Antarctic sea ice appears to have reached its annual maximum extent on October 6. The maximum occurred relatively late compared to past years. In contrast to the past three years, the 2015 maximum did not set a new record high for the period of satellite observations, but was nevertheless slightly above the 1981 to 2010 average.

Antarctic sea ice extent for October 6, 2015 was 18.83 million square kilometers (7.24 million square miles). The orange line shows the 1981 to 2010 median extent for that day. The black cross indicates the geographic South Pole. Credit: National Snow and Ice Data Center
Antarctic sea ice extent for October 6, 2015 was 18.83 million square kilometers (7.24 million square miles). The orange line shows the 1981 to 2010 median extent for that day. The black cross indicates the geographic South Pole. Credit: National Snow and Ice Data Center

The sea ice cover of the Southern Ocean reached its yearly maximum extent on October 6. At 18.83 million square kilometres, the new maximum extent falls roughly in the middle of the record of Antarctic maximum extents compiled during the 37 years of satellite measurements - this year's maximum extent is both the 22nd lowest and the 16th highest. More remarkably, this year's maximum is quite a bit smaller than the previous three years, which correspond to the three highest maximum extents in the satellite era, and is also the lowest since 2008.
The growth of Antarctic sea ice was erratic this year: sea ice was at much higher than normal levels throughout much of the first half of 2015 until, in mid-July, it flattened out and even went below normal levels in mid-August. The sea ice cover recovered partially in September, but still this year's maximum extent is 1.33 million square kilometres below the record maximum extent, which was set in 2014.

The graph above shows Antarctic sea ice extent as of October 13, 2015, along with daily ice extent data for four previous years. 2015 is shown in blue, 2014 in green, 2014 in orange, 2012 in brown, and 2011 in purple. The 1981 to 2010 average is in dark grey. The grey area around the average line shows the two standard deviation range of the data. Credit: National Snow and Ice Data Center
The graph above shows Antarctic sea ice extent as of October 13, 2015, along with daily ice extent data for four previous years. 2015 is shown in blue, 2014 in green, 2014 in orange, 2012 in brown, and 2011 in purple. The 1981 to 2010 average is in dark grey. The grey area around the average line shows the two standard deviation range of the data. Credit: National Snow and Ice Data Center

Scientists believe this year's strong El Niño event, a natural phenomenon that warms the surface waters of the eastern equatorial Pacific Ocean, had an impact on the behaviour of the sea ice cover around Antarctica. El Niño causes higher sea level pressure, warmer air temperature and warmer sea surface temperature in the Amundsen, Bellingshausen and Weddell seas in west Antarctica that affect the sea ice distribution.

The images compare Antarctic sea ice concentration for Septembers during two strong El Niño events (2015, left; 1997, right) to 1981 to 2010 averages. Colours show percent difference from average sea ice concentration surrounding Antarctica. Oranges and reds indicate concentrations higher than average; greens and blues indicate concentrations lower than average. (Credit: National Snow and Ice Data Center)
The images compare Antarctic sea ice concentration for Septembers during two strong El Niño events (2015, left; 1997, right) to 1981 to 2010 averages. Colours show percent difference from average sea ice concentration surrounding Antarctica. Oranges and reds indicate concentrations higher than average; greens and blues indicate concentrations lower than average. (Credit: National Snow and Ice Data Center)

"After three record high extent years, this year marks a return toward normality for Antarctic sea ice," said Walt Meier, a sea ice scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "There may be more high years in the future because of the large year-to-year variation in Antarctic extent, but such extremes are not near as substantial as in the Arctic, where the declining trend towards a new normal is continuing." This year's maximum extent occurred fairly late: the mean date of the Antarctic maximum is Sept. 23 for 1981-2010.

Source: Maria-José Viñas NASA's Earth Science News Team, NASA/Goddard Space Flight Center and National Snow and Ice Data Center