Abstract/Summary

Melt water from the surface of Arctic sea ice fills hollows in the topography of the ice, forming pools of sea water that can cover up to 50% of the surface of the ice in the summer months. Since ice is a porous medium, water can percolate down from these melt ponds to the base of the sea ice, where it gathers in pools between the relatively cold, saline ocean water and the sea ice. These ponds are known as under-ice melt ponds. Freezing can occur at the interface between the under-ice melt pond and the oceanic mixed-layer, forming a sheet of ice called a false bottom. False bottoms insulate the true base of the sea ice from the ocean and their formation is a significant mechanism of Arctic sea ice summer growth. By changing the rate of basal ablation, under-ice melt ponds alter the fresh water and salt fluxes from the sea ice into the ocean. Here, we evaluate the impact of under-ice melt ponds and false bottoms on the mass balance of the sea ice and its thermohaline interaction with the oceanic mixed layer below. We present a one-dimensional thermodynamic model of the evolution of under-ice melt ponds and false bottoms, with which we found that their development insulates the sea ice above them, increasing ice thickness. We couple this to a simplified ocean mixed layer model and found that the ablation of false bottoms rapidly transfers stored fresh water to the mixed layer. Our model indicates that under-ice melt ponds could lead to sea ice several tens of centimetres thicker if they form annually, and that false bottom ablation could lead to a sudden Arctic ocean mixed layer freshening and a resulting shallowing of the order of ten metres in areas of high under-ice melt pond fraction.