Laboratory experiments on intrusive flows and internal waves in a pycnocline

A laboratory study has been performed to simulate intrusive hows generated by internal wave-breaking activity in the oceanic pycnocline. Two different cases were considered. In the first set of experiments a short-duration source of motion was modelled by creating a finite region of well-mixed fluid. The collapse of this region resulted in intrusive flows and internal waves in the pycnocline. Attention was focused on the formation and subsequent evolution of solitary 'bulges' in the intrusion. Detailed flow measurements have revealed that the weak motion inside these bulges (which contain well-mixed fluid from the source) is organized in a four-vortex structure. Numerical flow simulations provided important information about the dynamics of this four-cell structure: the outer cells are associated with baroclinic generation of vorticity, while the inner cells are characterized by a balance between the advective and the viscous terms in the vorticity equation. In the second set of experiments continuous mixing was induced by a vertically oscillating, horizontal grid centred in the pycnocline. The mixed region collapses thus forming an intrusive flow into the pycnocline and internal waves that propagate along the pycnocline at higher speed than the intrusion. It was found that the velocity of the intrusive flow is approximately constant and that its dynamics is controlled by an inertial-buoyancy balance. The parameters of the internal waves in both cases were compared with theory.