Gap-Leaping Western Boundary Current in a Circular Tank Model

Abstract

An oceanographically generic problem of the interaction of a boundary current with bathymetric features such as a gap in the ridge or a strait between two islands is considered. Multiple flow patterns (penetrating or leaping the gap) and hysteresis (dependence on prior evolution) may exist in such systems. Examples include the Gulf Stream leaping from the Yucatan to Florida and the Kuroshio leaping from Luzon to Taiwan. Using numerical analysis, Sheremet earlier found that multiple steady states can be explained by variation in the balance between the inertia (which promotes leaping state) and the ? effect (which promotes penetrating state). In the present work a verification of the multiple states and hysteresis in a laboratory model are offered. To set up a gap-leaping current, a circular tank with a sloping bottom (simulating the ? effect) is used, and the flow is driven using a new method of pumping fluid through sponges (thus generating a Sverdrup flow in the interior). A semicircular ridge with a gap is inserted into the western part of the tank. Using a dye release flow visualization method, the existence of multiple flow patterns over varying boundary current transport values differing by a factor of more than 2 are dramatically shown. An associated numerical model in bipolar curvilinear coordinates, which allows for the matching of all the boundaries, reproduces the laboratory results very well. This idealized problem offers a very useful geophysical test case for numerical models involving flow separation and reattachment.