Etude de la circulation résiduelle lagrangienne en mer d'Iroise, Study of lagrangian residual circulation in the Iroise sea

The aim of this work is to assess the lagrangian residual circulation in the Iroise sea and to evaluate the relative contribution of the different physical processes at play. We calculate water displacements with a diagnostic lagrangian tool that computes the full trajectories of Active particles advected in a given velocity field. The lagrangian residual circulation is inferred from trajectories integrated over an appropriate period. This analysis is applied both to ocean model ouputs and genuine, gridded current measurements. These two sources of data are indeed complementary to study the lagrangian residual circulation according to the impact of the physical processes at play. We used sea surface currents measured by HF radars and surface currents computed with a 3D ocean model, forced by high resolution meteorological data from an atmospheric model. The lagrangian residual currents are compared to genuine movements of drifters. Our analyses and diagnoses provide a realistic description of the lagrangian residual circulation in the Iroise sea and evaluate the influence of the different driving mechanisms that govern it. We determine the contribution of tides, wind and density gradients by analysing HF radars surface currents during winter and summer, weak and strong wind conditions. idealized numerical simulations that switch on and off each individual physical processes help us refine our conclusions. To exploit this process study in terms of operational oceanography, we draw maps of the lagrangian residual circulation over five M2 tidal cycles in the Iroise sea. We provide representative samples of various meteorological and oceanic situations.