Impacts of the shelf bathymetry on coastal current instabilities

The development of high-resolution models is needed to reproduce a reliable cross-shelf transport which governs the heat and the salinity budgets, the dispersion of pollutants and the redistribution of nutrient-rich coastal waters toward the oligotrophic open sea. Indeed, such chaotic mixing and stirring is very sensitive to the resolution and the accuracy of the dynamical fields. The forecasts quality is then related to the precise description of unstable coastal processes and the wide variety of vortices evolving along or across the steep shelf topography. We study the impact of smooth to steep shelf topography on the destabilization of a frontal coastal current by means of numerical simulations and laboratory experiments. We used an idealized configuration of the primitive equation model NEMO to reproduce the unstable evolution of a coastal current and the eddy generation in a circular basin. The sensitivity to the spatial resolution and the dissipative scheme were tested. High and low Ekman numbers cases were studied in order to perform a quantitative comparison with laboratory experiments or the inviscid stability theory. The laboratory experiments were conducted on the UME/ENSTA turntable at Palaiseau. Non-intrusive PIV measurements using high resolution camera provide the surface velocity and vorticity fields for quantitative comparisons. Both the numerical and the laboratory studies have shown that the ratio of the shelf slope over the isopycnal slope of the coastal current is a main control parameter of the baroclinic instability. When this topographic parameter increases the most unstable wavelength decreases leading to the formation of smaller vortices. In a second stage theses surface vortices which transport large fluid parcel are affected by the deep shelf topography which prohibit cross shelf trajectories. For a steep enough shelf slope the coastal current could be completely stabilized.