Ninety per cent of the world‟s ice volume and 70% of the world's fresh water are found in Antarctica, mainly in the east Antarctic ice sheet, limited by Transantarctic Mountains (Figure 3.15).
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Figure 3.15 Ice Shelves in Antarctica (credit from Landsat Image Mosaic of Antarctica30 2003 n.p.)
Antarctica has a global area of 14,000,000 km². About 98% of Antarctica, is covered by the Antarctic ice sheet, which represents 13,720,000 km² and has an average thickness of 1.6 km (CIA, 201031). An interesting feature of the submarine topography off Antarctica is the occurrence of an inner shelf depression up to 1,700 m deep, separated from the outer continental shelf by sills about 500 m deep. On the base of the continental sheet, lies a plain with depths ranging from 3,700 to 5,000 m, while the South Sandwich Trench descends to 8,620 m. Most of the Antarctic coastline is buried under ice flowing over the continental margins. Littoral regions are common on the west coast of West Antarctica and the islands of the Scotia Arc, but rare in East Antarctica. In Antarctica the continental shelf break occurs on average at about 400 m deep, while in the Ross Sea it is up to 800 m deep. Visible areas of shoreline may be abraded by fast ice pack, brash ice and bergy bits, which may scour the littoral region to a depth of 15 m. The local characteristics of the shore determine the
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duration of ice cover. Fast ice and pack ice affect the majority of continental shores for more than six months a year, and in the Ross Sea fast ice persists for 11 months a year. In the Southern Hemisphere the sea ice extent varies from 3 million km² in February to 20 million km² in September (Gloersen et al., 1992). According to Hughes (2009 n.p.):
Most of the discharge of the West Antarctic Ice Sheet happens via the major ice streams entering the Ross Ice Shelf, the Rutford Ice Stream entering the Ronne and Filchner shelf of the Weddell Sea and the Thwaites Glacier or the Pine Island Glacier entering the Amundsen Ice Shelf.
In Antarctica, beyond the yearly variability of iceberg distribution, it appears that icebergs are mainly produced in summer in the Ronne Filchner Ice Shelves zones, in the Amery Ice Shelf zone, and at a smaller degree in the Ross Ice Shelf zone. From October to March the iceberg concentration is important in the Ronne Filchner Ice Shelves zones, in the Amery Ice Shelf zone (Figure 3.16).
Figure 3.16 Number of Iceberg around Antarctica Deduced from Computer Models; colour scale: red
100 icebergs, orange 50 icebergs, blue green 10 icebergs, blue 5 icebergs (credit Ifremer - French
Institute for Sea Research, n.d.n.p.)
In summer time (January to March), more than 100 icebergs are calved in the Weddell Sea, Ross Sea and Ronne-Filchner Ice Shelf. In winter time (July to September) less than 10
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icebergs are calved. The icebergs drift more from April to June. The calving begins to increase in spring, from October to December. Large tabular icebergs originate in the Weddell and Ross Seas (Gladstone and Bigg, 2002). In particular a region of interest for iceberg transportation projects is the Amery ice Shelf (AIS) located around 71° S, 70° E in East Antarctica between the Australian stations of Davis and Mawson (Janssen and Rachael, 200832). The Lambert Glacier Basin system supports the AIS (Allison, 1991; AAD, 201133). With 60,000 km2, it represents a major ice shelf in East Antarctica and is the third largest ice shelf in Antarctica (Budd et al., 1967). The calving front is about 550 km long. When analysing the balance of the AIS, it appears that most of the lost occurs through basal melting processes and icebergs calving production 44.6 +/- 9.3 Gt per year (Young and Hyland, 2002; Galton-Fenzi et al., 2009; Figure 3.17).
Figure 3.17 The AIS Mass Balance (credit Galton-Fenzi et al., 200934).
Large iceberg discharges occur on a 60 year cycle basis (Coleman and Young, 2003): in 1963/1964 a 10,000 km2 iceberg broke off and recently in the years 2000s a 500 km2 tabular iceberg was breaking off.
From the AIS, icebergs naturally go about 1,700 km south-southwest of W A, north of 56 degrees south. However sometimes they can get closer as in 2009 when a 20 billion tonne iceberg B17B (19 km long and 8 km wide), arrived at 48.8º S and 107.5º E. This
32
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iceberg calved from the Ross Ice Shelf 10 years before stayed for about five years close to the Mertz Glacier and drifted through currents and winds around Antarctica for five more years. Recently in 2006 and 2009 2 billion tonnes iceberg originating from the Ronne or the Ross Ice shelves passing off Australia's Macquarie Island and naturally drifting to New Zealand coasts were spotted (Figure 3.18a and b).
Figure 3.18a Iceberg Drift (13,500 km) from Ronne Ice Shelf to New Zealand (credit Helicopters
Otago Ltd. 200635 n.p) Legend: A group of 200 tabular icebergs arrived from the Ronne Ice Shelf in
the Weddell Sea to the South East coasts of New Zealand in November 2006
Figure 3.18b iceberg Drift (13,500 km) from Ross Ice Shelf to New Zealand (credit Dailymail36, 2009)
35 www.newzeal.com/theme/heli/IcebergMap.jpg 36 http://www.meteorologynews.com/2009/12/15/huge-icebergs-drifing-toward-australia-new-zealand/, http://www.dailymail.co.uk/news/article-1227228/Giant-iceberg-spotted-Australia.html
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The icebergs are carried northward and westward at up to 8 km/day in the Easterly wind and currents around the continent until they reach the Antarctic Convergence, where they melt rapidly in warmer water. The icebergs are present in a band from 150 to 300 km from the Antarctic costs, particularly in the Weddell Sea and in the Ross Sea, near the Kerguelen Islands, flowing in the Western currents of circumpolar ocean. They are moved by the western currents of the Southern Ocean and for a transportation from the Amery ice shelf located in the AAT to WA, these currents are favourable. From the Weddell Sea they reach South Africa, Argentina, Australia or New Zealand and usually melt between 58° to 63° S (Figure 3.19a).
Figure 3.19a Antarctic Circumpolar Currents (credit from Antarctica cup37 n.d. n.p.)
These types of event are rare even if Tasmanian Aboriginal legends evoke "white islands" floating (Young in AAD, 200938). For the iceberg transportation project studied in this thesis
37
http://www.antarcticacup.com/images/currentsxl.gif see also GRID, New-Zealand, UNEP,
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the region of iceberg capture being around 2,000 km away from WA, the iceberg concentration is relatively high and about 100 of large icebergs per year can reach this zone. However their origins, sizes, shapes and states vary. In this zone, icebergs mainly come from the Ross, the Ronne or the Amery ice shelves and generally head towards the ACC with more southerly path ways (Figure 3.19b). This figure will be later used for transportation analyses to put them in parallel with icebergs natural pathways.
Figure 3.19b The Major Ocean Currents in the Australian Region (credit from Edmonds, 200739 n.p.)
In details, the AIS currents circulation is characterised by a ice front circulation 72.5° 8 and 74.08E which splits into a weak north eastern gyre of 72.08° E under the ice shelf, and continues to an east circulation (70.78° S, 70.48° E) and a strong main gyre 70.08° S, 71.08° E (2.8-Sv), to the west with southern gyre centred at 71.68° S, 69.88° E. regions of freezing,
38 http://www.antarctica.gov.au/media/news-archive/2009/giant-iceberg-heading-towards-australia, http://www.theaustralian.com.au/news/nation/giant-iceberg-heading-for-western-australia/story- e6frg6nf-1225808821794 39 www.waterengineeringaustralia.com.au/pdf/wea_0809.pdf
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in the northwest corner of the domain, and around 69.88° S, 69.98° E (Warner and Budd, 1998; Williams, Warner and Budd, 200240).