Arctic sea ice extent continued to increase through the month of February, as it approaches its annual maximum. Ice extent averaged for February 2009 is the fourth-lowest February in the satellite record. From February 18 to 22, ice extent declined slightly, primarily because of weather conditions off the coast of Alaska; ice extent then rebounded.
Overview of conditions
Arctic sea ice extent averaged for the month of February was 14.84 million square kilometers (5.73 million square miles). February extent was 800,000 square kilometers (309,000 square miles) less than the 1979 to 2000 average, and 140,000 square kilometers (54,000 square miles) less than for February 2008.
During the month of February, Arctic sea ice extent increased by 520,000 square kilometers (201,000 square miles), an average increase of 19,000 square kilometers (7,300 square miles) per day. These values are based on data from the F13 Special Sensor Microwave/Imager (SSM/I) sensor, which NSIDC is once again using because of problems with the sensor on the F15 satellite. See our February 26 post for details.
Conditions in context
Arctic sea ice extent continued to climb through the month of February; NSIDC scientists expect Arctic sea ice to reach its annual maximum extent sometime in March. The date of the maximum can vary by as much as 6 weeks. The average date of the maximum is March 6, based on the satellite record from 1979 to 2008.
From February 18 to 22, ice extent declined from 14.89 million square kilometers (5.75 million square miles) to 14.80 million square kilometers (5.71 million square miles), before rebounding at the end of the month. Such ups and downs are not unusual at this time of year, as ice extent nears its annual maximum.
February 2009 compared to past Februaries
Monthly average ice extent for February 2009 was the fourth lowest in the satellite record. February 2005 had the lowest ice extent for the month; February 2006 was the second lowest; and February 2007 is in third place. Including 2009, the downward linear trend in February ice extent over the satellite record stands at –2.8% per decade.
Short-term changes in winter ice extent
The temporary decline in ice extent from February 18 to 22 illustrates the sensitivity of Arctic sea ice extent to transient weather conditions. Conditions along southern boundary of the ice cover, such as in the Bering Sea, are typically just barely cold enough for ice to exist, and the ice there can quickly expand or retreat in response to changes in temperature and winds.
The decline in mid-February appears to have been caused by the combination of low pressure centered in the western Bering Sea and high pressure centered in the western Gulf of Alaska. This weather pattern caused warm, southerly winds between the low and high pressure cells, which pushed the ice edge to the north and promoted melt. Air temperatures in the region at the 925 millibar level (approximately 915 meters [3,000 feet] above the surface) were up to 8°C (14°F) above average.