AN EXPERIMENTAL-STUDY ON THE FORMATION AND SURVIVAL OF STRATIFIED SUBSURFACE EDDIES

We report the results of laboratory experiments on the formation and survival of internally stratified subsurface eddies in a rotating fluid. The eddies were created by injecting a dense turbulent plume at the surface of a linearly stratified environment. The relative vorticity of the lenses was always negative but larger than that of homogeneous lenses created by laminar injection. During the first 100 revolutions, the eddies shed fluid in two symmetric arms. The shedding which is believed to result from shear instabilities always resulted in a stationary axisymmetric eddy. After the eddy had spun down, the remnant fluid persisted for thousands of rotations as a circular feature with internal stratification identical to that of the environment. We created eddies with and without double diffusive convective instabilities and compared the volume of dyed fluid and the evolution of their aspect ratios. Sugar and salt were used as laboratory analogues of salt and heat, respectively. The Burger number of the lenses decreased rapidly within the first 200 revolutions and then much more slowly to reach a value between 0.2 and 0.4. These latter values are larger than those predicted by Gill (1981) for a homogeneous lens due to the internal stratification of the lenses. Radial spreading of the lens due to double diffusive intrusions was found to be larger, but of the same order of magnitude, as that induced by the vertical exchange of momentum in the absence of double diffusive convection. We formed eddies internally stratified in the diffusive sense (stable sugar gradient and unstable salt gradient) or doubly stable (stable sugar and salt gradients) by changing the ratio of the volume flux at the source to the volume flux at the spreading level as described by Bormans and Turner (1990). When the stratification in the eddies was doubly stable, three distinctive regions were observed: a region of convective layers and diffusive density interfaces at the top, a central region with no apparent structure and a bottom region dominated by fingers sheared by rotation. This type of eddy is compared to the particular case of a Meddy.