Influence of the Aral Sea negative water balance on its seasonal circulation patterns: use of a 3D...

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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: [email protected] (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).


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).


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


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.


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)


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)


46.5

46

45.5

45

44.5

44

43.5

Lati

tud

e (d

ecim

al

deg

ree,

nort

h)


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.


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.


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).


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).


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).


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).


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.


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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