SHALLOW, BRINE-DRIVEN FREE-CONVECTION IN POLAR OCEANS - NONHYDROSTATIC NUMERICAL PROCESS STUDIES

A three-dimensional nonhydrostatic convection model, which accounts fo r small-scale ice-ocean interactions, is used to study convection in s hallow sea (coastal) ice formation regions which contribute significan tly to water mass formation in both the Arctic and Antarctic Ocean. Fo r certain conditions the results presented in this paper are also tran sferable to shallow open ocean convection. The model is applied to an initial well-mixed ocean at rest with a temperature close to the freez ing point. The ocean, initially free of ice, is exposed to cold and dr y polar air. We consider situations in which the mean wind stress is n egligible but wind fluctuations result in (small) sensible and latent heat fluxes corresponding to a wind speed of 2 m s(-1). Cellular conve ction patterns develop in the ocean, finally occupying a mean aspect r atio of 2. Convection is driven by salt release during frazil ice form ation due to supercooling. Newly forming sea ice is collected along co nvergent (downwelling) regions at the surface, thus showing also cellu lar structures. Because the area of insulating sea ice remains small, new ice can be formed continuously, and the surface buoyancy forcing r emains large. This collection of ice in small fractions of the sea sur face results in a latent heat polynya type, which is very effective in terms of dense water mass formation. A comparison of the three-dimens ional model and a two-dimensional (slice) model shows that key results can be reproduced with the slice model. In summary, the results of th e process studies indicate that cellular features in the sea ice cover , which may be detectable by remote sensing techniques, are closely re lated to active brine-driven convection.