On stratified flow over a topographic ridge in a rotating annulus

Interactions between the rotating stratified oceanic and atmospheric flows and topography play a fundamental role in Earth's climate. Here we use laboratory experiments in a differentially-heated rotating annulus to explore stratified flow-topography interactions in a dynamical regime of strong background geostrophic turbulence. A localised small-scale topographic ridge is differentially-rotated at a range of angular velocities around the base of the annulus to impose a relative velocity between the stratified fluid and the small ridge. Considering the idealised setup of the laboratory configuration, the experiments exhibit rich dynamics that include, but are not limited to, lee waves, internal bores, baroclinic instabilities, boundary currents, large-scale gyres, blocking, and geostrophic eddies. Despite the complicated nature of the circulations, several bulk properties and features of the system are able to be characterised globally by relatively simple parameters. We find that the most useful parameter for describing the flows is the internal Froude number (Fr-n), which is the ratio between the imposed ridge velocity and the internal wave phase speed in the stratified fluid. Standing features that are predominantly barotropic and stationary relative to the ridge are only able to occur when the imposed ridge velocity is less than the internal wave phase speed (|Fr-n| < 1|). This implies that the flow-topography interactions leading to stationary dynamics are primarily internal stratified processes, which steer the flow and shape the background geostrophic turbulence such that it can support standing barotropic features.