Calving of Iceberg B15 from Ross Ice Shelf
Neal Young, Antarctic CRC and Australian Antarctic Division 28 March 2000 Introduction The calving of an immense iceberg from the Ross Ice Shelf has been reported by the Antarctic Meteorological Research Center at the University of Wisconsin – Madison. Matt Lazzara identified the calving in images acquired by the AVHRR instrument on NOAA’s meteorological satellites, on 20-24 March 2000. An image received at McMurdo station shows the calving had probably already commenced by 17 March 2000. More information Various images can be seen here Various press releases have been posted including one from the US National Science Foundation The iceberg has been named “B15” by the (US) National Ice Center. Some information on Antarctic icebergs can be found at the National Ice Centre Dimensions of B15 and largest icebergs The dimensions of iceberg B15 have been assessed using the AVHRR images as 170 miles long by 25 miles wide, which translates to 295 km long, 37 km wide (on average). These are approximate dimensions. The area is estimated to be about 11,000 sq. km. This is certainly the longest iceberg on record and probably the largest iceberg by area in the modern, satellite era where images from satellites are used to monitor the distribution of icebergs. The previous largest iceberg was A20 (approx 95 km by 95 km) that calved from southern Larsen Ice Shelf in early 1986. Thickness of B15 is estimated to range between about 200 m toward the old front and 350 m on the inland margin. Thickness immediately at the old front is probably less than 200 m. Height above sea level of the old front of the Ross Ice Shelf, which nows forms one of the long sides of B15, has been measured to be between 20 m and 40 m. The other long side of B15, which was along the rift in the interior of the shelf, could be as much as 40 to 60 m in height above the ocean surface. Ross Ice Shelf front and previous calving events The front of the Ross Ice Shelf, the seaward margin of the ice shelf, is currently at its most advanced state for any time this past century, and, except for the more easterly section, the most advanced since 1841. A review of this information can be found in Keys, et al. in Annals of Glaciology Vol. 27:93-98 (1998). In the sector where B15 is calving, the front is advancing outwards with the flow and spreading of the ice at a rate of about 900 m/year. So, the 37 km width is equivalent to about 40-45 years of advance of the ice shelf front. The most recent major calving was of iceberg B9, 154 km by 35 km, from the sector east of Roosevelt Island, neighbouring the Bay of Whales, and immediately east of the calving site of B15. B9 calved in 1987, and took 20 months to drift away from the front of the ice shelf, and then across the Ross Sea. See: Keys et al. Antarctic Science 2(3):243-257 (1990). Cause of calving The calving of B15 is a natural consequence of the development of an ice shelf. Snow which has fallen on the surface of the ice cover, compacts and forms ice as further snow accumulates on top. The ice gradually flows outwards till it crosses the margins of the grounded ice cover. In many areas this ice flows into ice shelves which are the floating parts of the ice cover. Ice is lost from the ice sheet by calving of icebergs from the outer margins and by melting from the basal surface. The rate of loss from the margins roughly balances the input of snow to the surface. In satellite images it is possible to see rifts forming in the Ross Ice Shelf many kilometres inland from the margin, and running parallel to the margin. These rifts typically develop and extend over time till an iceberg breaks off. The rifts which form the “calving front” for B15 could be clearly seen in images acquired in September 1997 over a length of about 240 km. The precursors to these rifts were identifiable many years before this. The calving of B15 could thus be anticipated, but the exact timing is very difficult if not impossible to predict. The calving of B15 is the consequence of a natural progression of events that occur in ice shelves, and is quite unrelated to “Global warming” or “Greenhouse” effects. Calving events on ice shelves and glacier tongues may occur frequently and produce a few or many small icebergs, or occur rarely and produce one or a few very large icebergs. It is common for many smaller fragments to be produced in a calving event. The average annual turnover of ice in the Antarctic continental ice cover is of the order of 2,500 Giga-tonnes (2,500 cu. km of water). Th volume of water in B15 is roughly estimated to be equivalent 70% of the average annual mass input/loss terms. B15 drift track and likely behaviour Based on the drift track of B9, the most likely scenario is that B15 will gradually move away from the ice shelf and drift north-west across the Ross Sea, passing Cape Adare, then drifting with the “East Wind Drift” to the west round Antarctica close to the coast, probably in the continental slope current which is located roughly above the upper break in the continental slope. It is likely that B15 will break into smaller sections, each immense in itself. Any of these may become grounded on shallower parts of the continental shelf. The passage of B15 around Antarctica may take many years. Eventually it will join an ocean current moving northwards away from Antarctica and into the southern part of the Antarctic Circumpolar Current which will carry it eastwards. A second, but less likely, scenario is for B15 to drift mainly northwards through the Ross Sea then progressively eastwards, joing the ACC and eventually passing through the Drake Passage into the South Atlantic. From there it may drift more northwards or even continue to drift around Antarctica with the ACC. Iceberg B10A followed a path like this over a period of six years, from its source at the Thwaites Glacier (approx 110W) passing through Drake Passage in 1999. In March 2000, it is north of the island of South Georgia. Once it moves into warmer waters the iceberg may take many months or even a year or two to break up and melt. This estimate of the timing is quite speculative because it depends on many factors, including ocean temperature, etc. It is unlikely that there will be considerable melt while the iceberg(s) remains in the cold waters close to the coast. The immense length of B15 makes it quite unlike other icebergs, and difficult to predict its behaviour. Iceberg naming convention used by NIC The nomenclature applied by NIC to naming of icebergs divides the Antarctic region into four quadrants, and designates each quadrant with a letter A to D. The A quadrant is from the Greenwich Meridian to 90W, and spans the Atlantic sector; quadrant B, Pacific sector from 90W to 180; quadrant C Australian sector from 180 to 90E; quadrant D, Indian sector from 90E to Greenwich Meridian (0). The NIC tracks each iceberg greater than 10 nautical miles in length. Each iceberg is designated by the letter corresponding to the sector of its first sighting, and a number which is the next in the sequence. So B15 is the fifteenth iceberg satisfying the NIC criterion to be first identified in the B sector (90W-180). The assumption is that the iceberg has been sighted soon after it has calved from the margins of the ice cover. When one of the tracked icebergs splits into two or more fragments, each fragment is given the designation of the parent iceberg and an additional letter, so B9 split into three icebergs satisfying the criterion: B9A, B9B, B9C.