Eddy Formation by Overflows in Stratified Water

Abstract

The formation of eddies by dense overflows in stratified water is examined by laboratory experiments. The dense fluid initially flows down the slope but turns (under the influence of rotation) to flow along the slope. The inviscid alongslope flow is continuously drained by a viscous Ekman layer that flows more directly downslope. In some cases this Ekman layer flow becomes unstable to growing waves. Under certain conditions, strong cyclonic vortices form in the ambient fluid above the alongslope flow due to vortex stretching, causing the dense fluid to break up into a series of domes. There are three main mechanisms for this: first, the initial downslope flow of the current (before it turns under the influence of rotation) may take ?captured? upper-layer fluid with it out into deeper water; second, adjustment of the current to geostrophic balance stretches the fluid column above the current; and, finally, the continuous viscous draining from the current (and later from the domes) also causes stretching in the ambient fluid. The vertical extent of the influence of the overflow (and thus the initial effective height of these columns of ambient fluid) is controlled by the stratification in the ambient fluid. Two types of stratification are examined: a two-layer ambient fluid with an interface above the overflow and a linearly stratified ambient fluid. For the two-layer ambient fluid the relevant vertical scale is simply the height of the interface above the overflow, dl, while for the linearly stratified case a height scale based on the strength of the stratification is derived, dN. The stretching of the columns of ambient fluid is measured by the parameter Γl = Lα/dl or ΓN = Lα/dN, where L is the Rossby deformation radius and α is the bottom slope. The frequency at which the eddy/dome structures are produced increases with the stretching parameter Γ, while the speed at which the structures propagate along the slope depends on viscous effects. The behavior is very similar to that for flow into an unstratified ambient fluid where the stretching parameter Γ = Lα/D, where D is the total fluid depth, except that the propagation speed of the eddies along the slope is slower in the stratified case by a factor of approximately 0.7. The flow of dense fluid on slopes is a very important part of the global ocean circulation system, and the implications of the laboratory experiments for oceanographic flows are discussed particularly for Denmark Strait.