In the Greenland and Iceland Seas, the low-pressure system that sits over Iceland in the winter draws deep water up to the surface, creating a convection current. This action cools the deeper parts of the ocean to form a low-temperature, high-salinity water. The deep water of the Greenland and Iceland Seas form a North Atlantic deep water that extends past the Straits of Denmark. The entire cycle encompassing all of the world's oceans takes 2,000 years to complete one full cycle of water supply through deep-water circulation.
3.2.3 Sea Ice
The ice that floats on the ocean's surface is known by the generic name of "floating ice." Floating ice includes ice from frozen seawater, ice that flowed into the ocean from rivers and tides, icebergs from glaciers and ice islands that break off of various ice shelves. The number of icebergs and ice islands is much smaller in the Arctic Ocean than in the Antarctic Ocean.
Terminology and classification of sea ice is given in the appendices at the back of this book. Initial ice formation occurs at or near the surface of the seawater in the form of small platelets and needles called frazil. Continued freezing results in the production of grease ice, a soupy mixture of seawater and unconsolidated frazil crystals. Under quiescent conditions the frazil crystals rapidly freeze together to form a solid, continuous ice cover with thickness between 1 and 10cm. Wind-induced turbulence in the water, however, often inhibits immediate development of a solid cover. In the presence of a sustained wave field, circular masses of semiconsolidated frazil called pancakes form, ranging from 03 to 3.0m in diameter. Eventually these pancakes freeze together to form a continuous sheet of ice, called first-year ice, thicker than 30cm. The thickness of sea ice during its formation is known to be roughly proportional to the square root of the cold sum; also called the accumulated freezing index, this value is a summation of the absolute value for average daily air temperature below the freezing temperature. When first-year ice survives one summer it is called second-year ice, and if it lasts for two summers or more it is called multi-year ice. Multi-year ice varies greatly in thickness according to the degree of annual melting and accumulation, but is usually 3-6m thick.
Sea ice contains salt. Although the salt is not trapped in the crystal lattice of the ice when it freezes, water with concentrated salinity (brine) is trapped between the ice plates. Every kilogram of sea water contains roughly 35g of salt, 3.5%." The salinity of sea ice is defined as the salinity of the fully melted sea ice. Immediately after formation, sea ice has a salinity of about 1%, which indicates that one-third of the brine is trapped in sea ice during the freezing process, while two-thirds is returned to the ocean. With the passage of time, the salt in the sea ice drains out, yielding a salinity of 0.4-0.5% in 1m-thick one-year ice, almost no salinity in the surface layer of multi-year ice and a decreased salinity of 0.2-0.3% in the lower layer of multi-year ice. The brine in sea ice maintains an equilibrium concentration value with respect to temperature: As temperature decreases, ice crystals separate from the sea water; as temperature increases, the ice melts to dilute the brine. Changes with time in the structure of sea ice occur mainly in response to temperature changes in the ice.
