Etudes et évaluation de processus océaniques par des hiérarchies ...

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16 CHAPTER 3. PHYSICAL PROPERTIES OF SEA WATER
tel-00545911, version 1 - 13 Dec 2010
(ii) reduces the transfert of heat, isolating the ocean, (iii) actes as a thermal resevoir due to the
latent heat associated with melting and freezing and (iv) changes the salinity and buoyancy,
through melting and freezing. The first point is responsible for the fact that polar regions are
very sensitive to global climate change. A little change in temperature can freeze or melt sea
ice, the positive feed back of the albedo will then amplify the initial temperature change.
The Arctic Ocean is almost completely surrounded by land, while the Antarctic Ocean is
completely open towards lower latitude. Arctic sea ice is thus hindered to travel towards lower
latitude and typically survives several summer periods and has a typical thickness from two to
three meters. Only about 10 % of the arctic ice travels south through the Fram Strait every
year. Whereas the Antarctic ice is mostly seasonal, with 80 % disappearing by the end of the
austral summer and has a typical thickness from one to two meters.
During freezing salt becomes trapped in the ice forming brine pockets which have a size
around 10 −4 m. The amount of salt trapped in the ice during freezing increases with the growth
rate of the sea ice and the salinity of the seawater. Newly formed ice has a typical salinity
around 14psu which is roughly half of the salinity of seawater. Within the sea ice the brine
moves downward and leaves the ice at its lower boundary. The overall salinity of sea ice
decreases with its age, leading to different salinities of the seasonal ice in the Antarctic and the
multi-year ice in the Arctic. The salinity of ice has important influence on its thermal properties
such as heat capacity, thermal conductivity and latent heat content. Sea ice also contains air
bubbles, their volume typically icreases with age, reducing the ice density. A typical value for
air bubble volume of multi-year ice is 15%. Sea ice is a multi-component multi-phase material.
The fraction of each component and phase is subject to change due to exterior forcings.
Ice bergs are broken off (calved) parts of land glaciers and ice shelves, land glaciers that
have migrated into the ocean, and do not contain significant amounts of salt. Ice bergs calved
from ice shelves can have a horizontal extent of 100km.