Cold Oceans: Ice, Currents, Icequakes and Icebergs

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McMurdo Iceberg

There are places on Earth that are so cold that water is frozen solid. These areas of snow or ice, which are subject to temperatures below 0°C 32°F for at least part of the year, compose the cryosphere. The term “cryosphere” comes from the Greek word, “krios,” which means cold. [Source: NOAA]

Ice and snow on land are one part of the cryosphere. This includes the largest parts of the cryosphere, the continental ice sheets found in Greenland and Antarctica, as well as ice caps, glaciers, and areas of snow and permafrost. When continental ice flows out from land and to the sea surface, we get shelf ice.

The other part of the cryosphere is ice that is found in water. This includes frozen parts of the ocean, such as waters surrounding Antarctica and the Arctic. It also includes frozen rivers and lakes, which mainly occur in polar areas.

The components of the cryosphere play an important role in the Earth’s climate. Snow and ice reflect heat from the sun, helping to regulate our planet’s temperature. Because polar regions are some of the most sensitive to climate shifts, the cryosphere may be one of the first places where scientists are able to identify global changes in climate.

Websites and Resources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium

Ocean Ice

Ocean water freezes just like freshwater, but at lower temperatures. Fresh water freezes at 0 degrees Centigrde (32 degrees Fahrenheit) but seawater freezes at -2 degrees Centigrade (about 28.4 degrees Fahrenheit), because of the salt in it. When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water. [Source: NOAA]

At least 15 percent of the ocean is covered by sea ice some part of the year. On average, sea ice covers almost about 26 million square kilometers (10 million square miles) of the Earth. Sea water becomes more and more dense as it becomes colder, right down to its freezing point. Fresh water, on the other hand, is most dense while still at 39.2 degrees Fahrenheit, well above the freezing point. The average temperature of all ocean water is about 38.3 degrees Fahrenheit.

Ilulissat Iceberg

Almost completely surrounded by Eurasia and North America, the Arctic Ocean is partly covered by sea ice throughout the year (and almost completely in winter). The Arctic Ocean's surface temperature and salinity vary seasonally as the ice cover melts and freezes. The summer shrinking of the ice has been quoted at 50 percent. The US National Snow and Ice Data Center (NSIDC) uses satellite data to provide a daily record of Arctic sea ice cover and the rate of melting compared to an average period and specific past years. [Source: Wikipedia]

Arctic ice is always moving. Large expanses of water doesn’t necessarily mean that ice has melted. Often it has simply been pushed in one direction or another by winds or currents. Ice flows that endure through several summers are called multi-year ice flows. They can be more than ten feet thick. In the summer they can melt so blue melt ponds are produced on the top of the ice. A phenomenon called elastic gravity waves and sonar from submarines can be used to measures the thickness of ice.

Arctic Ocean and Southern Ocean

The Arctic Ocean (also known as the Northern Ocean) is located in the Northern Hemisphere and mostly in the Arctic north polar region. the smallest and shallowest of the world's five major oceanic divisions, it is recognized by the International Hydrographic Organization (IHO) as an ocean, although some oceanographers regard it as an estuary of the Atlantic Ocean or a water body like the Mediterranean Sea. The salinity of the Arctic Ocean is the lowest on average of the five major oceans, due to low evaporation, heavy fresh water inflow from rivers and streams, and limited connection and outflow to surrounding oceanic waters with higher salinities. [Source: Wikipedia]

The Arctic Ocean is the smallest of the world's five ocean basins. With an area of about 15.8 million square kilometers (6.1 million square miles), the Arctic Ocean is about 1.5 times as big as the United States. It is bordered by Greenland, Canada, Norway, Alaska, and Russia. The average depth of the Arctic Ocean is 1,205 meters (3,953 feet) and it is 5,567 meters (18,264 feet) deep at its deepest point. [Source: NOAA]

The Southern Ocean is the 'newest' named ocean. It is recognized by the U.S. Board on Geographic Names as the body of water extending from the coast of Antarctica to the line of latitude at 60 degrees South. The boundaries of this ocean were proposed to the International Hydrographic Organization (IHO) in 2000. However, not all countries agree on the proposed boundaries, so this has yet to be ratified by members of the IHO. The U.S. is a member of the IHO. [Source: NOAA]

Also known as the Antarctic Ocean, the Southern Ocean is comprised the southernmost waters of the world ocean, generally taken to be south of 60° S latitude and encircling Antarctica. Covering of 20,327,000 square kilometers (7,848,000 squar miles), it is considered the second-smallest of the five principal oceans — smaller than the Pacific, Atlantic, and Indian oceans but larger than the Arctic Ocean. Since the 1980s, the Southern Ocean has been subject to rapid climate change, which has led to changes in the marine ecosystem. [Source: Wikipedia]


Sea Ice and Global Climate

Sea ice affects both global ocean temperatures and the global movement of ocean waters. The ocean is salty and when sea ice forms, much of the salt is pushed into the ocean water below the ice, although some salt may become trapped in small pockets between ice crystals. Water below sea ice has a higher concentration of salt and is denser than the surrounding ocean water, so it sinks and moves from the surface. In this way, sea ice contributes to the circulation of the global ocean conveyor belt. Cold, dense polar water descends from the surface and circulates along the ocean bottom toward the equator, while warm water from mid-depth to the surface travels from the equator toward the poles. [Source: NOAA]

Sea ice is frozen water that forms, expands, and melts in the ocean. It is different from icebergs, glaciers, ice sheets, and ice shelves, which originate on land. For the most part, sea ice expands during winter months and melts during summer months, but in certain regions, some sea ice remains year-round. About 15 percent of the world's oceans are covered by sea ice during part of the year.

While sea ice exists primarily in the polar regions, it influences the global climate. The bright surface of sea ice reflects a lot of sunlight out into the atmosphere and, importantly, back into space. Because this solar energy "bounces back" and is not absorbed into the ocean, temperatures nearer the poles remain cool relative to the equator.

When warming temperatures gradually melt sea ice over time, fewer bright surfaces are available to reflect sunlight back into the atmosphere. More solar energy is absorbed at the surface and ocean temperatures rise. This begins a cycle of warming and melting. Warmer water temperatures delay ice growth in the fall and winter, and the ice melts faster the following spring, exposing dark ocean waters for a longer period the following summer.

Changes in the amount of sea ice can disrupt normal ocean circulation, thereby leading to changes in global climate. Even a small increase in temperature can lead to greater warming over time, making the polar regions the most sensitive areas to climate change on Earth.


To be classified as an iceberg, the height of the ice must be greater than five meters (16 feet above sea level and the thickness must be 30 to 50 meters (98 to 164 feet) and the ice must cover an area of at least 500 square meters (5,382 square feet). [Source: NOAA]

There are smaller pieces of ice known as “bergy bits” and “growlers.” Bergy bits and growlers can originate from glaciers or shelf ice, and may also be the result of a large iceberg that has broken up. A bergy bit is a medium to large fragment of ice. Its height is generally greater than three feet but less than five meters (16 feet) above sea level and its area is normally about 100 to 300 square meters (1,076-3,229 square feet). Growlers are smaller fragments of ice and are roughly the size of a truck or grand piano. They extend less than one meter (three feet) above the sea surface and occupy an area of about 20 square meters (215 square feet).

Icebergs are also classified by shape, most commonly being either tabular or non-tabular. Tabular icebergs have steep sides and a flat top. Non-tabular icebergs have different shapes, with domes and spires.

Icebergs are monitored worldwide by the U.S. National Ice Center (NIC). NIC produces analyses and forecasts of Arctic, Antarctic, Great Lakes, and Chesapeake Bay ice conditions. NIC is the only organization that names and tracks all Antarctic Icebergs.

Color of Iceberg Ice

The colorful hues of icebergs result from the interplay of light with dense ice, ice crystals, and algal growth. Most people would say that icebergs are white — and most of them are. But did you know that icebergs can also appear in spectacular shades of blue and green? An iceberg looks white because compressed snow on its surface contains large numbers of tiny air bubbles and crystal edges that equally reflect all wavelengths of visible light. [Source: NOAA]

As more and more heavy snow accumulates atop an iceberg, the air bubbles get compressed, forcing the smaller ice crystals to grow together and merge into larger grains. When the iceberg is underwater, the air bubbles are squeezed out and washed away. Then, when light encounters the dense, compressed ice, much of the light penetrates it. The ice absorbs longer wavelengths of colors, such as red and yellow. Colors of shorter wavelengths, like green and blue, reflect the light. This "leftover" blue-green light is what gives some icebergs their remarkable colors.

Additionally, algae often grow on the underwater sides of icebergs, producing beautiful green stripes in the ice. These are readily seen when an iceberg rolls over and sections that were previously underwater are exposed.

Cold Water Ocean Currents and Stratification

Blue Greenland ice

The world’s deep ocean currents play a critical role in transporting heat and carbon around the planet, which helps regulate the Earth's climate.

The movements of ice, surface water and near surface water in the Arctic Ocean are driven by the clockwise Beaufort Gyre. Below this is a contraclockwise current that moves water primarily of Atlantic origin to the east along the continental margin. In some places in the Arctic Ocean the water is highly stratified with relatively fresh water supplied by river water on top and more saline water in the lower depths. In some basins the water has remained undisturbed and isolated for more than 500 years. Rivers supply about a third of the fresh water that enters the Arctic Ocean. Scientist can track down the source of these water by the unique chemical signature of the water.

Sarah Gibbens wrote in National Geographic: While the other oceans are defined by the continents that fence them in, the Southern Ocean is defined by a current. Scientists estimate that the Antarctic Circumpolar Current (ACC) was established roughly 34 million years ago, when Antarctica separated from South America. That allowed for the unimpeded flow of water around the bottom of the Earth. [Source: Sarah Gibbens, National Geographic, June 8, 2021]

“The ACC flows from west to east around Antarctica, in a broad fluctuating band roughly centered around a latitude of 60 degrees south—the line that is now defined as the northern boundary of the Southern Ocean. Inside the ACC, the waters are colder and slightly less salty than ocean waters to the north.

“Extending from the surface to the ocean floor, the ACC transports more water than any other ocean current. It pulls in waters from the Atlantic, Pacific, and Indian Oceans, helping drive a global circulation system known as the conveyor belt, which transports heat around the planet. Cold, dense water that sinks to the ocean floor off Antarctica also helps store carbon in the deep ocean. In both those ways, the Southern Ocean has a crucial impact on Earth’s climate.

Coldest, Deepest Ocean Water Mysteriously Disappears

The coldest deep ocean water that flows around Antarctica in the Southern Ocean has been mysteriously disappearing at a high rate over the last few decades, scientists have found. NBC News reported: This mass of water is called Antarctic Bottom Water, which is formed in a few distinct locations around Antarctica, where seawater is cooled by the overlying air and made saltier by ice formation (which leaves the salt behind in the unfrozen water). The cold, salty water is denser than the water around it, causing it to sink to the sea floor where it spreads northward, filling most of the deep ocean around the world as it slowly mixes with warmer waters above it. Previous studies had indicated that this deep water has become warmer and less salty over the past few decades, but a new study has found that significantly less of this water has also been formed during this time.[Source: OurAmazingPlanet NBC News, March 23, 2012]

Oceanographers examined temperature data collected from 1980 to 2011 at about 10-year intervals by an international program of repeated ship-based oceanographic surveys in the Southern Ocean. They found that Antarctic Bottom Water has been disappearing at an average rate of about 8 million metric tons per second over the past few decades, equivalent to about 50 times the average flow of the Mississippi River, according to statement from the National Oceanic and Atmospheric Administration (NOAA), which helped fund the data collection.

"In every oceanographic survey repeated around the Southern Ocean since about the 1980s, Antarctic Bottom Water has been shrinking at a similar mean rate, giving us confidence that this surprisingly large contraction is robust," said lead author of the study Sarah Purkey, a graduate student at the University of Washington in Seattle.



"The Bloop" is the given name of a mysterious underwater sound recorded in the 90s. Years later, NOAA scientists discovered that this sound emanated from an iceberg cracking and breaking away from an Antarctic glacier. Shown here: a NASA Landsat mosaic image of Antarctica. [Source: NOAA]

In 1997, researchers listening for underwater volcanic activity in the southern Pacific recorded a strange, powerful, and extremely loud sound. Using hydrophones, or underwater microphones, that were placed more than 3,219 kilometers apart across the Pacific, they recorded numerous instances of the noise, which was unlike anything they had heard before. Not only was it loud, the sound had a unique characteristic that came to be known as “the Bloop.”

Scientists from NOAA’s Pacific Marine Environmental Laboratory (PMEL) were eager to discover the sound's origin, but with about 95 percent of the ocean unexplored, theories abounded. Was the Bloop from secret underwater military exercises, ship engines, fishing boat winches, giant squids, whales, or a some sea creature unknown to science?

As the years passed, PMEL researchers continued to deploy hydrophones ever closer to Antarctica in an ongoing effort to study the sounds of sea floor volcanoes and earthquakes. It was there, on Earth’s lonely southernmost land mass, that they finally discovered the source of those thunderous rumbles from the deep in 2005. The Bloop was the sound of an icequake — an iceberg cracking and breaking away from an Antarctic glacier! With global warming, more and more icequakes occur annually, breaking off glaciers, cracking and eventually melting into the ocean.

PMEL’s Acoustics Program develops unique acoustics tools and technologies to acquire long-term data sets of the global ocean acoustics environment, and to identify and assess acoustic impacts from human activities and natural processes on the marine environment.

Image Sources: Wikimedia Commons; YouTube, NOAA

Text Sources: National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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