<P> As noted, the process of ice formation and movement is a key driver in Arctic Ocean circulation and the formation of water masses . With this dependence, the Arctic Ocean experiences variations due to seasonal changes in sea ice cover . Sea ice movement is the result of wind forcing, which is related to a number of meteorological conditions that the Arctic experiences throughout the year . For example, the Beaufort High--an extension of the Siberian High system--is a pressure system that drives the anticyclonic motion of the Beaufort Gyre . During the summer, this area of high pressure is pushed out closer to its Siberian and Canadian sides . In addition, there is a sea level pressure (SLP) ridge over Greenland that drives strong northerly winds through the Fram Strait, facilitating ice export . In the summer, the SLP contrast is smaller, producing weaker winds . A final example of seasonal pressure system movement is the low pressure system that exists over the Nordic and Barents Seas . It is an extension of the Icelandic Low, which creates cyclonic ocean circulation in this area . The low shifts to centre over the North Pole in the summer . These variations in the Arctic all contribute to ice drift reaching its weakest point during the summer months . There is also evidence that the drift is associated with the phase of the Arctic Oscillation and Atlantic Multidecadal Oscillation . </P> <P> Much of the Arctic Ocean is covered by sea ice that varies in extent and thickness seasonally . The mean extent of the ice has been decreasing since 1980 from the average winter value of 15,600,000 km (6,023,200 sq mi) at a rate of 3% per decade . The seasonal variations are about 7,000,000 km (2,702,700 sq mi) with the maximum in April and minimum in September . The sea ice is affected by wind and ocean currents, which can move and rotate very large areas of ice . Zones of compression also arise, where the ice piles up to form pack ice . </P> <P> Icebergs occasionally break away from northern Ellesmere Island, and icebergs are formed from glaciers in western Greenland and extreme northeastern Canada . These icebergs pose a hazard to ships, of which the Titanic is one of the most famous . Permafrost is found on most islands . The ocean is virtually icelocked from October to June, and the superstructure of ships are subject to icing from October to May . Before the advent of modern icebreakers, ships sailing the Arctic Ocean risked being trapped or crushed by sea ice (although the Baychimo drifted through the Arctic Ocean untended for decades despite these hazards). </P> <P> Under the influence of the Quaternary glaciation, the Arctic Ocean is contained in a polar climate characterized by persistent cold and relatively narrow annual temperature ranges . Winters are characterized by the polar night, extreme cold, frequent low - level temperature inversions, and stable weather conditions . Cyclones are only common on the Atlantic side . Summers are characterized by continuous daylight (midnight sun), and temperatures can rise above the melting point (0 ° C (32 ° F). Cyclones are more frequent in summer and may bring rain or snow . It is cloudy year - round, with mean cloud cover ranging from 60% in winter to over 80% in summer . </P>

Why arctic ocean is a grave to many ships
find me the text answering this question