Oceanography (2+1)

Sea ice formation

The formation of sea ice depends mainly on sea water temperature, surface salinity, vertical distribution of salinity and water depth. Sea ice forms at higher latitudes due to extremely low water temperature. As soon as the temperature of the sea surface reaches the freezing point, the formation of ice begins. The freezing point of water with a salinity of 35 is -1.91^oC. as water freezes at the surface of the ocean, the dissolved solids are left out of the crystalline structure of the ice. Hence, the salinity of the sourrounding water increases. This higher salinity causes the lowering of the freezing point of the remaining water. Thus, surface salinity influences the freezing point. The vertical distribution of salinity as well as the depth of the water determine the thickness of the water layer the is involved in the vertical convection during the cooling processes.

Different types of ice are formed depending on the wind, current and sea state condition. when the winds are very weak and sea state is calm, clear crystalline “new ice” or “young ice” forms, while with the strong winds and rough seas, “grease ice” or “slush ice” forms, which looks turbid due to the enclosure of air bubbles within the ice crystals. If the winds continue to be weak during the freezing process, a primary thin ice called “nilas” forms from the new ice. When the grease ice begins to grow into disk’s, primary “pack ice” develops. The “pan cake” ice represents the transition from grease ice to pack ice. Sometimes, “hummocked ice” and “pressure ridges” form over depths of 8 – 10 m or even in 20m depth in the sea.