Investigation on passive open boundary conditions adapted to the conjunction of strong currents, standing tidal wave and high stratification: Application to the French Guiana Continental Shelf

Hydrodynamics on the French Guiana Continental Shelf are the result of the conjunction of a strong long-shore current, a highly stratified water column, a semi-diurnal tide and trade-wind-induced circulation. The accurate modelling of all these phenomena needs the use of well-suited open boundary conditions (OBC), especially for passive boundaries. A large panel of OBC is implemented: Neumann condition, Orlanski conditions, gravity wave conditions, Roed and Smedstadʹs condition, Raymond and Kuoʹs condition, characteristic methods. Experiments are conducted with the three-dimensional, primitive equation, code MOBEEHDYCS and are shared in two parts. First, OBC concerning only barotropic variables are tested in two different situations. Then, combinations of OBC are experimented within a three-dimensional case. An inter-comparison of numerical simulations for a realistic situation is presented to validate the choice of OBC. According to our results, the behaviour of OBC is directly influenced by the tidal circulation, which is tangential to the passive boundary under study. Characteristic method and Neumann condition give the best results for the barotropic mode whereas radiation conditions involve a phase lag of the tidal velocity component normal to the boundary: the direction of the tidal wave propagation induces numerical instabilities in the phase speed computation. In such a hydrodynamic situation, characteristic method and Neumann OBC for baroclinic velocities combined with simplified advection condition for tracers provide reasonable behaviour of three-dimensional numerical solutions. But these OBC combinations seem to fail when used with radiation conditions for baroclinic variables. This problem is felt to be related to the presence of different wave velocities.