Multiyear ice

Multiyear ice remains frozen year-round, but still goes through a cycle of changes. The surface layer of this ice melts in the summer and the water then drains through pores in the ice to the bottom. This salt-free meltwater remains floating beneath the ice atop the denser salt water. In the winter, the meltwater refreezes as a layer on the bottom of the pack ice. One year at a time, the layers travel upward in a continuous cycle of loss and replacement. This cycle prevents the multiyear ice from thickening indefinitely. Multiyear ice forms the permanent polar pack in the centre of the Arctic Ocean, as well as the summer ice pack that surrounds the permanent polar ice pack, and breaks up in the summer. Unlike annual ice, the summer ice pack never completely melts.

The surface layer of multiyear ice melts in the summer, flows downwards, and sits atop the denser salt water, which ...
... Then refreezes during the winter, creating a cycle of melting and freezing that creates a permanent ice field.