Shallow, brine-driven free convection in polar oceans: Nonhydrostatic numerical process studies

Abstract

A three-dimensional nonhydrostatic convection model, which accounts for small-scale ice-ocean interactions, is used to study convection in shallow sea (coastal) ice formation regions which contribute significantly to water mass formation in both the Arctic and Antarctic Ocean. For certain conditions the results presented in this paper are also transferable to shallow open ocean convection. The model is applied to an initial well-mixed ocean at rest with a temperature close to the freezing point. The ocean, initially free of ice, is exposed to cold and dry polar air. We consider situations in which the mean wind stress is negligible but wind fluctuations result in (small) sensible and latent heat fluxes corresponding to a wind speed of 2 m s-1. Cellular convection patterns develop in the ocean, finally occupying a mean aspect ratio of 2. Convection is driven by salt release during frazil ice formation due to supercooling. Newly forming sea ice is collected along convergent (downwelling) regions at the surface, thus showing also cellular structures. Because the area of insulating sea ice remains small, new ice can be formed continuously, and the surface buoyancy forcing remains large. This collection of ice in small fractions of the sea surface results in a latent heat polynya type, which is very effective in terms of dense water mass formation. A comparison of the three-dimensional model and a two-dimensional (slice) model shows that key results can be reproduced with the slice model. In summary, the results of the process studies indicate that cellular features in the sea ice cover, which may be detectable by remote sensing techniques, are closely related to active brine-driven convection.