Estimating Lagrangian Residual Circulation (LRC) in the Iroise Sea

The Iroise Sea is located at the western end of Brittany (France), along the route of low-pressure atmospheric systems. The area is shallow (mean depth 110 m) and exhibits strong tidal currents (up to 8 knots in springtime). In summer, tidal fronts are generated in transition zones between the coastal area, where strong tidal currents efficiently mix the whole water column, and the offshore ocean, where tidal mixing does not reach the surface and induces stratification. Frontal regions are subject to intense biological activity. Due to the intense maritime traffic and the permanent threat of pollution, it is essential to monitor the Lagrangian residual circulation in the Iroise Sea and to evaluate the relative contribution of different physical processes involved (tides, atmospheric forcing, density gradients). To evaluate Lagrangian residual currents, the trajectories of Active particles were computed using ARIANE, a diagnostic Lagrangian tool (Blanke,1997) and infer the Lagrangian residual circulation from these trajectories. The analysis is performed for both, an ocean model output (3D regional model MARS, forced by high-resolution meteorological data from the regional atmospheric model WRF) and high-frequency radar current measurements (SURLITOP experiment, by ACTIMAR,2005). In the latter case, Open-boundary Modal Analysis (Lekien 2004) was used to overcome the spatial and temporal variations of the radars range and coverage, by interpolating, extrapolating and filtering the measurements. Estimated Lagrangian residual currents are compared against Lagrangian drifter tracks from a field experiment. This study was preceded by validation of the instantaneous currents from a comparison between HF radar data, real drifter data and model outputs.