Some aspects of the deep abyssal overflow between the middle and southern basins of the Caspian Sea

Abstract. This study investigates the deep gravity current between the middle and southern Caspian Sea basins, caused by density difference of deep waters. Oceanographic data, numerical model and dynamic models are used to consider the structure of this Caspian Sea abyssal overflow. The CTD data are obtained from UNESCO, and the three-dimensional ocean model COHERENS results are used to study the abyssal currents in the southern basin of the Caspian Sea.

The deep overflow is driven by the density difference mainly due to the temperature difference between the middle and southern basins especially in winter. For this reason, water sinks in high latitudes and after filling the middle basin it overflows into the southern basin. As the current passes through the Absheron Strait (or sill), we use an analytic model for the overflow gravity current with inertial and frictional effects to consider its structure. The dynamical characteristics of this deep baroclinic flow are investigated with different initial and boundary conditions. The results show that after time passes, the flow adjusts itself, moving as a deepening gravity driven topographically trapped current. This flow is considered for different seasons and its velocity and width are obtained. Because of the topography of the Southern Caspian basin, the flow is trapped after the sill; thus, another simple dynamical model of the overflow, based on potential vorticity conservation similar to that of Bidokhti and Ezam (2009) but with the bottom friction included, is used to find the horizontal extent of the outflow from the western coast. The result of this model shows that the Rossby length (deformation radius) of the flow decreases when drag coefficient increases. Because of the importance of the overflow in deep water ventilation, a simple dynamical model of the boundary currents based on the shape of strait is used to estimate typical mass transport and flushing time which is found to be about 15 to 20 years for the southern basin of the Caspian Sea. This time scale is important for the possible effects of pollutions due to oil exploration on the ecosystem of this water body.