Now that's a positive temperature anomaly!
Bob Henson:
Cold comfort: Canada's record-smashing mildness: Some fascinating weather has unfolded across the Northern Hemisphere over the last month... heavy snow that battered the mid-Atlantic and New England states in late December... 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.... 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.... After New Year’s Day, the town went 11 days without getting down to its average daily high. On the 6th of the month, the low temperature was –3.7°C (25.3°F). That’s a remarkable 30°C (54°F) above average. On both the 5th and 6th, Coral Harbor inched above the freezing mark. Before this year, temperatures above 0°C (32°F) had never been recorded in the entire three months of January, February, and March....
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.... Farther west, a separate monster high developed over Alaska last week....
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... 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....
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...
