ARCHIVED - CHAPTER 3: Ice and Weather Environment
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3.3 Icebergs, Bergy Bits, and Growlers
Bergy bit: A piece of glacier ice, generally showing 1 to less than 5 m above sea-level, with a length of 5 to less than 15 m. Normally about 100-300 sq. m in area.
Calving: The breaking away of a mass of ice from a glacier, ice wall, ice front, or iceberg.
Growler: Smaller piece of glacier ice than a bergy bit, often transparent, but appearing green or almost black in colour, extending less than 1 m above the sea surface. It has a length of less than 5 m and normally occupies an area of about 20 sq. m.
Iceberg: A massive piece of ice of varying shape, protruding 5 m or more above sea-level, which has broken away from a glacier, and which may be afloat or aground. May be described as tabular, domed, pinnacled, wedged, drydocked, or blocky. Sizes of icebergs are small, medium, large, and very large.
Ice island: Floating piece of ice broken from an ice shelf, often 40-50 m in thickness, with a higher freeboard than sea ice, and exhibiting undulating surface rolls.
Icebergs and ice islands differ from sea ice in that they represent extreme local hazards to navigation, rather than the limited but widespread problem offered by sea ice. Severe damage can result from hitting glacial ice.
WARNING: THE GLACIAL ICE OF ICEBERGS AND ICE ISLANDS IS VERY HARD. THEY SHOULD BE GIVEN A WIDE BERTH.
Icebergs are a common feature of Arctic waters, along the Labrador coast, and on the Grand Banks of Newfoundland. Icebergs (Figure 14) differ from sea ice in that they are formed from freshwater ice originally on land. They form when pieces of glacier ice break off or calve into the sea. A second type of floating glacial ice is created when fragments calve from ice shelves along the northern coast of Greenland and the Arctic Archipelago, particularly Ellesmere Island. The floating pieces of ice are known as ice islands. They are mainly found in the Arctic Ocean, Beaufort Sea, and channels of the Archipelago. Occasionally ice islands have reached the eastern Arctic.
Almost all icebergs found along the east coast of Canada originate from the glaciers of west Greenland (Figure 14). Most of the active glaciers along the west Greenland coast are located between Smith Sound and Disko Bay. Melville Bay, from Cape York to Upernavik, is a major source of icebergs; it is estimated that 19 active glaciers produce 10,000 icebergs annually. A second area of importance is Northeast Bay, including Karrats and Umanak Fiords, where about 5,000-8,000 icebergs are calved from 10 major glaciers each year. Disko Bay also produces a small number of icebergs from two glaciers.
A few Canadian glaciers on Baffin, Bylot, Devon, Coburg, and southern Ellesmere Islands calve icebergs, but only in small numbers. The annual production of icebergs from Canadian glaciers is estimated to be about 150.
Total annual production of icebergs in Baffin Bay is estimated to be 25,000-30,000, although some estimates are as high as 40,000. More than 90 per cent of the icebergs come from west Greenland glaciers.
The size of icebergs calved varies from growler size (about 20 sq. m with 1 m above water) to icebergs 1 km long and over 200 m high. The height-to-draught ratio of an iceberg varies from 1:1 to 1:3 for pinnacled icebergs, to 1:5 for blocky, steep-sided tabular icebergs. A study of icebergs in Davis Strait suggested that a ratio of 1:4 was a good approximation for estimating iceberg size. If the height of an iceberg is 100 m it would not be unreasonable to expect a draught of 300 to 500 m. As a result of their substantial draught, even smaller icebergs frequently become grounded in coastal waters and on shoals.
An important consequence of the substantial draught of an iceberg is that its drift is strongly influenced by ocean currents, as well as winds. The relative importance of winds and currents on iceberg drift depends on the area and mass exposed to winds and currents and the relative strength of each. Icebergs calved from glaciers on the west Greenland coast usually drift northward (see Figure 14) at a rate of 3 to 5 nautical miles per day, before being carried westward across northern Baffin Bay. From there, currents along east Baffin Island carry the icebergs south to the Labrador Sea and onto the Grand Banks of Newfoundland. Along Labrador, drift rates of 10 nautical miles per day are not uncommon.
Whereas the main drift path is anticlockwise in Baffin Bay, it is not uncommon for icebergs to be carried westward across Baffin Bay by smaller current streams which branch off from the West Greenland current. Iceberg drift is seldom direct, with icebergs frequently following lesser currents into bays and inlets. In particular, numerous icebergs are drawn into Lancaster Sound, moving westward through the Sound as far as 85°W. Icebergs also drift southward into Navy Board Inlet and eastward to Pond Inlet. Similarly, icebergs are sometimes carried into Hudson Strait south of Baffin Island. Icebergs have been observed as far west as Big Island, probably in response to strong tidal flows.
Occasionally icebergs enter the Gulf of St. Lawrence, passing through the Strait of Belle Isle. These icebergs are generally small, as the water depths in the Strait (55 m) limit iceberg draught. Most icebergs entering the Gulf tend to go aground along the Quebec shore, east of Harrington Harbour, although a few have been observed as far west as Anticosti Island and in the Bay of Islands area along the west Newfoundland coast. A considerable number of icebergs can remain grounded in the Strait of Belle Isle.
It is estimated that an iceberg travels between 2,700 and 3,700 km from its place of calving to reach the Grand Banks of Newfoundland. Based on estimated current speeds, an iceberg calved in Melville Bay could complete the trip in one year. It is more likely that it would not remain in the main current and a more realistic estimate of the travel time is two to three years.
As icebergs drift, they become smaller through melting and calving of ice fragments. Calving is frequent and, by exposing more ice surface to the water, encourages greater melting. Melting occurs both above and below the waterline. As water temperatures vary with depth, it is possible to have melting of the iceberg near the water surface, but not melting at greater depths, where temperatures may be lower than the 0°C required to melt freshwater ice. Combined with surface melting, an iceberg's centre of buoyancy can change, resulting in unstable conditions and rolling of the iceberg. Icebergs encountered off Newfoundland are generally more deteriorated and unstable than icebergs further north. It is not uncommon for an iceberg to roll up to several times a day. Therefore, it is very important for vessels to steer a wide berth around an iceberg in case it rolls.
Based on studies of decaying icebergs, the U.S. Coast Guard International Ice Patrol has developed simple approximations of the deterioration times for icebergs of different sizes, under various water temperature conditions. These are shown in Table 7.
a Small iceberg - under 15 m high, under 45 m long
b Medium iceberg - 15-30 m high, 45-90 m long
c Large iceberg - over 30 m high, over 90 m long
d For tabular bergs the height limits differ: less than 6 m for small, 6-15 m for medium, and more than 15 m for large icebergs.
The melt rate for icebergs in Arctic waters is slow, but, even so, it is unlikely that more than 20 to 25 per cent of the icebergs calved from Greenland glaciers reach western Baffin Bay. It is estimated that half of these melt before entering Davis Strait, and only 20 per cent of the remainder will complete the drift to the Grand Banks.
In an average year, about 300 icebergs drift south of 48°N, but there is considerable year-to-year variation in this number. Based on International Ice Patrol observations, the total number of icebergs crossing 48°N has varied from a high of 1587 icebergs in 1984 to a low of no icebergs in 1966. Figure 15 shows the annual variability between 1955 and 1984.
Icebergs drift all year, although when in winter pack ice their drift rate is slowed. As the sea-ice cover along the Labrador and Baffin coasts deteriorates, icebergs move more freely. Within a given year, most icebergs cross 48°N between March and June. On average in past years, almost two-thirds of the icebergs have been observed in April.
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