Arctic sea ice has retreated significantly in recent years, reaching a record low in September 2007. It is known that the seasonal cycle in Arctic sea ice extent is not symmetric—seasonal ice retreat proceeds gradually during early summer and then accelerates toward end of summer, while in winter, ice growth is rapid at first and then slows later in the season. Scientists have observed that ice cover has retreated far more rapidly in September than during other times of the year.
Some scientists have suggested that this seasonal asymmetry is due to factors such as temperature changes. However, Eisenman finds that the seasonal differences in rate of ice growth or retreat are caused by the geometry of the landmasses surrounding the Arctic Ocean. Because the Arctic Ocean is mostly surrounded by land, coastlines block the southward extent of sea ice growth during the winter, but coastlines have little effect on the extent of ice during the summer.
The author suggests that to better interpret changes in Arctic sea ice, instead of considering sea ice areal extent, scientists should track the line marking the latitude of the Arctic sea ice edge, averaged zonally over locations where the edge is free to move. He finds that this line moves northward or southward at a steady pace over the course of the year, with no seasonal asymmetry. In recent years, this line has been migrating northward at a rate of about 8 kilometers (5 miles) per year, consistent with overall ice loss. The study explains some aspects of the seasonal Arctic sea ice cycle and could help scientists better interpret sea ice evolution in the future.
Title: Geographic muting of changes in the Arctic sea ice cover
Author: Ian Eisenman: Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA and Department of Atmospheric Sciences, University of Washington, Seattle, Washington, USA.
Source: Geophysical Research Letters (GRL) paper 10.1029/2010GL043741, 2010 http://dx.doi.org/10.1029/2010GL043741
Here’s the abstract:
GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L16501, 5 PP., 2010
Geographic muting of changes in the Arctic sea ice cover
Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA
Department of Atmospheric Sciences, University of Washington, Seattle, Washington
The seasonal cycle in Arctic sea ice extent is asymmetric. Its amplitude has grown in recent decades as the ice has retreated more rapidly in summer than in winter. These seasonal disparities have typically been attributed to different physical factors operating during different seasons. Here we show instead that the seasonal asymmetries in Arctic sea ice extent are a geometric consequence of the distribution of continents. Coastlines block southward ice extension during winter, thereby muting changes in ice extent, but they have relatively little effect at the time of summer minimum extent. We suggest that the latitude of the Arctic sea ice edge, averaged zonally over locations where it is free to migrate, is the most readily interpretable quantity to describe the Northern Hemisphere sea ice cover. We find that the zonal-mean sea ice edge latitude during the 1978–present era of satellite measurements has been following an approximately sinusoidal seasonal cycle that has been migrating northward at an approximately annually constant rate of 8 km/year. These results suggest a change in perspective of the most critical quantities for understanding changes in Arctic sea ice.