Sea Ice Cover
D. Perovich1, R. Kwok2, W. Meier3, S. Nghiem2, J. Richter-Menge1
1ERDC-Cold Regions Research and Engineering Laboratory, Hanover, NH
New! Monthly Sea Ice Outlook from SEARCH/Arcus
One of the most dramatic signals of the general Arctic-wide warming trend in recent years is the continued significant reduction in the extent of the summer sea ice cover and the decrease in the amount of relatively older, thicker ice. The extent of the 2009 summer sea ice cover was the third lowest value of the satellite record (beginning in 1979) and >25% below the 1979 - 2000 average.
Sea ice extent
Sea ice extent is the primary parameter for summarizing the state of the Arctic sea ice cover. Microwave satellites have routinely and accurately monitored the extent since 1979. There are two periods that define the annual cycle and thus are of particular interest: March, at the end of winter when the ice is at its maximum extent, and September, when it reaches its annual minimum. Maps of ice coverage in March 2009 and September 2009 are presented in Figure S1, with the magenta line denoting the median ice extent for the period 1979 – 2000.
On September 12, 2009 sea ice extent reached a 2009 minimum of 5.1 million km2. The 2009 summer minimum is the third-lowest recorded since 1979. It was 0.6 million km2 greater than 2008 and 1.0 million km2 above the record low in 2007. Surface air temperatures through the 2009 summer were relatively cooler, particularly in the Chukchi and Beaufort seas. Winds in 2009 also tended to disperse the ice pack over a larger region. While the 2009 minimum was an increase over the two previous years, it was still 1.6 million km2 below the 1979 to 2000 average minimum. The March 2009 ice extent was 15.2 million km2, the same as in 2008 and only 4% less that the 1979 – 2000 average of 15.8 million km2.
The time series of the anomalies in sea ice extent in March and September for the period 1979–2009 are plotted in Figure S2. The anomalies are computed with respect to the average from 1979 to 2000. The large interannual variability in September ice extent is evident. Both winter and summer ice extent exhibit a negative trend, with values of -2.5 % per decade for March and -8.9 % per decade for September over the period 1979-2009.
Sea ice age and thickness
The age of the ice is another key descriptor of the state of the sea ice cover, since older ice tends to be thicker and more resilient than younger ice. A simple two-stage approach classifies sea ice into first year and multiyear ice. First-year is ice that has not yet survived a summer melt season, while multi-year ice has survived at least one summer and can be several years old. Satellite derived maps of ice age for March of 2007, 2008, and 2009 are presented in Figure S3.
In the past decade, the extent of multiyear sea ice rapidly reduced at a rate of 1.5 x 106 km2 per decade, triple the reduction rate during the three previous decades (1970-2000). Springtime multiyear ice extent was the lowest in 2008 in the QuikSCAT data record since 2000. QuikSCAT results in March 2009 showed a multiyear ice extent of 3.0 ± 0.2 million km2. This was 0.3 million km2 larger than the multiyear ice extent on the same date in 2008, even though the total sea ice extent was similar in the spring of 2008 and 2009. While the multiyear ice extent was similar in March 2008 and 2009, its distribution was quite different. More specifically, in 2008 there was a significant amount of multiyear ice the Beaufort Sea and in 2009 there was a large amount of multiyear ice the central Arctic Ocean.
Recent estimates of Arctic Ocean sea ice thickness from satellite altimetry show a remarkable overall thinning of ~0.6 m in ice thickness between 2004 and 2008 (Figure. S4a). In contrast, the average thickness of the thinner first-year ice in mid-winter (~2 m), did not exhibit a downward trend. Seasonal ice is an important component covered more than two-thirds of the Arctic Ocean in 2008. The total multiyear ice volume in the winter experienced a net loss of more than 40% in the four years since 2005 while the first year ice cover gained volume due to increased overall coverage of the Arctic Ocean. The declines in total volume and average thickness (black line in Figure S4a) are explained almost entirely by thinning and loss of multiyear sea ice due to melting and ice export. These changes have resulted in seasonal ice becoming the dominant Arctic sea ice type, both in terms of area coverage and of volume.
The recent satellite estimates were compared with the longer historical record of declassified sonar measurements from US Navy submarines (Figure S4b). Within the submarine data release area (covering ~38% of the Arctic Ocean), the overall mean winter thickness of 3.6 m in 1980 can be compared to a 1.9 m mean during the last winter of the ICESat record—a decrease of 1.7 m in thickness. This combined submarine and satellite record shows a long-term trend of sea ice thinning over submarine and ICESat records that span three decades. The contribution of the increasing fraction of first year ice to the long term thickness trend remains to be determined.
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