Alignment of lenses: laboratory and numerical experiments

Abstract

This paper addresses the question of what happens when two anticyclonic lens-like eddies with different densities come in contact with each other by, say, an advective current. We use laboratory and numerical experiments to demonstrate that, in a similar fashion to lenses with identical densities which often merge, lenses with unequal densities often align (i.e. the lenses “strive” toward a state where one lens is situated on top of the other). The two initially isolated eddies are formed in the laboratory by continuously injecting salty water through small filters situated in the interface separating two environmental layers. Hence, there is a total of four different fluids with graduated densities in the tank. The heaviest water is the lowest layer. One of the lenses is slightly lighter than this heaviest water and the other is still lighter than the first lens. Finally, the upper layer is the lighest of the four. Since the lenses are formed by injection, weak anticyclones which extend beyond the boundary of the lenses are formed above and below the lenses. Due to the continuous injection of fluid into the lenses they grow slowly so that after some time their edges meet. As in merging of lenses with identical densities, arms are then extended from one vortex to the other. However, in the present case the arms are situated above the heavier vortex and below the lighter vortex instead of being at the same level. At this point, the lenses are locked together and the arms continue to propagate and grow until the lenses ultimately align. This process is accompanied by filamentation where fluid is expelled from both lenses to accommodate the change in angular momentum. Comparable numerical experiments also are discussed. Here Gaussian lenses in a four-layer fluid are studied; the lenses occupy the two intermediate layers. The physical parameters of these experiments are chosen to resemble the now famous case of observed eddy merging off East Australia. In contrast to the laboratory study, the numerical lenses have no initial anticyclones located in the layers surrounding them. The results indicate alignment in a manner similar to the laboratory experiments, provided the eddies are initially sufficiently close.