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Temperature
Both air and sea temperature are vital for the freeze/melt processes. Ice will form if air temperature is lower than the freezing point of the water below. The greater the temperature gradient between air and sea is, the more efficient the heat transfer. If the difference between air and water is not large enough, heat conduction within the water column itself will keep the surface water from freezing. Salinity Surface salinity is also of importance. Freshwater freezes more easily than salt water - zero salinity requires a temperature of zero degrees Celsius, whereas "normal" sea water with salinity around 34 psu (practical salinity units, comparable to parts per mille (thousand)) must be cooled down to almost -2°C. Areas with large amounts of excess melt-water from the previous season, or extensive freshwater input from rivers and precipitation are well disposed for ice production. In the central Polar Seas, a deep layer of freshwater also tends to isolate the sea ice from warmer water below. Wind Wind is another crucial factor. Strong winds enhance the effectivity of heat exchange across the air-sea interface. Wind forcing also transports ice away from the freezing areas, allowing for more ice to be produced efficiently. When sea ice is pushed together by convergent wind fields, ridges are formed. This way, the ice becomes thicker than by freezing alone. Mean thickness of up to 7-8 meters is observed north of Greenland/Eastern Canada. Wind is also responsible for moving ice out of the Polar Sea and into regions of higher temperature, where it melts. Surface Currents Upper layer surface currents have similar effects as wind. Ice can be forced together, or out of freezing regions as is the case with the EGC. The NAC brings warm water into the seas around Svalbard, impeding ice formation in an area that would otherwise have air temperatures low enough for freezing. Snow Snow acts as an insulator between air and sea, and reduces heat transfer. Freezing is thus slowed down considerably by moderate amounts of snow. On the other hand, a sufficiently thick snow cover will impede melting, in certain places enough for the sea to remain ice covered throughout the melting season. Cloud Cover Clouds have different effects on the ice cover. During summer, thick clouds will protect the ice from solar heating. In winter, when there's no radiation from the sun anyway, the effect is the contrary. Long-wave emission from the sea surface/ice is absorbed and reflected by clouds. This increases surface temperature, and slows down the freezing process. A thick cloud cover will usually also yield more precipitation in the form of snow, which in most cases acts to reduce ice thickness. |
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