Sea

The seasonal formation of vast areas of sea ice (a 400-2,000 km wide belt with an area of up to 19x106 km2, which completely encircles the continent in September; Gloersen et al. 1992) is a predominant feature of the Southern Ocean. Unlike in the Arctic, more than 85 % of Antarctic sea ice melts in summer; most of it is therefore usually much thinner than Arctic ice. Ice formation in the Southern Ocean increases in autumn (from April to June) and progressively extends northwards until September-October. Melting is relatively rapid (with a rate more than double that of freezing) in spring, and the most extensive areas of ice remaining at the end of summer are those along the coasts of the Bellingshausen and Amundsen Seas and in the western Weddell Sea (Fig. 23). The sea-air heat exchange and upwelling of relatively warm water (about 2 °C), which resides below the Southern Ocean pycnocline (Gordon 1981), probably contribute to the rapid break-up and melting of Antarctic sea ice. The sea-ice zone in the Arctic varies by less than 25 %, and freezing is much faster than the retreat of ice. According to Gordon (1981), this reverse pattern is due to the large input of freshwater, which induces a strong pycno-cline in the seawater column and consequent rapid freezing mainly determined by the ocean-atmosphere heat flux.

Surface seawater in the Southern Ocean has a salinity of about 34% and reaches the freezing point at -1.91 °C. When the surface layer of the sea reaches this point, additional heat loss determines a slight supercooling of

Fig. 23. Average sea-ice cover variations around Antarctica in winter and summer

water; ice crystals of pure water begin to form and salt is expelled into the surrounding water. At first, a floating suspension of small ice crystals or platelets (called frazil ice) occurs at or near the water surface to produce a soupy, unconsolidated mixture (grease ice). If freezing continues and ice formation exceeds 30-40 %, the transition to a solid cover begins. Wave fields break this thin layer of ice into small, circular pieces (termed pancake ice) with raised rims due to rubbing against other pieces. Pancake ice eventually coalesces to form a composite and continuous sheet (Maykut 1985; Wad-hams 1991). As seawater freezes, brine-rich streamers form and sink, carrying away heat from the ice formation zone. Long, columnar ice crystals begin to form below (congelation ice) in response to conductive heat loss along the temperature gradient. Less saline water rises up, and frazil ice or platelets may form within the zone of convection; these float up, accumulate and fuse together beneath congelation ice to form a porous layer (bash ice) containing pockets of brine. Platelets may also form at great depth in supercooled water streaming out from under ice shelves or at the seafloor in inshore zones of convective circulation (anchor ice). Although anchored to the bottom, the anchor ice can move in response to thermal or mechanical stress and, where it adjoins land or ice sheets, it shows typical parallel tide cracks. The annual ice sheet forms early in the autumn near the coastline (fast ice); although it usually begins to detach from the shoreline in late summer, it may also remain for some years, giving rise to multiyear ice with a thickness of several metres. The ice sheet formed later and seawards can break up under the influence of storms, and pack ice can move considerable distances before re-freezing into solid pack (Fig. 24).

Depending on physical and hydrodynamic conditions, sea-ice formation and structure can vary considerably according to location and from one year to the next. The ice formed in leads and polynyas often consists of frazil crystals quickly piled by wind to form thick layers. In the Weddell Sea, frazil ice layers are often sandwiched between congelation ice (Knox 1994). Sea ice is usually overlain by a variable amount of snow, which ensures that a large proportion of incoming solar radiation is reflected. The snow-ice interface may become flooded with seawater along leads in the pack ice, especially in the marginal ice zone. The ice edge is the transitional zone between the ice cover and open waters, and forms a complex frontal system affected by dynamic interactions among ice, seawater and the atmosphere. The ice edge is characterised by large spatio-temporal variations - while the boundary between consolidated pack ice and open water is sometimes sharply delineated (hundreds of metres or a few kilometres), at other times the transition zone with loose pack may be tens or some hundred kilometres wide.

In springtime, sea ice is fragmented by the wind and waves, and melting is favoured by the north-easterly component of the ACC, which constantly pushes the ice to lower latitudes. In areas of the Southern Ocean such as the Weddell and Ross seas, where semi-enclosed gyres occur, sea ice is obviously

Fig. 24. Schematic representation of coastal and open ice polynyas and sea-ice formations in the Southern Ocean

more likely to become multiyear ice. Ice melting releases water with a low salt content, which floats to the sea surface and promotes the stability of the water column. Like sea ice, great tabular icebergs calved from the fronts of ice shelves or the snouts of tidewater glaciers melt below the waterline as they float away from the continent. However, they seem to have an opposite effect on the water column because they generate vertical convection and promote the upwelling of nutrient-rich deep water (Allison et al. 1985).

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