Evolution of two-layer thermohaline systems under surface cooling

This paper presents the results of a series of laboratory experiments aimed at understanding the processes associated with surface freezing of a two-l ayer fluid. The flow configuration consists of a layer of cold, salty water overlying a relatively deep bottom layer of warm, saltier water. This situ ation is common in high-latitude oceans during periods of rapid ice formati on. The experiments were conducted in a tank with well-insulated side and b ottom walls, placed in a walk-in freezer with air temperatures from -12 to -20 degrees C. A system of thermocouples was used to measure the temperatur es at fixed levels in water, ice and air. Microscale conductivity and tempe rature probes were used to obtain vertical profiles of temperature and sali nity in the water. In general, when external fluxes of heat and salt are ab sent, such a system enhances static stability, in the sense that the net de nsity difference between the layers increases with time. When external flux es of heat (because of surface cooling) and salt (rejected during ice forma tion) are applied, however, this fluid system may become unstable and overt urning of fluid layers is possible. In addition, heat transport from the wa rmer bottom layer to the colder upper layer may be important, possibly lead ing to a reduction in the rate of ice formation compared to that of a homog eneous fluid with temperature and salinity identical to the upper layer. De scriptions of such physical processes are given using laboratory experiment s, and quantitative measurements of salient parameters are compared with th e predictions of a theoretical model developed to explicate the flow evolut ion.