Evolution of two-layer thermohaline systems under surface cooling

Abstract

This paper presents the results of a series of laboratory experiments aimed at understanding the processes associated with surface freezing of a two-layer fluid. The flow configuration consists of a layer of cold, salty water overlying a relatively deep bottom layer of warm, saltier water. This situation is common in high-latitude oceans during periods of rapid ice formation. The experiments were conducted in a tank with well-insulated side and bottom walls, placed in a walk-in freezer with air temperatures from −12 to −20°C. A system of thermocouples was used to measure the temperatures at fixed levels in water, ice and air. Microscale conductivity and temperature probes were used to obtain vertical profiles of temperature and salinity in the water. In general, when external uxes of heat and salt are absent, such a system enhances static stability, in the sense that the net density difference between the layers increases with time. When external uxes of heat (because of surface cooling) and salt (rejected during ice formation) are applied, however, this fluid system may become unstable and overturning of fluid layers is possible. In addition, heat transport from the warmer bottom layer to the colder upper layer may be important, possibly leading to a reduction in the rate of ice formation compared to that of a homogeneous fluid with temperature and salinity identical to the upper layer. Descriptions of such physical processes are given using laboratory experiments, and quantitative measurements of salient parameters are compared with the predictions of a theoretical model developed to explicate the flow evolution.