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Monster icebergs calve off Antarctica's
Ross Ice Shelf
Event not linked to global warming
say leading glaciologists
April, 2000
The largest iceberg to calve off the Ross Ice Shelf in the history of modern satellite imaging is making heads turn right 'round the world, while several of its smaller brethren are joining the drift around Antarctica.
Named B15 by the US National Ice Centre, it measures 37km wide, on average, by 290 km long and spans 11,000 square kilometres in area. That's as big as Jamaica. Yet, the great white wonder represents just two per cent of the total Ross Ice Shelf.
But don't expect it to melt any time soon. B15's older brother, B9 (154km by 35 km), calved off the Ross Ice Shelf 12 years ago and bits - large bits - are still grounded or drifting around Antarctica today.
B15 is being called the largest berg on record, although some argue such a title belongs to an unsubstantiated report from a US icebreaker on an expedition to the Antarctic in 1956. The USS Glacier reported a monster berg 208 miles long and 60 miles wide in November of that year.
Within three weeks of calving from the shelf, beginning on March 17, B15 was joined by three others: B16, B17 and B18. Within days B17 began to break up into smaller fragments. The four bergs removed more than 400 km of ice along the shelf's northern edge. Considering the main front of the Ross Ice Shelf totals about 750km, the calving event of late is significant indeed.
Scientists, however, say these events do not herald a new phase in global warming.
"The word before these bergs calved was that the Ross Ice Shelf was at its most northerly advanced state ever seen over the past century," said Neil Young, Australian glaciologist with the Antarctic Co-operative Research Centre. "And it is known that the ice front was further south during that time."
It is normal, he says, for the ice shelf to grow over time and it is normal for icebergs to break off along its edge.
"Ice shelves are forever renewing themselves through ice flowing out from the grounded ice sheet," Young said. "And it is typical of the Ross Ice Shelf that large icebergs calve at frequent intervals."
The recent activities of the shelf stand in contrast to the retreat and breakup of ice sheets on the Antarctic Peninsula, eight degrees or more further north, where regional warming is being blamed for the irreversible
decline of permanent ice. In polar regions, eight degrees of latitude accounts for significant temperature differences.
Which way will it go?
First of all, no one with a Ph.D. is game to say anything for certain. Work on B15 and its siblings is in its infancy, with Professor Doug MacAyeal at the University of Chicago involved in
number-crunching predictions.
Based on previous bergs - such as B9 which calved in 1987, and the much smaller B10a, which threatened shipping in the Drake Passage last year - these new bergs will be influenced somewhat by wind, but predominately by ocean currents. Don't forget that for bergs of the size we are talking about, average depth is between 200 and 300
metres!
Young suggests that the recent bergs will eventually drift north west across the Ross Sea and around Cape Adare moving westwards along the coast with the East Wind Drift.
This current is driven in part by a combination of the dominant, anti-clockwise-spinning high pressure cell over the Antarctic continent and the circumpolar trough to the north, with its countless clockwise spinning low pressure cells. Together they create easterly winds along the Antarctic coast which help to drive the upper part of the ocean there.
Within the East Wind Drift, over the edge of the continental shelf, is a continental slope current - 'like a core moving faster than the rest', explains Young. If bergs get in that, they can move up to 30km a day, traveling west around Antarctica.
And to the north of the East Wind Drift lies the Antarctic Divergence: A boundary where cold polar water meets warmer northern waters. To the north of that, is the Antarctic circumpolar current. The largest current system in the world's oceans.
If some of the bergs get caught in that powerful stream they will drift through the Drake Passage and into the Atlantic.
Whatever happens, scientific eyes on the ground will be using their satellite 'eyes' in the sky to tell them what the heck is going on.
This satellite image shows B15 and
the much smaller B16, to the left, soon after calving in late march.
Eye-in-the-sky imagery, such as this, has enabled scientists to view calvings
like never before.
By early April, the movement of
B15 has helped neighbouring bergs B17 and B18 to calve from the shelf as well.
B16 has already moved to the north west with tidal currents.
Did you know?
-
Ice bergs roam the Southern Ocean like a plague of locusts, with 200 to 300,000 of them in existence. They range from monsters like B15 to 'small' ones just 50 metres across. Below that size are many, many more nasty chunks called
'bergy bits' - typically the size of a small Japanese car - which can cause terminal damage to yachts and even small ships because they float just below the water's surface. The keenest of lookouts are unlikely to spot a bergy bit until it is too late. However, most bits float downwind of larger icebergs, so skippers look for the bergs and keep them to leeward.
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B15 is named as such because B is the quadrant of the Antarctic facing the Pacific and 15 represents the fifteenth berg classified by the US National Ice
Centre. To be classified bergs must be longer than 10 nm (18km) and satisfy certain criteria. 'A' quadrant faces the Atlantic, 'C' quadrant faces Australia and 'D' faces the Indian Ocean.
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B15 is ten times larger than B10a,
which threatened shipping off Cape Horn last year. B10a has been on the move for six years, since it calved from Thwaites Glacier, and as of March 2000 was to the north of South Georgia.
-
The shelf represents a mixing of many glacial sources, coalescing to form a continuous mass. From space it appears smooth - certainly smoother than the continental ice 'pouring' down from the Antarctic plateau, but on the surface it is varied, containing pressure ridges and large cracks. Below the surface its depth varies from 800m near the edge of land to 150m at its seaward edge.
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In the section of the ice shelf where B15 calved, the front is advancing at a rate of about 900 m/year, says Neil Young. As a result, the 37km width of berg B15 is equivalent to about 40-45 years of movement at the ice shelf front.
For more information:
The University of Wisconsin at Madison has a heap of great photos and information
tid-bits. It is a great starting point.
Neil Young has written an excellent story about B15 and the Ross Ice Shelf in the
Antarctic Co-operative Research Centre's web
page, based at the University of Tasmania. There are many links to other interesting sites as well.
The US National Ice Center is in charge of naming large bergs.
The US National Science Foundation is
at the core of this project.
© weather-wise.com 2000
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The arrow points to the approximate position
of icebergs recently calved from the Ross Ice Sheet
This satellite image shows B15 soon after it
calved in March.
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