Recent quasi-synoptic seasonal observations (May 1995-February 1996) carried out within the framework of the national programme PRISMA (Programma di Ricerca e Sperimentazione del Mare Adriatico) have significantly contributed to a better understanding of longitudinal water exchanges across four sections of the Adriatic Sea. The observations show the complex variability due to forcing mechanisms (i.e. the atmospheric forcing, the fresh-water buoyancy input and the inflow of warm and highly saline water originating in the Eastern Mediterranean Sea) and the natural seasonal variability, as an inherent component of the system. During winter, a homogeneous cold and dense water mass is formed in the northern shelf area (sigma (theta) congruent to 29.40 kg .m(-3)), while open-ocean deep convective movements lead to dense-water formation in the southern cyclonic gyre (sigma (theta) congruent to 29.18 kg .m(-3)). The Northern Adriatic dense water (NADW) flows southward along the western continental shelf. A part of NADW flows over the Pelagosa sill with a potential density of about 29.14 kg/m(3) however, without filling the deepest part of the Southern Adriatic basin. The latter fraction reaches the Otranto Strait at the shelf break and intrudes into the Northern Ionian Sea. The Adriatic deep water (ADW), formed through open-ocean convection in the Southern Adriatic Sea, flows over the sill of Otranto to form the main component of the Eastern Mediterranean deep water. The Adriatic Sea is defined on the yearly scale as a dilution basin; thus, an inflow of water from the Ionian Sea, saltier and warmer than the Adriatic waters, i.e. the Levantine intermediate water (LIW), occurs. The LIW enters the Otranto Strait and penetrates, diluted, towards the eastern portion of the Southern Adriatic Sea where it is entrained cyclonically in the gyre. A branch of modified LIW extends over the Pelagosa sill and intrudes into the northern shelf regions. The maximum northward flow of the LIW was observed during the autumn, probably compensating the fresh water buoyancy input in the north that had occurred mainly in spring and early autumn.
