Overview of conditions
Average ice extent for May 2011 was 12.79 million square kilometers (4.94 million square miles). This is 210,000 square kilometers (81,000 square miles) above the previous record low for the month, set in May 2004, and 810,000 square kilometres (313,000 square miles) below the average for the reference period of 1979 to 2000.
Ice extent remained lower than average in the Atlantic sector of the Arctic, including the Kara and Barents seas and the Labrador Sea. During May, areas of open water known as polynyas continued to develop in the Beaufort, Chukchi, and Laptev seas, and Hudson Bay. Those open water areas absorb the sun’s energy, which will likely help to hasten further ice melt.
Conditions in context
Ice extent during May 2011 declined at an average rate of 50,720 square kilometers (19,580 square miles) per day. That was close to the average decline rate for May 1979 to 2000 of 46,000 square kilometers (18,000 square miles) per day. Ice extent declined more slowly than in May 2010, but faster than in May 2007, the year when September sea ice extent reached the lowest in the satellite record.
At the end of May, Arctic sea ice extent was 11.99 million square kilometers (4.63 million square miles), 175,000 square kilometers (67,600 square miles) less than the ice extent on May 31, 2007 and 48,790 square kilometers (18,800 square miles) above the previous record low for May 31, set in 2010.
Air temperatures averaged for the month of May were 4 to 5 degrees Celsius (7 to 9 degrees Fahrenheit) warmer than normal in the Beaufort and Chukchi seas, western Siberia, and in the Kara Sea. The areas with high air temperatures correspond to locations where ice retreated and polynyas formed. By contrast, temperatures were 2 to 5 degrees Celsius (4 to 9 degrees Fahrenheit) colder than normal in Baffin Bay and the East Siberian Sea.
Figure 3. Monthly May ice extent for 1979 to 2011 shows a decline of 2.4% per decade. —Credit: National Snow and Ice Data Center
May 2011 compared to past years
Arctic sea ice extent in May 2011 was the third lowest in the satellite record, continuing the overall downward trend of the past thirty years. The two lowest years for May were 2004 and 2006, respectively. In 2004, average May ice extent fell below 13 million square kilometres (5 million square miles) for the first time. Since then there have been five years when ice averaged less than that benchmark in May. The long-term rate of decline for May now stands at -2.4% per decade.
Figure 4. This map of sea level pressure for May 2011 shows unusually high pressure over the Canadian Archipelago and Greenland, as well as central Siberia (green and yellow).—Credit: NSIDC courtesy NOAA ESRL PSD
Atmospheric pressure and ice loss
Atmospheric pressure patterns for the month of May contributed to the formation of open water areas in some regions of the Arctic. During May, high pressure settled over northeastern Canada and Greenland, with low pressure over the Chukchi and East Siberian seas. This pattern helped push ice in the Beaufort Sea away from the coast and to the west, helping create areas of open water north of Tuktoyaktuk, Canada. Higher-than-normal pressure over central Siberia helped bring unusually warm temperatures over western Siberia and the Kara Sea, along with winds that pushed ice away from the coast in the Laptev Sea near the New Siberian Islands. This pressure pattern also led to above-average temperatures in the Beaufort Sea, and unusually cold conditions over the East Siberian Sea. Lower-than-normal pressure also dominated the Norwegian Sea and the northwestern Pacific Ocean.
The last four summers have been dominated by an atmospheric pattern known as the Arctic dipole anomaly, which has been associated with low sea ice extent at the end of summer. This pattern features unusually high pressure over the Beaufort Sea and unusually low pressure over the Kara and Laptev Seas, which promote warm southerly winds along the Siberian coast, helping to melt ice and push it away from the coasts and out of the Arctic Basin through Fram Strait.
While the atmospheric pattern for May 2011 bears some resemblance to the Arctic dipole anomaly pattern, the centers of the pressure anomalies are in different locations this year, and it is not yet clear whether the pattern will persist through the summer and contribute to low ice extent.
Figure 5. Far fewer storms are found in the Arctic during May, June and July in years with low sea ice at the end of summer (left) than in years with high sea ice at the end of summer (right). —Credit: NSIDC courtesy James Screen, Unversity of Melbourne
Importance of storms
Arctic weather in the next few months will be a critical factor in how much ice remains at the end of the melt season. New research led by James Screen at the University of Melbourne shows that the storms that move northwards into the Arctic from the lower latitudes during summer strongly influence sea ice extent at the end of summer. Years with dramatic ice loss, such as 2007, have been associated with comparatively warm, calm, and clear conditions in summer that have encouraged ice melt. Summers with slow melt rates are opposite and tend to be stormier than average. The number of storms influences how warm, windy and cloudy the Arctic summer is.
Stroeve, J.C., M.C. Serreze, M.M. Holland, J. Kay, J. Maslanik, A. P. Barrett. 2011. The Arctic’s rapidly shrinking sea ice cover: a research synthesis, Climatic Change, in press.
Wang, J., J. Zhang, E. Watanabe, M. Ikeda, K. Mizobata, J.E. Walsh, X. Bai, and B. Wu. 2009. Is the Dipole Anomaly a major driver to record lows in Arctic summer sea ice extent?, Geophys. Res. Letts., doi:10.1029/2008GL036706.
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