The layered structure in exchange flows between two basins (Middle and Southern basins of the Caspian Sea)

Ocean waters often show layered structures especially where exchange flow between two basins occurs. These structures are often attributed to processes such as double.diffusive convection, internal waves, turbulent modulated mixing. In this paper by examining the vertical structures of temperature, salinity and density, of the middle parts of Caspian Sea it is shown that such layered structure may be due to the normal modes of the internal waves as double diffusive convection (the density ratio is often negative) does not often occur in these waters. Contours of isotherm, isohaline and isopycnal show the existence of rather regular wavy structures, which may indicate that internal waves, which are produced of exchange, flow between two basins; as a result of horizontal density gradients (usually from middle basin to southern basin) may generate these layers. The flow velocity associated with this gravity driven flow is about 0.1 to 0.15 m/s which gives a Froude number of about one. The wave lengths of these wavy structures are about 200 km and hence, the frequencies of them are of order of inertial frequency at these latitudes. The normal modes of these waves appear to have a near steady structure and can fold the inflow front from the Northern part of the Caspian Sea to the Southern deep basin. The thickness of formed layers is found be about 10-20 m which is in agreement with the values predicted by the model of Wong et al. (2001) and is less likely to be produced by double diffusive convection as the density ratio is often not appropriate for this process.