Lagrangian and Eulerian observations of the surface circulation in the Tyrrhenian Sea

This paper focuses on the study of the Tyrrhenian Sea, a subbasin of the western Mediterranean whose surface and near-surface dynamics are still relatively poorly known, in particular, its southern region. Its circulation is described first by a set of 53 surface drifters deployed in the area between December 2001 and February 2004. In order to supplement the drifter data with continuously and uniformly sampled observations and to characterize the seasonal, as well as higher-frequency variability of the surface circulation, the Lagrangian analysis was associated to simultaneous satellite remotely sensed altimeter, covering the period 2001-2004. The investigation was based on trajectory analysis and on the computation of the pseudo-Eulerian statistics using the same binning and space-time averaging for drifter and altimeter data. The data reveal a complex pattern of the circulation, especially in the southern region of the Tyrrhenian, dominated by semipermanent recirculations and transient features, which sometimes makes it difficult to identify a consistent mean flow, while the northern subbasin is characterized by a pair composed of cyclonic and anticyclonic circulations known in the literature as the North Tyrrhenian Cyclone and North Tyrrhenian Anticyclone. The pseudo-Eulerian statistics computed with the two data sets evidenced the representativeness of a joint analysis of altimeter and drifter data and yielded useful indications about proper preliminary preprocessing and resampling procedures, so as to make the comparison statistically sound.