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Winter's freezing, so what's with Arctic sea ice?

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In January, Arctic sea ice covered 13.6 million square miles of ocean, nearly 20,000 square miles below the previous record low in January 2006 and some 490,000 square miles below the 1979-2000 average.

The drivers for January's record low extent, according to the National Snow and Ice Data Center, included a natural climate pattern known as the Arctic Oscillation, as well as residual heat from the Arctic Ocean, captured and retained during the previous melt season.

When the Arctic Oscillation is in a negative phase, wind patterns change in ways that can permit frigid Arctic air to plunge farther south than usual, accounting for below-normal winter temperatures in much of the US in January, including the South.

At the same time however, Arctic temperatures can run above normal during a negative phase. In January, much of the region experienced temperatures between 4 and 11 degrees Fahrenheit above normal, according to the NSIDC.

As February began the oscillation switched to a positive phase, which could speed ice growth for a period, according to the center.

But the prognosis for ice extent during the upcoming melt season isn't good, according to Mark Serreze, who heads the NSIDC.

Even if the ice were to reach a winter expanse nearer normal, "a lot of that ice is thin, first-year stuff, and it's going to tend to melt out easily" come spring, he says.

Indeed, he adds, the thickness of the ice heading into the melt season is a bigger factor than overall winter extent in determining how severe the spring melt-back is likely to be. Researchers over the past several years have documented a decline in older ice and an increase in thinner ice at the start of the melt season.

"We know right now, that we'll be continuing that pattern" heading into the 2011 melt season, he says.

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