Surface circulation and Lagrangian transport in the SE Bay of Biscay from HF radar data

Two HF Radar CODAR SeaSonde stations are working operationally in the south-eastern Bay of Biscay since 2009 (Fig. 1). The HF radar stations are emitting at a central frequency of 4.86 MHz and provide hourly surface currents with 5 km radial and 5 degrees angular resolutions and a 150 km mean radial coverage. The examination of 3 year HF radar data reveals a well-defined seasonal variability of sea surface currents and the circulation along the slope in the area. In this contribution different Lagrangian exercises have been performed to evaluate the skills of the HF radar system to reproduce the drifter trajectories available in the study area at different periods. The mean distance observed between real and radar derived trajectories after 6 hours of simulation is of 4.6 km. Moreover, a description of the surface circulation in the area from a Lagrangian point of view is provided, with regard to the variability observed at different time-space scales. In addition to a well-defined seasonality, the presence of mesoscale cyclonic and anticyclonic eddies is recurrent in the area covered by the HF radar. Mesoscale variability is observed to have a substantial impact on the Lagrangian transport in the area.