Drifter observations of surface circulation in the Adriatic Sea between December 1994 and March 1996

The trajectories of satellite-tracked drifters are used to describe the characteristics of the subtidal surface circulation in the Adriatic Sea between December 1994 and March 1996. The mean surface circulation inferred from the drifter velocities consists of an elongated basin-wide cyclonic gyre with northward flow on the eastern side and return southward currents near the Italian coast (western side). This global circulation feature is composed of two sub-basin cyclonic, circulatory velocity patterns around the two main deeps of the Adriatic, i.e., the Jabuka and the South Adriatic Pits. The mean currents are maximum (greater than 40 cm s−1) near the outside (coastal) perimeter of these features. Specific zones of horizontal convergence of the mean flow were identified by converging drifters. Other areas in the open sea appeared to be diverging as drifters were reluctant to enter them. The seasonal modulation of the surface circulation was resolved in the lower southern Adriatic Sea and in the Strait of Otranto. An enhanced horizontal shear (northward flow on the eastern flank and southward currents on the western side) in the strait and an increased cyclonic gyre circulation around the South Adriatic Pit were observed in winter. The inflow of Ionian water and the subsequent cyclonic veering around the South Adriatic Pit are minimum in spring. In summer, the southward currents outflowing on the Italian shelf are maximum. Qualitative comparison between the drifter-inferred velocities and contemporaneous moored current observations discloses substantial vertically coherent current variations within the top 50 m of water that give rise to significant transport variability. In addition to the seasonal variations, surface subtidal velocity fluctuations with scales ranging from a few days to a few weeks are ubiquitous in both the drifter and moored observations. They are due to transient wind events, to changes in the buoyancy input (river runoff) and to instability of the mean flow in the form of mesoscale eddies, jets and filaments.