Bringing Sea Ice Microphysics and Biogeochemstistry into Global Climate Models

Salt in sea ice is captured in liquid inclusions known as brine pockets or pores. The brine volume adjusts so the brine salinity depresses the freezing point to the brine temperature, provided brine transport in the pore structure is not too great. The same principle of freezing point depression is exploited when salting icy roads. In summer pores can grow large enough to permit melt water to drain that might otherwise pond on the surface, strongly absorb sunlight, and create a hummocky surface. Pores in sea ice also provide a conduit for chemical transport and gas exchange between air and sea. Certain algal species are adapted to the high brine salinity and low light levels in the sea ice, where the pores offer protection from grazing zooplankton. Desalination of sea ice introduces high salinity water to the ocean surface and promotes ocean mixing. These properties and processes influence future climate change and therefore it is desirable to model desalination and the porosity of sea ice in earth system models. During sea ice growth, the ice fraction at the advancing ice-ocean front approaches zero continuously, which indicates salt is not rejected immediately upon freezing across the ice-ocean front. Instead desalination of the sea ice results from brine transport through the pore structure, which is controlled predominantly by buoyancy-driven convection in winter and flushing in summer. Both processes also supply nutrients to algal communities in the sea ice. The entire sea ice slab can be considered a mushy layer, whith halo and thermodynamics that are modeled with equations of heat and salt conservation. Brine transport dynamics have substantial influence on both equations and require milimeter scale resolution to model pores explicitly. This is impossible in global models where horizontal length scales are tens of kilometers at best, so instead clever parameterizations must be constructed. Convection proves to be the most challenging to model. We propose a method based on mixing-length theory and assume the eddy velocity can be modeled from Darcy's law. Hence we treat convection as a diffusive process with a diffusivity that is a function of the sea ice Rayleigh number.