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Bob Henson | 18 January 2011 • Some fascinating weather has unfolded across the Northern Hemisphere over the last month, but you may have only heard about part of it. The media dutifully reported on the heavy snow that battered the mid-Atlantic and New England states in late December. It was also the United Kingdom’s coldest December in at least the last century. Meanwhile, the sparsely populated Canadian Arctic basked in near-unprecedented mildness.
It’s the second chapter of a tale that began a year ago, when Canada as a whole saw the warmest and driest winter in its history. Much of the blame went to El Niño, which typically produces warmer-than-average weather across Canada. So far, so good—but similar things are happening this winter, even with a La Niña now at the helm.
Just how mild has it been? The map at right shows departures from average surface temperatures for the period from 17 December 2010 to 15 January 2011, as calculated by NOAA’s Earth Systems Research Laboratory. The blue blip along the southeast U.S. coast indicates readings between 3°C and 6°C (5.4–10.8°F) below average for the 30-day period as a whole. That’s noteworthy—and in fact, it was the coldest December in more than a century of record-keeping across south Florida (see PDF summary). Blue also shows up across the UK, where December averaged 5.2°C (9.4°F) below normal.
What really jumps out, though, is a blob of green, yellow, orange, and red covering a major swath of northern and eastern Canada. The largest anomalies here exceed 21°C (37.8°F) above average, which are very large values to be sustained for an entire month.
To put this picture into even sharper focus, let’s take a look at Coral Harbour, located at the northwest corner of Hudson Bay in the province of Nunavut. On a typical mid-January day, the town drops to a low of –34°C (–29.2°F) and reaches a high of just -26°C (–14.8°F). Compare that to what Coral Harbour actually experienced in the first twelve days of January 2011, as reported by Environment Canada (see table at left).
The extremes have been just as impressive when you look high in the atmosphere above these areas. Typically the midpoint of the atmosphere’s mass—the 500-millibar (500 hPa) level—rests around 5 kilometers (3 miles) above sea level during the Arctic midwinter. In mid-December, a vast bubble of high pressure formed in the vicinity of Greenland. At the center of this high, the 500-mb surface rose to more than 5.8 kilometers, a sign of remarkably mild air below. Stu Ostro (The Weather Channel) found that this was the most extreme 500-mb anomaly anywhere on the planet in weather analyses dating back to 1948. Details are at the conclusion of Ostro’s year-end blog post.
Farther west, a separate monster high developed over Alaska last week. According to Richard Thoman (National Weather Service, Fairbanks), the 500-mb height over both Nome and Kotzebue rose to 582 decameters (5.82 km). That’s not only a January record: those are the highest values ever observed at those points outside of June, July, and August.
Why so freakishly mild? One factor that both feeds and is fed by the warmth is the highly unusual amount of open water across seas that are normally frozen by late November. On the winter solstice (December 21), Hudson Bay was little more than half frozen (see map at right).
Similarly, a large swath of the Baffin/Newfoundland Sea fell weeks behind schedule in freezing up. As evident in the charts at bottom, these bodies of water remain in catch-up mode. Around the south part of Baffin Island, “the boats were still in the water during the first week of January,” says David Phillips, a senior climatologist with Environment Canada. “The Meteorological Service of Canada was still writing marine forecasts as of 7 January, well beyond anything we have ever done.”
Storm after storm sweeping up the East Coast in recent weeks has pumped warm Atlantic air across eastern Canada, helping postpone the freeze-up even further and allowing temperatures over land to soar far above average.
According to Philips, the implications for people in the far north have been widespread. Nunavut’s capital, Iqaluit, had to cancel its year-end snowmobile run on Frobisher Bay for the first time. “Last New Year’s Eve, the big story was ice breaking up,” says Phillips. “This year there was no ice to break up.” Worst of all, he adds, “it’s impossible for many people in parts of the eastern Arctic to safely get on the ice to hunt much-needed food for their families—for the second winter in a row. Never before have we seen weather impact a way of life in so many small and big ways.”
The extraordinary Arctic warmth and the midlatitude chill and snow bear the fingerprints of a negative North Atlantic Oscillation (NAO), the pattern that prevailed for much of last winter as well. As opposed to a positive NAO, where the jet stream whisks mild air across the Atlantic, a negative NAO—which has predominated since October—features a blocked-up jet stream that allows cold air to plunge more easily southward and mild air to take hold in the Arctic.
It seems plausible that the open water between Greenland and Canada has played a role in the record warmth observed at the surface and aloft and the associated negative NAO. However, the NAO’s causes remain mysterious, and its future is impossible to predict beyond a few days. Clearly, the back-to-back punch of two winters with heavy snows in populated areas gives researchers added incentive to examine how the NAO might evolve in a warming climate. We’ll look at some of this work in an upcoming UCAR Magazine article.
Correction: A reference to Barrow, Alaska, in the eighth paragraph above was amended to refer to Nome and Kotzebue.