The Antarctic ice pack differs from the sea ice in the Arctic. Arctic sea ice generally is confined to the landlocked Arctic basin. This protects the ice and keeps it from melting back, which means the ice thickens over several years. Most of the sea ice that forms the following season is thin first year ice. Winds, currents, and storms work to break up the ice pack and open up new leads and polynyas.
Leads are essentially trails of open water through the ice.
Polynyas are areas of open water or little sea ice that occur in the middle of more concentrated pack ice. Katabatic winds or upwelling currents may work to keep the polynyas open, and may also explain why polynyas occur in the same place year after year. The polynyas and ice leads make the ice pack more navigable to ice-strengthened vessels. Several ships passing through leads within the sea ice have become entrapped and even crushed when the ice shifted. The ship reverses, and then powers ahead and again runs up onto the ice to repeat the process.