In most sea-ice models the bulk salinity is assumed to be uniform or is prescribed empirically as a function of ice thickness. Therefore, the temporal evolution of both the salinity field within the ice and the salt fluxes into the ocean can not be studied in great detail. The reason for this lies partly in the lack of a concise theoretical understanding and a lack of high resolution measurements of the temporal and spatial evolution of the salinity field. To guide the development of alternative models, we present results from an enthalpy-based numerical model in which the full coupled thermodynamical and fluid-dynamical equations for the evolution of the two-phase, two-component system sea ice are solved. The model results are in very good agreement with similarity solutions and show for example that so-called brine expulsion does not lead to any salt loss from the ice, but only to a re-distribution of brine within the ice itself. Brine expulsion is the movement of brine within the ice caused by the change in density during solidification.

To test the model results against measurements we have developed an instrument that is capable of measuring the salinity and solid fraction of growing sea ice with a spatial resolution of up to 5 mm and a temporal resolution of up to fractions of a second. The technique is based on measurements of electrical impedance. The measurements confirm the prevalent understanding that the bulk salinity in ice-core measurements of sea ice is usually significantly underestimated in the lower parts of the core. Using the data in connection with our sea-ice model allows for the quantification and thus parametrisation of gravitational effects of brine loss from sea ice.