Thermohaline strucutre and circulation of

the Western Large Aral Sea in the beginning

of XXI century

A. S. Izhitskiy1, P.O. Zavialov1 and E. Roget2

Girona, 27 of June 2014

1 Shirshov Institute of Oceanology RAS, Moscow, Russia2 University of Girona, Catalonia, Spain

Motivation

The fourth largest lake on Earth in 1960, the Central Asia’s Aral Sea has lost over 80% of its area and over 90% of its

volume because of severe desiccation, associated mainly with unsustainable diversions of water for irrigation from the

tributary rivers.

The changes in physical and geographical conditions occurred in past 50 years have led to considerable rebuilding of the

entire hydrological structure of the sea.

In comparison with pre-desiccation state the overall level drop was about 27 m.

the significance of the ongoing changes of the Aral Sea is not limited to the applied, regional aspects. The lake can be

thought of as a “natural laboratory”, where the evolution of a large inland water body under anthropogenic intervention

through diversions of the river runoff can be investigated.

The shrinkage resulted in profound changes of the lake’s ecosystem and desertification of the surrounding areas.

main objective

investigation of the hydrological regime and

circulation of the today’s Aral Sea

methods

direct field observations

numerical modelling

Thermohaline structure – “natural” conditions

Salinity (ppt) distribution in summer, typical for the predesiccation

Aral Sea. Longitudinal vertical section through the western deep

basin [Zavialov , 2009].

Surface salinity (ppt) distribution in summer,

typical for the predesiccation Aral Sea

[Zavialov, 2009].

• the slim spatial and vertical variability of salinity, except the

estuaries regions

• the relatively small vertical density stratification

• the vertical stratification was governed mainly by

temperature

• thermal and haline mixing of the water column, full

ventilation through the year

Water circulation – “natural” conditions

the peculiar feature of the water circulation in the Aral Sea in its predesiccation

state:

• previous studies [e.g., Berg, 1908, Simonov, 1954, Kosarev, 1975, Bortnik and Chistyaeva,

1990] have revealed the anticyclonic character of the surface circulation in the Aral Sea under

predominant winds, as opposed to the Black Sea, Caspian Sea, Azov Sea, and other seas of the

same latitudinal belt

reasons:

• the anticyclonic character of the surface circulation

has been hypothetically attributed to the combined

effect of the inhomogeneous wind stress distribution

over the Sea and the specific bottom topography of

the basin

question:

Has the water circulation character

changed during the desiccation process?

Area of observations

• 15 field studies were carried

out by Shirshov Institute of

Oceanology to western basin

and another parts of the Aral

sea since 2002 year

• five surveys to the

Aktumsuk region of the

western basin of the sea in:

August, 2009

September, 2010

November, 2011

September, 2012

Осtober-November, 2013

2013

201220112010

2009

Thermohaline structure

Depth, m

� three-layered pattern

� two local salinity maxima

� temperature inversion in the bottom layer

2009

2009 2010

2010

Maximum salinity - 113,8 g/kg

Minimum salinity – 110,1 g/kg

132,2 g/kg

112,5 g/kg

Thermohaline structure

Maximum salinity – 128,5 g/kg

Minimum salinity – 116 g/kg

2012

De

pth

, m

2011

2012

126,9 g/kg

119,9 g/kg

Thermohaline structure

Temperature, оС

Salinity, g/kg

� three-layered pattern

� two local salinity maxima

� temperature inversion in the bottom layer

1

1 1

1

2

2

2

2 3

3

3

3

Upper mixed layer, surface local maximum of salinity1

2 Intermediate layer, minimum of temperature and salinity

Bottom layer, bottom maximum of salinity, temperature inversion3

Thermohaline structure

Salinity, g/kg

Tem

pe

ratu

re, оС

2009 2010 2011 2012 2013

ΔS (g/kg) 0,2 14,7 11,3 3,9

ΔT (oC) 1,1 6,0 4,0 1,6 0.4

Estimation of dependence of T,S-characteristics of the western basin on the interbasin exchange

ΔS - difference between values of surface and bottom salinity maxima

ΔT - differece between maximum temperature in the inversion layer and

minimum temperature in the intermediate layer

Thermohaline structure

Measurements:

acoustic doppler currentmeters Nortek Aquadopp and SonTek - surface currents speed and

direction, discreteness 1 min

mechanic currentmeters SeaHorse - bottom currents speed and direction, discreteness 10

min

portable Drahtlose Weatherstation - variability of wind and principal meteorological

parameters, discreteness 10 min

Water circulation: direct observations

201320122011

20102009 Depth, mmooring station meteostation

Water circulation – direct observation, September 2010

Western slopeEastern slope

meteostation mooring station

Depth, m

Northerly wind event

(a) – development of southward currents along the eastern

shore

(b) – development of northward surface compensative

current along the western shore 30-35 hrs

(c,d) – forming of sea level tilt in the cross-basin directrion

(30 hrs)

(f) – organization of surface currents as an anti-cyclonic gyre

(35 – 40 ч)

(e) – organization of bottom currents as an cyclonic gyre ( 40

hrs)

Basing on measurements of 2010, the complete period of

the establishment of the anti-cylonic flow in the western

basin of the Aral Sea following a northerly wind event can be

estimated as about 40 hrs

Water circulation: direct observations

Water circulation – numerical modellingФактическая (слева) и инвертированная (справа) батиметрии западного бассейна Большого Арала.

Глубины соответствуют уровню моря, зафиксированному в сентябре 2010 г

Water circulation – model specifics

Equation of state for the Aral Sea [Gertman, Zavialov,

2011]:

SIGMA-t = A0+AtTw+AttTw2+AsS+AssS2+AtsTS

• Princeton Ocean Model (POM)

• Grid: NS 211, resolution of 967 m x EW 173, resolution 569 m

• 17 sigma-levels

• Forcing: constant and spatially uniform NE wind 3 m/s

•Duration of experiment: 12 days

• Stratification: conditions of 2010

• Non-stratified basin: 15oC, 106 g/kg

• Result: mean values computed between the days 9 and 12

Four model experiments:

1) real bathymetry, non-stratified conditions,

2) real bathymetry, stratified conditions,

3) inverted bathymetry, non-stratified conditions,

4) inverted bathymetry, stratified conditions

Water circulation - modelling results

Non-stratified conditions

Real bathymetry Inverted bathymetry

Water circulation - modelling results

Stratified basin

Real bathymetry Inverted bathymetry

Water circulation - modelling results

Non-stratified conditions

Real bathymetry Inverted bathymetry

Water circulation - modelling results

Stratified basin – bottom layer

Real bathymetry Inverted bathymetry

Conclusions

• Shrinkage of the Aral Sea resulted in development of strong vertical stratification of the water column and

forming of “three-layred” pattern with surface and bottom salinity maxima, separated by the relatively fresh

intermediate layer. Such a pattern is a result of combine action of two forming mechanisms – convective

and advective. The latter, connected with the water exchange between the western and the eastern basins

of the Large Aral Sea, is subject to significant interannual changes.

• Despite the profound changes in geographical and hydrological conditions of the lake, water circulation in

the deep western basin still retains its peculiar character, typical for the Aral Sea in its pre-dessication state.

According to direct observations, water circulation in the surface layer has anti-cyclonic character, while

circulation in the bottom layer is more complex but generally has cyclonic sign under the predominant

northerly winds.

• Model experiments helped to elucidate the origins of the anti-cyclonic circulation establishment in the

surface layer of the lake: the main reason for this feature is the “asymmetric” bathymetry with broad

shallow area along the eastern coast and relatively steep and deep western part. Simulation experiments

demonstrated clearly that surface circulation switches to the opposite sign as soon as the bathymetry is

inverted with respect to the longitudinal axis of the basin.

• A.S. Izhitskiy, P.O. Zavialov, E.Roget, H.P. Huang, A.K. Kurbaniyazov. On thermohaline structure

and circulation of the Western Large Aral Sea from 2009 to 2011: Observations and modeling, J.

Mar. Syst. 2014. V. 9. P. 234-247.

Thank you!