1997,
1998 and 1999, both studies showed good consistency
in the simulation of the pre-1960 hydrodynamics of
Fig. 9. Seasonal variations of the vertically integrated circu
the Aral Sea. Estimation of the (anticyclonic) trans-
port of the main gyre for the month of June is of the
similar order to that given by this earlier 2D study, but
a little higher (around 21,000 vs. 17,000 m3/s). very
similar estimates of about 40,000 m3/s were given for
the month of August by both models. Barth 2D model
indicated a transport of ‘‘� 21,000 m3/s’’ for the
month of January, suggesting a reversal of the circu-
lation of the main gyre, becoming cyclonic. A sharp
winter modification of the circulation was also sug-
gested by our results, but they showed a reduction and
displacement of the anticyclonic gyre towards the
south, and the appearance of two interconnected
cyclonic gyres, instead of a simple circulation rever-
sal. Thus, the notion of ‘‘main gyre’’ transport was not
estimated for the winter months of November to
January (Fig. 13). Good correspondence of sea level
elevation could also be noticed with the two simu-
lation (m2/s) corresponding to the period 1981–1985.
Fig. 10. Seasonal variations of the horizontal flow across a longitudinal section centered on the main gyre (latitude 45j16Vnorth), for the period1956–1960 (flow in m3/s, counted positively towards the north).
Fig. 11. Seasonal variations of the horizontal flow across a longitudinal section centered on the main gyre (latitude 45j16Vnorth), for the period1981–1985 (flow in m3/s, counted positively towards the north).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 61
0.8 m/s
b)a)
Fig. 12. Dominant anticyclonic surface circulation of the Aral Sea according to (a) Blinov (in Barth, 2000) and (b) 2D simulation realized for the
year 1960 with anticyclonic wind forcing (Barth, 2000).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6662
lations realized by Barth (2000) with typical wind
forcing corresponding to the months of June and
September.
Subsequent to the 10-m sea level drop observed
between the two periods considered, the 1981–1985
simulation (Figs. 9 and 11) suggests an intensification
of the previously described seasonal changes. The
October division of the Barsakelmes Current is sharp-
er, with the main part of the flow clearly passing south
of Barsakelmes Island. In December, cyclonic circu-
Fig. 13. Seasonal total transport of the main ant
lations are more widely installed than for the 1956–
1960 period, dramatically reducing the anticyclonic
circulation to the sole southern sub-gyre (the northern
sub-gyre disappeared). For both simulations, the sea-
sonal variability of the total water transport of the
main gyre is characterized by two maxima (Fig. 13),
one in the spring (April) and the other in the late
summer (August and September). This phenomenon
is linked to the combined occurrence of higher wind
speed and a more pronounced anticyclonic wind
i cyclonic gyre for each simulated period.
Fig. 14. Cross section of horizontal flows through Berg and Kokaral straits for the months of August and December (flow in m3/s, counted
positively towards the north).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 63
forcing pattern during these months. As logically
expected, our simulations indicate that between the
periods 1956–1960 and 1981–1985, the total water
transport of the main gyre was reduced, as a result of
the sea level drop, by a minimum of 30% in May and
up to 54% in September.
Fig. 15. Seasonal fluctuations of net horizontal flows through Berg and
1956–1960 (flow in m3/s, counted positively towards the north).
3.2. Changes in the water mass exchange between the
Small and Large Aral Seas
For the period 1956–1960, the water mass ex-
change between the Small and the Large Seas was
studied by computing the seasonal variations of
Kokaral straits and of their north/south components for the period
Fig. 17. Typical cross section of horizontal flows through Berg strait
observed during the 1981–1985 simulation, for the month of
September (flow in m3/s, counted positively towards the north).
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–6664
northerly and southerly components of the flows
crossing the Berg and Kokaral straits. The cross
sections of the horizontal flow through these straits
are illustrated in Fig. 14 for the months of August and
December, whereas the seasonal fluctuations of their
components are plotted in Fig. 15. The analysis of
these fluctuations was done in terms of exchange due
to global anticyclonic circulation around Kokaral
Island and of exchange due to locally compensated
circulation at the scale of each strait (Fig. 16). This
approach reveals that, in fall and winter, important net
transports lead to a mean global anticyclonic ex-
change of 1356 m3/s (from September to February).
This exchange flow contributes to force an anticy-
clonic circulation in the Small Sea, resulting in an
even larger gyre around Kokaral Island in December.
Net flows are only briefly reversed in the spring,
leading to a mean exchange due to global cyclonic
circulation of 316 m3/s (for April and May). In
summer, the net flows through the straits are low
(mean anticyclonic exchange of 222 m3/s for July and
August) and the water exchange by local circulation at
the scale of each strait is relatively more important
than in winter. The mean water exchange due to local
circulation was estimated to be 289 m3/s for the
Kokaral strait and 759 m3/s for the Berg strait.
After about 20 years of negative water balance
(period 1981–1985), the western Kokaral strait was
dried up and the depth of Berg strait was reduced from
15 to 5 m. Simulation indicated a quasi null net
transport, except at the occasion of the seasonal
Fig. 16. Water mass exchange between Large and Small Aral Seas throu
circulation around Kokaral Island (a) and to locally compensated circulat
alteration of the circulation pattern, in February and
October. A limited, but stable, water exchange of
about 100 m3/s remains throughout the year, as a
result of the permanent superposition of opposite
currents (as illustrated in Fig. 17 for the month of
September).
3.3. Synthesis of the dynamical balances causing the
observed flows and their variability
The main observed flow (large anticyclonic gyre)
is mainly the result of a geostrophic equilibrium.
gh Berg and Kokaral Straits: exchange due to global anticyclonic
ion at the scale of Berg (b) and Kokaral (c) straits.
D. Sirjacobs et al. / Journal of Marine Systems 47 (2004) 51–66 65
Upon this, seasonal variation of wind fields is most
probably the main responsible of the variability of the
general circulation, although differing water balances
for each basin may also contribute to observed sea-
sonal variations. Concerning the variability of the
flow between the two periods, the main factors acting
are the drastic changes of the morphology of the water
body (it is shallower, only one communication sub-
sists between the north and south basins through Berg
Strait, the area exposed to wind forcing is much
reduced), and also the lost of the freshwater input
from rivers. Generally, further modeling results are
required in order to provide more dynamical interpre-
tation, mainly when considering that the next steps of
the research should include baroclinic effects, as well
as ice cover formation and salt balance, both playing a
fundamental role in the water mass formation.
4. Conclusions
The general anticyclonic circulation suggested by
our seasonal simulation of the original situation (be-
fore 1960) is in good agreement with previous inves-
tigations. In addition, the main anticyclonic gyre was
shown to be stable and clearly established from Feb-
ruary to September, while winter winds (particularly
southeasterly winds in December) lead to another
circulation scenario. In winter, the main anticyclonic
gyre is considerably limited, and cyclonic circulations
appeared in the deep western basin and in the northeast
of the shallow basin. In contrast, stronger anticyclonic
circulation was observed in the Small Aral Sea during
winter. As a result of the 10-m sea level drop observed
between the two periods considered, the 1981–1985
simulation suggests an intensification of the previously
described seasonal changes. For both simulations, the
main gyre total water transport is characterized by two
maxima (one in the spring and the other in the late
summer). Total water transport of the main gyre was
reduced with sea level drop, by a minimum of 30% in
May and up to 54% in September. The study of the net
flows through Berg and Kokaral straits before 1960
allowed estimation of the component of water ex-
change between the Small and the Large Seas linked
with the general anticyclonic circulation around Koka-
ral Island. This exchange was lowest in summer (with
a mean anticyclonic exchange of 222 m3/s for July and
August), most important in fall and winter (with a
mean value of 1356 m3/s from September to February)
and was briefly reversed in spring (mean cyclonic
circulation of 316 m3/s for April and May). Mean
annual water exchanges due to local circulation at the
scale of each strait were estimated to be 289 m3/s for
Kokaral Strait and 759 m3/s for Berg Strait. In summer,
this type of exchange was comparatively more impor-
tant, as net flows through the straits were low. After
about 20 years of negative water balance, the western
Kokaral strait was dried up and the depth of Berg strait
was reduced from 15 to 5 m. Simulation indicated a
quasi-null net transport, except on the occasion of the
seasonal modification of the circulation pattern, in
February and October. A limited but stable water
exchange of about 100 m3/s remained throughout the
year, as a result of the permanent superposition of
opposite currents.
Acknowledgements
This research was supported by the European
Community INCO-Copernicus grant N ICA2-CT-
2000-10023: ARAL-KUM: ‘‘Desertification in the
Aral Sea Region: A Study of the Natural and An-
thropogenic Impacts’’.
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