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
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.
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.
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 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.
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).
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)
Monthly
September ice extent for 1979 to 2011 shows a decline of 12.0% per decade.
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.
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
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.
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
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.
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
