Adriatic Sea surface circulation as derived from drifter data between 1990 and 1999

Abstract

The Adriatic Sea surface circulation for the period 1990-1999 is studied using the data of more than 200 satellite-tracked drifters. The spatial structure snd the tempporal variability of the surface currents, at meso- to seasonal scales, are described in terms of Eulerian and Lagrangian statistics estimated from the low-pass filtered drifter velocities. Maps of mean currents, subtidal velocity variance and mean kinetic energies were produced using a 40-km averaging scale. The mean flow map confirms that the global cyclonic circulation in most of the Adriatic basin is broken into three re-circulation cells in the nothern, central, and southern sub-basins (the latter two being controlled by the bathymetyry of the Jabuka and South Adriatic Pits, respectively). An isolated cyclonic gyre prevails near the head of the basin. Mean velocities in the cyclonic gyres can exceed 25 cm s-1 in the coastal areas where the velocity variance is also maximum (reaching 500 cm 2s-2). Values near 2 X 107 cm 2-1, 2 days and 18 km were obtained for the diffusivity and the Lagrangian integral time and spatial scales in the along-basin direction, respectively. In the across-basin direction, the statistics are typically 50% of the above values. Geographical and seasonal of the Lagrangian statistics can be substantial. It was found that the fluctuating velocities (or the mesoscale eddies) has a preferential cyclonic sense of rotation. The gyres and the coastal currents are mostly prevailing in summer and fall. In winter and spring, they are less intense but the southern one tends to re-circulate more around the South Adriatic Pit. The mean eddy kinetic energy is maximum in winter and fall throughout the central and southern sub-basins. The drifters showed that the southeastward flow along the Italian Peninsula has a width varying between 45 and 70 km and a mean core speed of 25-35 cm s-1. In the northern and central sub-basins, maximum velocities are found within 5-10 km off the coast in winter and sprong, while a weaker maximum is seen more offshore (15-25 km) during the other seasons. In the southern Adriatic, the current is wider in summer, fall and winter, whereas in spring, it becomes thinner (maximum core speed near 10 km from shore and width of about 50 km). The maximum core speed is generally larger in summer than in winter.