Laboratory Modeling of the Downwelling Coastal Current and the Related Bottom Ekman Layer above the Inclined Bottom in the Rotating Homogeneous and Stratified Fluid

摘要

Laboratory experiments on studying the downwelling coastal current above the inclined bottom are conducted in the tank placed at a rotating platform. The current is formed by a source of constant flow of water with the same density (barotropic case) or the lower one (baroclinic case) as compared to the water in the tank. It is revealed that both in the barotropic and the baroclinic (under certain conditions) cases, the bottom Ekman layer formation implies sinking of the downwelling coastal current water. Some regularities in dynamics of the downwelling coastal current and the associated bottom Ekman layer in the barotropic and baroclinic cases are determined. It is found that in the barotropic case, the water in the bottom Ekman layer spreads from the source down the slope. It is shown that in the baroclinic case when the densities of water in the source and in the tank do not differ significantly, the water with lower density from the source enters the bottom Ekman layer which undergoes convective instability. The criteria for transition from the barotropic current regime to the baroclinic one, i. e. to the convective instability of the bottom Ekman layer are determined. It is revealed that convection in the bottom layer develops at the Rayleigh number values considerably exceeding its critical values characteristic of the laminar boundary layer in a rotating fluid. Actually, the bottom Ekman layer in the experiments is not purely laminar; its thickness is much higher than that of the laminar flow. Consequently, the coefficient of effective viscosity in this layer is also higher than the kinematic viscosity value which is used for calculating the critical Rayleigh number. When the densities of water in the source and in the tank differ significantly, the water with lower density propagates near the surface, does not enter the bottom Ekman layer and does not undergo convective instability.