A fractured winter

Arctic sea ice is nearing its winter maximum and will soon begin its seasonal decline. Ice extent remains below average, in part a result of the persistence of the negative phase of the Arctic Oscillation that has kept winter temperatures warmer than average. The Antarctic passed its summer minimum ice extent, reaching the second highest level in the satellite record at this time of year, primarily due to continued higher-than-average ice in the Weddell Sea.

Overview of conditions

Figure 1. Arctic sea ice extent for February 2013 was 14.66 million square kilometers (5.66 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data

Credit: National Snow and Ice Data Center
High-resolution image

Average sea ice extent for February 2013 was 14.66 million square kilometers (5.66 million square miles). This is 980,000 square kilometers (378,000 square miles) below the 1979 to 2000 average for the month, and is the seventh-lowest February extent in the satellite record. Since 2004, the February average extent has remained below 15 million square kilometers (5.79 million square miles) every year except 2008. Prior to 2004, February average extent had never been less than 15 million square kilometers. Ice extent remains slightly below average everywhere except the Bering Sea.

Conditions in context

Figure 2. The graph above shows Arctic sea ice extent as of March 3, 2013, along with daily ice extent data for the 2012, the record low year. 2013 is shown in blue, and 2012 in green. The gray area around the average line shows the two standard deviation range of the data. Sea Ice Index data.

Credit: National Snow and Ice Data Center
High-resolution image

Through the month of February, the Arctic gained 766,000 square kilometers of ice (296,000 square miles), which is 38% higher than the 1979 to 2000 average for the month. Air temperatures at the 925 hPa level were 2 to 5 degrees Celsius (4 to 9 degrees Fahrenheit) higher than average across the Atlantic sector of the Arctic, especially near Iceland and in Baffin Bay. Temperatures were lower than average by 2 to 6 degrees Celsius (4 to 11 degrees Fahrenheit) north of Greenland and the Canadian Archipelago, and in the Beaufort, Chukchi and East Siberian seas, linked to anomalously low sea level pressure over Alaska and Canada. The dominant feature of Arctic sea level pressure for February 2013 was unusually high pressure over the East Greenland and Barents seas, consistent with a predominantly negative phase of the Arctic Oscillation.

February 2013 compared to previous years

Figure 3. Monthly February ice extent for 1979 to 2012 shows a decline of -2.9% per decade.

Credit: National Snow and Ice Data Center
High-resolution image

Average Arctic sea ice extent for February 2013 was the seventh lowest for the month in the satellite record. Through 2013, the linear rate of decline for February ice extent is -2.9% per decade relative to the 1979 to 2000 average. Although the relative reduction in winter sea ice extent remains small compared to reductions in summer, the linear trend represents an overall reduction of more than 1.57 million square kilometers (606,000 square miles) from 1979 to 2013.

Persistence of the negative phase of the Arctic Oscillation

Figure 4. These ice motion images for November 2012 (left) and December 2012 (right) show strong export of ice through the Fram Strait in November, while in December ice export through the Fram was about average.

Credit: National Snow and Ice Data Center
High-resolution image

As discussed in the January and February posts, sea level pressure in the Arctic has remained higher than average, resulting in persistence of the negative phase of the Arctic Oscillation (AO). During the negative phase of the Arctic Oscillation, enhanced poleward transport of warm air tends to keep temperatures in the Arctic above average. At the same time, the negative phase of the Arctic Oscillation allows for more cold Arctic air to intrude or mix with air at lower latitudes. These cold air outbreaks can result in low temperatures and increased storminess in mid latitudes.

The Arctic Oscillation also impacts sea ice movement in the Arctic. The negative phase of the Arctic Oscillation is linked to an increase in the strength of the Beaufort Gyre and reduced outflow of ice through Fram Strait. A negative AO used to help promote ice survival through summer by strengthening the Beaufort Gyre and thereby increasing the distribution of old, thick ice along coastal Alaska and Siberia. However, the location and strength of positive sea level pressure anomalies has varied throughout winter, with varied impacts on ice motion.

For example, during November (weak AO index of -0.111) positive sea level pressure anomalies were centered over the Bering Sea and Alaska, resulting in strong ice motion from the central Arctic towards coastal Canada and north of Greenland outwards towards Fram Strait. In December, the strong negative AO index of -1.749 was reflected in positive sea level pressure anomalies centered over the Kara and Barents seas, enhancing ice motion from the southern Beaufort into the Chukchi sea and out towards the Bering Sea. Export of ice out of Fram Strait was about average. Similar variations in positive sea level pressure anomalies have continued, with the largest positive anomalies over the central Arctic in January, and over the Barents Sea in February.

This pattern is similar to that observed during the extreme negative Arctic Oscillation year of 2009/2010, when old ice was transported into the southern Beaufort and Chukchi seas where it then melted out during summer 2010, further depleting the Arctic of its store of old, thick ice.

Ice fracture

Figure 5. In this series of images from February 13 to March 2, from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS), a large crack expands in the sea ice near the coasts of Canada and Alaska. Black areas indicate where the satellite instrument did not collect data due to lack of sunlight. The dark area decreases as the sun rises in the Arctic. Rapid Response imagery was obtained from the NASA Land Atmosphere Near-real time Capability for EOS (LANCE) system.

Credit: NASA LANCE/National Snow and Ice Data Center
View the image series

During the last couple of weeks of February, a broad area of sea ice has fractured off the coast of Alaska and Canada, extending from Ellesmere Island in the Canadian Arctic to Barrow, Alaska. This fracturing event appears to be related to a series of storms that moved across central Alaska starting on February 10, 2013, causing intense easterly winds along the coast and strong off-shore ice motion.* The large area of fractured ice is located in predominantly first-year ice, which is thinner and easier to fracture than thick, multiyear ice. Similar patterns were observed in early 2011 and 2008, but the 2013 fracturing is quite extensive.  The animation (Figure 5) shows the progress of the fracturing, and the general strong rotation of the Beaufort Gyre ice motion pattern during late February. (See also this animation of the fracture from the AVHRR instrument, posted on the Arctic Sea Ice Blog.)

* Note: We originally attributed the fracturing event to a storm that passed over the North Pole, and stated “This fracturing event appears to be related to a storm that passed over the North Pole on February 8, 2013, creating strong off-shore ice motion.” We corrected this sentence after reexamining weather charts. The updated version now reads, “This fracturing event appears to be related to a series of storms that moved across central Alaska starting on February 10, 2013, causing intense easterly winds along the coast and strong off-shore ice motion.”

Antarctic sea ice extent continues above average

Figure 6. Antarctic sea ice extent for February 2013 was 3.83 million square kilometers (1.48 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic South Pole. Sea Ice Index data. About the data

Credit: National Snow and Ice Data Center
High-resolution image

The Antarctic sea ice minimum extent appears to have passed, on February 20. Ice was quite extensive throughout the austral summer period. Monthly average sea ice extent for February 2013 was 3.83 million square kilometers (1.48 million square miles) and minimum daily sea ice extent for the Antarctic region was 3.68 million square kilometers (1.42 million square miles) on February 20. Unusual circulation patterns, likely resulting from higher-than-average pressure in the Bellingshausen Sea, pushed sea ice in the northwestern Weddell Sea far to the north, as we mentioned in our February post. NASA’s Earth Observatory posted this image of ice in the Weddell Sea as Image of the Day for March 1st, 2013. Extent was also well above average for the Ross Sea region relative to the entire 1979 to 2013 satellite record.

The Odden

Figure 7. This image shows sea ice cover in early May, 2012 in the east Greenland Sea. Sea ice extent is provided at 4 kilometer resolution by the NSIDC/NIC multi-sensor MASIE product and sea ice concentration (varying from 0 to 1) at 25 kilometer resolution by NSIDC’s Near-Real Time Passive Microwave product. The red dot shows the estimated position of an ARGO profiling float deployed as part of a NASA-sponsored project led by Michael Steele  and Patricia Matrai. This float is capable of storing ocean data while under the ice pack, which are then received via satellite when the ice recedes. Ongoing analysis of these data indicates that cold, fresh surface water lies just under the ice extension along the Jan Mayen Ridge, a signature of Arctic waters.

Credit: M. Steele, University of Washington and P. Matrai, Bigelow Lab/National Snow and Ice Data Center
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Within the East Greenland Sea, an ice tongue about 1,300 kilometers (807 miles) in length, referred to as “The Odden” (Norwegian word for headland), would regularly form during winter months eastwards from the main East Greenland ice edge. The Odden would form in winter because of an eastward flow of very cold ocean waters in the Jan Mayen current and may have played an important role in winter ocean convection as new ice would form. It would form as early as December and as late as April and was present during the 1980s, a few times in the 1990s, and very rarely since 2000. While the Odden rarely formed in last two decades, there is frequently a small extension of ice along the Jan Mayen Ridge, which may indicate that eastward flow of cold ocean water is still occurring.

February ice extent low in the Barents Sea, high in the Bering Sea

As in January, sea ice extent in February was low on the Atlantic side of the Arctic, but unusually high on the Pacific side of the Arctic, remaining lower than average overall. At the end of the month, ice extent rose sharply, as winds changed and started spreading out the ice cover.

Sea ice extent in late winter can go up and down very quickly, getting pushed together or dispersed by strong winds. Ice extent usually reaches its annual maximum sometime in late February or March, but the exact date varies widely from year to year.

Arctic sea ice extent for February 2012 was 14.56 million square kilometers (5.62 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. Sea Ice Index data.

Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions
Arctic sea ice extent in February 2012 averaged 14.56 million square kilometers (5.62 million square miles). This is the fifth-lowest February ice extent in the 1979 to 2012 satellite data record, 1.06 million square kilometers (409,000 square miles) below the 1979 to 2000 average extent.

Continuing the pattern established in January, conditions differed greatly between the Atlantic and Pacific sides of the Arctic. On the Atlantic side, especially in the Barents Sea, air temperatures were higher than average and ice extent was unusually low. February ice extent for the Barents Sea was the lowest in the satellite record.  Air temperatures over the Laptev, Kara and Barents seas ranged from 4 to 8 degrees Celsius (7 to 14 degrees Fahrenheit) above average at the 925 hectopascal (hPa ) level (about 3000 feet above sea level).  In contrast, on the Pacific side, February ice extent in the Bering Sea was the second highest in the satellite record, paired with air temperatures that were 3 to 5 degrees Celsius (5 to 9 degrees Fahrenheit) below average at the 925 hPa level.

graph showing years and ice extent

The graph above shows daily Arctic sea ice extent as of March 5, 2012, along with the ice extents for the previous four years. 2011 is shown in light blue, 2010 is in pink, 2009 in dark blue, 2008 is in purple, and 2007, the year with the record low minimum, is dashed green. The gray area around the average line shows the two standard deviation range of the data. Sea Ice Index data.

Credit: National Snow and Ice Data Center
High-resolution image

Conditions in context

Overall, the Arctic gained 956,000 square kilometers (369,000 square miles) of ice during the month. This was 486,000 square kilometers (188,000 square miles) more than the average ice growth for February 1979 to 2000. The overall low ice extent for the month stemmed mostly from the low ice extent in the Barents Sea: the extensive ice in the Bering Sea was not enough to compensate. On average, the Barents Sea has 865,000 square kilometers (334,000 square miles) of ice for the month of February. This year there were only 401,000 square kilometers (155,000 square miles) of ice in that region, the lowest recorded in the satellite data record.

At the end of February, ice extent rose sharply. Data from the NSIDC Multisensor Analyzed Sea Ice Extent (MASIE) showed that the rise came mainly from the Bering Sea and Baffin Bay. In the Bering Sea and Baffin Bay, winds pushed the ice extent southward. Ice growth in the Kara Sea also contributed to the rise in ice extent. In the Kara Sea, westerly winds that had been keeping the area ice-free shifted, allowing the open water areas to freeze over. During late winter, ice extent can change quickly as winds push extensive ice cover together, or spread out ice floes over a greater area.

Monthly February ice extent for 1979 to 2012 shows a decline of 3.0% per decade.

Credit: National Snow and Ice Data Center
High-resolution image

February 2012 compared to past years
Arctic sea ice extent for February 2012 was the fifth lowest in the satellite record. Including the year 2012, the linear rate of decline for February ice extent over the satellite record is 3.0% per decade. Based on the satellite record, through 2003, average February ice extent had never been lower than 15 million square kilometers (5.79 million square miles). February ice extent has not exceeded that mark eight out of the nine years since 2003.

This photograph of sea ice near Greenland was taken on March 18, 2011 from the NASA P3 aircraft. The IceBridge mission is collecting data on ice thickness, an important measure of the health of sea ice.

Credit: NASA/ATM automatic Cambot system
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IceBridge thickness data
Measuring ice thickness is critical to assessing the overall health of Arctic sea ice. The passive microwave data that NSIDC presents here provide only ice extent, a two-dimensional measure of ice cover. But ice can vary in thickness from a few centimeters to several meters, and scientists want to know if the ice pack is thinning overall as well as declining in extent. A new study by NASA scientist Ron Kwok compared ice thickness data collected by airplanes during the ongoing Operation IceBridge with thickness data from the NASA Ice, Cloud and Land Elevation Satellite (ICESat), which ended its mission in 2009. IceBridge is an airborne data-collection mission that started in 2009, in order to bridge the data gap between the first ICESat and ICESat-2, which is scheduled to launch in 2016.

Kwok found good agreement between simultaneous IceBridge and ICESat freeboard measurements made in 2009. Freeboard is the elevation of sea ice above the ocean surface, and provides a measure of ice thickness. These results show that IceBridge measurements will be able to bridge the gap between the ICESat and ICESat-2 satellite missions and add to other ice thickness data from the European Space Association (ESA) Cryosat-2. Satellite measurements of ice thickness provide a third dimension of information on the changing sea ice cover, helping scientists to more accurately assess the amount of sea ice in the Arctic.

Data collected by the IceBridge mission is archived and distributed by the NSIDC IceBridge Data program.

These images show the general effects of the positive phase (left) and negative phase (right) of the NAO. Red dots show the location of harp seal breeding grounds.

Credit: Johnston, et. al., 2012
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Regional ice conditions and harp seals
Many animals rely on sea ice as part of their habitat. Harp seals, for example, give birth to and care for their young on floes of sea ice. Recent research by David Johnston and colleagues at Duke University showed that harp seals in the northwest Atlantic have higher mortality rates during years when the North Atlantic Oscillation (NAO) is in its negative phase, a pattern that favors low ice cover in the Labrador Sea and Gulf of St. Lawrence, where harp seals breed. 

This winter, the NAO has mostly been in a positive phase and ice conditions in the Labrador Sea and Gulf of St. Lawrence have been at near-normal levels. However, in recent years, ice conditions in the region have been very low. The study showed a longer-term decline in sea ice cover of up to 6% per decade across all North Atlantic harp seal breeding grounds since 1979. While harp seals are well-suited to deal with natural short-term shifts in ice conditions, they may not be able to adapt to the combined effects of both short-term variability and long-term climate change.

Further reading
Friedlaender, A.S., D.W. Johnston and P.N. Halpin. 2010. Effects of the North Atlantic Oscillation on sea ice breeding habitats of harp seals (Pagophilus groenlandicus) across the North Atlantic, Progress in Oceanography, 86, 261-266. 

Johnston DW, Bowers MT, Friedlaender AS , Lavigne DM. 2012. The Effects of Climate Change on Harp Seals (Pagophilus groenlandicus). PLoS ONE, 7(1): e29158. doi:10.1371/journal.pone.0029158.

Kwok, R., G.F. Cunningham, S.S. Manizade and W.B. Krabill. 2012. Arctic sea ice freeboard from IceBridge acquisitions in 2009: Estimates and comparisons with ICESat, Journal of Geophysical Research, Vol. 117, C02018, doi:10.1029/2011JC007654.

Arctic ice extent low overall, high in the Bering Sea

Overall, Arctic sea ice extent remained lower than average in January. However, in the Bering Sea, ice extent was much greater than normal. The heavy ice cover caused problems for fishermen and made for an arduous late-season resupply mission to Nome, Alaska. The Arctic Oscillation, which had been in its positive phase most of the winter so far, switched to a negative mode, bringing cold weather to Europe and changing the direction of sea ice movement.

Figure 1. Arctic sea ice extent for January 2012 was 13.73 million square kilometers (5.30 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. Sea Ice Index data.

Credit: National Snow and Ice Data Center
High-resolution image

Overview of conditions
Arctic sea ice extent in January 2012 averaged 13.73 million square kilometers (5.30 million square miles). This is the fourth-lowest January ice extent in the 1979 to 2012 satellite data record, 1.10 million square kilometers (425,000 square miles) below the 1979 to 2000 average extent.

As in December, ice extent was lower than normal on the Atlantic side of the Arctic, especially in the Barents Sea. However, on the other side of the Arctic, ice extent in the Bering Sea was much greater than average, reaching the second-highest levels for January in the satellite record. The greater-than-normal ice extent in the Bering Sea partly compensated for low ice extent on the Atlantic side of the Arctic Ocean, but ice extent as a whole remained far below average.

Figure 2. The graph above shows daily Arctic sea ice extent as of February 5, 2012, along with the ice extents for the previous four years. 2011 is shown in light blue, 2010 is in pink, 2009 in dark blue, 2008 is in purple, and 2007, the year with the record low minimum, is dashed green. The gray area around the average line shows the two standard deviation range of the data.
Sea Ice Index data.

Credit: National Snow and Ice Data Center
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Conditions in context
The growth rate for Arctic sea ice in January was the slowest in the satellite record. After growing relatively quickly early in January, ice extent declined briefly in the middle of the month, and then grew more slowly than normal for the rest of the month. The slow growth likely stemmed from winds from the south and west that compressed the sea ice in the Barents Sea, and above-average temperatures and winds that limited ice growth in the Sea of Okhotsk.

Overall, the Arctic gained 765,000 square kilometers (295,000 square miles) of ice during the month. This was 545,000 square kilometers (210,000 square miles) less than the average ice growth rate for January 1979 to 2000.

Figure 3. Monthly January ice extent for 1979 to 2012 shows a decline of 3.2% per decade.

Credit: National Snow and Ice Data Center
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January 2012 compared to past years

Arctic sea ice extent for January 2012 was the fourth lowest in the satellite record. Including the year 2012, the linear rate of decline for January ice extent over the satellite record is 3.2% per decade.
Based on the satellite record, before 2005 average January ice extent had never been lower than 14 million square kilometers (5.41 million square miles). January ice extent has now fallen below that mark six out of the last seven years.

Figure 4. Monthly average sea ice motion for December 2011 (top) and January 2012 (bottom), derived from the DMSP Special Sensor Microwave Imager and Sounder (SSMIS) shows how the changing Arctic Oscillation (AO) phase affects the movement of ice. In January, the AO switched from a positive phase to a negative phase.

Credit: National Snow and Ice Data Center
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The Arctic Oscillation turns negative

From November through the first part of January, the Arctic Oscillation (AO) was in a generally positive phase, which tends to bring warm conditions to the United States and Europe, and colder conditions to the Arctic. However, in the middle of the month, the AO shifted back to a negative phase. This shift helped bring cold air outbreaks over middle latitudes, notably a record cold snap throughout much of Europe.

The Arctic Oscillation also affects how sea ice moves in the Arctic, which can affect how much ice melts in the summer months. In December, when the AO was in its positive phase, ice was flowing from Siberia toward North America, and also south out of the Arctic through Fram Strait. That pattern favors a thinner, younger ice cover in the summer. In mid-January, the AO switched to its negative phase. In general, the negative phase of the AO tends to retain ice in the Arctic Ocean, leading to a stronger, more resilient summer sea ice cover. Ice motion charts for January confirm that in January, there was less motion across the pole and through Fram Strait but a stronger clockwise motion in the Beaufort Sea called the Beaufort Gyre.

For background on the Arctic Oscillation, see the NSIDC Icelights post: The Arctic Oscillation, winter storms and sea ice.

Figure 5. This NASA Moderate Resolution Imaging Spectroradiometer (MODIS) image, acquired in mid-January, shows heavy sea ice conditions in Bristol Bay and the Bering Sea, off the western coast of Alaska.

Credit: NASA Earth Observatory courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC.
High-resolution image

Ice-covered Bering Sea
Arctic sea ice extent in the Bering Sea was the second highest in the satellite record for the month of January. Ice extent in the Bering Sea was 562,000 square kilometers (217,000 square miles), which is 104,600 square kilometers (40,400 square miles) above the 1979 to 2000 average. The record high ice extent for the month occurred in January 2000, at 629,000 square kilometers (242,900 square miles).

The above-average sea ice extent in the Bering Sea stemmed from a weather pattern that brought cold air from the Arctic into the Bering Sea, driving sea ice southwards. The weather pattern, which has persisted since November, features unusually low surface pressure south and east of the Alaskan coast, which leads to winds from the north or northeast that blow into the Bering Sea region. This weather pattern also brought moist air from the Pacific Ocean to the southern Alaska coast, helping to explain record snowfalls in towns such as Cordova, Alaska, which received over 15 feet of snow between early November and mid-January.

The extensive sea ice impeded winter fishing in the Bering Sea and slowed an important fuel resupply mission to Nome, on the west coast of Alaska. More information on sea ice conditions off southwestern Alaska, and a full-resolution version of the image at left, are available on the NASA Earth Observatory Web site.

Positively Arctic: Arctic Oscillation switches phase

Arctic sea ice extent remained unusually low through December, especially in the Barents and Kara seas.  In sharp contrast to the past two winters, the winter of 2011 has so far seen a generally positive phase of the Arctic Oscillation, a weather pattern that helps to explain low snow cover extent and warmer than average conditions over much of the United States and Eastern Europe.  In Antarctica, where summer is beginning, sea ice extent is presently above average.

Figure 1. Arctic sea ice extent for December 2011 was 12.38 million square kilometers (4.78 million square miles). The magenta line shows the 1979 to 2000 median extent for that month. The black cross indicates the geographic North Pole. Sea Ice Index data.

Credit: National Snow and Ice Data Center
High-resolution image


Overview of Conditions

Arctic sea ice extent in December 2011 averaged 12.38 million square kilometers (4.78 million square miles). This is the third lowest December ice extent in the 1979 to 2011 satellite data record, 970,000 square kilometers (375,000 square miles) below the 1979 to 2000 average extent.

Ice extent was particularly low on the Atlantic side of the Arctic, most notably in the Barents and Kara seas. The eastern coast of Hudson Bay did not freeze entirely until late in the month: normally, Hudson Bay has completely frozen over by the beginning of December. In the Bering Sea, ice extent was slightly above average.

Figure 2. The graph above shows daily Arctic sea ice extent as of January 3, 2012, along with the ice extents for the previous four years. 2011 is shown in light blue, 2010 is in pink, 2009 in dark blue, 2008 is in purple, and 2007, the year with the record low minimum, is dashed green. The gray area around the average line shows the two standard deviation range of the data. Sea Ice Index data.

Credit: National Snow and Ice Data Center
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Conditions in context
For the Arctic as a whole, ice extent for the month remained far below average. The Arctic sea ice cover grew slightly faster than average, but December started out with a lower-than-average ice extent.

Overall, the Arctic gained 2.37 million square kilometers (915,000 square miles) of ice during the month. The average ice gain for December was 1.86 million square kilometers (718,000 square miles). On December 31, Arctic sea ice extent was 13.25 million square kilometers (5.12 million square miles), 561,000 square kilometers (217,000 square miles) more than the ice extent on December 31, 2010, the lowest extent on December 31 in the satellite record.

Figure 3. Monthly December ice extent for 1979 to 2011 shows a decline of 3.5% per decade.

Credit: National Snow and Ice Data Center
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December 2011 compared to past years
Arctic sea ice extent for December 2011 was the third lowest in the satellite record. The five lowest December extents in the satellite record have occurred in the past six years. Including the year 2011, the linear rate of decline ice December ice extent over the satellite record is -3.5% per decade.

Figure 4. This map of air temperature anomalies at the 925 hPa level (approximately 3000 feet) for December 2011 shows near-normal or lower-than-average temperatures over much of the Arctic basin, while air temperatures over the Kara and Barents seas were warmer than normal.

Credit: NSIDC courtesy NOAA/ESRL PSD
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Arctic temperatures
Air temperatures in December were lower than average over much of the Arctic Ocean, but higher than average over the Kara and Barents seas. Higher-than-average temperatures in these regions stemmed from two major factors. First, where sea ice extent is low, heat can escape from areas of open water, warming the atmosphere. Second, surface winds in the Kara and Barents Sea ice blew persistently from the south, bringing in heat from lower latitudes. This imported heat also helped to keep sea ice extent low in this area. Conditions over Canada were also unusually warm during December, but conditions over southeast Greenland have been 6 to 8 degrees Celsius (11 to 14 degrees Fahrenheit) colder than average, partly because of northerly winds in the area.

Figure 5. This graph shows daily Arctic Oscillation Index values from the NOAA Climate Prediction Center for early September 2011 to early January 2012. The index is a normalized (unitless) index of relative pressure anomalies between polar and mid-latitude regions.

Credit: NSIDC courtesy NOAA NWS Climate Prediction Center
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Positive phase of the Arctic Oscillation
The past two Arctic winters were dominated by a negative phase of the Arctic Oscillation, a large-scale weather pattern that brings generally warm conditions to the Arctic and colder conditions to Europe and North America. In contrast, the winter of 2011 has so far seen a mostly positive phase of the Arctic Oscillation. While temperatures were above normal in the Kara and Barents seas, the positive phase of the Arctic Oscillation tends to keep the coldest winter air locked up in the Arctic, which keeps the middle latitudes free of frigid Arctic temperatures and strong snowstorms. This weather pattern helps to explain the low snow cover and warm conditions over much of the United States and Eastern Europe so far this winter.

Several studies have shown that during the positive phase of the Arctic Oscillation, thick ice tends to move out of the Arctic through Fram Strait, leaving the Arctic with thinner ice that melts out more easily in summer. Scientists will be watching closely for this connection if the positive phase of the Arctic Oscillation continues through the winter.

Some scientists have speculated that the negative Arctic Oscillation pattern of the last two winters was in part driven by low sea ice extent. The recurrence of the positive phase of the Arctic Oscillation so far this winter, following a near-record low summer sea ice extent, suggests that other factors play an important role.

Figure 6. This full-year graph puts 2011 sea ice extent in context. The gray line shows the 1979 to 2000 climatology, thick blue-gray indicates the 1981 to 2010 (30-year) climatology. NOAA this year switched to using the period 1981 to 2010 for their thirty-year climate comparison period. Sea Ice Index data.

Credit: National Snow and Ice Data Center
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2011 year in review
Arctic sea ice extent fell to its seasonal minimum on September 9, 2011, falling just short of the record low set in September 2007, when summer weather conditions were extremely favorable for ice loss. This summer, the weather was not as extreme as 2007, so it was surprising that ice extent dropped so low. The low ice extent, along with data on ice age, suggests that the Arctic ice cover remains thin and vulnerable to summer melt.

Northern Hemisphere snow cover retreated very rapidly last spring, with record and near-record low snow cover extents in May and June despite higher-than-average winter snow extent as of February and March.

Figure 7. This graph shows Antarctic sea ice extent as of January 3, 2011. This year’s sea ice extent is shown in light blue, and 2010 is shown in dark blue. The gray area around the average line shows the two standard deviation range of the data. Sea Ice Index data.

Credit: National Snow and Ice Data Center
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A quick look at Antarctica
After a particularly slow ice loss rate at the beginning of the month, Antarctic sea ice extent was unusually high through much of December, in particular in the northern Ross Sea and the eastern Weddell Sea. December and November have had a markedly strong Amundsen Sea Low, an atmospheric pressure pattern that tends to spread the sea ice cover northward in the Ross Sea. Low pressure over the eastern Weddell Sea later in the month had the same effect there. Sea ice for the Antarctic in December 2011 was the fifth-highest for that month in the satellite record; the highest December extent occurred in 2007. Overall though, Antarctic sea ice extent remained below or near normal for most of 2011. Antarctic sea ice data are available on the Sea Ice Index Web site.

References
Rigor, I. G., and J. M. Wallace. 2004. Variations in the age of Arctic sea-ice and summer sea-ice extent, Geophys. Res. Lett., 31, L09401, doi:10.1029/2004GL019492.