Multi-year ice is old ice up to 10 ft (3 m) or more thick that has survived at least two summers’ melt. Hummocks even smoother than in second-year ice, and the ice is almost salt free. The color, where snow free, is usually blue. The melt pattern consists of large interconnecting irregular puddles and a well developed drainage system. Undeformed multiyear ice is distinguished by its undular surface (remnants of drained or refrozen melt ponds). Multiyear ridges are distinct from first-year ridges in that they are typically smaller, more rounded, nearly solid ice and are therefore a serious impediment to surface ships. Multi-year ice is less common in the Antarctic than the Arctic, and is usually confined to the western Weddell Sea and isolated embayments at other locations around the coast. Ocean currents and the atmospheric circulation result in a net divergence of sea ice around the continent, causing most of the ice to melt in the summer as it drifts into warmer waters, or as the upper ocean heats up as the open water areas within the pack absorb solar radiation. The Weddell Sea accounts for about 80% of the multi-year ice in the Antarctic. The clockwise circulating current known as the Weddell Gyre is responsible for trapping sea ice along the eastern side of the Antarctic Peninsula, allowing it to survive for more than one year. Eventually the current transports the ice further north where it dissipates into the ocean and melts. Melt patterns on multi-year ice are a feature most commonly observed in the Arctic. The ablation season of Antarctic sea ice is rarely associated with the presence of melt water on the surface of the ice.
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