Sea Ice Extent and Variability
Arctic Sea Ice Extent and Thickness.
(links included below)

Arctic Sea Ice Extent and Thickness

Arctic sea ice extent over the past 30 years has been widely reported to be diminishing (Vinnikov et al. 1999). This decrease has also been accompanied by a thinning of the ice.
The reductions in sea ice extent that have been so widely reported are regionally averaged determinations (Parkinson 2000a, 2000b) which apply to combined analyses from the eastern, western and central Arctic. Examination of individual regional trends (Parkinson, 2000b) reveals that sea ice extent histories over the past 20 years vary markedly from one part of the Arctic region to another. The focus of the research presented here is on the western Arctic, so discussion of sea ice behavior over the past 50 years will focus on two areas: Bering Sea and Arctic Ocean.

One of the longest sea ice extent records for the Bering and Chukchi seas is that of Niebauer (1998) who examined sea ice extent for the period 1947-1996 (Fig. 4). This record displays considerable interannual variability over the length of the record, but most significantly displays a step decrease of some 5% in the late 1970s. While both periods are marked by considerable inter annual variability in ice extent (Cavalieri et al. 1997; Parkinson 2000a) related locally to trends in the Aleutian Low and regionally in El Nino patterns, the last 20 years of data show a weak overall trend toward very slightly increasing ice extent (Parkinson 2000b). This trend however is not statistically significant. Whereas some parts of the Arctic as a whole also show statistically significant changes in sea ice extent patterns over the last decade, no such changes are noted for the Bering Sea. In the Arctic Ocean, the last 20 years reveals a slight overall trend of diminishing ice extent (Johannessen et al. 1995; Bjorgo et al. 1997). Examination of the last 10 years reveals a slight increase in ice extent. However, this apparent reversal in trend in not statistically significant (Parkinson 2001) (Fig. 5). It is apparent that generalized statements about sea ice extent need to be made with reference to specific areas of the arctic and subarctic realms as considerable temporal and spatial variability is to be found in the region. Clearly, impacts of spatial variability in ice extent impact whaling communities in different ways within the same season.

The length of the sea ice season, defined by Parkinson (1992) as the number of days during the year when sea ice concentration exceeds 15%, also shows considerable variability both spatially and temporally. Over the past 20 years, for the Arctic as a whole, sea ice season length has decreased, with the number of regions within the Arctic showing season shortening exceeding those showing season lengthening, while in the western Arctic, the trend is considerably more variable. Parkinson (2000a) finds that in the Chukchi Sea there is a shortening of sea ice season over the past two decades, however, in the Bering Sea and the eastern Beaufort Sea there has been a lengthening of the sea ice season. Again, length of ice season displays considerable temporal and spatial variability over a relatively small geographic area. This clearly has quite variable implications for whale hunting communities of northwestern Alaska.

Driving Mechanisms

Ice variability in the Bering Sea is strongly tied to atmospheric circulation. The ice forms in shore leads, primarily in Norton Sound and off the coast of St. Lawrence Island. Northerly winds then move the ice southward, where it melts in the warmer waters near the edge of the Bering shelf. This pattern of ice formation, migration, and melt is driven by a constant stream of low pressure systems that track across the northern North Pacific Basin, producing the Aleutian Low. Day-to-day variability in the ice cover reflects changes in the atmospheric circulation caused by low pressure systems moving through the region, while the interannual variability depends on the preferred location of the storm tracks during the season. A high frequency of storms tracking northward along the western margin of the Bering sea produce southerly winds that push the ice margin northward. Conversely, a storm track that moves low pressure systems across the southern edge of the Bering Sea, or a northward track along the eastern margins of the Sea, produces northerly winds that result in extensive ice cover.

Interannual variability in the storm tracks is related to larger-scale features of the ocean-atmosphere system in the Pacific, most significantly the position of the Aleutian Low and hemispheric patterns of El Nino-Southern Oscillation (ENSO). Prior to 1978 there is a clear, and regular, relationship between alternating patterns of El Nino-La Nina events, circulation over the North Pacific, and the lagged responses of sea ice cover in the Bering Sea. In the late 1970s the circulation switches to an almost continuous El Nino mode, there has been a shift in the mean location of the Aleutian Low, and reduction in Bering Sea ice extent (Niebauer1998). This switch may be best explained by the influence of the Pacific Decadal Oscillation.

Changes in the extent, distribution, and thickness of Arctic Ocean sea ice have been largely attributed to increases in the Arctic Oscillation Index (Thompson and Wallace 1998, Morison et al 2000). Increases in this index lead to decreases in the strength of the Beaufort High which ultimately affects Arctic Ocean circulation patterns. Reduced convergence within the Beaufort Gyre leads to more open water, increased ocean heating and therefore increased ice melting (Morison et al. 2000).

REFERENCES

Bjørgo, E., Johannessen, O.M., and Miles, M.. 1995. Analysis of merged SMMR- SSMI time series of Arctic and Antarctic sea ice parameters 1978-1995. Geophysical Research Letters,24, 413-416.

Cavalieri, D.J., Parkinson, C.L., Gloersen, P., Comiso, J.C., and Zwally,H.J. 1999. Deriving long-term time series of sea ice cover from satellite passive-microwave multisensor data sets. Journal of Geophysical Research, 104, (C7), 15803-15814.

Johannessen, O.M., Miles, M., and Bjørgo, E. 1995. The Arctic's shrinking sea ice. Nature, 376, 126-127.

Morison, J., Aagaard, K., and Steele, M. 2000. Recent environmental changes in the Arctic: A review. Arctic, 53, 359-371.

Niebauer, H.J. 1998. Variability in Bering Sea ice cover as affected by a regime shift in the north Pacific in the period 1947-1996. Journal of Geophysical Research, 103 (C12) 27717-27737,

Parkinson, CL 1992. Spatial patterns of increases and decreases in the length of the ice season in the north polar region, 1979-1986. Journal of Geophysical Research, 97 (C9), 14377-14388.

Parkinson, CL 2000a. Variability of Arctic sea ice: The view from space, an 18-year record. Arctic, 53, 341-358.

Parkinson, CL 2000b. Recent trend reversals in Arctic sea ice extents: Possible connections to the North Atlantic Oscillation. Polar Geography, 24, 1-12.

Thompson, W.J. And Wallace, J.W. 1998. The Arctic oscillation signature in the wintertime geopotential height and temperature fields. Geophysical Research Letters, 25, 1297-1300.

Vinnikov, K.Y and 8 others. 1999. Global warming and northern hemisphere ice extent. Science. 286, 1934-

http://nsidc.org/
National Snow and Ice Data Center.

http://neonet.nlr.nl/ceos-idn/datasets/FA00435.html
Arctic and Antarctic Sea Ice Database Since 1972

http://www.engr.uaa.alaska.edu/ice/
Alaska Ice Studies

http://aol.wff.nasa.gov/html/graphics_library/aoltm_science_data/icesheet.html
Arctic Ice mapping

http://polar.wwb.noaa.gov/seaice/Welcome.html
A great source for current ice conditions. Sea ice coverage is also archived here.

http://psc.apl.washington.edu/thinning/thinning.html
The Thinning of the Arctic Ice Cover

http://www.natice.noaa.gov/westarct.htm
Current Conditions

http://polar.wwb.noaa.gov/seaice/alaska.html
Sea Ice conditions for AK, images from the last 14 days

http://polar.wwb.noaa.gov/seaice/ak.html#click
Clickable sea ice map from NOAA

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