|
|
News  Giant iceberg B-15A edges past floating ice pier
Envisat radar imagery confirms that the B-15A iceberg – the world's largest floating object – is adrift once more after two months aground on a shallow seamount. This latest development poses a renewed threat to the nearby pier of land-attached ice known as the Drygalski ice tongue. The sheer scale of B-15A is best appreciated from space. The bottle-shaped Antarctic iceberg is around 120 kilometres long, with an area exceeding 2500 square kilometres, making it about as large as the entire country of Luxembourg.
Back in Janauary the iceberg appeared to be drifting towards the 70-kilometre-long Drygalski ice tongue in McMurdo Sound on the Ross Sea, and an unprecedented ice collision looked imminent.
In early March local tides and currents lifted B-15A free from its temporary resting place, an event coinciding with numerous fragments of ice seen breaking off from the centre of its coastward side as the iceberg was worked loose.
Now prevailing currents are transporting it into deeper and out of McMurdo Sound, right past the far end of the Drygalski ice tongue. The latest Envisat satellite image shows the two ice masses only a few kilometres apart.
"It was now achieved a critical overlap with the end of the Drygalski ice pier, so far without touching. It would now appear that any contact – if at all – between the drifting iceberg and the land-fast floating ice tongue is likely to be a consequence of being 'brushed' or 'bumped' by the broader trailing end of the iceberg, much like the wide turns made by a long trailer behind a truck or the stern of a ship."
"As long as the rear end of the iceberg remains pinned to its west by the shallow bottom topography, a collision may remain less likely," Drinkwater states. "A lot now depends on the ability of the tides and local currents to free the southernmost end of the iceberg, and to close the gap between the iceberg and the ice tongue."
B-15A is the largest remaining section of the even larger B-15 iceberg that calved from the Ross Ice Shelf in March 2000. Equivalent in size to Jamaica, B-15 had an initial area of
Since then, B-15A has found its way to McMurdo Sound, where its presence has blocked ocean currents and led to a build-up of sea ice. With the Antarctic summer now at an end and in-situ observations therefore limited, the ASAR instrument aboard Envisat becomes even more useful for monitoring changes in polar ice and tracking icebergs.
The Drygalski ice tongue is located at the opposite end of McMurdo Sound from the US and New Zealand bases.
Large and (considered) permanent enough to be depicted on standard atlas maps of the Antarctic continent, the long narrow tongue stretches out to sea as an extension of the land-based David Glacier, which flows through coastal mountains of Victoria Land.
Envisat's ASAR instrument monitors Antarctica in two different modes: Global Monitoring Mode (GMM) provides 400-kilometre swath one-kilometre resolution images, enabling rapid mosaicking of the whole of Antarctica to monitor changes in sea ice extent, ice shelves and iceberg movement. Wide Swath Mode (WSM) possesses the same swath but with 150-metre resolution for a detailed view of areas of particular interest.
ASAR GMM images are routinely provided to a variety of users including the US National Oceanic and Atmospheric Administration (NOAA) National Ice Centre, responsible for tracking icebergs worldwide.
The Northern View recently extended this service to the Southern Ocean for the first time, to ensure that participants in the Oryx Quest 2005 yacht race stayed safely away from icebergs and ice fields.
CryoSat, in connection with regular Envisat ASAR GMM mosaics and SAR interferometry – a technique used to combine radar images to measure tiny millimetre-scale shifts between acquisitions - should answer the question of whether the kind of ice-shelf calving that gave rise to B-15 and its descendants are a consequence of ice sheet dynamics or other factors.
Together they will provide insight into whether such iceberg calving occurrences are becoming more common, as well as improving our understanding of the relationship between the Earth's ice cover and the global climate.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
