Malki-Epshtein, L; (2005) Growth and structure of double-diffusive intrusions. Doctoral thesis , UNSPECIFIED.

Abstract

An experimental and theoretical study on the formation and growth of double-diffusive intrusions occurring at a cooled side wall in stably-stratified salt water is described. Observations are made of growth rates of the intrusions, of internal velocities estimated by particle tracking, and of temperature and salinity distributions. As the experiment begins, fluid is cooled by a temperature difference along the side wall and sinks a distance where its density, increased due to cooling is matched by the density of the stratified ambient fluid. Temperature and salinity measurements reveal ‘stepped’ density profiles in the intrusions and the decrease of the salinity at the bottom of an intrusion over time. The formation of the intrusions is found to be governed by heat transfer, an inertia-buoyancy balance along the cold side wall and the evolution of the ambient fluid. Internal waves are shown to propagate from the cold wall to the far end of the tank and establish horizontal selective withdrawal layers, which extend throughout the ambient fluid to the far end of the tank. As the intrusions grow, the density structure of the ambient fluid decreases by a process of continuous hydrostatic adjustment, to match the ‘stepped’ density structure inside them. This process is confirmed by measurements in the ambient fluid of a small upward drift which transports saltier fluid from the bottom of a layer to the withdrawal layer at the top, and of ‘stepped’ density gradients in the ambient fluid which decrease with time. A simple two-dimensional model is developed to describe the growth rate of the intrusions, which is found to depend on the length of the tank and decrease as a function of time. This surprising effect of a finite tank on slowly extending intrusions does not seem to have been recognized before. The internal buoyancy frequency and the mean length of the intrusions are given by , and , where is a time scale for the development of layering and is determined by the length of the tank , the initial buoyancy frequency , the height of the intrusions and a heat transfer parameter . The model accurately describes experiments with a range of initial conditions. Future application of the results of this study to the large scale could have significant implications for the modelling of oceanic double-diffusive processes, which are thought to drive large vertical and lateral fluxes of heat and salt.

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