Glaciers are formed by thousands of years of snowfall forming ice. More than 10000 icebergs calve each year, mainly from about 20 major glaciers between the Jacobshaven and Humboldt Glaciers of western Greenland. These glaciers account for 85% of the icebergs which reach the Grand Banks of Newfoundland. Other sources of icebergs are the East Greenland glaciers, and ice shelves of northern Ellesmere Island.
Snow falls on the ice cap of Greenland. Then over the course of several months it changes into firn. Later it is compressed into very dense ice by the weight of the firn and snow. Gradually, the ice begins to flow seaward through openings in the fringe of the mountains. This force moves the rivers of ice, known as glaciers very slowly (a few inches per hour), eventually pushing the ice to Greenland’s western coast.
At the glacier’s terminus or end, huge slabs of ice are weakened and then broken by the action of the rising and falling tides and the gravity force of ice berg itself. This process is called calving and results in the birth of an iceberg.
Journey of Iceberg
This is a long trip and most icebergs never make it till the end (ice limit). Most icebergs melt well before entering the Atlantic Ocean. One estimate is that of the 15,000 to 30,000 icebergs produced annually by the glaciers of Greenland only one percent (150 to 300) ever make it to the Atlantic Ocean. When an iceberg does happen to reach the Atlantic its long and traveled life quickly comes to an end, melting rapidly in the warm waters. At most it will take two months to melt unlike icebergs stuck in parts of Baffin Bay where it may take about four years to melt. The mean number of icebergs passing south of 480 N is about 475 icebergs. Therefore, yearly totals are highly variable and are subject to highly variable climatic factors.
The ‘Iceberg Alley’ is located about 250 miles east and southeast of the island of Newfoundland, Canada. Iceberg Alley is usually considered to be that portion of the Labrador Current, which flows southward from the Flemish Pass, along the eastern edge of the Grand Banks of Newfoundland to the Tail of the Banks. This area extends approximately from 480 to 430 North Latitude at 480 degrees West longitude. Icebergs and sea ice flowing south from Iceberg Alley caused the Titanic disaster in 1912.
Icebergs from Antarctic
These calve from floating ice shelves and are a magnificent sight, forming huge, flat ‘tabular’ structures. A typical newly calved iceberg of this type has a diameter that ranges up to tens of kilometres, a thickness of 200-400 metres, and a freeboard, or the height of the ‘berg’ above the waterline, of 30-50 metres. The mass of a tabular iceberg is typically several billion tonnes. Floating ice shelves are a continuation of the flowing mass of ice that makes up the continental ice sheet. Floating ice shelves fringe about 30% of Antarctica’s coastline, and the transition area where floating ice meets ice that sits directly on bedrock is known as the grounding line.
Under the pressure of the ice flowing outward from the centre of the continent, the ice in these shelves moves seaward at 0.3-2.6 km per year. The exposed seaward front of the ice shelf experiences stresses from subshelf currents, tides and ocean swell in the summer and moving pack ice during winters. Since the shelf normally possesses cracks and crevices, it will eventually fracture to yield freely floating icebergs. Some minor ice shelves generate large iceberg volumes because of their rapid velocity; the small Amery Ice Shelf, for instance, produces 31 cubic km of icebergs per year as it drains about 12% of the east Antarctic Ice Sheet.
In the Antarctic, a freshly calved iceberg is led westward by the Antarctic Coastal Current. Since, its trajectory is also turned to the left by the Coriolis force, it may run aground and remain stationary for years before moving on. For instance, Trolltunga was calved from the Fimbul Ice Shelf in 1967 and it became grounded in the southern Weddell Sea for five years before continuing its drift. If a berg can break away from the coastal current (as Trolltunga had done by late 1977), it enters the Antarctic Circumpolar Current, or West Wind Drift. This eastward-flowing system circles the globe at latitudes of 400 – 600S. Icebergs tend to enter this current system at four well-defined longitudes or ‘retroflection zones’: the Weddell Sea, east of the Kerguelen Plateau at longitude 900E, west of the Balleny Islands at longitude 1500E and in the northeastern Ross Sea. These zones reflect the partial separation of the surface water south of the Antarctic Circumpolar Current into independently circulating gyres, and they imply that icebergs found at low latitudes may originate from specific sectors of the Antarctic coast.
Once in the Antarctic Circumpolar Current, the iceberg’s track is generally eastward, driven by both the current and the wind. Also, the Coriolis force pushes the berg slightly northward. The berg will then move in a northeasterly direction so that it can end up at relatively low latitudes and in relatively warm waters before disintegrating. Under extreme conditions, such as its capture by a cold eddy, an iceberg may succeed in reaching extremely low latitudes. For example, clusters of bergs with about 30 metres of freeboard were sighted in the South Atlantic at 350 50’S, 180 05’E in 1828. The icebergs have also been known to sink the ships off Cape Horn.
Icebergs in the Antarctic area sometimes have stripes, formed by layers of snow that react to different conditions Blue stripes are often created when a crevice in the ice sheet fills up with melt water and freezes so quickly that no bubbles form. When an iceberg falls into the sea, a layer of salty seawater can freeze to the underside. If this is rich in algae, it can form a green stripe. Brown, black and yellow lines are caused by sediment, picked up when the ice sheet grounds downhill towards the sea. The iceberg at certain stage must have been horizontal along the coloured layers.
The sinking of the Titanic led to many safety related issues including formation of ‘International Ice Patrol’ (IIP). IIP monitors the icebergs in the Northern Atlantic Ocean and reports their movement, providing a very useful service towards safety and is usable by the ships voyaging between Europe and the major ports of the United States and Canada. The service was initially managed by U. S. and the expenses were to be shared by the thirteen nations interested in Trans Atlantic voyages. From its inception until the beginning of World War II, the Ice Patrol was conducted from two surface patrol cutters alternating surveillance patrols of the southern ice limits. In the earlier phase, the ships were assigned to Ice Patrol to perform oceanographic observations in the vicinity of the Grand Banks. After World War II, aerial surveillance became the primary ice reconnaissance method with surface patrols phased out except during unusually heavy ice years or extended periods of reduced visibility.
Today the International Ice Patrol is located at the Coast Guard Research and Development Center in Groton, Connecticut.
IIP receives funding from Canada, US, Japan & European governments. IIP monitors iceberg danger in the vicinity of the Grand Banks and broadcasts the southeastern, southern and southwestern Limits of All Known Ice (LAKI). Because of frequent fog and poor visibility over the Grand Banks, IIP relies heavily on radar onboard the aircrafts for iceberg reconnaissance. Since 1983, IIP’s primary detection radar has been the aircraft with Side-Looking Airborne Radar (SLAR). In 1993, IIP added the aircraft with Forward-Looking Airborne Radar (FLAR) as an additional sensor. FLAR has a special ability to distinguish between icebergs and ships. Sometimes, FLAR fails to detect small and medium icebergs at ranges from which SLAR had routinely detected targets. Therefore, Ice Patrol uses the two radars together to form a much-improved sensor suited for iceberg reconnaissance. In 1983, Ice Patrol began to use the aircraft will Side-Looking Airborne Radar-an X-band (9250 MHz), for iceberg reconnaissance. The image was not available to the operator in real time because it required approximately five minutes to process the film. In 2001, the Coast Guard implemented a digital user-interface for the SLAR called the Maritime Surveillance System 5000. The MSS 5000 dramatically improved the usefulness of SLAR for iceberg reconnaissance by providing real-time return. Though, the resolution and clarity of the film image was considerably lost through data compression. A more than 10 knot speed on radar screen is a definite indication of a ship in comparison to an iceberg.
SLAR though, has an all-weather capability, it has a drawback about uncertainties in respect of detection and target-discrimination.
Ice Patrol searches for icebergs using a long-range surveillance aircraft that operates out of St. John’s, Newfoundland, Canada for seven days every other week. It takes approximately four flight days to investigate a 120 miles swath along the entire LAKI. Daily patrols are conducted using a parallel search pattern with 30 miles track spacing and the SLAR range set at 27 miles. Thus, SLAR gets two looks at most of the search area.
Later, Coast Guard began using the Forward-Looking Airborne Radar as a search and rescue target detector. FLAR is a X-band air-to-surface radar capable of Inverse Synthetic Aperture Radar (ISAR) mode and was ideally suited for the Ice Patrol environment. It is a high-power radar that integrates long-range detection and target-imaging capabilities into one system.
Combined FLAR/SLAR operations:
IIP uses a combined SLAR-FLAR-visual reconnaissance strategy. Essentially, Ice Patrol relies on SLAR (and the FLAR search mode to a lesser extent) for wide-area searching and employs the ISAR mode of FLAR to positively identify as many targets as possible. Of course, Ice Patrol prefers visual confirmation, but FLAR’s image mode alone enables Ice Patrol to identify most targets. However, in the event of no visual confirmation and ambiguous radar information, a patrol may choose to divert from searching and descend to facilitate visual identification. Currently, the two radars operate independently. The FLAR and SLAR radar repeaters are located next to each other, allowing for easy correlation of the radar information. This combination has been very successful and is a very effective iceberg reconnaissance system.
Sea ice charts are maps with areas containing ice of similar concentration and type outlined and labeled with a code. This code (known as the egg code, for its shape) has information on total sea ice concentration, ice type, information on ice stage of development, etc. A typical egg code used by Canadian Ice Service (CIS), Manual of Standard Procedures for Observing and Reporting Ice Conditions or MANICE is described below:
Total concentration: The ice coverage of an area is determined by its concentration and is expressed in tenths (in this example 9/10th).
Partial concentration: The break-down of the total ice coverage is expressed in tenths and is graded by thickness. The thickest starting from the left. In this example, 1/10th is the thickest.
Stage of development: The type of ice in each of the grades is determined by its age, that is 1/10th is medium first-year ice (1), 6/10th is grey-white ice (5) and 2/10th is new ice (1). Trace of old ice is represented on the left hand side (outside the egg) by the number 7.
Floe Size: The form of the ice is determined by its floe size for each section. In this example, big floes (5) for medium first-year ice (1); small floes (3) for grey-white ice (5); and undetermined, unknown or no form floes (x) for new ice (1).
Arctic vs. Antarctic
One interesting difference between the two icebergs types is that polar bears live only in the Arctic, and penguins live only in the Antarctic. Although sea ice moves around the Arctic basin, it normally remains in the Arctic waters. Floes tend to pile up into thick ridges. These converging floes make Arctic ice thicker. The presence of ridge ice and its higher thickness leads to ice that stays frozen longer during the summer melt. So, some Arctic sea ice remains through the summer and continues to grow the following autumn. Of the 15 million square kilometers of sea ice that exists during winter, on average, 7 million square kilometers remain at the end of the summer melt season.
Arctic icebergs are pyramid or hill shaped whereas the Antarctic icebergs are generally tabular. This is because the northern icebergs are of land origin formed after calving of river glaciers. The Antarctic glaciers generally form after a bay is frozen. Antarctic sea ice forms ridges much less often than sea ice in the Arctic. There is no land boundary to the north, the sea ice is free to float northward into warmer waters where it tends to melts.
Antarctic icebergs generally are symmetric around the pole, forming a circle around Antarctica. In contrast, the Arctic the pattern is asymmetric, with much more ice in some longitudes than others. For example, sea ice off the eastern coast of Canada extends south of Newfoundland to 500 north latitude, and ice off the eastern coast of Russia extends to Bohai Bay, China, at about 380 north latitude. Conversely, in western Europe, the northern coast of Norway at 700 north latitude.
Antarctic sea ice does not reach the South Pole, extending only to about 750 south latitude (in the Ross and Weddell Seas), because of the Antarctic continent. However, Arctic sea ice can extend all the way to the North Pole.
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