<P> The density of sea water, in contrast to fresh water, increases as it nears the freezing point and thus it tends to sink . It is generally necessary that the upper 100--150 m (330--490 ft) of ocean water cools to the freezing point for sea ice to form . In the winter the relatively warm ocean water exerts a moderating influence, even when covered by ice . This is one reason why the Arctic does not experience the extreme temperatures seen on the Antarctic continent . </P> <P> There is considerable seasonal variation in how much pack ice of the Arctic ice pack covers the Arctic Ocean . Much of the Arctic ice pack is also covered in snow for about 10 months of the year . The maximum snow cover is in March or April--about 20 to 50 cm (7.9 to 19.7 in) over the frozen ocean . </P> <P> The climate of the Arctic region has varied significantly in the past . As recently as 55 million years ago, during the Paleocene--Eocene Thermal Maximum, the region reached an average annual temperature of 10--20 ° C (50--68 ° F). The surface waters of the northernmost Arctic Ocean warmed, seasonally at least, enough to support tropical lifeforms requiring surface temperatures of over 22 ° C (72 ° F). </P> <P> Endangered marine species in the Arctic Ocean include walruses and whales . The area has a fragile ecosystem which is slow to change and slow to recover from disruptions or damage . Lion's mane jellyfish are abundant in the waters of the Arctic, and the banded gunnel is the only species of gunnel that lives in the ocean . </P>

What is the average temperature of the arctic ocean
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