The Caspian Sea and adjacent regions received an increased attention from geophysical science due to their unique natural characteristics. Being a closed basin with no surface outlets, the Caspian Sea shows high sensitivity to climatic variations. Its level depends on the balance between precipitation and evaporation over a large area including the sea and its watersheds. It effectively filters high frequency variations of net water balance and serves as a good indicator of long-term climatic trends through observed changes of its water level.
The AMIP programme presents a unique opportunity to assess the ability of the current GCMs to simulate the water cycle and heat balance of the atmosphere for the Caspian Sea region. An analysis of precipitation, evaporation, runoff, snow mass, radiation balance, surface temperature etc. was conducted for most of the models participating in the 10-years integrations with prescribed SST for the period 1979-1988. The models employ a large range of spatial resolutions and incorporate different parameterizations of physical processes. Regional analysis is made separately for models having low and high resolutions.
The computed seasonal cycle of precipitation over the Caspian Sea and its watershed show large spread for the low resolution as compared to that of the high resolution models. In about half of the models the annual precipitation is underestimated over the watershed and the maximum of the summer precipitation occurs 1-2 months later as compared to the observation. The seasonal cycle of the surface temperature is well simulated with high resolution models. The maximum snow mass accumulated over the watershed in winter has a large range of variations. Because integration in some models started from initial conditions with large snow mass, it took about one year to attain an equilibrium for the main heat and water balance components in such models.
Most of the high resolution models indicate the existence of a signal in precipitation over the watershed that decreased in 1983; this relationship is also found in the index of drought. The annual sea level derived from the net water balance over the Caspian Sea and its watershed increases during 1979-1988 in the majority of the models. However the sea level trend varies significantly.
The annual precipitation over watersheds of the Volga and Ural rivers and evaporation from the Caspian Sea provide the main contribution to the sea level change. The correct size of the Caspian sea presented in the models is crucial for the calculation of the water balance and this depends on the horizontal resolution. This implies that the net water balance may vary significantly among the models even if the net annual runoff is the same. For this reason, preference was made in the analysis to models having a realistic size to the Caspian Sea. The high resolution models usually satisfy such conditions.