Growlers, Bergy Bits, and Behemoths: The Many Types of Icebergs and How They Are Predicted

April 15, 2020, 1:24 AM EDT

Above: Icebergs and ice float in the Ilulissat Icefjord on August 04, 2019, near Ilulissat, Greenland. (Sean Gallup/Getty Images)

Icebergs—the huge chunks of ice that peek above the surface of the ocean water—are the stuff of legend. One of the greatest nautical disasters, the sinking of the HMS Titanic back on April 14-15, 1912, was a result of a collision with an iceberg. But did you know there is an iceberg season, and there are “iceberg alleys”?

There are favored locations at high latitudes in both the Northern and Southern Hemisphere where icebergs are “born” and begin their journey in the world’s oceans. For the region along the east coast of North America from Greenland to Newfoundland, the ice season runs from about February 1 through July 3. That region, including the west coast of Greenland, is the birthplace of most of the icebergs that find their way to the main transatlantic shipping lanes between North America and Europe.

Before I delve further into all of that. however, let’s review what an iceberg is. According to the U.S. Coast Guard, “an iceberg is a floating mass of fresh water ice extending more than 5 m [16.4 feet] above the sea surface. It may originate from a glacier flowing directly to the sea, such as the tidewater glaciers of Greenland, or from an ice shelf, such as those found in Antarctica.” Icebergs are different from sea ice. The most basic difference is that sea ice forms from salty ocean water, whereas icebergs and their parent glaciers form from fresh water or snow.

The life cycle of an iceberg is fascinating. It begins with snowfall that builds up on an ice cap like the one over Greenland. Over several decades the snow gets compressed by layer upon layer that accumulate to form very dense ice. As a result of all of that weight, the ice begins to move under the forces of gravity, from the high plateau toward the sea through the various mountain passes. That slow flow of ice toward the sea is what we know as a glacier. At the edge of the glacier is where the birthing process or calving of the iceberg occurs, as it breaks off the edge and drops into the sea. This entire process may take as much as 3000 years.

Icebergs are part of the big picture of ice balance for the Greenland Ice Sheet. Huge amounts of ice are lost from Greenland by calving as well as melting, and huge amounts are added by snowfall. It's the balance between these that determines how much ice is gained or lost in a given year. A project led by NASA and the European Space Agency called IMBIE (Ice Sheet Mass Balance Inter-comparison Exercise) used data from 13 NASA and ESA satellite missions from 1992 to 2018 to create the most accurate measurements of ice loss to date. NASA reported in December that "half of the loss is tied to surface ice melting in warmer air. The rest of the loss is the result of factors such as warmer ocean temperatures, iceberg calving, and the ice sheet shedding ice into the ocean more quickly."

The range of iceberg size

Icebergs cover a huge spectrum of sizes, from the tiny “growlers” (less than 3 ft tall and 15 ft across, or smaller than the U.S. Coast Guard definition above) and “bergy bits” (up to the size of a small cottage) to very large icebergs that can be bigger than a Caribbean island. Back in August 2010, a huge piece of Greenland's Petermann Glacier broke off. This iceberg, or ice island as it was called, was over 4 times the size of Manhattan Island, with an area of about 95 sq mi. It gradually moved and eventually broke into several pieces over the course of the next six weeks.

Below I have an animation of MODIS polar orbiter satellite Arctic Mosaic images on days when it was clear enough to see the huge iceberg. The August 2010 Petermann Glacier calving event created the largest iceberg observed in the Arctic since 1962, when the Ward Hunt Ice Shelf on the north coast of Canada's Ellesmere Island calved off a massive 230-sq-mi chunk.

Antarctic icebergs are often much bigger than those in the Arctic. The iceberg referred to as B15 calved from the Ross Ice Shelf in 2000 and was estimated to have an area of roughly 4000 sq. mi. (about twice the size of Delaware). This is the largest iceberg measured by satellite on record. Prior to the satellite era, when iceberg sizes were more difficult to establish, an Antarctic iceberg in November 1956 was estimated as encompassing an area of roughly 12,000 sq mi—larger than Belgium. This iceberg was sighted in the South Pacific Ocean in November 1956 by the aptly named USS Glacier!

Once an iceberg enters the ocean, its journey is driven primarily by ocean currents. This occurs because roughly 7/8 of a typical iceberg’s mass is below the waterline. The other 1/8 of the iceberg sits above the surface, and therefore winds have much less effect on its movement compared to the ocean currents.

When you observe an iceberg, it typically looks white in color. It takes on that color because sunlight is scattered by the millions of air bubbles frozen into the ice itself. Sunlight can’t penetrate very far into the iceberg before all of those bubbles scatter the light through all of its visible wavelengths, resulting in the white appearance to the human eye. However, icebergs can appear in shades of blue or even green, especially older icebergs, often composed of ice that has been under pressure for some time. The pressure removes any air bubbles in the ice that could reflect sunlight. As a result, the solid ice absorbs the red color of the visible light spectrum and greens and blues are reflected back to the human eye.

There are favored paths that icebergs traverse as they calve from glaciers on both the Northern and Southern Hemisphere and head to warmer ocean waters where they eventually melt. Scientists refer to these areas as “iceberg alleys.” In the Northern Hemisphere, Iceberg Alley refers to the region of the Atlantic that runs from the coastal waters off Labrador south through Newfoundland and further south into the region known as the Grand Banks, which coincidentally runs right through the major transatlantic shipping lanes from North America to Europe.

Many of the icebergs that calve off the Greenland glaciers take a very interesting, and often long-lived, journey before they end up off Canada’s Newfoundland Coast and into the major transatlantic shipping lanes. Off the west coast of Greenland they are first steered northward by the West Greenland Current. Then they reverse course through the Davis Strait toward Baffin Island, as shown below, and continue south along the east coast of Labrador, steered by the aptly named Labrador Current toward Newfoundland and those major transatlantic shipping lanes.

The overall trip from Greenland down to the shipping lanes can take up to three years to complete. It’s an arduous journey as well. Many icebergs break up in churning sea long before they make it to the shipping lanes, grinding against sea ice and shorelines and melting at an accelerating pace as they get farther south. According to the U.S. Coast Guard Navigation Center (NAVCEN), “It has been estimated 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 North Atlantic shipping lanes.”

As noted above, glaciers in the Northern Hemisphere’s Iceberg Alley that move from Greenland past Newfoundland and to the transatlantic shipping lanes follow an “ice season”. The Newfoundland ice season runs from February 1 through July 31. During this time the U.S. Coast Guard International Ice Patrol monitors the area of the Grand Banks of Newfoundland for icebergs. The story of the beginnings of the International Ice Patrol goes back to that cold, moonless night of April 14–15, 1912, when the Titanic sailed into the path of an iceberg about 400 miles south of Newfoundland. Soon after the Titanic sank, the International Ice Patrol (IIP) was established to track icebergs and warn ships, and that patrol continues today.

The IIP uses a combination of observations from satellites and Coast Guard aircraft to monitor the movement of icebergs. The North American Ice Service (NAIS)—a partnership comprised of the IIP, the Canadian Ice Service, and the U.S. National Ice Center—provides safety information on iceberg and sea ice conditions in the North Atlantic Ocean. This includes a chart of the daily NAIS Iceberg Limit as well as a text Iceberg Bulletin and a graphic Iceberg Chart to advise mariners of the estimated iceberg extent within the region.

On the charts below, numbers within each grid sector inside the Iceberg Limit show the relative density of icebergs along with a host of other information. You can find these charts at the NAVCEN website. The animation below, showing the ice charts during the 2019 season on a weekly basis from February through June, shows the evolution of the iceberg migration and subsequent drop-off as the season moves into summer.

Forecasting icebergs

The number of icebergs that make it south across the Grand Banks and into the transatlantic shipping lanes varies significantly from one year to the next. In fact, in some years virtually no icebergs will cross 48°N, which is a rough demarcation for the shipping lanes, yet in others over 1000 icebergs cross that latitude. In fact, in 2014 the Coast Guard estimated that over 1,500 icebergs made it into the shipping lanes. It is imperative, therefore, to strive to develop methods to predict these seasonal hazards.

The year-to-year variations in iceberg migration rely on several parameters. These include the rate of calving of icebergs from Greenland’s ice sheet, as well as ocean currents, prevailing winds, and air and sea temperatures.

Researchers have shown that calving rates from Greenland are directly related to the number of icebergs that drift south of 48°N, as one might expect, but there are many other important factors as well. If those icebergs get chewed up by the rocky shorelines of Labrador, or move too slowly, or if the water is too warm to keep them from melting, then they may not complete the journey south to the shipping lanes.

With 7/8 of a typical iceberg below the surface of the ocean, the strength of the Greenland, Baffin, and eventually the Labrador Currents steers them and determines how quickly they reach the shipping lanes.

On a lesser but still important scale, the prevailing wind direction plays a similar role, also steering the icebergs. Strong northwest to west (offshore) winds such as those during the winters of 2013–14 and 2014–15 resulted in significant cooling across much of the area icebergs traversed on their way south. The anomalously strong offshore flow was associated with a particular phase of the North Atlantic Oscillation (NAO). The NAO quantifies a pattern that can produce significant impacts to winter weather in Eastern North America and the adjacent ocean.

The NAO fluctuates between negative and positive phases. In the negative phase, persistent onshore (easterly) winds and warmer maritime air often occur during the winter along the coast of Labrador. This results in less buildup of sea ice, which in turn exposes icebergs to wave-induced deterioration, and the onshore wind moves them toward the shallower waters near the coast, where they can run aground or become trapped in bays. In contrast, positive NAO phase is often accompanied by strong, persistent northwest winds along the Labrador coast during the winter. The cold offshore flow builds up more sea ice, protecting icebergs during the journey south toward the shipping lanes, and that journey is also aided by the longshore wind.

The trajectory that icebergs take in their long journey from Greenland to Newfoundland is also related to the air and sea surface temperatures, which modulate the melting of the icebergs. Observations suggest that heavier iceberg conditions are associated with colder surface temperatures and stronger northwesterly winds, so iceberg conditions are closely linked to the state of the climate on a seasonal timescale. Therefore, if one can forecast the overall state of the NAO, there may be some skill in tying that to the forecast of iceberg conditions. Furthermore, researchers may soon look forward to useful seasonal forecasts for spring/summer iceberg conditions in the northwest Atlantic.

The winter of 2019-20 brought mostly positive NAO conditions. This would imply a greater-than-usual chance of plentiful icebergs, all else being equal. However, the other factors I outlined above are also important. Recently, researchers from the University of Sheffield in England have developed a computer model using artificial intelligence analysis to predict the iceberg season for the northwest Atlantic. The results of their modelling are sent to the IIP to help them gauge the potential for just how busy the iceberg season may get. The group has issued seasonal forecasts since 2018, but this year will be the first time they have used artificial intelligence to help predict the total number of icebergs passing 48°N and the rate of change in this number across the season. The Sheffield group’s seasonal outlook for 2020, issued in January, is for a relatively low number of icebergs. They predict that between 479 and 1,015 icebergs will move south of 48°N in 2020, compared with 1,515 last year. The group’s modeling will be taken into account by IIP when they issue their weekly iceberg forecasts.

In Part II of this post, I will look at how climate change relates to icebergs and how iceberg tourism is on the rise, especially off the coast of Labrador and Newfoundland.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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Tom Niziol

Tom Niziol recently retired as winter weather expert for the Weather Channel after a 32-year career as a forecaster, science and operations officer, and meteorologist-in-charge at the National Weather Service office in Buffalo, NY. Tom has published several papers and taught forecasters around the world through the COMET Program. His keenest winter weather interest is lake-effect snow.

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