A study of the roles of important forcing mechanisms on the circulation of the Caspian Sea using numerical simulation

In this study, the roles of different driving forces of circulation (or flow) pattern of the Caspian Sea surface have been evaluated using COHERENS (a three-dimensional hydrodynamic model) for the year 2004. The model is based on the hydrostatic version of the Navier-Stokes equations. The hydrodynamic part of the model uses the equations of temperature and salinity, and the momentum equations use the Boussinesq approximation, an assumption of vertical hydrostatic equilibrium, and the continuity equation. The equations of the model are discretised on an Arakawa C-grid. The equations of momentum and continuity that are solved numerically use the mode-splitting technique. In order to simulate the circulation of the Caspian Sea, the gridded fields were chosen as 0.046 × 0.046 degrees along the horizontal directions, which gave a grid size of about 5 km, and 30 sigma layers along the vertical axis. The model was set up for three different forcing configurations. First, the effects of only wind forcing were evaluated using some field observations of wind-driven currents. In the second cofiguration, only the river driving-force was evaluated by the model and the flow fields were obtained. Finally, in the last configuration, all driving-forces such as wind forcing, air pressure, air temperature, precipitation rate, cloud cover and humidity along the initial conditions including temperature and salinity of the basin were examined in order to calculate the overall circulation of the Caspian Sea. The outputs and results showed that the approximate mean current created only by rivers was 1/10 of the circulation velocity created by the wind driving force and this was about 1/3 in May and June due to an increase in the discharge of the Volga River. However, the peak velocity of the wind-driven current was much less than that of the currents caused by the river Volga near its entrance. Because the wind forcing also plays an effective role in evaporation over the water surface, and changes the density of water masses, it could be considered a factor that indirectly contributed to the formation of currents as a result of the density gradient. Also the rivers, due to their low salinity and a different temperature, can change the water density creating currents resulted from the density gradient.Our results showed that the mean surface current speed for most of the year, regardless of the wind effect on the formation of currents, caused by density gradients, is caused mainly by the wind stress. Therefore, it could be concluded that the wind-driven forcing near the surface was the main cause of surface current formations of the Caspian Sea. Our results also showed that with all forcing thermohaline circulation in the northern part of the Caspian Sea in cold seasons and deep basin water, circulations during the year were the main components of abyssal flows in the Caspian Sea. The interesting feature of the deep flow was the abyssal flow over the Abshooran sill (between the middle and southern basins of the Caspian Sea) that as it entered the southern basin it generated an isobathic flow in the deeper part of this basin.