Sea Ice Cover

J. Richter-Menge¹, S. Nghiem², D. Perovich¹, I. Rigor³

¹ERDC-Cold Regions Research and Engineering Laboratory, Hanover, NH
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA
³Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, WA

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Extent and Thickness

Satellite-based passive microwave images of the sea ice cover have provided a reliable tool for monitoring changes in the extent of the ice cover since 1979. During 2006 the minimum ice extent, typically observed in September, reached 5.9 million km² (Figure I1, bottom left panel). This marked a slight recovery from the record minimum of 5.6 million km² for the period 1979-2006, observed in 2005. Consistent with the past several years, the summer retreat of the ice cover was particularly pronounced along the Eurasian coastline. A unique feature was the sizeable isolated region of open water apparent in the Beaufort Sea.

The 2007 summer sea ice extent marked a new record minimum, with a dramatic reduction in area of coverage (4.3 million km²) relative to the previous record set just 2 years ago in 2005 (Figure I1, bottom right panel). At the end of the 2007 melt season, the sea ice cover was 23 percent smaller than it was in 2005 and 39 percent below the long-term average from 1979 to 2000.

The maximum ice extent is typically observed in March. In 2006, the maximum extent was 14.4 million km² and set a record minimum for the ice-extent maximum for the period 1979-2006 (Figure I1, top left panel). It is notable that in March 2006 the ice extent fell within the mean contour at almost every location. In March 2007, the maximum ice extent was 14.7 million km² (Figure I1, top right panel).

For comparison, the mean ice extent for March and September, for the period 1979-2007, is 15.6 million km² and 6.7 million km², respectively.

Figure I1. Sea ice extent in March and September 2006 and 2007, when the ice cover was at or near its maximum and minimum extent, respectively. The magenta line indicates the median maximum and minimum extent of the ice cover, for the period 1979-2000. The March 2006 maximum extent and the September 2007 minimum extent established new records as the lowest extents for the period 1979-2007. (Figures from the Sea Ice Index, nsidc.org/data/seaice_index)

To put the 2006 and 2007 minimum and maximum ice extent into context, the time series of the anomaly in ice extent in March and September for the period 1979-2007 is presented in Figure I2. In both cases, a negative trend is apparent with a rate of 2.8% per decade for March and 11.3% per decade for September relative to the 1979 values. The summers of 2002-2007 have marked an unprecedented series of extreme summer ice extent minima.

Ice thickness is intrinsically more difficult to monitor. With satellite-based techniques (Laxon et al., 2003; Kwok et al., 2004) only recently introduced, observations have been spatially and temporally limited. Data from submarine-based observations indicate that the ice cover at the end of the melt season thinned by an average of 1.3 m between the period 1956-1978 and the 1990s, from 3.1 m to 1.8 m (Rothrock et al., 1999). Measurements of the seasonal and coastal ice cover do not indicate any statistically significant change in thickness in recent decades (Melling et al., 2005; Haas, 2004; Polyakov et al., 2003).

Figure I2. Time series of the difference in ice extent in March (the month of ice-extent maximum) and September (the month of ice-extent minimum) from the mean values for the time period 1979-2007. Based on a least squares linear regression, the rate of decrease for the March and September ice extents was 2.8% per decade and 11.3% per decade, respectively.

Perennial and Seasonal Ice

The Arctic sea ice cover is composed of perennial ice (the ice that survives year round, generally located towards the center of the Arctic basin) and seasonal ice (the ice around the periphery of the Arctic basin that melts during the summer). Consistent with the diminishing trends in the extent and thickness of the cover is the observation of a significant loss of the older, thicker perennial ice in the Arctic (Figure I3). Results from a simulation using drifting buoy data and satellite-derived ice concentration data to estimate the age distribution of ice in the Arctic Basin (Rigor and Wallace, 2004) indicate that the March ice cover has experienced a significant decline in the relative amount of perennial ice over the period 1958-2006, from approximately 5.5 million km² to 3.0 million km². While there is significant interannual variability, a generally downward trend in the amount of perennial ice begins in the early 1970s. This trend appears to coincide with a general increase in the Arctic-wide, annually averaged surface air temperature, which also begins around 1970 (Figure A2).

Results from a new technique employing data acquired by the U.S. National Aeronautics and Space Administration (NASA) SeaWinds scatterometer on board the QuikSCAT satellite (QSCAT) have recently become available (Nghiem et al. 2005; Nghiem et al.; 2006, Nghiem and Neumann, 2007). In the half decade of overlap with the buoy-derived results, which presently begins in 2002 and represents the period of data reprocessed to date by the QSCAT project, the two products provide consistent estimates of perennial ice in March and suggest a precipitous decrease in the perennial ice extent in the last few years.

Figure I3. Time-series of the area of perennial sea ice extent in March estimated by a drift age model and satellite-derived ice concentration data and observed by the QuikSCAT scatterometer within the drift age model domain.

Figure I4 presents a comparison of the ice distribution derived from the drift age model and observed by QSCAT in March 2006. The two products provide similar results. Both indicate that the older, thicker ice is concentrated in the western Arctic basin. This result is consistent with the dominant ice circulation patterns in the Arctic (see Figure O1). Ice residence times are typically longer in the western Arctic in the region of the Beaufort Gyre. The eastern Arctic is dominated by the Trans Polar Drift, which carries sea ice out of the Arctic Basin via the Fram Strait.

Figure I4. Comparison of sea ice distribution estimated using the drift-age model (March average, left panel) with QSCAT observations (21 March 2006, right panel). The red line in both panels indicates ice age older than 1 year (i.e. perennial ice) as estimated by the drift age model.

The development of a relatively younger, thinner ice cover coincided with a strong, persistent positive pattern in the AO from 1989 to 1995 (see Figure A1). These characteristics make the current ice cover intrinsically more susceptible to the effects of atmospheric and oceanic forcing. It is of crucial importance to observe whether the sea ice cover will continue its decline or recover under the recent more neutral AO conditions (Lindsay and Zhang, 2005).

References

Haas, C. (2004) Late-summer sea ice thickness variability in the Arctic Transpolar Drift 1991-2001 derived from ground-based electromagnetic sounding. Geophys. Res. Lett., 31, L09402, doi: 10.1029/2003GL019394.

Kwok, R., H.J. Zwally, and D. Yi (2004) ICESat observations of Arctic sea ice: A first look. Geophys. Res. Lett., 31, L16401, doi: 10.1029/2004GL020309.

Laxon, S., N. Peacock, and D. Smith (2003) High interannual variability of sea ice thickness in the Arctic Region. Nature, 425, 947-950.

Lindsay, R.W., and J. Zhang (2005) The Thinning of Arctic Sea Ice, 1988-2003: Have We Passed a Tipping Point? J. Climate, 18, 4879-4894.

Melling, H., D.A. Riedel, and Z. Gedalof (2005) Trends in the draft and extent of seasonal pack ice, Canadian Beaufort Sea. Geophys. Res. Lett., 32, L24501, doi:10.1029/2005GL024483.

Nghiem, S.V., M.L. Van Woert, and G. Neumann (2005) Rapid formation of a sea ice barrier east of Svalbard. J. Geophys. Res., 110, doi:10.1029/2004JC002654.

Nghiem, S.V., Y. Chao, G. Neumann, P. Li, D. K. Perovich, T. Street, and P. Clemente-Colon (2006) Depletion of perennial sea ice in the East Arctic Ocean. Geophys. Res. Lett., 33, L17501, doi:10.1029/2006GL027198.

Nghiem, S.V., and G. Neumann, Arctic Sea-Ice Monitoring (2007) 2007 McGraw-Hill Yearbook of Science and Technology, New York, in press.

Polyakov, I., G.V. Alekseev, R.V. Bekryaev, U. Bhatt, R. Colony, M.A. Johnson, V.P. Karklin, D. Walsh, and A.V. Yulin (2003) Long-term ice variability in arctic marginal seas. J. Climate, 16(12), 2078-2085.

Rothrock, D.A., Y. Yu, and G.A. Maykut (1999) Thinning of the Arctic sea-ice cover. GRL, 26, 3469-3472.

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