Convectively-driven migration of brine enclosures in sea ice

Brine pocket migration in sea ice is the process by which an isolated, brine-filled enclosure migrates in the direction of an imposed temperature gradient due to melting at the warmer end of the pocket, and freezing at the cooler. In sea ice this is considered a very slow mechanism of brine movement, limited by the molecular diffusion of salt in the brine. In this study brine pocket migration is re-examined in circumstances in which the fluid enclosed in the pocket is likely to convect; that is, in the case where the long axis of a slender, cylindrical brine enclosure is tilted at an angle to gravity and to the temperature gradient. For tilted enclosures of nominal diameter in the range 1 to 5 mm, we measured vertical migration velocities that are an order of magnitude larger than would be expected if the phenomenon was controlled solely by the molecular diffusion of salt. At constant tilt angle the migration velocity depends primarily on brine pocket diameter and on the temperature gradient imposed on the ice. We interpret the data in terms of a published theoretical investigation of low Rayleigh number natural convection in a slender,tilted, cylindrical cavity embedded in a solid that is subjected to a uniform vertical temperature gradient. Our experiments suggest that the migration velocity of tilted, cylindrical brine enclosures in sea ice varies as the 1/3 power of the predicted convection velocity within the enclosure. High migration rates due to convection could affect the evolution of the pore space in sea ice, potentially altering sea ice permeability and hence the optical, mechanical and microwave properties.