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Sea Ice

D. Perovich1,2, S. Gerland3, S. Hendricks4, W. Meier5, M. Nicolaus4, M. Tschudi6

1ERDC-Cold Regions Research and Engineering Laboratory, Hanover, NH, USA
2Thayer School of Engineering, Dartmouth College, Hanover, NH, USA
3Norwegian Polar Institute, Fram Centre, Tromsø, Norway
4Alfred Wegener Institute, Bremerhaven, Germany
5NASA Goddard Space Flight Center, Greenbelt, MD, USA
6Aerospace Engineering Sciences, University of Colorado, Boulder, CO, USA

December 2, 2014

Highlights

  • The September 2014 Arctic sea ice minimum extent was 5.02 million km2, slightly less than the 2013 minimum, but 1.61 million km2 greater than the record minimum of 2012. The sixth smallest ice extent of the satellite record (1979-2014) occurred in 2014.
  • The coverage of multiyear ice in March 2014 increased to 31% of the ice cover from the previous year's value of 22%.
  • Satellite observations indicated an increase of mean thickness in the multi-year sea ice zone north-west of Greenland, from 1.97 m in March 2013 to 2.35 m in March 2014.

The Arctic sea ice cover plays an important role in the global system. From a climate perspective, it serves as both an indicator and an amplifier of climate change. Sea ice is a barrier limiting the exchange of heat, moisture, and momentum between the atmosphere and the ocean, and is host to a rich marine ecosystem. Changes in ice cover affect a wide range of human activities from hunting to shipping to resource extraction.

Sea Ice Extent

There are three key variables used to describe the state of the ice cover; the ice extent, the ice age, and the ice thickness. Sea ice extent is used as the basic description of the state of the Arctic sea ice cover. Satellite-based passive microwave instruments have been used to determine sea ice extent since 1979. There are two months each year that are of particular interest: September, at the end of summer, when the sea ice reaches its annual minimum extent, and March, at the end of winter, when the ice is at its maximum extent. The Arctic sea ice extents in March 2014 and September 2014 are presented in Fig. 4.1.

Figure - Sea ice extent in March 2014 and September
Fig. 4.1. Sea ice extent in March 2014 (left) and September 2014 (right), illustrating the respective monthly averages during the winter maximum and summer minimum extents. The magenta lines indicate the median ice extents in March and September, respectively, during the period 1981-2010. Maps are from NSIDC at nsidc.org/data/seaice_index.

Based on estimates produced by the National Snow and Ice Data Center (NSIDC) the sea ice cover reached a minimum annual extent of 5.02 million km2 on September 17, 2014. This was just 80,000 km2 below the 2013 minimum, but substantially higher (1.61 million km2) than the record minimum of 3.41 million km2 set in September 2012 (Fig. 4.2). However, the 2014 summer minimum extent was still 1.12 million km2 (23%) below the 1981-2010 average minimum ice extent. In March 2014 ice extent reached a maximum value of 14.76 million km2 (Fig. 4.2), 5% below the 1981-2010 average. This was slightly less than the March 2013 value, but was typical of the past decade.

Figure - Time series of Arctic sea ice extent anomalies in March and September
Fig. 4.2. Time series of Arctic sea ice extent anomalies in March (the month of maximum ice extent, black symbols) and September (the month of minimum ice extent, red symbols). The anomaly value for each year is the difference (in %) in ice extent relative to the mean values for the period 1981-2010. The thin black and red lines are least squares linear regression lines. The slopes of these lines indicate ice losses of -2.6% and -13.3% per decade in March and September, respectively.

Sea ice extent had decreasing trends in all months and virtually all regions, the exception being the Bering Sea during winter. The September monthly average trend is now -13.3% per decade relative to the 1981-2010 average (Fig. 4.2). The trend is smaller during March (-2.6% per decade), but is still decreasing at a statistically significant rate.

There was a loss of 9.48 million km2 of ice between the March and September average extents. This is the smallest seasonal decline since 2006, but is still over 500,000 km2 higher than the average seasonal loss. After reaching the March 2014 maximum extent, the seasonal decline began at a rate comparable to the 30-year average, which continued through mid-June 2014. Then, for a few weeks in late-June and early-July, the decrease in ice extent accelerated. Subsequently, the 2014 ice extent tracked the shape of the average ice extent curve for the remainder of the summer melt season, but at a value about one million km2 less than the average curve. The retreat of sea ice in summer 2014 and comparisons to previous years and the long-term record are illustrated in the September 2014 report of the Arctic Sea Ice News and Analysis (NSIDC 2014).

Age of the Sea Ice

The age of the sea ice is another descriptor of the state of the sea ice cover. It serves as an indicator for the ice physical properties including surface roughness, melt pond coverage, and thickness. Older ice tends to be thicker and thus more resilient to changes in atmospheric and oceanic forcing compared to younger ice. The age of the ice can be determined using satellite observations and drifting buoy records to track ice parcels over several years (Tschudi et al. 2010, Maslanik et al. 2011). This method has been used to provide a record of age of the ice since the early 1980s (Fig. 4.3).

Figure - Age of the sea ice in March 1988, 2012, 2013 and 2014
Fig. 4.3. Age of the sea ice in March 1988, 2012, 2013 and 2014, determined using satellite observations and drifting buoy records to track the movement of ice floes. The dark red line denotes the median multiyear ice extent for the period 1981-2010.

The coverage of multiyear ice in March 2014 increased from the previous year, approaching the median multiyear ice extent for 1981-2010. There was a fractional increase in second-year ice, from 8% to 14%. This increase offset the reduction of first-year ice, which decreased from 78% of the pack in 2013 to 69% this year, indicating that a significant portion of first-year ice survived the 2013 summer melt. The oldest ice (4+ years) fraction has also increased, comprising 10.1% of the March 2014 ice cover, up from 7.2% the previous year. Despite these changes, there is still much less of the oldest ice in 2014 compared to, for example, 1988 (Fig. 4.3). In the 1980's the oldest ice made up 26% of the ice pack.

After winter 2014, multiyear ice continued to drift through the Beaufort Sea, and remained along the coasts of northwest Greenland and northern Canada. Melt out in the Laptev and Kara Seas occurred, but first-year ice, with a tongue of second-year ice, remained in the East Siberian Sea, as of August. The nature of this sea ice cover suggests that it will retain older ice as we enter freeze-up in autumn 2014.

Sea Ice Thickness

Ice thickness is an important descriptor of the state of the Arctic sea-ice cover. The CryoSat-2 satellite of the European Space Agency has now produced a time series of radar altimetry data for four successive seasons, with sea ice thickness information available between October and April. However, the algorithms for deriving freeboard (the height of the ice surface above the water level) and its conversion into sea-ice thickness are still being improved (Kurtz et al. 2014, Ricker et al. 2014, Kwok et al. 2014). Recent studies of the impact of snow layer properties on CryoSat-2 freeboard retrieval conclude that radar backscatter from the snow layer may lead to a bias in sea ice freeboard if it is not included in the retrieval process (Ricker et al. 2014, Kwok et al. 2014). Current sea-ice thickness data products from CryoSat-2 are, therefore, based on the assumption that the impact of the snow layer on radar freeboard is constant from year to year and snow depth can be sufficiently approximated by long-term observation values.

With these assumptions, updated radar freeboard and sea-ice thickness maps of the CryoSat-2 data product from the Alfred Wegner Institute (Fig. 4.4) show an increase in average freeboard of 0.05 m in March 2014 compared to the two preceding years (2012: 0.16 m, 2013: 0.16 m, 2014: 0.21 m). This amounts to an increase of mean sea-ice thickness of 0.38 m (2012: 1.97 m, 2013: 1.97 m, 2014: 2.35 m). The mean values were calculated for an area in the central Arctic Ocean where the snow climatology is considered to be valid. Excluded are the ice-covered areas of the southern Barents Sea, Fram Strait, Baffin Bay and the Canadian Arctic Archipelago. The main increase of mean freeboard and thickness is observed in the multi-year sea ice zone north-west of Greenland, while first year sea ice freeboard and thickness values remained typical for the Arctic spring.

Figure - Arctic sea ice freeboard and thickness maps
Fig. 4.4. Arctic sea ice freeboard (left) and thickness (right) maps for March retrieved from the ESA CryoSat-2 satellite for the period 2012-2014. The areas with the darkest shading, west and east of Greenland, the Canadian Arctic Archipelago and the Kara Sea, are outside the valid region for long-term snow observations. Freeboard is the height of the ice surface above the water level.

Regionally, thicker sea ice than in previous years has also been observed by airborne electromagnetic survey by the Norwegian Polar Institute in Fram Strait in late summer 2014. Preliminary results show a modal total sea ice thickness of 1.6 m, while the mean total ice thickness is 2.0 m (J. King et al. unpublished data). In surveys in the three years 2010-12, the modal thickness values for the same region and method were between 1.0 m and 1.4 m, and mean thickness values were between 1.1 m and 1.4 m (Renner et al. 2014).

References

Kurtz, N. T., N. Galin, and M. Studinger, 2014: An improved CryoSat-2 sea ice freeboard retrieval algorithm through the use of waveform fitting. The Cryosphere, 8, 1217-1237, doi:10.5194/tc-8-1217-2014.

Kwok, R., 2014: Simulated effects of a snow layer on retrieval of CryoSat-2 sea ice freeboard, Geophys. Res. Lett., 41, 5014-5020, doi:10.1002/2014GL060993.

Maslanik, J., J. Stroeve, C. Fowler, and W. Emery, 2011: Distribution and trends in Arctic sea ice age through spring 2011. Geophys. Res. Lett., 38, doi:10.1029/2011GL047735.

NSIDC, 2014: Arctic sea ice reaches minimum extent for 2014. In Arctic Sea Ice News and Analysis, National Snow and Ice Data Center (NSIDC), Boulder, CO, http://nsidc.org/arcticseaicenews/2014/09/.

Renner, A. H. H., S. Gerland, C. Haas, G. Spreen, J. F. Beckers, E. Hansen, M. Nicolaus, and H. Goodwin, 2014: Evidence of Arctic sea ice thinning from direct observations. Geophys. Res. Lett., 41, 5029-5036, doi:10.1002/2014GL060369.

Ricker, R., S. Hendricks, V. Helm, H. Skourup, and M. Davidson, 2014: Sensitivity of CryoSat-2 Arctic sea-ice freeboard and thickness on radar-waveform interpretation, The Cryosphere, 8, 1607-1622, doi:10.5194/tc-8-1607-2014.

Tschudi, M. A., C. Fowler, J. A. Maslanik, and J. A. Stroeve, 2010: Tracking the movement and changing surface characteristics of Arctic sea ice. IEEE J. Selected Topics in Earth Obs. and Rem. Sens., 3, doi: 10.1109/JSTARS.2010.2048305.