Sea Ice Presented by: Dorothy Gurgacz
What is Sea Ice? It occurs in both the Arctic and Antarctic
Sea ice is formed from freezing ocean water It occurs in both the Arctic and Antarctic In the Northern Hemisphere it can exist as far as 38°N In the Southern Hemisphere it only occurs around Antarctica and can reach as far north as 55°S
Arctic Sea Ice on 11/30/10
Antarctic Ice on 11/30/10
Formation Formation of ice occurs at approximately -1.8°C (28.76°F)
As the water begins to freeze, small needle-like crystals called frazil form. These crystals are typically 3-4 mm in diameter. During their formation salt is expelled into the surrounding water, frazil crystals are nearly pure fresh water. Sheets of ice are able to form when the frazil crystals float to the top, accumulate, and bond together. The formation and melting of ice is a seasonally variable process.
Sea Ice Forming
Arctic vs. Antarctic The Arctic is a semi-enclosed ocean, surrounded by land. The Antarctic is a land mass, surrounded by ocean. Mobility of the ice in these regions are very different. In the Antarctic the ice can move freely, and eventually travel into warmer waters. Although the Arctic ice does move, it tends to stay within Arctic waters. This causes the ice masses to bump into each other and accumulate ice ridges at a higher frequency. These ice ridges make Arctic ice more thick. The presence of ridge ice and its longer life cycle leads to ice that stays frozen longer during the summer melt. So some arctic sea ice remains through the summer and continues to grow the following autumn Minimum and maximum sea ice cover for the Arctic and Antarctic. Winter in the Southern Hemisphere is opposite that of the Northern Hemisphere, which explains why Antarctica has less sea ice during February. The black circles in the center of the Northern Hemisphere images are areas lacking data due to limitations in satellite coverage at the North Pole. Image courtesy of the National Snow and Ice Data Center, University of Colorado, Boulder, Colorado. (Source: NSIDC)
Albedo Albedo is a unit-less quantity that indicates how well a surface reflects solar energy. Sea ice has a much higher albedo compared to other earth surfaces Typical albedo for bare sea ice is between 30-40% and can be as high as 95% for snow covered ice High albedo serves to insulate the sea ice, maintaining cold temperatures and delaying ice melt in the summer Incoming radiation that is not reflected is absorbed in the top few millimeters of ice. The only way heat can be transferred to the ocean is by conduction from the ice to the water; but the ice is always colder than the water underneath, the conduction of heat is from the water to the ice.
Why is Sea Ice Important?
Sea ice is a critical component in the earth’s climate system. Sea ice reflects the Sun's radiation more compared with open water and also acts like a lid on the ocean trapping heat According to scientific measurements, both the thickness and extent of summer sea ice in the Arctic have shown a dramatic decline over the past thirty years. This is consistent with observations of a warming Arctic. This trend is a major sign of climate change in the polar regions and may be an indicator of the effects of global warming Global warming has caused sea ice to melt at faster rates then ever before, this can cause the sea level to rise significantly. Because sea ice is largely fresh, greater rates of melting means greater input of freshwater into the ocean. This can affect density driven processes.
Sea Ice Extent During the Summer Melt Season
When sea water freezes to sea ice a great deal of the salt in the water is separated out. This means that the saline content of the surface water in polar regions increases where sea ice is formed. Water with a higher salinity has higher density than water with lower salinity. Thus the salty water sinks to lower levels in the ocean, while warmer water from the south replaces what used to be surface water. In this way the ice contributes to ocean currents that transport considerable amounts of water between the different oceans.
Many animals and plants depend on the sea ice
Many animals and plants depend on the sea ice. Among these are polar bears, seals, seabirds, crustaceans and algae. As a result of the greatly reduced sea ice extent in summer over the last decades, polar bears were recently registered as an endangered species.
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