In the Arctic, the overlying snow layer typically begins to melt in mid-June and is gone by early July. The meltwater from the snow gathers to form a network of meltwater pools over the surface of the ice. On first year ice, which has a smooth upper surface at the end of winter (except where ridged), the pools are initially very shallow, forming in minor depressions in the ice surface, or simply being retained within surviving snow pack as a layer of slush. As summer proceeds, however, this initial random structure becomes more fixed as the pools melt their way down into the ice through preferential absorption of solar radiation by the water, which reflects only 15-40% of the radiation falling on it compared to 40-70% for bare ice.
As the melt pools grow deeper and wider they may eventually drain off into the sea, over the side of floes, through existing cracks, or by melting a thaw hole right through the ice at its thinnest point or at the melt pool's deepest point. The downrush of water when a thaw hole opens may be quite violent, and on very level ice, such as fast ice, a single thaw hole may drain a large area of ice surface. From the air such thaw holes give the appearance of "giant spiders", with the "body" being the thaw hole and the "legs" channels of melt water draining laterally towards the hole.
The underside of the ice cover also responds to the surface melt. Directly underneath melt pools the ice is thinner and is absorbing more incoming radiation. This causes an enhanced rate of bottom melt so that the ice bottom develops a topography of depressions to mirror the melt pool distribution on the top side. In this way an initially smooth first-year ice sheet acquires by the end of summer an undulating topography both on its top and bottom sides. Some of the drained melt water may in fact gather in the underside depressions to form under-ice melt pools, which refreeze in autumn and partially smooth off the underside, leaving it with bulges but not depressions.
A final and most important role of the melt water is that some of it works its way down through the ice fabric through minor pores, veins and channels, and in doing so drives out much of the remaining brine. This process, called flushing, is the most efficient and rapid form of brine drainage mechanism, and it operates to remove nearly all of the remaining brine from the first-year ice. The hydrostatic head of the surface meltwater provides the driving force, but an interconnecting network of pores is necessary for the flushing process to operate. Given that the strength properties of sea ice depend on the brine volume, this implies that the flushing mechanism creates a surviving ice sheet which during its second winter of existence has much greater strength than in its first winter.
What happens to the ice that survives?
Ice which has survived one or more summer seasons of partial melt is called multi-year ice. In the Arctic, sea ice commonly takes several years to either make a circuit within the closed Beaufort Gyre surface current system (7-10 years) or else be transported across the Arctic Basin and expelled in the East Greenland Current (3-4 years). More than half of the ice in the Arctic is therefore multi-year ice. Growth continues from year to year until the ice thickness reaches a maximum of about 3 metres, at which point summer melt matches winter growth and the thickness oscillates through an annual cycle. This old, multi-year ice is much fresher than first-year ice; it has a lower conductivity and a rougher surface. The low salinity of multi-year ice makes it much stronger than first-year ice and a formidable barrier to icebreakers.