Identification of a cusp catastrophe in a gap-leaping western boundary current

The Luzon Strait is ail example of a location where a western boundary current must negotiate a gap in bathymetry. In the gap region, the current can exhibit multiple steady states (leaping the gap or penetrating the gap) and hysteresis (dependence oil past flow state). Laboratory experiments on such flows are presented in order to investigate the system behavior in a two-dimensional parameter space of varying flow rate and platform rotation rate. The experiments were performed in a cylindrical tank on a one-meter rotating table. A semi-circular ridge with a gap was inserted over sloping bottom topography in the active region, and the flow was driven by pumping water through sponges. The flow was visualized with the Particle Image Velocimetry method. By varying the flow rate (strength of current), we were able to identify transitions between leaping and penetrating flow states. These transitions bound a region of multiple steady states where hysteresis is present. The dynamics of the system is shown to exhibit a cusp catastrophe classified as A(3). The scaling dependencies of some critical properties of the flow are analyzed.