Sea Ice

SEA ICE IS frozen ocean water. It forms primarily in and near the polar regions, though it can grow closer to the equator as far as 40 degrees N latitude and 55 degrees S latitude. Sea ice has a strong seasonal variability. In the Northern Hemisphere, the annual maximum extent occurs in late winter (March), covering about 5,791,532 sq. mi. (15 million sq. km.) on average. It then melts during spring and summer to an annual minimum extent of about 2,702,715 sq. mi. (7 million sq. km.) in September. In the Antarctic, the annual maximum is about 7,335,941 sq. mi. (19 million sq. Ion.) during September, and the annual minimum is about 1,158,306 sq. mi. (3 million sq. km.) in February or March. Overall, roughly 10 percent of the worlds ocean area is covered with sea ice at some point during the year.

The point at which the ocean begins to freeze is a function of the salt content of the water, which for typical ocean salinities is around 29 degrees F (minus 1.6 degrees C). The saline nature of the ocean makes the formation of sea ice distinct from freshwater ice growth in lakes and rivers. Freshwater becomes less dense as it approaches the freezing point, keeping the coldest water at the surface and allowing ice to form as soon as the surface cools to the freezing point. However, the presence of salt changes the charac-ter [Page 888]of near-freezing water such that as saline water nears the freezing point, it continues to increase in density. Thus, cooling surface waters will become denser and sink. This means that there is overturning, and subsurface waters must also cool before ice can begin to form.

Sea ice typically grows to an average level thickness of 3 to 6 ft. (1 to 2 m.) in the Antarctic and 10 to 13 ft. (3 to 4 m.) in the Arctic. The ice is thinner in the Antarctic because most ice melts during the austral summer, whereas in the Arctic a significant fraction (−40 percent) remains through the summer and can grow over several years. A larger ocean heat flux at the bottom of the ice in the Antarctic also keeps the ice thinner.

However, thicker ice is not uncommon because of the effect of ice motion. Most sea ice is almost constantly in motion mainly because of the force of winds and ocean currents (other factors include the Coriolis effect, the slope of the ocean surface, and the internal structure of the ice). The speed of sea ice motion varies considerably; it can move 31 mi. (50 km.) or more in a day, though 1.2 mi. (2 km.) per day is typical. The motion of the ice can result in convergence between different parts of the ice cover, causing the ice to pile up into features called ridges. Ridges may easily rise 16 to 33 ft. (5 to 10 m.) above the surrounding level ice (and many tens of m. below the surface).

Sea ice plays an important role in climate. It has a much higher albedo than the unfrozen ocean, meaning that 60 to 70 percent of the sun's energy is reflected by the sea ice surface, whereas the unfrozen ocean reflects less than 10 percent of the sun's energy, resulting in much less energy absorption where ice is present. Sea ice is also a physical barrier between the ocean and atmosphere. This prevents the transfer of heat and moisture between the two and during winter. Thus, sea ice keeps the polar regions cooler and drier than they would be without ice. Sea ice also reduces fetch and dampens waves, limiting coastal erosion.

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