Laboratory experiments on mesoscale vortices colliding with a seamount

Abstract

Interaction between a vortex and a right vertical cylinder was investigated in the laboratory for both a self‐propagating vortex and one advected by a background uniform flow. In the former case, experiments were carried out with a sloping bottom in order to simulate the β plane. In the latter case the bottom was flat and a cylinder was towed, with a uniform speed, through a fluid otherwise at rest and into a stationary vortex. In both cases, after a cyclonic vortex came in contact with the cylinder, fluid peeled off the outer edge of the vortex and went around the cylinder with a counterclockwise velocity v s as predicted by the circulation equation. This fluid formed a new cyclonic vortex in the wake of the cylinder, and bifurcation of the original vortex into two vortices occurred provided 400 ≤ Re ≤ 1100, where the Reynolds number Re = v s L max /ν and L max is the larger of the vortex or the cylinder diameter. This result is in agreement with previous studies of uniform flow past a cylinder in a rotating environment, and therefore we suggest that the new vortex in the wake of the cylinder was formed like those in the well‐known Karman vortex street. Experiments have been carried out systematically by varying D / d , the ratio of the cylinder diameter to the vortex diameter and the geometry of the encounter. The results suggest that the presence of a background flow enhances the bifurcation mechanism. A good agreement between the laboratory experiments and the observation of a meddy bifurcating after collision with the Irving Seamount in the Canary Basin, suggests that the oceanic vortex‐bifurcation process is similar to that observed in the laboratory experiments.