| | | 18.3 Southern and central basins |
In the deeper basins the water column is divided into three layers: the superficial one is constituted by waters of low density, influenced along the western side by the indirect effects of riverine dilution; the intermediate one, with waters of high salinity of Ionian origin; and the deep one, constituted of dense waters, generated during the wintertime.
The intermediate layer is strongly influenced by Levantine Intermediate Water (LIW). This water mass originates in the eastern Mediterranean during the wintertime, by vertical convective mixing of water of low temperature (15 °C) and high salinity (39.1‰), and occupies a layer between 200 to 600 m depth. This water layer, which is entrained by the circulation in the Otranto Channel, enters into the southern basin, and is propagated along the whole Adriatic Sea, by different mixing and transport processes. The ingression of saline waters is highly variable, showing pluriannual fluctuations.
The circulation is always cyclonic, with a northward flow along the eastern side, and a southward one along the western coast. Both the extension and the intensity of flow in the opposite sides of the basin show large seasonal variations (Figure).
In summer the general circulation shows a cyclonic gyre extending to both the central and southern basins, with a noticeable transversal flow in the former. The gyre is vertically strongly stabilized by the thermal fluxes at the surface, and is affected by the advection of western diluted waters, deflected towards the east by the irregularities of the coastline, and of southern waters from the intermediate layer.
During the winter the circulation generates two semi-stationary cyclonic gyres, situated in the northern area of the southern basin and in the central basin. In both areas the isopycnal surfaces assume a dome-like shape: the intense evaporative thermal fluxes at the surface, driven by the NE wind (bora), generate high-density water in the central zone of the gyres and vertical convection. Afterward this dense water flows along the western side of the Otranto Channel, feeding the deep water of the Ionian Sea.
The dense water generated in similar conditions in the central and the northern basin, characterized by higher densities (up to 29.5 kg m-3), fills the deepest part of the central basin.
The distribution of dissolved matter is driven by the vertical and horizontal density and motion fields described above. Nutrients are found in low concentrations in the surface layer, and in relatively greater one in the saline intermediate water inflowing into the southern basin, and in the dense deep water. The transfer by turbulent diffusion from the intermediate water core advected from the Ionian basin is the major source of nutrients for the surface layer of the southern Adriatic, coastal runoff playing a secondary role. By the dense water outflow, nutrients produced by mineralization processes in the lower part of the water column are brought into the Ionian and eastern Mediterranean deeper basins.
| | | 18.3 Southern and central basins |