An experimental study of a mesoscale vortex colliding with topography of varying geometry in a rotating fluid

Abstract

The interaction of a self-propagating barotropic cyclonic vortex with an obstacle has been investigated and the conditions for a vortex to bifurcate into two vortices determined. As in a previous study, after a self-propagating cyclonic vortex came into contact with the obstacle, fluid peeled off the outer edge of the vortex and a so-called "streamer" went around the obstacle in a counterclockwise direction. Under certain conditions, this fluid formed a new cyclonic vortex in the wake of the obstacle, causing bifurcation of the original vortex into two vortices. In the present study we performed three sets of idealized laboratory experiments with the aim of investigating the importance on the bifurcation mechanism of the obstacle's horizontal cross sectional geometry, the influence of the height of the obstacle, and the importance of the slope of the obstacle sidewalls. The present results suggest that bifurcation occurs only when the obstacle height is equal or larger than 85% of the vortex height and that steep sloping sidewalls do not influence the bifurcation mechanism. In addition, experiments performed using an obstacle with an elliptical horizontal cross section revealed that the relevant parameter governing the occurrence of bifurcation is the length which the "streamer" has to travel around the obstacle, and not the dimension of the obstacle in the direction orthogonal to the motion of the vortex. Collisions of oceanic mesoscale vortices with seamounts often result in major modifications of their structure, having significant impacts on the redistribution of water properties. Observations of a "Meddy" bifurcating after colliding with the Irving Seamount in the Canary Basin show behavior similar to these idealized laboratory experiments. This suggests that these results could be used to explain and predict the outcome of a vortex colliding with seamounts of varying geometry in the ocean.