Iceberg B17B in the Southern Ocean

Latest known position of largest section of B17B: 31-Dec-2009 Latitude: 49 35.2 S Longitude: 113 57.5 E Area: About 43 sq km Length: 14 km Width: 3.2 km B17B, a very large Antarctic iceberg, has drifted north and east across the Southern Ocean into the south Indian Ocean on a path from near Antarctica towards Western Australia during 2009. It is now heading eastwards in mid-latitudes, about 1700 km from the Australian coastline. Over the year, it has reduced in size from more than 400 sq km to 200 sq km, and now (December 2009) to about 115 sq km, by some large, and many smaller, icebergs calving from its edge. Its drift track has taken it across the Antarctic Circumpolar Current from cold Antarctic waters to much warmer waters. The actual track roughly follows the pattern of the ocean current, but its movement is also influenced by the effect of eddies in the current and on the boundaries of the current streams, as well as the cumulative effect of wind which itself varies with the passage of storms. Sighting of a very large iceberg at mid-latitudes is very rare for the Indian and Pacific sectors of the Southern Ocean. But, it is not unusual in that icebergs do drift into the area from time to time. We are seeing icebergs now at mid-latitudes in the Indian Ocean sector because the parent iceberg that drifted away from Antarctica, B17B, is so large and has stayed largely intact over the many months of its slow progress to lower latitudes. Its sheer size has probably contributed to its longevity by acting as a self-preservation mechanism. Its initial large thickness of perhaps 300+ m, and thus its deep draft, has meant that the base of the iceberg has been in contact with relatively cold water for much of its drift track. Its very large area may also have helped by limiting the mixing of water in the thin layer immediately under the berg which could help to insulate it for a time from warmer water. But it has been slowly thinning and so its base is becoming shallower and shallower. The warmer water temperature it now encounters also causes the melting rate of ice from the base of the iceberg to increase markedly, such that the berg is thinning faster. At some stage, as the ice thins, the main berg will breakup into many smaller bergs and these will rapidly dissipate. This will also occur for other large bergs that have calved from this parent iceberg. All these icebergs that have calved from the main sections of B17B have dispersed across an extensive area of the ocean of perhaps 1500 km east-west by about 500 km north-south. The ultimate breakup and dissipation of the icebergs can be anticipated, but the actual timing of these events cannot be predicted. So how long the icebergs will last and where their drift tracks will take them is largely unknown and very dependent on how long the larger bergs remain whole. By the 31 December 2009, the main part of B17B has broken into three large sections and several smaller sections. The breakup process appears to be well underway, but how long it will ultimately take is still a guess. History of B17B The iceberg B17B began its life in early 2000 when massive icebergs calved from the front of the Ross Ice Shelf. One of these was B17, which came from the eastern end of the shelf front. Although it was massive in itself, it was dwarfed by its much larger cousin, B15, which was frequently reported in the media. About two months after calving, B17 split into two large sections: B17A and B17B. It is the remnant smaller section, B17B, that is now of interest. There are other massive bergs that came from the Ross Ice Shelf, either by calving in 2000 or 2002, or forming when these parent icebergs split. All these massive bergs and their progeny have slowly drifted out of the Ross Sea and then to the west round Antarctica close to the coast in a current (the "coastal" or "slope current" that sits above the outer edge of the continental shelf) or more broadly in the "East Wind Drift". Most of these bergs spent up to several years locked together in fast ice (meaning "land-fast" ice) in an area to the east of Mertz Glacier situated directly south of Tasmania. Gradually, indivdual bergs eventually got free of the fast ice and re-commenced their westward drift round the continent. B17B spent about 5 years locked in the ice. By early 2008 it had drifted a third of the way round Antarctica past the West Ice and into Prydz Bay north of Amery Ice Shelf and the Australian Antarctic station, "Davis", where it wandered about for a year before heading north away from these seas adjacent to Antarctica. B17B has exhibited a twisting and looping path as it has responded to the effects of current, eddies, and wind. By early 2009, B17B had an area of 450 sq km. In April, it split into several sections, the largest of about 200 sq km. It retained that size for another 6-7 months till 05 November when it again began to calve icebergs from its margins. Iceberg "names" Very large icebergs with a length greater than 10 nautical miles (about 18 km) are given a designation or label by the (US) National Ice Center. It is a nomenclature they adopted when they started regular  reporting of iceberg positions in the Antarctic over two decades ago. Icebergs are identified and positions are assessed using satellite images usually on a once weekly basis. The first letter of a  name indicates which of four quadrants where the iceberg was first sighted and hence where it is assumed to have initially formed/calved. The number indicates the "nth" iceberg to have been identified for that quadrant. The trailing letter indicates sections that have calved from a parent iceberg that still satisfy the length criterion. Thus B17A and B17B are the two sections calved from B17.  Thereafter, when a larger berg such as B15A splits, the largest remaining section retains the parent name, and smaller sections get the next letter in the alphabetic sequence for the original iceberg (e.g. B15). Iceberg B15, the longest ever observed, has split into many sections: B15A, ¼, B15R and so on. B15A still exists as a massive iceberg and the largest in Antarctica. The NIC ceases to track an iceberg once its length is less than 10 nautical miles, or it drifts out of the Antarctic region, or it is not observed for some period and not re-identified.

Authorised by the CEO of the Antarctic Climate and Ecosystems Cooperative Research Centre October 2019.

The ACE CRC was established and supported under the Australian Government’s Cooperative Research Centres Program.

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