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Open water means a warm Arctic

Natasha Vizcarra — Mon, 15 Oct 2012 18:00:57 +0000

Autumn over the Arctic Ocean is a season of falling temperatures and rapid growth of sea ice. However, as in recent years, low sea ice extent at the beginning of autumn means large transfers of heat to the atmosphere from open water areas, keeping the Arctic warmer than usual. Despite this warmth, freeze up is in high gear.

Overview of conditions

Figure 1. Arctic sea ice extent for October 15, 2012 was 5.18 million square kilometers (2.00 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data

Credit: National Snow and Ice Data Center
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As of October 15, sea ice extent stood at 5.18 million square kilometers (2.00 million square miles). This is 3.49 million square kilometers (1.35 million square miles) below the 1979 to 2000 mean for this time of year and 70,000 square kilometers (27,000 square miles) below the same date in 2007. Although it is still at record low levels, extent is climbing fast.

Large areas of the Kara, Laptev, East Siberian, Chukchi, and Beaufort seas remain open for hundreds of kilometers offshore. East of Greenland, ice extent is near average.

Conditions in context

Figure 2. The graph above shows Arctic sea ice extent as of October 15, 2012, along with daily ice extent data for the previous five years. 2012 is shown in blue, 2011 in orange, 2010 in pink, 2009 in navy, 2008 in purple, and 2007 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
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Arctic sea ice extent is increasing rapidly, at about 100,000 square kilometers (38,600 square miles) per day, expanding southward at the ice edge, as well as northward from the Arctic continental coasts. However, sea ice extent is still at record low levels for the date, compared to the satellite measurement period (1979 to 2012). On October 6, daily sea ice extent climbed above the 2007 record daily minimum (4.17 million square kilometers or 1.61 million square miles), having spent forty days below that level. On October 14, extent went above 5.0 million square kilometers (1.93 million square miles) for the first time since mid-August.

Open water warms the lower atmosphere

Figure 3. This figure shows air temperatures as a function of height and longitude at 75 degrees north latitude. Temperatures are for the period September 20 to October 10, 2012 compared to averages for the years 1981 to 2010. Between longitudes 120 degrees west to 150 degrees west, temperatures more than 4 degrees Celsius (7 degrees Fahrenheit) above normal are found up to the 850 hPa level (roughly 4500 feet above the surface), with temperatures near the surface, in closer proximity to the warming effects of the ocean, more than 6 degrees Celsius (11 degrees Fahrenheit) above normal.

Credit: NSIDC courtesy NOAA/ESRL PSD
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The past decade has seen much stronger increases in air temperature in the Arctic compared to the globe as a whole. While this strong warming, known as Arctic amplification, has a number of causes, a primary one is declining summer sea ice extent. Through summer, solar energy is readily absorbed within expanding areas of dark, open water. When the sun sets, this heat stored in the uppermost layers of the Arctic Ocean is released upwards, keeping the overlying atmosphere unusually warm.

This warming effect has been especially pronounced during the early autumn of 2012. Air temperatures averaged over the most recent thirty days of data are above average over nearly all of the Arctic Ocean. The warmest conditions compared to average are over northwestern Canada and extending into the Beaufort Sea. Here, the effect of heat released from the ocean is augmented by warm, southerly winds, and is linked to a pattern of unusually high pressure centered over the Gulf of Alaska.

Arctic wind patterns support melting, ice export

Figure 4. Sea level pressure for June 2007 to 2012 compared to averages over the period 1981 to 2010. The Arctic dipole anomaly refers to the combination of unusually high pressure over the northern Beaufort Sea and Greenland and unusually low pressure over northern Eurasia.

Credit: NSIDC courtesy NOAA/ESRL PSD
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Recent research led by James Overland of the National Oceanic and Atmospheric Administration (NOAA) and Jennifer Francis of Rutgers University shows that the Arctic dipole anomaly, featuring unusually high pressure over the northern Beaufort Sea and Greenland and unusually low pressure over northeastern Eurasia, has become more common in the early summer of recent years.

As discussed in previous posts, this pattern brings in warm southerly winds along the shores of the East Siberian and Chukchi seas. It favors strong ice melt in these sectors and pushes the ice away from the coast, leaving open water. The pressure pattern also favors the transport of ice out of the Arctic Ocean and into the North Atlantic through Fram Strait.

The Arctic dipole anomaly was very well developed throughout the summer of 2007 and was in part responsible for the very low September ice extent recorded that year (the second lowest in the satellite record). According to Overland and colleagues, no other six-year period matches the intensity and persistence of the June pattern for 2007 to 2012 in the past sixty-three years. The pattern is linked to the general weakening of the circumpolar jet stream and the greater meandering of this wind flow.

Recall from our previous post that the summer of 2012 as a whole saw a somewhat different pattern. Unusually low pressure was present along the Eurasian coastal seas and extended eastward into the Beaufort Sea, most prominently over the East Siberian Sea, with unusually high pressure centered over Greenland and the northern North Atlantic.

Antarctic sea ice extent remains above average

Figure 5. This graph shows the highest extent years for Antarctic sea ice. The blue line shows 2012, the orange line shows 2011, the green line shows 2007, and the pink line shows 2006. The 1979 to 2000 average is in dark gray. The gray area around this 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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Antarctic sea ice extent dropped below record levels seen for the date but remains well above average. Journalists inquired about the cause of gradual growth in wintertime Antarctic sea ice and the record high Antarctic sea ice extent in mid-September despite general climate warming and the dramatic sea ice losses in the Arctic. As we noted in the previous post, researchers cite stronger westerly winds surrounding Antarctica, in contrast to the Arctic. See also the following news stories on Antarctic sea ice, to which NSIDC scientists contributed, under “Further reading” below.

Reference

Overland, J. E., J. A. Francis, E. Hanna, and M. Wang. 2012. The recent shift in early summer Arctic atmospheric circulation. Geophysical Research Letters 39, L19804, doi: 10.1029/2012GL053268.

Summer 2011: Arctic sea ice near record lows

admin — Wed, 05 Oct 2011 02:43:17 +0000

The summer sea ice melt season has ended in the Arctic. Arctic sea ice extent reached its low for the year, the second lowest in the satellite record, on September 9. The minimum extent was only slightly above 2007, the record low year, even though weather conditions this year were not as conducive to ice loss as in 2007. Both the Northwest Passage and the Northern Sea Route were open for a period during September.

Figure 1. Arctic sea ice extent for September 2011 was 4.61 million square kilometers (1.78 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
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Overview of conditions

Average ice extent for September 2011 was 4.61 million square kilometers (1.78 million square miles), 2.43 million square kilometers (938,000 square miles) below the 1979 to 2000 average. This was 310,000 square kilometers (120,000 square miles) above the average for September 2007, the lowest monthly extent in the satellite record. Ice extent was below the 1979 to 2000 average everywhere except in the East Greenland Sea, where conditions were near average.

As in recent years, northern shipping routes opened up this summer. The Northern Sea Route opened by mid August and still appeared to be open as of the end of September. The southern “Amundsen Route” of the Northwest Passage, through the straits of the Canadian Arctic Archipelago, opened for the fifth year in a row. Overall, sea ice in the wider and deeper northern route through Parry Channel reached a record low, according to Stephen Howell of Environment Canada, based on Canadian Ice Service analysis. Parry Channel had a narrow strip of ice that blocked a short section of the channel, but it did appear to open briefly in early September.

For additional numbers for previous years, see Table 1.

Figure 2. The graph above shows daily Arctic sea ice extent as of October 1, 2011, along with daily ice extents for the previous three lowest years for the minimum ice extent. Light blue indicates 2011, dashed green shows 2007, dark blue shows 2010, purple shows 2008, and dark gray shows the 1979 to 2000 average. 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

While the melt season in 2011 got off to a slow start, the ice loss pace quickened during June. Ice retreated quite rapidly in the Kara and Barents seas, with rates more than double the average rate. Rapid ice loss continued during the first half of July but then slowed considerably as a series of low pressure systems moved over the central Arctic Ocean. By the end of July, ice extent was slightly above that seen in 2007.

Ice extent stayed above 2007 for the remainder of the melt season, reaching its minimum of 4.33 million square kilometers (1.67 million square miles) on September 9, 2011. Since the minimum, a rapid freeze-up has begun. On October 1, the five-day average extent rose above 5 million square kilometers (1.93 million square miles).

Figure 3. Monthly September ice extent for 1979 to 2011 shows a decline of 12.0% per decade.—Credit: National Snow and Ice Data Center
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September 2011 compared to past years

Ice extent for September 2011 was the second lowest in the satellite record for the month. The last five years (2007 to 2011) have had the five lowest September extents in the satellite record. The linear rate of decline is now -84,700 square kilometers (-32,700 square miles) per year, or -12% per decade relative to the 1979 to 2000 average. In contrast to 2007, when a “perfect storm” of atmospheric and ocean conditions contributed to summer ice loss, this year’s conditions were less extreme. From the beginning of the melt season in March, to the minimum extent on September 9, the Arctic Ocean lost 10.3 million square kilometers (4.0 million square miles) of sea ice. It was the fifth year in a row with more than 10 million square kilometers of ice extent change from maximum to minimum. In comparison, the average seasonal ice loss during the 1980s was 9.0 million square kilometers (3.5 million square miles)

Figure 4. Ice motion charts for August 2011 show different movement patterns for this summer compared to 2007. The arrows show the direction of ice motion, with larger arrows indicating stronger motion. In 2007, northward ice motion helped push the ice together and flush it out of the Arctic.—Credit: National Snow and Ice Data Center
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Atmospheric conditions

In 2007, a persistent dipole anomaly weather pattern, with unusually high pressure over the Beaufort Sea and unusually low pressure over the Kara Sea, helped contribute to the record ice loss. This pattern resulted in strong southerly winds from the Bering Strait region across the North Pole, which brought warmer winds and ocean waters northward to melt the ice edge and push the ice northward. In addition, especially strong high pressure over the Beaufort and Chukchi Seas in June 2007 resulted in less than average cloudiness, allowing more sunlight to reach the ice.

The Arctic saw a similar weather pattern this summer, but not as strong and persistent as in 2007. The location of the high and low pressure centers was also shifted, so that the winds blew east to west instead of toward the north as in 2007. This shift is reflected in the movement of the sea ice, particularly during August.

Patterns of air temperatures (measured at the 925 millibar level or about 1,000 meters or 3,000 feet above the surface) were also quite different this year compared to 2007. In summer 2007, temperatures in the Beaufort and Chukchi Seas were 5 degrees Celsius (9 degrees Fahrenheit) above average. This year, temperatures in that region were near average, but north of Greenland and in the Canadian Archipelago, conditions were even warmer than in 2007. These high temperatures likely played a role in the opening of the Northwest Passage.

Figure 5. Sea surface temperatures this year were generally lower than in 2007, although some areas of the ocean surface still had higher than average temperatures.—Credit: NSIDC courtesy M. Steele and W. Ermold, Univ. Washington PSC, and NOAA
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Sea surface temperatures

Ocean sea surface temperatures (SSTs), based on National Oceanographic and Atmospheric Administration (NOAA) data provided by Michael Steele and Wendy Ermold of the University of Washington Polar Science Center, indicate above normal temperatures on the surface of the Arctic Ocean. However, the temperatures anomalies were not as extreme as in 2007 and were comparable to those recorded for 2009 and 2010. These lower temperatures may be the result of less solar heating of the exposed ocean surface or less transport of warm waters from the south. In 2007, ice retreated early from the shores of Alaska and Siberia, allowing the ocean mixed layer to heat up and enhance melting of the ice from below. In contrast, ice was slower to retreat in this region in summer 2011, and less bottom melt was observed.

Figure 6. Data on ice age show that coverage of the oldest, thickest ice types (ice four years or older) has declined over the past 28 years.—Credit: National Snow and Ice Data Center courtesy J. Maslanik, C. Fowler, and M. Tschudi, U. Colorado Boulder
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Ice remains younger, thinner

Why did ice extent fall to a near record low without the sort of extreme weather conditions seen in 2007? One explanation is that the ice cover is thinner than it used to be; the melt season starts with more first-year ice (ice that formed the previous autumn and winter) and less of the generally thicker multi-year ice (ice that has survived at least one summer season). First- and second-year ice made up 80% of the ice cover in the Arctic Basin in March 2011, compared to 55% on average from 1980 to 2000. Over the past few summers, more first-year ice has survived than in 2007, replenishing the younger multi-year ice categories (2- to 3-year-old ice). This multi-year ice appears to have played a key role in preserving the tongue of ice extending from near the North Pole toward the East Siberian Sea. However, the oldest, thickest ice (five or more years old) has continued to decline, particularly in the Beaufort and Chukchi Seas. Continued loss of the oldest, thickest ice has prevented any significant recovery of the summer minimum extent. In essence, what was once a refuge for older ice has become a graveyard.

**Table 1.** Previous Arctic sea ice extents
for the month of September
Year	Average Arctic Sea Ice Extent for September		Trend, in % per decade (relative to 1979-2000 avg.)
Year	in millions of square kilometers	in millions of square miles	Trend, in % per decade (relative to 1979-2000 avg.)
2007	4.30	1.66	-10.2
2008	4.67	1.80	-11.1
2009	5.36	2.07	-11.2
2010	4.90	1.89	-11.6
2011	4.61	1.78	-12.0
1979 to 2000 average	7.04	2.72
1979 to 2010 average	6.52	2.52

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.

Sea ice enters critical period of melt season

admin — Wed, 06 Jul 2011 20:25:44 +0000

Arctic sea ice extent for June 2011 was the second lowest in the satellite data record since 1979, continuing the trend of declining summer ice cover. Average ice extent fell below that for June 2007, which had the lowest minimum ice extent at the end of summer. However, ice extent this year was greater than in June 2010. The sea ice has entered a critical period of the melt season: weather over the next few weeks will determine whether the Arctic sea ice cover will again approach record lows.

Figure 1. Arctic sea ice extent for June 2011 was 11.01 million square kilometers (4.25 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
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Overview of conditions

Average ice extent for June 2011 was 11.01 million square kilometers (4.25 million square miles). This is 140,000 square kilometers (54,000 square miles) above the previous record low for the month, set in June 2010, and 2.15 million square kilometers (830,000 square miles) below the average for 1979 to 2000.

June ice extent was lower than normal in much of the Arctic, but the Kara Sea region had particularly low ice extent. Ice has also started to break up off the coast of Alaska in the Beaufort Sea. These open water areas absorb the sun’s energy, which will help to further ice melt through the summer.

Figure 2. The graph above shows daily Arctic sea ice extent as of July 4, 2011, along with daily ice extents for previous low-ice-extent years in the month of May. Light blue indicates 2011, dashed green shows 2007, dark blue shows 2010, and dark gray shows the 1979 to 2000 average. 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

Ice extent during June 2011 declined at an average rate of 80,800 square kilometers (31,200 square miles) per day, about 50% faster than the average decline rate for June 1979 to 2000. Ice extent declined more slowly than in June 2010, the year with the lowest average ice extent for the month. However, ice declined faster than in June 2007, the year when September sea ice extent reached the lowest in the satellite record. Ice loss in the Kara Sea was especially fast, more than double the average rate and close to double the rate of the past four years (2007 to 2010). Sea ice has largely disappeared in the southern Kara Sea, which normally still has considerable ice cover at this time of year.

At the end of June, Arctic sea ice extent was 9.54 million square kilometers (3.68 million square miles), 375,000 square kilometers (145,000 square miles) less than the ice extent on June 30, 2007 and 264,000 square kilometers (102,000 square miles) above the record low for June 30, set in 2010.

Figure 3. Monthly June ice extent for 1979 to 2011 shows a decline of 3.6% per decade. —Credit: National Snow and Ice Data Center
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June 2011 compared to past years

Arctic sea ice extent in June 2011 was the second lowest in the satellite record, consistent with the overall downward trend of the past thirty years. The lowest year for June was 2010. June average ice extent exceeded 12 million square kilometers (4.6 million square miles) 16 out of 21 years between 1979 and 1999, but has been below that value every year since.

Figure 4. This map of air temperature anomalies for June 2011 shows warmer than average temperatures over much of the Arctic Ocean, except in the Greenland and Beaufort seas, where temperatures were near and slightly below normal.

—Credit: NSIDC courtesy NOAA ESRL PSD
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Warmer than average temperatures continue

Air temperatures for June were 1 to 4 degrees Celsius (2 to 7 degrees Fahrenheit) warmer than average over most of the Arctic Ocean, except in the Beaufort and Greenland seas, where temperatures were near normal or slightly below normal. High pressure dominated most of the central Arctic, with the highest pressures over the Beaufort Sea. The monthly averaged pressure field shows a circulation pattern somewhat similar to a pattern known as the dipole anomaly, with unusually high pressure over the Beaufort Sea and unusually low pressure over central Siberia. Similar patterns have become common in recent summers.

Figure 5. This MODIS image from June 28 shows ice in the Beaufort Sea region off the coast of Barrow, breaking up into smaller floes and open water. But while open water is apparent, a layer of ice still clings to the coastline.—Credit: NSIDC courtesy MODIS Rapid Response System Arctic Mosaic
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A detailed view from MODIS data

Data from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) provide a detailed view of seasonal ice breakup. Along the Alaska coast, large ice floes are breaking away near the coast southwest of Barrow. However, in this image from June 28, a narrow strip of landfast ice remained anchored to the coast, bounded on the seaward side by grounded ridged ice. That last bit of ice broke up around July 3, according to the Geophysical Institute at the University of Alaska, Fairbanks

Sea ice breakup in Barrow is defined as the time when the landfast ice along the coast starts to move. The timing of this breakup is closely tied to the cumulative amount of solar energy input to the Barrow region—the amount of sunshine the area receives—after June 5. The Geophysical Institute uses this relationship to forecast the breakup: this year they predicted the breakup to occur on July 11.

Further ReadingOcean heat

An article published recently in the journal Science showed that the flow of ocean heat into the Arctic Ocean from the Atlantic is now higher than any time in the past 2000 years. The warm, salty Atlantic water flows up from the mid-latitudes and then cools and sinks below the cold, fresh water from the Arctic. The higher salt content of the Atlantic water means that it is denser than fresher Arctic water, so it circulates through the Arctic Ocean at a depth of around 100 meters (328 feet). This Atlantic water is potentially important for sea ice because the temperature is 1 to 2 degrees Celsius (1.5 to 3 degrees Fahrenheit) above freezing. If that water rose to the surface, it could add to sea ice melt.

Spielhagen, R.F., K. Werner, S. Sorensen, K. Zamelczyk, E. Kandiano, G. Budeus, K. Husum, T.M. Marchitto, M. Hald, 2011. Enhanced modern heat transfer to the Arctic by warm Atlantic Water, Science, vol. 331, pp. 450-453, 28.

New ice thickness data

The first preliminary map of sea ice thickness data from the European Space Agency’s Cryosat-2 was released in June. This radar altimeter measures the height of features such as snow and sea ice on the Earth surface. This initial map is very preliminary and considerable work will be required before the thickness retrievals are validated and useful for scientific study. Researchers expect that Cryosat-2 will eventually provide additional information about changes in sea ice thickness and volume. http://www.esa.int/esaLP/SEMAAW0T1PG_LPcryosat_0.html

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.

Low ice extent in May, but summer melt will depend on weather

admin — Mon, 06 Jun 2011 20:24:06 +0000

Arctic sea ice extent for May 2011 was the third lowest in the satellite data record since 1979, continuing the long-term decline. During the month of May, sea ice declined at a near average rate, while air temperatures in the Arctic remained generally above average. Although ice extent is low for this time of year, ice extent at the end of summer largely depends on weather over the next few months.

Figure 1. Arctic sea ice extent for May 2011 was 12.79 million square kilometers (4.94 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
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Overview of conditions

Average ice extent for May 2011 was 12.79 million square kilometers (4.94 million square miles). This is 210,000 square kilometers (81,000 square miles) above the previous record low for the month, set in May 2004, and 810,000 square kilometres (313,000 square miles) below the average for the reference period of 1979 to 2000.

Ice extent remained lower than average in the Atlantic sector of the Arctic, including the Kara and Barents seas and the Labrador Sea. During May, areas of open water known as polynyas continued to develop in the Beaufort, Chukchi, and Laptev seas, and Hudson Bay. Those open water areas absorb the sun’s energy, which will likely help to hasten further ice melt.

Figure 2. The graph above shows daily Arctic sea ice extent as of June 1, 2011, along with daily ice extents for previous low-ice-extent years in the month of May. Light blue indicates 2011, dashed green shows 2007, dark blue shows 2010, and dark gray shows the 1979 to 2000 average. 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

Ice extent during May 2011 declined at an average rate of 50,720 square kilometers (19,580 square miles) per day. That was close to the average decline rate for May 1979 to 2000 of 46,000 square kilometers (18,000 square miles) per day. Ice extent declined more slowly than in May 2010, but faster than in May 2007, the year when September sea ice extent reached the lowest in the satellite record.

At the end of May, Arctic sea ice extent was 11.99 million square kilometers (4.63 million square miles), 175,000 square kilometers (67,600 square miles) less than the ice extent on May 31, 2007 and 48,790 square kilometers (18,800 square miles) above the previous record low for May 31, set in 2010.

Air temperatures averaged for the month of May were 4 to 5 degrees Celsius (7 to 9 degrees Fahrenheit) warmer than normal in the Beaufort and Chukchi seas, western Siberia, and in the Kara Sea. The areas with high air temperatures correspond to locations where ice retreated and polynyas formed. By contrast, temperatures were 2 to 5 degrees Celsius (4 to 9 degrees Fahrenheit) colder than normal in Baffin Bay and the East Siberian Sea.

Figure 3. Monthly May ice extent for 1979 to 2011 shows a decline of 2.4% per decade. —Credit: National Snow and Ice Data Center
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May 2011 compared to past years

Arctic sea ice extent in May 2011 was the third lowest in the satellite record, continuing the overall downward trend of the past thirty years. The two lowest years for May were 2004 and 2006, respectively. In 2004, average May ice extent fell below 13 million square kilometres (5 million square miles) for the first time. Since then there have been five years when ice averaged less than that benchmark in May. The long-term rate of decline for May now stands at -2.4% per decade.

Figure 4. This map of sea level pressure for May 2011 shows unusually high pressure over the Canadian Archipelago and Greenland, as well as central Siberia (green and yellow).—Credit: NSIDC courtesy NOAA ESRL PSD
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Atmospheric pressure and ice loss

Atmospheric pressure patterns for the month of May contributed to the formation of open water areas in some regions of the Arctic. During May, high pressure settled over northeastern Canada and Greenland, with low pressure over the Chukchi and East Siberian seas. This pattern helped push ice in the Beaufort Sea away from the coast and to the west, helping create areas of open water north of Tuktoyaktuk, Canada. Higher-than-normal pressure over central Siberia helped bring unusually warm temperatures over western Siberia and the Kara Sea, along with winds that pushed ice away from the coast in the Laptev Sea near the New Siberian Islands. This pressure pattern also led to above-average temperatures in the Beaufort Sea, and unusually cold conditions over the East Siberian Sea. Lower-than-normal pressure also dominated the Norwegian Sea and the northwestern Pacific Ocean.

The last four summers have been dominated by an atmospheric pattern known as the Arctic dipole anomaly, which has been associated with low sea ice extent at the end of summer. This pattern features unusually high pressure over the Beaufort Sea and unusually low pressure over the Kara and Laptev Seas, which promote warm southerly winds along the Siberian coast, helping to melt ice and push it away from the coasts and out of the Arctic Basin through Fram Strait.

While the atmospheric pattern for May 2011 bears some resemblance to the Arctic dipole anomaly pattern, the centers of the pressure anomalies are in different locations this year, and it is not yet clear whether the pattern will persist through the summer and contribute to low ice extent.

Figure 5. Far fewer storms are found in the Arctic during May, June and July in years with low sea ice at the end of summer (left) than in years with high sea ice at the end of summer (right). —Credit: NSIDC courtesy James Screen, Unversity of Melbourne
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Importance of storms

Arctic weather in the next few months will be a critical factor in how much ice remains at the end of the melt season. New research led by James Screen at the University of Melbourne shows that the storms that move northwards into the Arctic from the lower latitudes during summer strongly influence sea ice extent at the end of summer. Years with dramatic ice loss, such as 2007, have been associated with comparatively warm, calm, and clear conditions in summer that have encouraged ice melt. Summers with slow melt rates are opposite and tend to be stormier than average. The number of storms influences how warm, windy and cloudy the Arctic summer is.

ReferencesScreen, J., I. Simmonds, and K. Keay. 2011. Dramatic inter-annual changes of perennial Arctic sea ice linked to abnormal summer storm activity, J. Geophys. Res., doi:10.1029/2011JD015847, in press.

Stroeve, J.C., M.C. Serreze, M.M. Holland, J. Kay, J. Maslanik, A. P. Barrett. 2011. The Arctic’s rapidly shrinking sea ice cover: a research synthesis, Climatic Change, in press.

Wang, J., J. Zhang, E. Watanabe, M. Ikeda, K. Mizobata, J.E. Walsh, X. Bai, and B. Wu. 2009. Is the Dipole Anomaly a major driver to record lows in Arctic summer sea ice extent?, Geophys. Res. Letts., doi:10.1029/2008GL036706.

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.

End of summer approaches for Arctic sea ice

admin — Tue, 07 Sep 2010 19:50:58 +0000

Arctic sea ice generally reaches its annual minimumextent in mid-September. This August, ice extent was the second lowest in the satellite record, after 2007. On September 3, ice extent dropped below the seasonal minimum for 2009 to become the third lowest in the satellite record.

The Northwest Passage and the Northern Sea Route are largely free of ice, allowing the potential for a circumnavigation of the Arctic Ocean. At least two expeditions are attempting this feat, the Norwegian explorer Borge Ousland and the Peter I yacht from Russia.

Figure 1. Arctic sea ice extent for August 2010 was 5.98 million square kilometers (2.31 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
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Overview of conditions

Average ice extent for August was 5.98 million square kilometers (2.31 million square miles), 1.69 million square kilometers (653,000 square miles) below the 1979 to 2000 average, but 620,000 square kilometers (240,000 square miles) above the average for August 2007, the lowest August in the satellite record. Ice extent remained below the 1979 to 2000 average everywhere except in the East Greenland Sea near Svalbard.

The minimum ice extent for the year will probably occur in the next two weeks. NSIDC scientists are closely monitoring conditions and will report the minimum when it occurs.

Figure 2. The graph above shows daily Arctic sea ice extent as of September 6, 2010, along with daily ice extents for years wtih the four lowest minimum extents. The solid light blue line indicates 2010; orange shows 2009, pink shows 2008; dashed green shows 2007; light green shows 2005; and solid gray indicates average extent from 1979 to 2000. 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

At the end of August, ice extent had fallen to the fourth lowest in the satellite record, behind the seasonal minima recorded for 2007, 2008, and 2009. On September 3, ice extent fell below the seasonal minimum for 2009 to claim third lowest on record, with perhaps one to two weeks left in the melt season.

The daily rate of decline for August was 55,000 square kilometers (21,000 square miles) per day, close to the 1979 to 2000 average of 54,000 square kilometers (21,000 square miles).

Figure 3. Monthly August ice extent for 1979 to 2010 shows a decline of 8.9% per decade. —Credit: National Snow and Ice Data Center
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August 2010 compared to past years

Ice extent for August 2010 was the second lowest in the satellite record for the month. The linear rate of decline of August ice extent over the period 1979 to 2010 is now 8.9% per decade.

Figure 4. This map of sea level pressure for August 2010 shows a return of the dipole anomaly, which was present in June but not in July.—Credit: NSIDC courtesy NOAA/ESRL PSD
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Return of the dipole anomaly

In August, a pattern of higher than average pressure over the northern Beaufort Sea and lower than average pressure over the Siberian side of the Arctic replaced the stormy and cool weather conditions that persisted through July. This atmospheric pattern, known as the dipole anomaly, brought relatively warm southerly winds into the Beaufort and Chukchi seas, where air temperatures were 1 to 3 degrees Celsius (1.8 to 5.4 degrees Fahrenheit) above normal for the month of August. The warmth enhanced melt in the region, and southerly winds contributed to ice loss by pushing the ice edge northward. This pattern is similar to the pattern at the end of the 2007 melt season, but not as pronounced. Air temperatures this August were also 1 to 3 degrees Celsius (1.8 to 5.4 degrees Fahrenheit) below normal over the Barents and Kara Seas.

Figure 5. This image from NASA’s MODIS sensor on the Aqua satellite on August 25, 2010, shows open water and low-concentration ice in the Beaufort Sea, the region where large amounts of rotten ice were observed last year.—Credit: National Snow and Ice Data Center courtesy NASA/GSFC MODIS Rapid Response
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Rotten ice in the Beaufort and Chukchi seas

Last year, Dave Barber, a researcher from the University of Manitoba, reported unusual conditions in the Beaufort Sea with large regions of rotten ice. Satellite imagery from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) sensors suggest similar conditions this year in the Beaufort and Chukchi seas, where there are large areas with unconsolidated ice floes and low ice concentration.

Figure 6. This graph of regional ice loss in the Arctic shows faster than normal ice loss in the Beaufort and Chukchi seas, and slower than normal ice loss in the East Siberian Sea and Central Arctic. The map in the bottom left corresponds to the regions plotted across the top of the graph. Colors in the bar graph correspond to August ice loss in different years.—Credit: National Snow and Ice Data Center
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Regional ice loss

The rate of ice loss in the summer varies from region to region depending on local air and ocean temperatures and wind patterns. This August, the decline in ice extent was unusually fast in the Beaufort and Chukchi Sea, likely because of the rotten ice that melted out completely. In addition, southerly winds linked to the dipole anomaly pattern brought warmer air into the region and helped push the ice edge northward.

However, the loss rate in the East Siberian Sea and the Central Arctic was slower than any of the past three years, and was also fairly slow (slower than the 1979 to 2000 average rate) in the Laptev and Kara Seas. The reason for slow ice loss in the Kara Sea, however, is that there was already very little ice in that region at the beginning of August. Such year-to-year variations demonstrate the importance of weather conditions in determining regional ice loss.

Further Reading

Barber, D. G., Galley, R., Asplin, M. G., De Abreau, R., K. A. Warner, M. Pucko, M. Gupta, S. Prinsenberg, and S. Julien, 2009: Perennial pack ice in the southern Beaufort Sea was not as it appeared in the summer of 2009, Geophysical Research Letters, 36, L24501, doi:10.1029/2009GL041434.

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.

A change in atmospheric circulation

admin — Tue, 20 Jul 2010 19:46:57 +0000

The rate of ice loss slowed in the first half of July, primarily because of a change in atmospheric circulation. The dipole anomaly, an atmospheric pattern that dominated the Arctic in June, broke down. It was replaced by a pattern of low-pressure systems tracking across northern Eurasia and then into the central Arctic Ocean.

Figure 1. Daily Arctic sea ice extent on July 15 was 8.37 million square kilometers (3.23 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole. Sea Ice Index data. About the data. —Credit: National Snow and Ice Data Center
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Overview of conditions

From July 1 to 15, Arctic sea ice extent declined an average of 60,500 square kilometers (23,400 square miles) per day, 22,500 square kilometers (8,690 square miles) per day slower than the 1979 to 2000 average and substantially slower than the rate of decline in May and June.

Ice extent remained lower than normal in all regions of the Arctic, with open water developing along the coasts of northwest Canada, Alaska and Siberia.

Figure 2. The graph above shows daily Arctic sea ice extent as of July 15, 2010. The solid light blue line indicates 2010; dashed green shows 2007; solid pink shows 2006, and solid gray indicates average extent from 1979 to 2000. 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

As of July 15, total extent was 8.37 million square kilometers (3.23 million square miles), which is 1.62 million square kilometers (625,000 square miles) below the 1979 to 2000 average for the same date, but 360,000 square kilometers (139,000 square miles) above July 15, 2007, the lowest extent for that date in the satellite record.

Figure 3. This map of sea level pressure for July 1 to 15, 2010 shows low pressure over the central Arctic Ocean, a pattern that brought cooler and cloudier conditions.—Credit: National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Division
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A change in circulation

Through much of May and June, high pressure dominated the Beaufort Sea with low pressure over Siberia. Winds associated with this pattern, known as the dipole anomaly, helped speed up ice loss by pushing ice away from the coast and promoting melt.

However, the dipole anomaly pattern broke down in early July. In the first half of July, cyclones (low pressure systems) generated over northern Eurasia tracked eastward along the Siberian coast and then into the central Arctic Ocean, where they tend to stall. This cyclone pattern is quite common in summer. The low-pressure cells have brought cooler and cloudier conditions over the Arctic Ocean. They have also promoted a cyclonic (anticlockwise) sea ice motion, which acts to spread the existing ice over a larger area. All of these factors likely contributed to the slower rate of ice loss over the past few weeks.

In the last few days, high pressure has started to build again in the Beaufort Sea, but whether this will continue remains to be seen.

Figure 4. In mid-summer, the NASA Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E) (left) may show areas of low ice concentration which are actually melt ponds or weather effects. Visible band images from the NASA Moderate Resolution Imaging Spectroradiometer (right) confirm areas of low-concentration sea ice in the interior pack ice. Both images are from July 12, 2010.—Credit: National Snow and Ice Data Center
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Areas of diffuse ice

Satellite images provided by the University of Bremen, from the NASA Advanced Microwave Scanning Radiometer – Earth Observing System (AMSR-E), show areas of low ice concentration over the central Arctic pack ice. While we normally report on the extent of area covered by at least fifteen percent sea ice, a more reliable measurement, it is also valuable to look at ice concentration values, which can reveal conditions in more detail. However, it can be difficult to interpret AMSR-E concentration data during the summer, because microwave signals associated with low ice concentration look very much like signals associated with surface melt. Weather effects can also cause false concentration signals.

By comparing AMSR-E data with data from other satellites, we can determine which areas of apparent low-concentration ice are real, and which appear to be low because of melt or atmospheric effects. Visible-band images from the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) sensor show that some of the areas of apparent low ice concentration within the central pack ice are actually melt and atmospheric effects. However, the MODIS data also confirm that there are substantial areas of open water within the pack ice, such as near the North Pole and in the Beaufort Sea.

Open water in the interior pack ice is not unprecedented. Winds can push the ice apart, creating openings in the pack ice. These areas of open water may close up quickly if the wind changes, but since the dark areas of open water readily absorb solar energy, they can also lead to more extensive melt.

Further Reading

Serreze, M., and A. P. Barrett. 2007. The Summer Cyclone Maximum over the Central Arctic Ocean. Journal of Climate 21, pp. 1048-1065. doi: 10.1175/2007JCLI1810.1

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Rapid ice loss continues through June

admin — Tue, 06 Jul 2010 19:45:53 +0000

Average June ice extent was the lowest in the satellite data record, from 1979 to 2010. Arctic air temperatures were higher than normal, and Arctic sea ice continued to decline at a fast pace. June saw the return of the Arctic dipole anomaly, an atmospheric pressure pattern that contributed to the record sea ice loss in 2007.

Figure 1. Arctic sea ice extent for June 2010 was 10.87 million square kilometers (4.20 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
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Overview of conditions

Arctic sea ice extent averaged 10.87 million square kilometers (4.20 million square miles) for the month of June, 1.29 million square kilometers (498,000 square miles) below the 1979 to 2000 average and 190,000 square kilometers (73,000 square miles) below the previous record low for the month of 11.06 million square kilometers (4.27 million square miles), set in 2006. In June, ice extent declined by 88,000 square kilometers (34,000 square miles) per day, more than 50% greater than the average rate of 53,000 square kilometers (20,000 square miles) per day. This rate of decline is the fastest measured for June.

During June, ice extent was below average everywhere except in the East Greenland Sea, where it was near average.

Figure 2. The graph above shows daily Arctic sea ice extent as of July 5, 2010. The solid light blue line indicates 2010; dashed green shows 2007; solid pink shows 2006, and solid gray indicates average extent from 1979 to 2000. 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

At the end of May 2010, daily ice extent fell below the previous record low for May, recorded in 2006, and during June continued to track at record low levels. By the 30th of June, the extent was 510,000 square kilometers (197,000 square miles) below the same day in 2006.

Weather conditions, atmospheric patterns, and cloud cover over the next month will play a major role in determining whether the 2010 sea ice decline tracks at a level similar to 2007, or more like 2006. Although ice extent was greater in June 2007 than June 2006, in July 2007 the ice loss rate accelerated. That fast decline led up to the record low ice extent of September 2007.

However, it would not be surprising to see the rate of ice loss slow in coming weeks as the melt process starts to encounter thicker, second and third year ice in the central Arctic Ocean. Loss of ice has already slowed in the Beaufort and Chukchi Seas due to the tongue of thicker, older ice in the region noted in our April update.

Figure 3. Monthly June ice extent for 1979 to 2010 shows a decline of 3.5% per decade. —Credit: National Snow and Ice Data Center
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June 2010 compared to past years

Average ice extent for June 2010 was190,000 square kilometers (73,000 square miles) less than the previous record low for June, observed in 2006; 620,000 square kilometers (240,000 square miles) below that observed in 2007; and 1.29 million square kilometers (498,000 square miles) below the average extent for the month.

The linear rate of monthly decline for June over the 1979 to 2010 period is now 3.5% per decade. This year’s daily June rate of decline was the fastest in the satellite record; the previous record for the fastest rate of June decline was set in 1999. This rapid decline was in part driven by ice loss in Hudson Bay.

Figure 4. This map of sea level pressure for June 2010 shows a return of the Arctic dipole anomaly pattern, with unusually high pressure (yellow and orange) over the northern Beaufort Sea and unusually low pressure (purple and blue) over the Eurasian coast.—Credit: National Snow and Ice Data Center courtesy NOAA/ESRL Physical Sciences Division
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The Arctic dipole anomaly

The record low ice extent of September 2007 was influenced by a persistent atmospheric pressure pattern called the summer Arctic dipole anomaly (DA). The DA features unusually high pressure centered over the northern Beaufort Sea and unusually low pressure centered over the Kara Sea, along the Eurasian coast. In accord with Buys Ballot’s Law, this pattern causes winds to blow from the south along the Siberian coast, helping to push ice away from the coast and favoring strong melt. The DA pattern also promotes northerly winds in the Fram Strait region, helping to flush ice out of the Arctic Ocean into the North Atlantic. The DA pattern may also favor the import of warm ocean waters from the North Pacific that hastens ice melt.

June 2010 saw the return of the DA, but with the pressure centers shifted slightly compared to summer 2007. As a result, winds along the Siberian coastal sector are blowing more from the east rather than from the south. Whether or not the DA pattern persists through the rest of summer will bear strongly on whether a new record low in ice extent is set in September 2010.

Figure 5. This satellite image, acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the NASA Terra satellite on June 30, 2010, shows that Nares Strait was open and sea ice was flowing through it. Normally Nares Strait remains plugged by an “ice arch” through early July, but this year it was clear by May.—Credit: National Snow and Ice Data Center courtesy NASA/GSFC MODIS Rapid Response
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Nares Strait

Ron Kwok of the Jet Propulsion Laboratory (JPL) reports that Nares Strait, the narrow passageway between northwest Greenland and Ellesmere Island is clear of the ice “arch” that usually plugs southward transport of the old, thick ice in the Lincoln Sea. Typically the ice arch forms in winter and breaks up in early July. This year the arch formed around March 15th and lasted only 56 days, breaking up in May. In 2007 the ice arch did not form at all, allowing twice as much export through Nares Strait than the annual mean. Although the export of sea ice out of the Arctic Ocean through Nares Strait is very small in comparison to the export through Fram Strait, the Lincoln Sea contains some of the Arctic’s thickest ice. For the ice flux rates out of Nares strait, see Figure 5a.

Figure 6. The graph above shows daily Antarctic sea ice extent as of July 5, 2010. The solid light blue line indicates 2010; dashed green shows 2007, and solid gray indicates average extent from 1979 to 2000. 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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Meanwhile, in Antarctica

At the end of June, Southern Hemisphere mid-winter, the sea ice surrounding Antarctica was more than two standard deviations greater than normal. On June 30, Antarctic sea ice extent was15.88 million square kilometers (6.13 million square miles), compared to the 1979 to 2000 average of 14.64 million square kilometers (5.65 million square miles) for that day.

While recent studies have shown that wintertime Antarctic sea ice has a weak upward trend, and substantial variability both within a year and from year to year, the differences between Arctic and Antarctic sea ice trends are not unexpected. Climate models consistently project that the Arctic will warm more quickly than the Antarctic, largely due to the strong climate feedbacks in the Arctic. Warming is amplified by the loss of ice cover in the Arctic Ocean in areas that had been ice-covered for decades, and by the warming of Arctic lands as snow cover is lost earlier and returns later than in recent decades.

Moreover, rising levels of greenhouse gases and the loss of stratospheric ozone appear to be affecting wind patterns around Antarctica. Shifts in this circulation are referred to as the Antarctic Oscillation (AAO). As greenhouse gases have increased, and especially when ozone is lost in spring, there is a tendency for these winds to strengthen (a positive AAO index). The net effect is to push sea ice eastward, and northward, increasing the ice extent. As the current sea ice anomaly has developed, the AAO index has been strongly positive. See the NOAA AAO Index Web site. For more information about the differences between sea ice dynamics in the Arctic and Antarctic, see the NSIDC All About Sea Ice Web site.

References

Arblaster, J. M., and G. A. Meehl. 2006. Contributions of External Forcings to Southern Annular Mode Trends. Journal of Climate, 19: 2896-2905.

Hall, A., and M. Visbeck. 2002. Synchronous Variability in the Southern Hemisphere Atmosphere, Sea Ice, and Ocean Resulting from the Annular Mode. Journal of Climate, 15: 3047-3053.

Kwok, R. 2005. Variability of Nares Strait ice flux. Geophys. Res. Lett., 32, L24502, doi:10.1029/2005GL024768

Kwok, R., L. Toudal Pedersen, P. Gudmandsen, and S. S. Pang. 2010. Large sea ice outflow into the Nares Strait in 2007. Geophys. Res. Lett., 37, L03502, doi:10.1029/2009GL041872.

Thompson, W. J., and S. Solomon. 2002. Interpretation of Recent Southern Hemisphere Climate Change. Science. 296, 895-899, doi:10.1126/science.1069270

Wang, J., J. Zhang, E. Watanabe, M. Ikeda, K. Mizobata, J. E. Walsh, X. Bai, and B. Wu. 2009. Is the Dipole Anomaly a major driver to record lows in Arctic summer sea ice extent? Geophys. Res. Lett., 36, L05706, doi:10.1029/2008GL036706.

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.

Arctic ice melts quickly through July

admin — Tue, 04 Aug 2009 17:36:19 +0000

Sign up for the Arctic Sea Ice News RSS feed for automatic notification of analysis updates. Updates are also available via Twitter.Arctic sea ice extent for the month of July was the third lowest for that month in the satellite record, after 2007 and 2006. The average rate of melt in July 2009 was nearly identical to that of July 2007. A strong high-pressure system, similar to the atmospheric pattern that dominated the summer of 2007, brought warm winds and clear skies to the western Arctic, promoting ice melt.

Figure 1. Arctic sea ice extent for July 2009 was 8.81 million square kilometers (3.40 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
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Overview of conditions

Sea ice extent averaged over the month of July 2009 was 8.81 million square kilometers (3.40 million square miles). This was 680,000 square kilometers (263,000 square miles) above the record low that occurred in July 2007, 250,000 square kilometers (97,000 square miles) below July 2008, and 1.29 million square kilometers (498,000 square miles) below the 1979 to 2000 average. Sea ice extent is unusually low in the Kara Sea, Baffin Bay, and along the Russian coast. The only area with significant above-average ice extent is southern Hudson Bay.

Figure 2. The graph above shows daily sea ice extent as of August 2, 2009. The solid blue line indicates 2009; the dashed green line shows 2007; and the solid gray line indicates average extent from 1979 to 2000. 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 average pace of ice loss during July 2009 was nearly identical to that of July 2007. Ice loss sped up during the third week of July, and slowed again during the last few days of the month.

Averaged for the month, July 2009 saw a decline rate in ice extent of 106,000 square kilometers (41,000 square miles) per day. For comparison, the rate of decline for July 2007 was 107,000 square kilometers (41,000 square miles) per day and the July 2008 rate of decline was 94,000 square kilometers (36,000 square miles) per day. The Arctic Ocean lost a total of 3.19 million square kilometers (1.23 million square miles) of ice during July 2009, and dropped below ice extent at this time in 2008.

Figure 3. Monthly July ice extent for 1979 to 2009 shows a decline of 6.1% per decade. —Credit: National Snow and Ice Data Center
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July 2009 compared to past years

Ice extent averaged for July 2009 was the third lowest in the satellite record for the month of July. The long-term trend indicates a decline of 6.1% per decade in July ice extent since 1979, relative to the 1979 to 2000 average, an average of 62,000 square kilometers (24,000 square miles) of ice per year.

Figure 4. The map of sea level pressure (in millibars) from July 1 to 31, 2009 shows a strong high-pressure cell over the Beaufort Sea. In 2007, a similar high-pressure cell, combined with unusually low pressure over eastern Siberia, contributed to the record melt.—Credit: From National Snow and Ice Data Center, courtesy NOAA/ESRL Physical Sciences Division
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Weather patterns bring clear skies, warmth

The atmospheric circulation pattern for summer 2009 has been similar to the pattern in summer 2007. As in 2007, an unusually strong high-pressure cell (an anticyclone) settled over the Beaufort Sea, bringing warm air into the Chukchi Sea. This year, the Beaufort Sea anticyclone, averaged for June and July 2009, was even stronger than the anticyclone in 2007. However, unlike 2007, this year the Beaufort Sea high-pressure cell is not paired with unusually low pressure over eastern Siberia, the “dipole” pattern that in 2007 promoted strong surface winds and extreme melt.

The strong Beaufort Sea high-pressure cell that occurred both this summer and in 2007 is part of a larger scale atmospheric pattern known as the Pacific North American (PNA) “teleconnection.” The airflow in the western hemisphere is usually characterized by a low pressure trough over the North Pacific, a ridge over western North America, and a trough over eastern North America. The PNA describes the strength of this pattern. When the PNA is positive, the normal pattern is amplified and the airflow becomes more “wavy” than usual. While the expressions of the PNA vary by season, the strong western North American ridge during the positive PNA favors a strong Beaufort Sea high pressure system. The stronger than usual trough over eastern North America also helps to explain the cool and rainy weather that has gripped this area much of the summer.

Figure 5. The Moderate Resolution Imaging Spectroradiometer (MODIS) image (top), acquired by the NASA Terra satellite on on July 26, 2009, shows unusually clear skies over much of the Arctic, in particular over the Beaufort Sea. Dark gray areas over the sea ice correspond to areas of high surface melt. Maps derived from CloudSat satellite data (bottom) compare total cloud fraction averaged for May 20 to July 20, in 2007, 2008 and 2009.—Credits: From National Snow and Ice Data Center courtesy A. NASA MODIS Rapid Response Arctic Mosaic and B. CloudSat Image courtesy Jennifer Kay, NCAR
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Clear skies favor melt in the Beaufort Sea

In 2007, unusually sunny skies throughout the summer melt season were one of the factors that helped lead to the record low ice extent. The clear skies allowed more of the sun’s energy to reach the surface, melting the ice and warming the ocean. This year, cloud fields provided by Jennifer Kay at the National Center for Atmospheric Research show fewer clouds over the Beaufort Sea than in 2007, leading to strong melt in that region. However, over the Chukchi and East Siberian Seas, the Arctic sky has been cloudier than 2007.

ReferencesKay, J. E., and A. Gettelman. 2009. Cloud influence on and response to seasonal Arctic sea ice loss. Journal of Geophysical Research, In press, doi:10.1029/2009JD011773.

L’Heureux, M. L., A. Kumar, G. Bell, M. Halpert, and R. Higgins. 2008. Role of the Pacific-North American (PNA) pattern in the 2007 Arctic sea ice decline. Geophys. Res. Lett., 45, L20701, doi:10.1029/2008GL035205.

Wang, J., J. Zhang, E. Watanabe, M. Ikeda, K. Mizobatu, J. E. Walsh, X. Bai, and B. Wu. 2009. Is the dipole anomaly a major driver to record lows in Arctic summer sea ice extent? Geophys. Res. Lett., 36, L05706, doi:10.1029/2008GL036706.

For previous analyses, please see the drop-down menu under Archives in the right navigation at the top of this page.