This thesis falls in a realistic ocean modelling framework, focused on the study of the Iroise Sea. The first two parts deals with the numerical set up and the validation compared with observations. The emphasis is then on the analysis of the main physical structures and process at work in Iroise Sea: first the Ushant tidal front and mixed layer connexion process (surface and bottom) associated, then we analysed the mean eulerian residual circulation, for which we tried to isolate the relative contributions of the different physical process acting on it. Then, we mapped the different dynamical regime at work: we observe cyclostrophic regime near islands and in shallow water area and a strong geostrophic at the tidal front, especially in summer. Nevertheless, in the main part of the study domain, the different dynamical terms are acting of an equivalent. To a thermal point of view, the analysis of heat budget shows that the air-sea flux dominate largely the advective flux, although these ones induce a high frequency modulation in the net heat gain. Then, a diagnostic parameter has been established and the role of different external forcings (air/sea heat fluxes, wind stress, spring/neap tidal cycles) upon the Ushant front variability has been highlighted through sensitivity experiments. Finally, the study of the cross-frontal mass transport, based on a lagrangian particles tracking technique, has been established. The methodology is reliable and the first results reveal that these exports take place through specific latitude "channel".
