Surface circulation in the Liguro-Provençal basin as measured by Lagrangian drifters (2007-2009)
Pierre-Marie Poulain, Riccardo Gerin, Annalisa Griffa, Nadia Pinardi
OGS
(Abstract received 08/13/2009 for session A)
ABSTRACT
The surface circulation in the Liguro-Provençal basin is studied using the data of Lagrangian CODE drifters deployed as part of Marine Rapid Environmental Assessment (MREA) exercises. The drifters were deployed in small-scale clusters in the open Ligurian Sea. Counting some drifters that stranded and were subsequently redeployed, the dataset used consists of 35 trajectories spanning the period 14 May 2007 to 23 January 2009. All drifters were tracked by, and transmitted data to, the satellite-based Argos system. They were also equipped with GPS receivers to increase the accuracy and frequency of their positions. The drifter positions were edited, interpolated and low-pass filtered to remove tidal/inertial currents.
First the surface circulation is described qualitatively. Complex circulation patterns prevail in the Ligurian Sea, before the drifters eventually joined the Northern Current and moved rapidly to the southwest from the Gulf of Genoa to the Gulf of Lion. There, some units were advected offshore towards the south, before heading to the east and approaching the Corsican coast, and re-entering the Ligurian to close a basin-wide cyclonic circulation. Only a few drifters continued moving towards the southwest in the Gulf of Lion and approached the Catalan Sea. The drifter trajectories were compared to the absolute dynamic topography derived from satellite altimeters and a remarkable match was found. Discrepancies only occur when strong Mistral winds blow and wind-driven currents and slippage become important.
Second, pseudo-Eulerian velocity statistics are calculated in the coastal region extending between the Gulf of Genoa and the Gulf of Lion, where the data are more abundant. Fast currents (mean ~60 cm/s) are evident on the shelf break in the Northern Current, especially off Imperia (Italy). In this area, low-pass filtered individual speeds can reach 90 cm/s. In contrast, a stagnation area near Fréjus (France) is characterized by little mean flow, low velocity fluctuations, and high probability of drifter stranding.
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