Double-diffusive intrusions into a density gradient


  • J. S. Turner


It is now accepted that double-diffusive processes play a part in the mixing of heat and salt in the ocean and that relevant information about them can be obtained from laboratory experiments. There is still a considerable gap, however, between the laboratory work and the conditions under which the phenomena occur in the ocean. Most of the laboratory experiments have been one dimensional, whereas in the ocean, strong double-diffusive layering is often associated with large horizontal gradients of T and S, especially where water with one set of T-S properties intrudes at its own density level into an environment with different properties. The main purpose of the present paper is to report a series of laboratory experiments which are related to the latter case. The sugar-salt system and shadowgraph photography have been used to explore various situations of increasing complexity. The double-diffusive effects are compared with the behavior of a simple source of salt solution, which intrudes horizontally as a thin layer into a salinity gradient at its own density level. In strong contrast to this a source of sugar in the same salinity gradient produces vigorous vertical convection near the source, followed by spreading at several levels. The extending fluid moves out at an angle to the horizontal, rising or falling across isopycnals in a manner which can be related to the composition of the environment and source fluids and the double-diffusive transports across the boundaries of the intrusions. When the ambient fluid is set up with opposing gradients, so that potential energy is stored in one of the components, there is a more rapid spreading both horizontally and vertically, and strong shears are produced. The laboratory results are compared with published oceanic data to suggest explanations of some existing observations and to predict what might be measurable in future work.