Sea ice is a year-round feature of the polar oceans, with seasonal ice cycles shaping the weather, climate, biology, and ecology of the polar regions. In the north, sea ice also shapes the social and cultural rhythms of indigenous people, through its influence on transportation and traditional fishing and hunting practices. Sea ice also extends to midlatitudes in the winter season, affecting shipping in protected near-shore waters such as the North Sea and Baltic Sea, offshore of Japan in the Sea of Okhotsk, and the Cabot Strait waters of eastern Canada.

The thermodynamics of sea ice are well represented by the lake ice processes discussed in chapter 4, with the addition of salinity effects. However, ocean currents and winds make sea ice a considerably more complex aspect of the cryosphere. Advection of ice floes, opening and closing of leads, and mechanical interactions between floes create an inhomogeneous ice cover with large variations in ice thickness and type. Ice rafting, ridging, shearing, and extensional failure require consideration of ice rheology: the way in which sea ice deforms, fractures, and fails under stress. This mechanical dissipation in turn feeds back on the ice thickness distribution, ice thermodynamics, and the momentum balance governing ice drift. All of this introduces multiyear memory in polar sea ice cover, where anomalies in the ice pack and associated ice-climate feedbacks can persist for many years. Chapters 8 and 9 examine aspects of sea-i ce-climate interactions in further detail.

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