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Subject: Re: Why Is Summer Antarctic sea-ice above normal Date: 3/16/2013 4:53 PM
Recommendations: 5
Ryan Maue: Why Is Summer Antarctic sea-ice above normal?

Fair question.

We know one reason that might appear obvious, but which is incorrect. One might think that summer Antarctic sea ice must be above normal because the surrounding oceans are colder. This is incorrect: the surrounding oceans have been warming at a net average rate of 0.17K per decade (for a gridded view of this warming, see Figure 9e in the Zhang article cited below).

Here is an explanatory quote from Dr. Stammerjohn of the University of Colorado (October 2012):

Both warming and ozone loss act to strengthen the circumpolar winds in the south. This is due primarily to persistently cold conditions prevailing on Antarctica year-round, and a cold stratosphere above Antarctica due to the ozone hole. Stronger winds generally act to blow the sea ice outward, slightly increasing the extent, except in the Antarctic Peninsula region, where due to geography, winds from the north have also increased, pushing the ice southward. Thus, sea ice extent near the northwestern Antarctic Peninsula continues to decline rapidly, while areas in the Ross Sea and the southern Indian Ocean show significant increases (Stammerjohn et al., 2012). Circumpolar-averaged sea ice extent changes nearly cancel each other out for all months of the year (Parkinson and Cavalieri, 2012). This winter, atmospheric conditions were near average overall, with roughly equal areas of cooler and warmer air temperatures over the sea ice.

I also found an older source (2007), a peer-reviewed article from the Journal of Climate, by Junlun Zhang of the University of Washington. The author looked at satellite data from 1979-2004, and compared this data to climate simulations of the Southern Ocean performed with real-world forcings. He found that Antarctic sea ice extent actually increased 0.2% per year over that period, while the simulations produced an increase of 0.6% per year. The author identifies fluctuations in the rate of upward oceanic heat transport as the primary reason for the discrepancy. In any event, it is clear that current models of global climate change accurately predicted the rise in Antarctic sea ice extent, even if they overstated the case.

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