The role of internal waves in the formation of layered structure at exchange flows between two closed basins (Middle and southern basins of the Caspian sea)

Layered structures in the oceans have always attracted the attention of oceanographers. The formation of these structures have been attributed to phenomena such as double – diffusive convection, internal waves and turbulent modulated mixing. In this paper, the vertical structures of temperature, salinity, density and the layered structure in the middle parts of Caspian Sea have been studied. Counters of iso-quantities of these physical properties, show the existence of regular structures, which indicate that internal waves which are produced by exchanging flow between two basins, as a result of horizontal density gradients (usually from north basin to south basin) may generate these layers. Froude number of this flow is about one. The length of wave of the internal waves is found to be about 200 km and the flow velocity associated with this gravity drive flow is about 0.2 m/s, the frequency of these waves is of order of inertial frequency. The normal modes of these waves have a near steady structure and can fold the inflow front from the North Caspian sea to South Caspian Sea basins, then the layered structure are formed. The thickness of these layers so formed is found to be about 10-20 m. These are in agreement with the values predicted by the model of Wong et al, (2001). In these waters density ratio is negative. Thus, double – diffusive convection does not often happen and cannot produce these layered structures.