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22
УДК 551.465.71(265-062.5)
V.I. Kuzin, E.N. Golubeva ICM&MG SD RAS, Novosibirsk
MODELING OF HYDRODYNAMICAL AND HYDROPHYSICAL PROCESSES IN THE ARAL SEA
Introduction
The problem of the Aral Sea is an example of the ecological hazard caused by the human activities. In the 1960-s the intensification of the Amu-Darya, the Syr-Darya water use for irrigation changed the water balance in the Aral Sea resulting exceeding the evaporation over the precipitation and the river runoff. In 1989 this caused the separation of the Northern and the Central Aral basins, and in 2000, the connection of the Lazarev, Vozrogdenija islands with mainland and the formation of a single Peninsula which separate the Western (deep) and Eastern (shallow) basins with connection only in the Northern part. As a result, the Aral Sea level height decreased from 53 m to 32.5 m (in the Baltic system), have lost the 80% of water volume and the 60% of sea surface. The salinity increased from 10 to 68-70 g/l thus destroying the fishing industry. The ecological and social consequences are also dramatic. This work is done within the range of the goals of the INTAS Grant 01-0511 - REBASOWS, The research objectives are as follows: the forecasting the future Aral Sea water and salt balance under different scenarios of the water inflow to the Aral coastal zone; the definition of sustainable ecological profile of close water body and selection of the strategy of possible ecosystem, biodiversity and bioproductivity restoration in the part of the Aral Sea.
The possible ways for rehabilitation of the Aral Sea are as follows:
- Reducing up to 70% of the Amu-Darya water amount for irrigation which will increase the Aral Sea level up to 38.5 m (an unrealistic variant);
- The separation of the Western and Eastern parts and keeping only one of them with 20-35% reduction of water for irrigation.
In this paper some results of the numerical modeling of the Aral Sea circulation as well as demineralization during the high-water period 1998 is presented. The 3D model used in the numerical experiment is the Novosibirsk Computing Center (now the ICM&MG) ocean circulation model [1, 2] adapted to the Aral Sea basin.
1. A 3D hydrodynamic model of hydrophysical processes in the Aral Sea
The general features of the models are as follows:
- Mathematical model is based on the complete “primitive” nonlinear equations of the thermo-hydrodynamics of the ocean;
- Temperature and salinity distribution are calculated;
- The model have a possibility to include the calculation of the pollutants;
- The interaction with the atmosphere is realized via the upper mixed layer with the ice formation;
- The model have a possibility to include the open boundaries with inflows and outflows from the basin;
- The numerical technique of the model are based on a combination of the finite element and splitting methods;
- The triangulated quasi-regular B-grid are used;
- The model has two variants of the vertical levels treating: the sigma-coordinate model and z-coordinate model.
2. Data sources
- The climatic meteorological data from for the calculations of the temperature and salinity fluxes;
- Amu-Darya river runoff from the observation for 1998 high water period;
- NCEP/NCAR reanalysis wind for wind-stress calculation.
3. Designing of the experiments and the analysis of the results
The Aral Sea domain for calculation of the 3D currents, thermodynamics as well as spreading of the fresh water as constructed on the basis of the bottom topography produced by CR2 team. The level 32.5 m is set as the initial value for the determination of the basin area. The bottom topography is presented on (Fig.1).
The essential difference in the bottom relief of the Western and Eastern parts of the Aral Sea gives some difficulties in the modeling on the unique version of the vertical grid. The cross-section across the basin alone the latitude is presented on Fig.2a. Because of this reason, in the Western Part of the Sea where the maximal value of the depth is 48 m the z-coordinate multilevel model is more preferable. The sigma-coordinate model was used for Eastern Part of the Aral basin with weakly changing depth with maximal value 14 m. Schematic representation of two coordinate systems in the model is shown on the Figs. 2 b, c. Two coordinate systems were connected on the basis of the domain decomposition technology.
The model has horizontal grid with resolution 1000*1000 m, which corresponds to array 172*390 nodes. By the vertical non-uniform grid is used (35 levels for maximal depth). In the regions with minimal depth 2 m, 5 levels are included.
Vertical grid has the levels: Z=0, 0.5, 1, 1.5, 2,3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37 ,39 ,41, 43, 45, 47.
Aral sea bathymetry
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Fig. 1. The levels of height Fig. 2. a) The vertical cross-section of Aral
Sea alone latitude; b) Schematic representation of the sigma-coordinate grid; c) Schematic representation of the z-grid
(the Baltic system) for Aral Sea basin in meters
4. Numerical experiment description and the results
At the numerical experiment the Aral Sea basin was taken in a total configuration without damn between the Eastern and the Western parts. The Amu-Darya river runoff was directed to the Eastern part as in nature.
At the initial step, we have no spatial distribution of the climatic data at the surface. So, the constant values of the temperature (25oC) and salinity (68 g/l) corresponding to the summer season were set in a hole basin. The integration of the model was carried out during the period of about two years 1998-1999 containing the high-water period of May-September 1988 with the wind-stress produced from the NCEP/NCAR wind. The ice sea model includes only the thermodynamic part without dynamics, reology and drift of ice. One hour was taken as the time step in the model. On each time step, the following 3D hydrophysical fields were calculated: temperature, salinity, velocity, fresh water as a tracer.
The results of the numerical experiment allow as obtaining some specific features of the Aral Sea circulation, thermodynamics and fresh water spreading.
The circulation
The circulation in the Aral Sea basin is highly varying and although there are no strongly dominated pictures of the main circulation, it is possible to separate some specific features. First, the circulation is very sensitive to the wind and is mainly derived by the wind, except a short period of the ice covering the sea surface. The Eastern part is very shallow and has the fastest feedback to the wind change. Wide area of the Eastern part allows a well-manifested cyclonic (Fig. 3) or anticyclonic circulation to be formed. The transition period between them is characterized by the dipole circulation (Fig. 4). The Western basin is more narrow and deep. So, the circulation consists of more local gyres, but the feedback to the
wind change is slower then this of the Eastern one. The circulation variations during the seasons of the integration period may roughly be described in the following way. In the summer of the integration period in the Eastern part there was a cyclonic circulation. In Fig. 3 velocity field at a depth of 2 m is presented. The velocity value reaches 30 cm/s. The circulation in the Eastern basin is cyclonic in the South and anticyclonic in the North. There also exist some local circulations caused by the bottom topography and the basin configuration. In autumn the circulation in the Eastern basin becomes anticyclonic via the dipole circulation, what is presented in Fig. 4. During the winter period, when the wind is blocked by the ice cover this circulation is weakening, until the ice cover disappears. In spring, the reconstruction of the circulation in some periods leads to the chaotic enough circulation certain periods, becoming sufficiently stable by May.
The thermodynamics
The seasonal variations of the Aral Sea are influenced by the seasonal cycle, but the processes of the seasonal variations in the Western and the Eastern parts are different. In the Eastern part, the temperature is determined by the processes of heating and cooling on the surface and mixing by wind. This brings about to the homogeneous distribution of the temperature in the Eastern part within the range from 27oC to 0oC. In contrast to this, the result of the calculations shows that the thermal conditions of the Western part of the Aral Sea, determined by the seasonal cycle are divided into two main states: summer stratification and winter homothermy. The summer distribution has the stratification in the temperature about 20 degrees, with strong well manifested thermocline. The horizontal distribution is characterized by the lower temperature values in the western deep part and higher values in the eastern part. The cooling in the autumn and the winter seasons results in the density convection and to the homothermy.
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Fig. 3. Velocity field at depth 2 m. June
Fig. 4. Velocity field at depth 2 m. December
The salinity distribution and the fresh water propagation The salinity conditions during the integration period are defined by the Amu-Darya inflow during May-September, 1998. In this period, the river inflow was extremely high. The pictures present the propagation of the fresh water through the Eastern basin. In Fig. 5 the horizontal pictures of the refreshing are presented. After the river inflow stops, the horizontal distribution becomes nearly uniform, but the salinity is lower than at the initial moment. One can see a well-manifested movement of the low saline water from the South to the North. The pool of the freshened water has a tendency to turn to the East under the influence of the circulation. The freshened water reaches the narrow straight between the Eastern and the Western basins and propagates to the Northern part of the Eastern basin.
Fig. 5. Desalinization of the Aral Sea during the period of V-IX 1998 - high Amu-Darya inflow and X-XII 1998 - low Amu-Darya inflow
Acknowledgements
This work was supported by INTAS Grant 01-0511. The authors wish thank to Dr. Sergey Stanichny for the help in the preparing the data.
REFERENCES
1. V.I. Kuzin. Finite Element Method in the modeling of the ocean processes. NCC Publisher, Novosibirsk, P.190.
2. E.N. Golubeva, Ju.A. Ivanov, V.I. Kuzin, G.A. Platov. Numerical modeling of the World Ocean circulation including upper ocean mixed layer. // Oceanology. - 1992. - Vol. 32. -No 3. - P. 395-405.
© V.I. Kuzin, E.N. Golubeva, 2005
