Ice on the Caspian Sea

The Caspian Sea isn’t really a sea but in fact a giant lake that spans roughly 1,000 kilometers (600 miles) from north to south. In the winter, ice often forms over the lake’s northernmost reaches, while the central and southern parts remain ice free. Temperatures are generally lower in the north, so you might guess that the ice owes its existence purely to the higher latitude. But the reality is more complex: From north to south, the Caspian Sea also exhibits differences in salinity and depth.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured a natural-color image of ice on the northern Caspian Sea on March 7, 2013 (top). The map (bottom) shows the bathymetry, or depth, of the northernmost part of the sea. Darker shades of blue indicate greater depth.
The differences in depth in the Caspian Sea are stark. Most of the northern part of the sea has a depth of 10 meters (30 feet) or less. Roughly west of Poluostrov Mangyshlak (the Mangyshlak Peninsula, or Tüpqaraghan Tübegi), depth begins to increase. In the southern part of the Capsian Sea, depth is measured in hundreds of meters, with some areas exceeding 1,000 meters (3,000 feet).

The Caspian Sea also shows varying levels of salinity. Overall, it is about one-third as salty as the ocean. The water is saltiest in the south and freshest in the north. Roughly 130 rivers feed the giant lake, and nearly all of them enter from the north or west. The biggest is the Volga, which enters the Caspian Sea from the northwest, bringing a generous supply of fresh water.

Fresh water freezes at 0 degrees Celsius (32 degrees Fahrenheit), but since the Caspian is brackish, temperatures must fall below that for salt water to freeze. For every five parts per thousand salt content (salinity), the freezing point drops by 0.28 degrees Celsius (0.5 degrees Fahrenheit).

A lower freezing point is not the only impediment to the freezing of salt water. Salty water behaves differently than fresh water, hampering the formation of an ice layer at the surface.

Ice floats because liquid water is denser than ice. But liquid water is not uniform in its density. Fresh water is densest at 4° Celsius (39° Fahrenheit), several degrees above its freezing. As it cools below 4°C, it starts to rise toward the surface. If a layer of ice has already formed at the lake surface, this cooling and rising water can easily freeze onto the bottom of that ice.

Salt water, in contrast, typically reaches its maximum density closer to its freezing point. Because salt water usually sinks just before it is about to freeze (rather than rising), more of the water column must cool down before salty water can form an ice layer.

In the shallower part of the Caspian basin, water more readily cools throughout the water column than in the deeper parts. So the shallower northern section facilitates freezing.

1. References

2. GRID-Arendal Caspian Sea Salinity. Accessed March 28, 2013.
3. International Institute for Caspian Studies Caspian Sea. Accessed March 28, 2013.
4. National Snow and Ice Data Center Salinity and Brine. Accessed March 28, 2013.

NASA Earth Observatory images by Jesse Allen and Robert Simmon, using data from the Level 1 and Atmospheres Active Distribution System (LAADS), and ocean bathymetry data from the British Oceanographic Data Center’s Global Bathymetric Chart of the Oceans GEBCO_08 Grid, Version 20100927. Caption by Michon Scott, with information from Walt Meier, National Snow and Ice Data Center.

Instrument: 

Terra - MODIS - NASA

Antarctic Sea Ice Reaches New Maximum Extent

Two weeks after a new record was set in the Arctic Ocean for the least amount of sea ice coverage in the satellite record, the ice surrounding Antarctica reached its annual winter maximum—and set a record for a new high. Sea ice extended over 19.44 million square kilometers (7.51 million square miles) in 2012, according to the National Snow and Ice Data Center (NSIDC). The previous record of 19.39 million kilometers (7.49 million square miles) was set in 2006.
The map above shows sea ice extent around Antarctica on September 26, 2012, when ice covered more of the Southern Ocean than at any other time in the satellite record. The map is based on an NSIDC analysis of data from the Special Sensor Microwave/Imagers flown in the Defense Meteorological Satellite Program. Land is dark gray, and ice shelves—which are attached to land-based glaciers but floating on the ocean—are light gray. The yellow outline shows the median sea ice extent in September from 1979 to 2000. Sea ice extent is defined as the total area in which the ice concentration is at least 15 percent.
The graph of NSIDC data shows the maximum extent for each September since 1979 in millions of square kilometers. There is a lot of variability from year to year, though the overall trend shows growth of about 0.9 percent per decade.
According to a recent study by sea ice scientists Claire Parkinson and Donald Cavalieri of NASA’s Goddard Space Flight Center, Antarctic sea ice increased by roughly 17,100 square kilometers per year from 1979 to 2010. Much of the increase, they note, occurred in the Ross Sea, with smaller increases in Weddell Sea and Indian Ocean. At the same time, the Bellinghausen and Amundsen Seas have lost ice. “The strong pattern of decreasing ice coverage in the Bellingshausen/Amundsen Seas region and increasing ice coverage in the Ross Sea region is suggestive of changes in atmospheric circulation,” they noted.
“The year 2012 continues a long-term contrast between the two hemispheres, with decreasing sea ice coverage in the Arctic and increasing sea ice coverage in the Antarctic,” Parkinson added. “Both hemispheres have considerable inter-annual variability, so that in either hemisphere, next year could have either more or less sea ice than this year. Still, the long-term trends are clear, but not equal: the magnitude of the ice losses in the Arctic considerably exceed the magnitude of the ice gains in the Antarctic.”
On their Arctic Sea Ice News and Analysis blog, scientists from the University of Colorado wrote: “Comparing winter and summer sea ice trends for the two poles is problematic since different processes are in effect. During summer, surface melt and ice-albedo feedbacks are in effect; winter processes include snowfall on the sea ice, and wind. Small changes in winter extent may be a more mixed signal than the loss of summer sea ice extent. An expansion of winter Antarctic ice could be due to cooling, winds, or snowfall, whereas Arctic summer sea ice decline is more closely linked to decadal climate warming.”

1. References

2. Parkinson, C.L., and D.J. Cavalieri (2012, August 15) Antarctic sea ice variability and trends, 1979-2010. The Cryosphere, Volume 6, pages 871-880.
3. NASA Earth Observatory (2009, April 20) Sea Ice.
4. NASA Earth Observatory (n.d.) World of Change: Antarctic Sea Ice.
5. NASA (2009, September 1) What's Holding Back Antarctic Sea Ice from Melting. Accessed October 10, 2012.
6. National Snow and Ice Data Center (2012, October 2) Poles Apart: A record-breaking summer and winter. Accessed October 10, 2012.
7. The New York Times (2012, October 3) Running the Numbers on Antarctic Sea Ice.

NASA Earth Observatory images by Jesse Allen, using DMPS SSMIS ice concentration data provided courtesy of the National Snow and Ice Data Center (NSIDC). Caption by Michael Carlowicz

Instrument: 

DMSP - SSMIS - NASA

Sea Ice Retreats in the Northwest Passage

Ice retreated rapidly in the Parry Channel—part of the famous and elusive Northwest Passage—between mid-July and early August 2012.

These images, acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, show significant changes over two weeks. The top image shows Parry Channel on July 17, 2012, when ice filled the channel. The bottom image shows the same region on August 3, when some ice was still clinging to the shores of Victoria and Melville Islands but open water otherwise dominated the region.

The Canadian Ice Service reported that ice cover in Parry Channel began to fall below the 1981–2010 median after July 16, 2012, and the loss accelerated over the following two weeks. On July 23, the percentage of ice cover in the channel was roughly 67 percent, compared to the median of 80 percent. On July 30, ice cover was roughly 33 percent, compared a median of 79 percent.

These photo-like images appear to confirm the Canadian Ice Service’s findings, given the widespread open water in early August. Walt Meier of the National Snow and Ice Data Center cautioned, however, that while the Parry Channel appeared almost entirely free of ice, it was not necessarily open for navigational purposes. Sea ice can be thin enough to avoid detection by satellite sensors such as MODIS yet still thick enough to impede ships. A close examination of the image from early August shows lingering patches of ice south of Melville Island.

Whether or not ships can easily pass, recent studies have suggested that certain organisms have begun to take advantage of the open water. The Northwest Passage opened in 2007, a year when there was record-low sea ice in the Arctic. A 2007 study on Neodenticula seminae—a type of plankton historically found in the Pacific Ocean—concluded that the species had turned up in the North Atlantic. The research suggested that the plankton’s route included the Canadian Arctic Archipelago. A 2012 study on bowhead whales, which tracked individuals with satellite transmitters, indicated that Pacific and Atlantic populations had begun to overlap in the Northwest Passage in August 2010.

Attempts to identify a shortcut between Europe and Asia across the Arctic date back to the late fifteenth century, just several years after Columbus journeyed to the Americas. For centuries, attempts to find the route were stymied by unfamiliar geography and unforgiving ice. The Northwest Passage was first successfully navigated by the Norwegian explorer Roald Amundsen between 1903 and 1906. He used the southern route through the Northwest Passage; Parry Channel is part of the northern or “preferred” route.
Wider views of the Northwest Passage, acquired on August 2, 2012, are available from the NASA Ocean Color Web and the Earth Observatory.

1. References

2. Canadian Ice Service. (2012, August 6) Animated map of the last 10 days and Weekly ice coverage for the season 2012: Northwest Passage, Parry Channel. Accessed August 6, 2012.
3. Heide-Jørgensen, M.P., Laidre, K.L., Quakenbush, L.T., Citta, J.J. (2012) The Northwest Passage opens for bowhead whales. Biology Letters, 8(2), 270–273.
4. NASA Goddard Space Flight Center. Ocean Color Web. Accessed August 7, 2012.
5. Princeton University Library. Of Maps and Men: In Pursuit of a Northwest Passage. Accessed August 6, 2012.
6. Reid, P.C., Johns, D.G., Edwards, M., Starr, M., Poulin, M., Snoeijs, P. (2007) A biological consequence of reducing Arctic ice cover: arrival of the Pacific diatom Neodenticula seminae in the North Atlantic for the first time in 800 000 years. Global Change Biology, 13(9), 1910–1921.
7. Roach, J. (2007, September 17) Arctic melt opens Northwest Passage. Accessed August 6, 2012.

NASA Earth Observatory images by Jesse Allen, using data from the Land Atmosphere Near real-time Capability for EOS (LANCE). Caption by Michon Scott, with information from Walt Meier, National Snow and Ice Data Center.

Instrument: 

Terra - MODIS- Nasa

Ice Island Drifts away from Petermann Glacier

The giant iceberg that broke off Greenland’s Petermann Glacier in mid-July 2012 continued moving down the fjord at the end of the month. The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image on July 30, 2012.
By the time MODIS acquired this image, the iceberg—named PII-2012—had traveled roughly 22 kilometers (14 miles) from the edge of the Petermann Glacier and had rotated counter-clockwise.
Andreas Muenchow, associate professor of physical ocean science and engineering at the University of Delaware, tracked the progress of the iceberg on his Icy Seas blog. On July 31, he reported that the iceberg’s speed had increased from 1 kilometer per day to 2 kilometers per day. He expected the iceberg to exit the fjord and enter Nares Strait around August 4.
In mid-July 2012, Muenchow stated that air temperatures in the region had increased by roughly 0.11 +/- 0.025 degrees Celsius (0.20 +/- 0.045 degrees Fahrenheit) per year since 1987, but that air temperatures have minimal effect on the Petermann Glacier. Ocean temperatures likely exert a greater influence, but the time-series data for ocean temperatures in this region is not old enough to establish a clear trend.

1. References

2. Bryant, T. (2012, July 16) Greenland glacier loses ice. University of Delaware. Accessed August 2, 2012.
3. Muenchow, A. (2012, July 31) Nares Strait 2012: First Challenges and Petermann Ice Island Coming. Icy Seas. Accessed August 2, 2012.

NASA image courtesy Jeff Schmaltz, LANCE MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott.

Instrument: 

Terra - MODIS - NASA

A Mélange of Ice

(Click on images for larger view.)

For the fourth consecutive year, NASA research aircraft are flying over the Arctic to assess the health of the ice in the fast-changing region. Global warming has had a particularly strong impact on the Arctic, yet the effects on the region’s ice have been anything but steady or predictable. Some glaciers are spitting out icebergs and draining the Greenland ice sheet at an alarming pace; others are barely moving; a few are growing thicker.

The flights are part of a six-year mission called IceBridge. The airborne campaign allows scientists to keep an eye on the complex dynamics of the ice, helping them project how much melting ice sheets might elevate sea level and how fast sea ice is likely to retreat. Funded and structured like a satellite mission, IceBridge continues measurements started by the Ice, Cloud, and Land Elevation Satellite (ICESat) in 2003.

ICESat stopped collecting data in 2009, making IceBridge critical for ensuring a continuous series of observations until ICESat-2 launches in 2016. IceBridge flights also collect data useful for climate modeling that ICESat could not, including measures of the land topography beneath the ice, grounding line position, and ice and snow thickness.

The top image, captured during an IceBridge flight, shows a glacier in eastern Greenland flowing through a long and narrow valley—a fjord—carved by the movement of ice. Where the edge of the glacier meets the sea, there’s a layer of floating ice dimpled with chunks of icebergs that have broken off from the glacier. Understanding ice mélanges, as these conglomerations are called, is important because there is evidence that they can slow the rate that glaciers like this slip into the sea. The photograph was taken with a camera aboard NASA’s P-3B aircraft on April 25, 2012.

The image below shows a closer view of a different ice mélange; the blue patch in the middle of the image is possibly the result of turbulence from a recent calving event. Because of the disturbance, the ice appears to be thinner and more transparent to the water below. In addition, the underside of ice from glaciers often has a blue color, so it’s not unusual for ice that recently broke off from a glacier to appear this color. This photograph, also captured during a P-3B flight, was taken on April 14.

This year’s Arctic campaign stands out for completing several more sea ice flights than in previous years and for covering a greater distance. Another highlight: the IceBridge team recently flew a set of coordinated flights with European Space Agency aircraft in order to verify ice thickness measurements made by CryoSat-2.

Throughout the campaign, scientists in the field have been sending back dispatches via the mission’s blog. “The once seemingly insignificant and remote Arctic region is now understood to be intimately connected to the rest of the planet,” noted Goddard Space Flight Center’s Nathan Kurtz in a post published on March 27. “Sea ice variability affecting the severity of snow storms in Europe, melting sea ice increasing the absorption of sunlight by the Earth, and melting ice sheets causing sea level rise are but a few of many such connections.“

References:
Riebeek, H. (2011) IceBridge: Building a Record of Earth's Changing Ice, One Flight at a Time. NASA’s Earth Observatory.
NASA. (2012) IceBridge: Arctic: 2012. Accessed May 2, 2012.
NASA. (2012) Operation IceBridge Blog. Accessed May 2, 2012.
Amundson, J. (2012). Ice Mélange Dynamics and Implications for Terminus Stability, Jakobshavn Isbræ, Greenland Journal of Geophysical Research Earth Surfaces. 001(405).
NASA images from the IceBridge Science Team and taken by Jefferson Beck and Maria-José Viñas. Image interpretation by Bob Bindschadler and Sophie Nowicki. Caption by Adam Voiland with reporting from George Hale and Holli Riebeek.

Instrument:
Photograph - NASA