Surface circulation in the central Mediterranean Sea as deduced from Lagrangian drifters in the 1990s

The surface circulation in the central Mediterranean, including the Sicily Channel, the southern Tyrrhenian and western Ionian seas, is studied using the data of more than 150 satellite-tracked drifters for the period spanning 1990-99. Pseudo-Eulerian maps of mean flow, eddy variability and energy levels are produced using all the data, the data sorted by seasons and the data divided in major wind categories, to study the circulation variability at meso to seasonal scales, and in terms of the local wind forcing. In general, the new results revealed by the drifter data confirm the general characteristics of the surface circulation as detected in previous Eulerian investigations. However, they provide a new broad quantitative description of the central Mediterranean surface circulation and they shed light on the dynamics of the Channel at large, including the Tunisian shelf and the area south of Malta, which shows unexpected characteristics of the surface velocity field, sometimes stagnant or even directed the opposite way with respect to the currents flowing further to the north. In addition, the drifter dataset allows to underline the seasonal character of the surface circulation in the area, and to assess the importance of the wind forcing. It was found that when winds blow from the northwestern sector (like the Mistral) the surface eastward transport in the Sicily Channel is enhanced. In contrast, for opposing wind conditions (blowing from the southeastern sector), the transport trough the Channel is significantly reduced. The quasi-Lagrangian nature of the drifters is also exploited to estimate Lagrangian statistics, such as the auto-covariance, the horizontal diffusivity and the integral time and space scales. Diffusivities and scales ranging in 1-5×107 cm2 s−1, 1-2 days and 10-30 km were found, respectively.