The Adriatic Sea is an interesting basin because of the different physical processes that characterize its dynamics. Early investigations of the physical processes that characterize the Adriatic Sea date back to the 19th century (Lorenz, 1863; Wolf and Luksch, 1887). Since then, several regional studies based on extensive open-sea measurements have been performed. More recently, remote sensing techniques based on satellite data have as well as analytical and numerical models have been used to interpret various aspects of the Adriatic Sea dynamics. Topography, meteorology, hydrography and hydrodynamics affect the Adriatic Sea dynamics and energetics.
The Adriatic Sea is a latitudinally elongated continental basin, with length and width of 800 km and 180 km respectively and with its major axis oriented along the northwest–southeast direction. It is located in the central Mediterranean, between the Appennines chain and the Balkans and is connected to the Ionian Sea by the 74 km wide Otranto strait. Marked morphological differences characterize the basin along its longitudinal transversal axes. Accordingly the Adriatic Sea can be divided into three sub-basins (Artegiani et al., 1996).
The northern sub-basin is characterized by a shallow average depth of ~ 35 m with a very weak bathymetric gradient toward south-east where the 100 m bathymetric line is met in front of Giulianova (Italy). Po and the other northern Italian rivers contribute to a strong river runoff (~ 3000 m3 s-1) and are believed to be the source of about 20% of the total Mediterranean river runoff (Hopkins, 1992).
The middle Adriatic is a transition zone between the northern and the southern sub-basin and is charcterized by fairly open sea conditions. It spans from the 100 m bathymetric line to the Pelagosa sill (~ 170 m depth), located around the line connecting Vieste and Split. The average depth of the middle sub-basin is ~ 140 m with the two Pomo and Jabuka depressions reaching ~ 270 m.
The southern sub-basin extends from Pelagosa sill to Otranto sill which divides it from the Ionian Sea. Each of the western and eastern coasts have a narrow continental shelf (20-30 km wide to the shelf break at 200 m depth), a steep continental slope reaching 1000 m and a fairly flat abyssal plain, with a maximum depth of 1270 m. The water exchange with the Mediterranean Sea takes place through the Otranto Channel, whose sill is 800 m deep.
The eastern coast of the Adriatic Sea is generally high and irregular and is characterized by the occurrence of many islands and a rocky steeply sloping bathymetry. The western coast is low, mostly sandy and generally regular with a gentle slope. A large number of rivers discharge into the basin, with significant influence on the circulation, particularly relevant being the Po River in the northern basin, and the ensemble of the Albanian rivers in the southern basin.
Several rivers with different estuarine structures and dimensions reach the Adriatic Sea coastlines. In particular, the northwest coast is characterized by the largest concentration of rivers. Here, in fact, temperature and salinity gradients that are created by the river discharge contribute to the general thermoaline circulation. The combined effect of medium size rivers (average discharge ranging from 40 to 100 m3s-1) and of the Po river induces a cyclonic circulation in the northwestern shelves.
The Po river (NE Italy) is the largest fresh water supply and is characterized by an annual mean runoff of about 1700 m3Js. The maximum peaks of freshwater runoff are associated with snow melt and rain precipitation and occur, respectively, in spring and autumn (Marchetti, 1984).
The annual mean runoff of the northwest coast rivers other than the Po is 350 m3Js; the rivers inflowing along the greater part of the eastern coast contribute about 700 m3/s, whereas the annual mean runoff concentrated along the 150 km of the eastern coast converging on the Otranto Strait amounts to about 1150 m3/s (Zore-Armanda, 1969 a).
Along the eastern coast there are numerous submarine springs (known as “vrulje”) which discharge fresh water originating in the littoral karst area (Alfirevié, 1969) but are probably not of major importance in the overall water budget.
Inputs from the Po and other Italian rivers are mostly confined near the western coast, but a certain quantity is transferred offshore by means of Ekman transport and mesoscale circulation.
The surface waters are freshened during spring and summer, as in the northern Adriatic, due to river runoff. Hence, the river runoff also exceeds evaporation during summer in the middle adriatic.