8/18 - NSIDC Update - "A Change In Ice Motion Slows Seasonal Decline"

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During the first half of August, Arctic ice extent declined more slowly than during the same period in 2007 and 2008. The slower decline is primarily due to a recent atmospheric circulation pattern, which transported ice toward the Siberian coast and discouraged export of ice out of the Arctic Ocean. It is now unlikely that 2009 will see a record low extent, but the minimum summer ice extent will still be much lower than the 1979 to 2000 average.

Note: This mid-monthly analysis update shows a single-day extent value for Figure 1, rather than the usual monthly average. While monthly average extent images are more accurate in understanding long-term changes, the daily images are helpful in monitoring sea ice conditions in near-real time.

Overview of conditions

On August 17, Arctic sea ice extent was 6.26 million square kilometers (2.42 million square miles). This is 960,000 square kilometers (370,000 square miles) more ice than for the same day in 2007, and 1.37 million square kilometers (530,000 square miles) below the 1979 to 2000 average. On August 8, the 2009 extent decreased below the 1979 to 2000 average minimum annual extent, with a month of melt still remaining.


Figure 1. Daily Arctic sea ice extent on August 17 was 6.26 million square kilometers (2.42 million square miles). The orange line shows the 1979 to 2000 median extent for that day. The black cross indicates the geographic North Pole.

EDIT

Ice motion changes in August

A recent atmospheric circulation pattern, which led to a change in ice motion, caused the ice loss rate to slow down significantly in the first two weeks of August. As discussed in the August 4 post, during much of June and July, a strong Beaufort Sea high-pressure pattern promoted winds that helped push ice out of the Siberian coastal seas, and also brought clear skies and warm temperatures that helped induce melt.

Toward the end of July, the atmospheric pattern changed. Averaged over the past two weeks, a high-pressure system has been centered over the Barents Sea, with low pressure centered over the Laptev Sea. In accordance with Buys Ballot's Law, this pattern led to winds that redirected the motion of the ice cover, pushing the ice edge outward toward the Siberian coast and discouraging ice from exiting the Arctic Ocean through Fram Strait.


Figure 3. Sea ice motion, derived from AMSR-E data and averaged for June, July, and the first week of August 2009 (the most recent data available), shows a recent change, with ice motion towards the eastern Siberian coast and little export of ice out of the Arctic Ocean via Fram Strait.

The Northwest Passage and Northern Sea Route

So far this year, neither the Northwest Passage nor the Northern Sea Route has opened. The Northern Sea Route appears likely to open soon, but ice still clogs many of the channels in the Northwest Passage.

Whether or not the navigational passages through the Arctic Ocean will open in a given summer depends on atmospheric circulation and ice thickness. For example, although 2007 was a record low extent in the Arctic and the Northwest Passage was nearly completely open, the Northern Sea Route was still choked with ice because of a circulation pattern that pushed a tongue of ice against the Siberian coast. Recent research by Stephen Howell at the University of Waterloo in Canada shows that whether the Northwest Passage clears depends less on how much melt occurs, and more on whether multi-year sea ice is pushed into the channels. Counterintuitively, as the ice cover thins, ice may flow more easily into the channels, preventing the Northwest Passage from regularly opening in coming decades.


Figure 4. The map of sea ice concentration from AMSR-E from August 16, 2009 shows ice clogging many of the channels of the Canadian Archipelago. The Northern Sea Route may be clear in the next few weeks. NASA AMSR-E data.

EDIT

http://nsidc.org/arcticseaicenews/index.html