Here’s one thing you probably know: Most Canadians are going without the sustained cold and blustering snow by which we define our winters, if not the nation.
But here’s another thing you probably don’t: The culprit is a mercurial weather pattern called the Arctic oscillation.
In Toronto, just 11 days have remained below freezing since the fall, compared with an average of 28 during the same period. The number of freezing days in Halifax has fallen by half, and even the Prairies, which recently experienced a sudden, week-long cold snap, are returning to warmer-than-average temperatures this week.
“It’s been a bit of a whipsaw from last winter to this winter,” NASA researcher Bill Patzert said. “You can definitely blame the Arctic oscillation for that.”
The complex weather pattern has held the polar jet stream at bay around the North Pole, preventing colder blasts of air from penetrating farther south.
When the index that measures the oscillation is positive, as it has been this winter, it tightens the jet stream. When it’s negative, the jet stream slackens, allowing pockets of cool air to travel south.
Notoriously unpredictable, the oscillation can switch with little warning from a positive to a negative value. But it has remained persistently positive this season, and that’s the reason most of the country has seen scant signs of a Great Canadian Winter.
“It's the closest we've come to cancelling winter,” Environment Canada climatologist David Phillips said. “I mean, you can count the number of nasty days on your hands, in various places.”
And it’s a far cry from the frigid weather meteorologists were telling Canadians to expect just a few months ago.
Last fall, a Pennsylvania-based company warned this could be one of the coldest winters in 20 years for Western Canada. While Environment Canada was more cautious, it, too, called for a colder-than-average winter in the north and west of the country.
The forecasters based their prediction on the knowledge that this would be another La Nina season. The more familiar weather pattern, which results from cooler-than-normal surface temperatures in the eastern Pacific Ocean, typically brings more precipitation and colder weather to parts of Canada and the northern United States.
Last year, La Nina contributed to the havoc wrought on some American cities, grounding flights and burying parts of the country in snow. Since the weather pattern is based on slow-moving ocean temperatures, the conditions it creates usually have staying power, making them among the safer bets in predicting long-term seasonal weather.
So it came as a surprise to many that this winter has turned out to be so moderate – and a significant disappointment for outdoor enthusiasts, who revelled in last year’s snow.
More than half of the snowmobile trails in Saskatchewan and Manitoba are still closed because there isn’t enough snow on the ground. And cross-country skiers in Alberta were forced to cancel one of their marquee ski events, the Alberta Cup, after 30 hours of non-stop rain turned the race course into a sheet of ice.
“It’s inconvenient,” Mike Neary, a manager at Cross Country Alberta, said with a sigh. “But there’s not much you can do besides be optimistic.”
Andrew Emsley, who owns an ice-fishing business on Lake Simcoe, near Barrie, Ont., got his company’s huts out a month later than usual, as temperatures in the region remained stubbornly high into January. It was the slowest start to a season he’s had in 10 years, Mr. Emsley said, and he counts himself among the lucky ones in the area.
“On Lake Simcoe, there are probably 12 operators, and four of us are open,” he said. “Lots of guys are sitting at home, holding down the couch.”
There have been exceptions to the balmy weather. In addition to the cold spell that hit Canada’s Prairie provinces last week, parts of Newfoundland and Labrador have had a normal snowfall and ski enthusiasts on British Columbia's coast say the powder has been excellent.
But the dominant pattern for most of Canada has remained warm winter weather punctuated by a few short bursts of cold.
André Viau, a climatologist at the University of Ottawa, describes the polar jet stream as similar to a ribbon that snakes across the continent, at the intersection of the colder air in the north and the warmer air that’s farther south.
When the oscillation is weak, or negative, the ribbon buckles, allowing colder Arctic air to penetrate farther south. Last year, the oscillation was more negative than positive, which helps explain Canada’s relatively colder, stormier winter.
This season, however, the oscillation has been almost exclusively positive. Strong polar winds have pulled the ribbon of the jet stream so taut, “it’s been almost straight,” Prof. Viau said, preventing Arctic air from escaping southward.
Last week, the strength of the oscillation finally relaxed, moving first to neutral and then to negative, allowing dense Arctic air to flow south and bring the first bone-chilling temperatures of the winter to the Prairies.
But climatologists don’t expect that change to last, and current forecasts suggest it could be on its way back to another positive phase.
“For the short range, you should see some action out of the Arctic,” Dr. Patzert said. “Will this get strong again and insulate you now from all that cold? At this point, the odds are it will.”
“Right now,” he added, “everyone’s eyeballs are fixed on the Arctic oscillation.”
But even with the recent dip in Western temperatures, Environment Canada's Mr. Phillips said it’s possible this winter will go on record as one of our warmest.
“Most Canadians' memories of this winter will be quite something: short and mild.”