Study of dispersion mechanisms in coastal areas Application to tidal fronts in the Iroise sea.

IN COASTAL AREAS, WHEN THE SUMMER STRATIFICATION SETTLES, THE BOTTOM AND SURFACE TURBULENT BOUNDARY LAYERS CAN OVERLAP AND DELINEATE THERMAL FRONTS. AN EXAMPLE IN THE IROISE SEA IS THE USHANT FRONT THAT DEVELOPS AROUND AREAS OF INTENSE MIXING (ZMP): THE "RAZ DE SEIN" AND THE AREA OF OUESSANT. ITS EXTENSION IS VARIABLE AND EXCEEDS THE AREAS OF MIXED WATERS PRODUCTION. BOTH THE EXTENSION OF THE FRONT AND ITS VARIABILITY ARE SENSITIVE TO MECHANISMS OF DISPERSION OF HOMOGENIZED WATERS. THOSE MECHANISMS OF DISPERSION ARE STUDIED USING ACADEMIC CONFIGURATIONS WHERE TIDAL MIXING IS REPRESENTED AS FIXED AREAS WHERE STRATIFIED WATERS ARE CONTINUOUSLY HOMOGENIZED. IT IS SHOWN THAT BAROCLINIC INSTABILITY DEVELOPING AT THE EDGE OF THE ZMPS REPRESENTS THE MOST EFFICIENT MECHANISM TO DISPERSE THE HOMOGENIZED WATERS. ANY PROCESS THAT MODIFIES THE CHARACTERISTICS OF BAROCLINIC INSTABILITY, OR THE AUTO-PROPAGATING EFFICIENCY OF EMERGING STRUCTURES, SIGNIFICANTLY IMPACTS THE DISPERSION RATE AND PATTERN. THE SENSITIVITY OF DISPERSIVE PROCESSES TO SEASONAL AND LOCAL PARAMETERS IS ANALYZED. Realistic configurations of the Iroise sea are then used to investigate the variability of thermal fronts when mixing processes are combined with dispersive mechanisms. Thermal gradients in the surface and subsurface layers are investigated considering extension patterns of the Ushant front during summer 2009. It is shown that thermal fronts are partly initiated by the presence of mixed waters produced in ZMPs and subducted in subsurface. Those anticyclonic structures thicken the subsurface layer, and can interact with surface turbulence, thus favoring the outcropping of cold to mild waters at the surface.