Laboratory studies of double-diffusive sources in closed regions

Various observations of layering and intrusions in the ocean strongly sugge st that such structures and motions are produced and driven by horizontal a nd vertical gradients of temperature and salinity, i.e. by double-diffusive processes. Much of the laboratory work in this field has concentrated on o ne-dimensional problems, with the neglect of two-dimensional phenomena. The latter are addressed explicitly in the present paper, using the salt-sugar analogue system in a simple geometry, but with the aim of establishing som e more widely applicable general principles. Two sources of salt or sugar s olution were fed in at opposite ends of a 750 mm long tank, with an overflo w tube drawing fluid from a point at the centre of the tank. With two salt sources of different concentrations and densities, a stratification built u p through the 'filling box' process, and the total density range lay within that of the input solutions. For one salt and one sugar source, a much lar ger density gradient could be set up, with the range lying outside that of the inputs. The flows were monitored using various experimental techniques: photographs of dye streaks with still and video cameras; a polarimeter to monitor sugar concentration; and the withdrawal of samples for the measurem ent of density and refractive index, from which the separate contributions of salt and sugar to the density could be calculated. Three related experiments with simple input conditions were particularly in structive, and these will be described first. Both inputs and the withdrawa l tube were located at mid-depth, and the tank fluid and the salt and sugar supplies had the same density. The only difference between runs was the in itial composition of the solution in the tank: pure salt, pure sugar, and a 50:50 mixture of the two. Following an initial transient response which wa s different in the three experiments, they all tended to the same asymptoti c distributions of salt, sugar and density after about 100 h, with a sharp central interface and weakly stratified upper and lower layers. This state corresponded approximately to the one-dimensional 'rundown' of a layer of s alt solution above sugar solution, with a slightly higher, unstable concent ration of salt in the top layer compared to the bottom and a very stable su gar distribution, with a much larger concentration in the bottom layer than in the top one. This distribution cannot be produced by 'finger' rundown, and it corresponds to the maximum release of potential energy. It was, howe ver, achieved through the action of many intrusions, which remained active in the dynamic final state, and maintained a strong communication between t he two ends of the tank. A comparable experiment was carried out using a ta nk 1820 mm long. With this larger aspect ratio there was a predominantly lo cal influence of the sources at each end of the tank. Other runs have explo red a variety of geometries of the sources and sinks, and the final state h as been shown to be sensitive to these boundary conditions.