Production and dispersion of mixed waters in stratified coastal areas

International audienceThis paper deals with the extension of mixed water plumes in a coastal environment. This problem is connected with the dynamics of tidal fronts and is studied using simplified academic configurations where tidal mixing is represented as fixed areas where stratified waters are continuously homogenized in the vertical. The sensitivity of dispersive processes to seasonal and local parameters is analyzed. Localized mixing produces baroclinically unstable structures which are shown to generate vortices that transport mixed waters trapped in their core away from the mixing area. New stratified waters then enter the mixing area so that the homogenization process can be pursued. The production rate of homogenized waters is thus closely related to the ability of vortices, or other dispersive effects, to propagate away from the mixing area and disperse homogenized waters. A way to quantify dispersion is then proposed, based on this principle. Several mechanisms leading to vortex propagation and dispersion of mixed waters are identified. We use a four layer configuration to study their sensitivity to different parameters such as topography, the presence of a coast near the mixed water plume, the vertical mixing rate, bottom friction, stratification or the existence of a background current. Potential vorticity anomaly is used as a tracer of the mixed waters and to calculate production rates as a function of the latter parameters, and to analyze the dispersive mechanisms. It is shown that: *Baroclinic instability represents the most efficient mechanism for dispersing the homogenized waters. The emerging vortices are indeed mainly constituted of baroclinic dipoles (or hetons) with self propagating capacities. *On the f -plane, a bottom slope perturbs baroclinic instability and reduces the production rate. The homogenized waters are also funneled in a plume along lines of constant depth. * When mixing is produced against a wall, smaller vortices emerge, which drastically reduces the baroclinic instability efficiency. Mirror effects and a thin coastal (Kelvin) current developing along shore also favor dispersion but are of weaker influence. They also funnel dispersion along the coast (an effect which is strengthened by a bottom slope) so that the mixed waters form a plume extending Northward when the wall is located on the Eastern boundary. However these effects also counteract baroclinic instability so that the production rate and dispersion efficiency globally decrease when homogenization occurs along a coast. *An enhancement of mixing has a limited effect on the production rate and dispersion of homogenized waters because the production of mixed waters is strongly constrained by dispersive processes rather than the ability of mixing to renew the homogenized plume. *A bottom friction establishes a reduced gravity dynamics and inhibits the barotropic mode. This induces a reduction of the size of vortices emerging from baroclinic instability, which strongly diminishes the production rate and dispersion efficiency. It also modifies the vertical structure of the coastal (Kelvin) current which can significantly alter the dispersion pattern, but the effect associated with this process remains difficult to predict. *A decrease of stratification has two counteracting effects: it reduces the size of the emerging vortices but also reinforces the coupling between layers. We have found that the latter effect is not strong enough to overcome the reduction of the size of the emerging vortices, so that baroclinic instability and dispersion of homogenized waters are inhibited when stratification is reduced. *Finally, when a background barotropic current is added and if mixing occurs away from boundaries, dispersion increases linearly with the velocity field above a background velocity threshold. However a tidal front developing along a coast exhibits strong asymmetries depending on the direction of the current: production and dispersion increases linearly (above a threshold) with a Southward current, whereas it is shown that a Northward current inhibits baroclinic instabilities so that the average production rate remains almost constant. In that case, dispersion exhibits specific patterns with periodic release of homogenized water