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

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 effects, bottom friction and entrainment, 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 similar to that of Bidokhti and Ezam (2009) but with the bottom friction and entrainment included, is used to find the horizontal extent of the outflow from the western coast. To estimate the changes of vorticity and potential vorticity of the flow over the Absheron sill, we use the method of Falcini and Salusti (2015), in this work, the effects of entrainment and friction are considered. 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 on the ecosystem here of pollution due to oil exploration.