Sea IceYes it’s just frozen sea water.

Freezing Point• Fresh water freezes at 0 degrees Celsius (32 degrees

Fahrenheit), but the freezing point of sea water varies. For every 5 ppt increase in salinity, the freezing point decreases by 0.28 degrees Celsius (0.5 degrees Fahrenheit); thus, in polar regions with an ocean salinity of 35 ppt, the water begins to freeze at -1.8 degrees Celsius (28.8 degrees Fahrenheit).

Sea Ice Formation• In calm waters, frazil crystals form a smooth, thin form of ice,

called grease ice for its resemblance to an oil slick. Grease ice develops into a continuous, thin sheet of ice called nilas. Initially, the sheet is very thin and dark (called dark nilas), becoming lighter as it thickens. Currents or light winds often push the nilas around so that they slide over each other, a process known as rafting. Eventually, the ice thickens into a more stable sheet with a smooth bottom surface, called congelation ice. Frazil ice cannot form in the relatively still waters under sea ice, so only congelation ice developing under the ice sheet can contribute to the continued growth of a congelation ice sheet. Congelation ice crystals are long and vertical because they grow much slower than frazil ice.

• If the ocean is rough, the frazil crystals accummulate into slushy circular disks, called pancakes orpancake ice, because of their shape. A signature feature of pancake ice is raised edges or ridges on the perimeter, caused by the pancakes bumping into each other from the ocean waves. If the motion is strong enough, rafting occurs. If the ice is thick enough, ridging occurs, where the sea ice bends or fractures and piles on top of itself, forming lines of ridges on the surface. Each ridge has a corresponding structure, called a keel, that forms on the underside of the ice. Particularly in the Arctic, ridges up to 20 meters (60 feet) thick can form when thick ice deforms. Eventually, the pancakes cement together and consolidate into a coherent ice sheet. Unlike the congelation process, sheet ice formed from consolidated pancakes has a rough bottom surface.

• Frazil ice

• After the ice sheet has formed, it will continue to grow through the winter, if the sheet gets thick enough it just thins during the summer. Otherwise it melts.

Salinity and Brine• When frazil ice crystals form, salt accumulates into droplets

called brine, which are typically expelled back into the ocean. This raises the salinity of the near-surface water. Some brine droplets become trapped in pockets between the ice crystals. These droplets are saline, whereas the ice around them is not. The brine remains in a liquid state because much cooler temperatures would be required for it to freeze. At this stage, the sea ice has a high salt content. Over time, the brine drains out, leaving air pockets, and the salinity of the sea ice decreases. Brine can move out of sea ice in diferent ways: Aided by gravity, the brine migrates downward through holes and channels in the ice, eventually emptying back into the ocean.

• The ice surrounding the brine compresses and breaks the brine pockets, allowing the brine to escape to the ocean.

• When the sea ice begins to melt during the summer, small freshwater ponds (called melt ponds) form on the top layer of the ice. This freshwater travels through the cracks and holes in the ice, washing out remaining brine.

• When the sea ice surface cools, brine increases in salinity to the point at which it can melt ice at its underside. This leads to a downward migration of brine droplets, ultimately allowing the brine to escape into the ocean below the ice sheet.

• The expulsion of salt from the freezing water makes the water under the ice denser, driving the more saline water to the bottom. This action is what drives ocean currents around the world.

Multi-year Ice• Multi-year ice has properties that distinguish it from first year

ice. Multi-year ice contains less brine and more air pockets, this makes the ice “stiffer.” Ice breakers find this “stiffer” ice harder to navigate.

• Hummocks of multiyear ice that are several years old are fresh enough that someone could drink their melted water. In fact, multiyear ice often supplies the fresh water needed for polar expeditions.

• First-year and multiyear ice have different electromagnetic properties that satellite sensors can detect, allowing scientists to distinguish the two.

• Multiyear ice is much more common in the Arctic than in the Antarctic. This is because ocean currents and atmospheric circulation move sea ice around Antarctica, causing most of the ice to melt in the summer as it moves into warmer waters, or as the upper ocean heats up due to absorption of solar heat by open water areas. Most of the multiyear ice that does occur in the Antarctic persists because of a circulating current in the Weddell Sea, on the eastern side of the Antarctic Peninsula. The Arctic Ocean, in contrast, is relatively land-locked, allowing extensive multiyear ice to form.

Features of Sea Ice• Sea ice is usually covered with snow, which insulates the ice

and delays melting in the summer. The snow also modifies the electromagnetic radiation signal detected by satellites. Except during a melt season, the snow is usually dry, wind-blown, and hard-packed. Wind from a consistent direction can blow snow into ridges parallel to the wind direction, just like small sand dunes. These complex, fragile shapes are called sastrugi.

• Other features that form on the surface of sea ice are frost flowers, crystals of ice deposited on the sea ice when water vapor bypasses the liquid phase and becomes a solid. Frost flowers roughen the surface and dramatically alter its electromagnetic signal.

• If snow cover is thick, especially over relatively thin sea ice, the weight of the snow can push the ice down into the water below. The salty ocean water floods the snow and creates a salty, slushy layer. This flooded sea ice is more common in the Antarctic than the Arctic because there is typically thinner ice and more snowfall in Antarctica.

• When wind, ocean currents, and other forces push sea ice around, ice floes (sheets of ice floating in the water) collide with each other, and ice piles into ridges and keels. Ridges are small "mountain ranges" that form on top of the ice; keels are the corresponding features on the underside of the ice. The total thickness of the ridges and keels can be several meters (in some cases, 20 meters, or 60 feet, thick), and the surface ridges can easily be 2 meters (6 feet) or higher. Ridges create significant obstacles to anyone trying to traverse the ice.

• Ridges are initially blocky with very sharp edges. Over time, especially during the summer melt, the ridges erode into smaller, smoother "hills" of ice called hummocks. This process is similar to the erosion of jagged mountain peaks into smooth, rolling hills, but at an accelerated pace. When keels erode into smooth features, they are called bummocks.

• During summer, as the snow on top of sea ice melts, the meltwater can accumulate in depressions on the sea ice surface called melt ponds. These ponds absorb more heat than the surrounding sea ice from sunlight, and they grow in area and depth. The fresh water in melt ponds appears blue because light reflects and scatters off the sea ice surface from the bottoms and sides of the melt pond. If a pond melts through the entire thickness of the ice, the pond's color turns dark, like the ocean. Melt ponds are more common in the Arctic than in the Antarctic, due to differences in relative humidity.

Leads and Polynyas• Leads and Polynyas are regions of open water, and they share

several characteristics. Both are regions of open water where we would expect to find sea ice; both can influence weather and climate in their immediate surroundings, and both play important roles in wildlife habitats. However, they are different in fundamental ways. Leads are narrow, linear features, while polynyas are generally more uniform in shape and larger in size. Leads form because of the motion of the ice, while polynyas form from either upwelling warm water or persistent winds. During winter, open water remains in leads for only a short time before it begins to refreeze, while polynyas usually remain unfrozen for long periods of time.

•http://nsidc.org/cryosphere/seaice

http://nsidc.org/cryosphere/seaice