Eddy formation by overflows in stratified water

The formation of eddies by dense overflows in stratified water is examined by laboratory experiments. The dense fluid initially flows down the slope b ut turns (under the influence of rotation) to flow along the slope. The inv iscid alongslope flow is continuously drained by a Viscous Ekman layer that flows more directly down slope. In some cases this Ekman layer flow become s unstable to growing waves. Under certain conditions, strong cyclonic vort ices form in the ambient fluid above the alongslope flow due to vortex stre tching, causing the dense fluid to break up into a series of domes. There a re three main mechanisms for this: first, the initial downslope flow of the current (before it turns under the influence of relation) may take "captur ed" upper-layer fluid with it out into deeper water; second, adjustment of the current to geostrophic balance stretches the fluid column above the cur rent; and, finally, the continuous viscous draining from the current (and l ater 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 st ratification in the ambient fluid. Two types of stratification are examined : a two-layer ambient fluid with an interface above the overflow and a line arly stratified ambient fluid. For the two-layer ambient fluid the relevant vertical scale is simply the height of the interface above the overflow, d (1), while for the linearly stratified case a height scale based on the str ength of the stratification is derived, d(N). The stretching of the columns of ambient fluid is measured by the parameter T-l = L alpha/d(l) or Gamma( N) = L alpha/d(N), where L is the Rossby deformation radius and alpha is th e bottom slope. The frequency at which the eddy/dome structures are produce d increases with the stretching parameter Gamma, while the speed at which t he structures propagate along the slope depends on viscous effects. The beh avior is very similar to that for flow into an unstratified ambient fluid w here the stretching parameter Gamma = L alpha/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 circula tion system, and the implications of the laboratory experiments for oceanog raphic flows are discussed particularly for Denmark Strait.