Reflection of barotropic vortices from a step-like topography

The motion of a barotropic vortex on a b-plane approaching a meridional, step-like topography is studied by means of laboratory experiments and numerical simulations. In the experimental arrangement, the north-south direction is defined by a uniform weakly sloping bottom in order to simulate the b-effect. The numerical simulations are performed by using a finite-differences code, which solves the barotropic non-divergent equation. Initially, the vortices are created at the eastern side of the step. Thus cyclonic (anticyclonic) vortices drift northwestward (southwestward) on the b-plane and eventually interact with the step. The study is focused on cases where v0DHD0, with v0 the initial vortex peak vorticity and DH the step height, defined positive for a step-down and negative for a step-up topography. In both situations (cyclones approaching a step-down and anticyclones approaching a step-up), the vortices are stopped from crossing the step. This dramatic effect on the vortex trajectory is due to the entrainment of fluid from the other side of the step. Due to potential vorticity conservation, these entrained fluid columns acquire oppositely signed vorticity (relative to the vortex) as they experience depth variations, and form together with the original vortex a dipolar structure that moves away from the step. This effect is referred to as “vortex reflection”. The reflection effect is enhanced either by increasing the Rossby number or by decreasing the step height. In cases where v0DHE0, cyclonic (anticyclonic) vortices are able to cross a step-up (down)