First of all, a quick reminder about sea ice in the Arctic Ocean:
Sea ice melt in the Arctic Ocean in 2012 smashed the previous record by 750,000 km2. The map below shows the observed sea ice extent in Sept 2012 (white area) compared to the 1979-2000 average extent (pink line) Wow!
Sea ice extent is defined as the area of the sea with at least 15% sea ice cover. The graph below shows the Arctic sea ice extent for the years 2007-2012 and the 1979-2000 average. Sea ice melt for all individual years shown (2007-2012) have minimum sea ice extents that are significantly (statistically) less than the 1979-2000 average (dark gray line; lighter gray area is + 2 standard deviations around the 1979-2000 average). The bottom line for the Arctic is that it is warming significantly, and much faster than even the fastest climate models developed to date.
Ok, let's take a look at what's happening in the Antarctic:The map below shows the Antarctic maximum sea ice extent (white area) for 2012 compared to the 1979-2000 average sea ice extent (orange line). 2012 sea ice extent in the Arctic set a new sea ice maximum record. The graph below the map shows the sea ice extent for 2012 compared to the 1979-2000 average.
The graph below shows average sea ice extent for the month of Sept for 1979-2000 and for selected individual years. Interestingly sea ice extent is increasing on average around Antarctica. When we look at data of sea ice cover in recent years in the Antarctic we see that 2006, 2007, 2011, and 2012 all had higher than average sea ice extent maxima but only 2006 and 2012 maximum extents were statistically higher (different) than the 1979-2000 average. With that being said, sea ice extent did exceed the + 2 standard deviation range in 2006 and 2012. So, what is going on in the Antarctic that is leading to increased sea ice cover?
A report by scientists at the National Snow and Ice Data Center explains very nicely what is happening with sea ice extent in the south (http://nsidc.org/arcticseaicenews/2012/10/poles-apart-a-record-breaking-summer-and-winter/).
Here's a brief summary of their report together with some additional information to increase clarity:
- Temperatures are warming in the Antarctic, just not as fast as in the Arctic (the NSIDC cites references you can refer to if you want more info on this.)
- Warming of the Pacific Ocean and ozone depletion over Antarctica combine to strengthen circumpolar winds.
- The strongest of these circumpolar winds blows east to west, and Coriolis Effect causes these winds to deflect to the left (north).
- The northerly flow of air around most of Antarctica causes sea ice to be pushed farther north than usual, spreading it out and increasing sea ice extent (remember, sea ice extent = 15% ice cover or more)
Sea ice extent around Antarctica is increasing, but it is not increasing because it is getting colder. It is increasing because winds blowing toward the north are dispersing sea ice farther away from the Antarctic coastline than usual. Don't forget that sea ice formation in the Antarctic winter is followed by nearly 100% sea ice melt in the Antarctic summer - this is different than in the Arctic where multiple-year sea ice has historically accumulated.