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Sea Ice

D. Perovich1,2, W. Meier3, J. Maslanik4, J. Richter-Menge1

1ERDC - CRREL, 72 Lyme Road, Hanover, NH 03755
2Thayer School of Engineering, Dartmouth College, Hanover NH 03755
3National Snow and Ice Data Center, University of Colorado, Boulder, CO 80309
4Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309

November 9, 2011

Highlights

  • Minimum Arctic sea ice extent in September 2011 was the second lowest recorded by satellite since 1979.
  • Based on passive microwave remote sensing data, the Northwest Passage (both the northern and southern routes) and the Northern Sea Route were open in 2011 for the second consecutive year.

Sea ice extent

Sea ice extent is the primary variable for summarizing the state of the Arctic sea ice cover. Passive microwave instruments on 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: September, at the end of summer, when the ice reaches its annual minimum extent, and March, at the end of winter, when the ice is at its maximum extent. Maps of ice extent in September 2011 and March 2011 are presented in Fig. SIO1.

Fig. 1 -- Sea ice extent in March 2011 and September 2011

Fig. SIO1. Sea ice extent in March 2011 (left) and September 2011 (right), illustrating the respective monthly winter maximum and summer minimum extents. The magenta line indicates the median maximum and minimum ice extents in the given month for the period 1979-2000. Maps are from the National Snow and Ice Data Center Sea Ice Index: nsidc.org/data/seaice_index.

On September 9, 2011, sea ice extent reached a minimum for the year of 4.33 million km2. The 2011 minimum is the second lowest, only 0.16 million km2 greater than the 2007 record minimum. Overall, the 2011 minimum was 31% (2.08 million km2) smaller than the 1979-2000 average. The last five summers (2007-2011) have experienced the five lowest minima in the satellite record, and the past decade (2002-2011) has experienced nine of the ten lowest minima.

The March 2011 maximum ice extent was 14.64 million km2, 7.7% less than the 1979-2000 average of 15.86 million km2. The yearly maximum sea ice extent occurred on March 7. This was about normal compared to the 1979-2000 average. As in 2010, the extent around the maximum data tended to be flat, undulating up and down in response to weather-induced variations in the ice edge.

The time series of the anomalies in monthly average sea ice extent in March and September for the period 1979-2011 are shown in Fig. SIO2. The anomalies are computed with respect to the average from 1979 to 2000. The large inter-annual variability in September ice extent is evident. Both winter and summer ice extent exhibit a negative trend, with values of -2.7 % per decade for March and -12.0% per decade for September over the period 1979-2011.

Fig. 2 -- Time series of the percentage difference in ice extent in March and September

Fig. SIO2. Time series of the percentage difference in ice extent in March (the month of ice extent maximum) and September (the month of ice extent minimum) relative to the mean values for the period 1979-2000. Based on a least squares linear regression for the period 1979-2011, the rate of decrease for the March and September ice extents is -2.7% and -12.0% per decade, respectively.

Monthly ice extents averaged for the periods 1979-2000 and 2001-2011 are plotted in Fig. SIO3. Ice extents for the period 2001-2011 are smaller than 1979-2000 for all months. For most months, the ice extent curves are more than a standard deviation apart. The decline in ice extent is greatest in August, September, and October. Those months also show an increase in inter-annual variability for the 2001-2011 period. The difference in ice extent between these two periods suggests a shift to a new regime of reduced sea ice.

Fig. 3 -- Mean monthly sea ice extent for 1979-2000 and for 2001-2011
Fig. SIO3. Mean monthly sea ice extent for the period 1979-2000 (thick black line) and for the period 2001-2011 (thick red line). The vertical bars represent one standard deviation about the mean.

Ice spatial distribution

The spatial distribution of September sea ice extent for the past five years is shown in Fig. SIO4. For reference, the ice extent for September 1980 is also shown. Large changes from 1980 are evident, with a pronounced retreat of ice around the periphery of the Arctic Basin and ice loss in the Canadian Archipelago. There is a similar pattern to ice distribution in the past three years. Ice remains in the central Arctic, along northeast Greenland, and adjacent to the northern edge of the Canadian Arctic Archipelago. North of Alaska and Siberian the ice edge forms an arc with lobes of ice extending from both edges. These lobes consist of a mixture of dispersed floes of first-year and multiyear ice amid considerable amounts of open water. This contrasts with many earlier years, including 2007, when the ice edge at the end of summer was compact and most of the remaining ice was a consolidated pack.

Fig. 4 -- Maps of September ice extent

Fig. SIO4. Maps of September ice extent for 1980, 2007, 2008, 2009, 2010 and 2011. The magenta line indicates the median September ice extent for the period 1979-2000. Maps are from the National Snow and Ice Data Center Sea Ice Index: nsidc.org/data/seaice_index.

Table SIO1 summarizes ice conditions in the southern and northern routes of the Northwest Passage and the Siberian Coast passage of the Northern Sea Route. Ice conditions were determined using passive microwave satellites. Open means the route appeared ice-free in the passive microwave imagery and closed means ice blocked passage somewhere along the route. Due to resolution limitations, passive microwave imagery may miss small pieces of ice and areas that appear "open" in satellite images may have ice covering as much as 15% of their surface (IICWG, 2011). All three routes have been open the past two years and the Northwest Passage southern route has been open every year since 2007. Additional information on the routes and on sea ice in the Greenland and Baltic Seas is available from the International Ice Charting Working Group news release (IICWG, 2011).

Table SIO1. Status of September ice conditions in the Northwest Passage and Northern Sea Route as determined from passive microwave satellite data.

Year Northwest Passage Southern Route Northwest Passage Northern Route Northern Sea Route
Siberian Coast
2011 Open Open Open
2010 Open Open Open
2009 Open Closed Open
2008 Open Closed Open
2007 Open Open Closed

Sea ice age

The age of the ice is another key descriptor of the state of the sea ice cover. Older ice tends to be thicker and thus more resilient than younger ice. The age of the ice is determined using satellites observations to track ice parcels over several years. This method has been used to provide a record of ice age since the early 1980s. Fig. SIO5 shows sea ice age derived from tracking ice parcels for 1979 through 2011. The distribution of ice of different ages illustrates the extensive loss in recent years of the older ice types (Maslanik et al., 2011). Analysis of the time series of areal coverage by age category indicates some recovery of 3rd-year ice coverage. However, the loss of the older (4+ year ice) that began in earnest in 2005 (Maslanik et al., 2011) has continued, reaching a record minimum in summer 2011, or 19% of the 1982-2005 mean. The net increase in 3rd year ice from 2010 to 2011 (0.36 million sq. km) is greater than the decrease in 4+ year ice (-0.11 million sq. km), which suggests a potential gain in ice mass within the multiyear ice coverage. However, given that the older ice types tend to be thicker (e.g., Maslanik et al., 2007a), this increase might be relatively slight.

Fig. 5 -- Sea ice age in the first week of March
Fig. SIO5. Sea ice age in the first week of March derived from tracking the drift of ice floes in 1988, 2009, 2010 and 2011. Figure courtesy of J. Maslanik and C. Fowler.

References

International Ice Charting Working Group (IICWG). 2011. News Release: International Ice Services Caution that "Open" Does Not Mean Ice-free. http://nsidc.org/noaa/iicwg

Maslanik J. A., C. Fowler, J. Stroeve, S. Drobot, J. Zwally, D. Yi, W. Emery, A younger, thinner Arctic ice cover: Increased potential for rapid, extensive sea-ice loss, Geophys. Res. Lett., 34, L24501, doi:10.1029/2007GL032043, 2007.

Maslanik, J., J. Stroeve, C. Fowler, and W. Emery. 2011. Distribution and trends in Arctic sea ice age through spring 2011. Geophysical Research Letters 38, L13502, doi:10.1029/2011GL047735.

National Snow and Ice Data Center, Web Site, http://nsidc.org/arcticseaicenews/2010/100410.html.

Stroeve, J. C., J. Maslanik, M. C. Serreze, I. Rigor, W. Meier, and C. Fowler. 2011. Sea ice response to an extreme negative phase of the Arctic Oscillation during winter 2009/2010. Geophysical Research Letter 38, L02502, doi:10.1029/2010GL045662.