Identification of typical scenarios for the surface Lagrangian residual circulation in the Iroise Sea

This paper describes the surface Lagrangian residual circulation (LRC) over 2.5 day intervals in the Iroise Sea, west of France, and evaluates, for operational purposes, the influence of the different physical mechanisms that govern it. The method consists of the calculation of water displacements with a diagnostic Lagrangian tool that computes the trajectories of numerical particles in a given velocity field. The LRC is inferred from trajectories integrated over five M2 tidal cycles. The analysis is applied to both gridded genuine current measurements and ocean model outputs: the sea surface currents are derived from high?frequency (HF) radar measurements and from MARS, a 3?D regional ocean model used here in idealized configurations. To substantiate the analysis, the Lagrangian residual currents are also compared to genuine movements of drifters released in the Iroise Sea in 2005 and 2007. The LRC is mapped for typical scenarios identified from the Lagrangian analysis of HF radar surface currents measured in winter and summer, under weak (7 m/s) and strong (>10 m/s) wind conditions, and in neap tide and spring tide seasons. Idealized numerical simulations that switch on and off each individual physical process are used to isolate in the LRC the patterns induced by the atmospheric forcing, tides, and density?driven currents.