Winter not what you expected? Blame Arctic oscillation, weather experts say

Steve Mertl
National Affairs Contributor
Daily Brew

Get ready to learn a new weather term. You already know a little something about La Nina and El Nino. Now here's another one: Arctic oscillation.

Apparently it's responsible for the nasty winter afflicting Western Canada in recent weeks while the the East Coast basks in unseasonably balmy weather.

According to the Globe and Mail, Toronto has experienced just 11 below-zero days, compared with an average 28 days for the period. Halifax's sub-zero temperature days were cut in half and even the Prairies are going back to above-average warm temperatures after its recent cold snap.

"It's been a bit of a whipsaw from last winter to this winter," NASA researcher Bill Patzert told the Globe. "You can definitely blame the Arctic oscillation for that."

The atmospheric circulation phenomenon affects atmospheric pressure over polar regions on a time scale that ranges from weeks to decades.

The complex weather pattern has helped keep colder Arctic air from pushing south. Arctic oscillation is notoriously unpredictable and can switch from positive, apparently as it is now, to negative without warning.

"It's the closest we've come to cancelling winter," Environment Canada climatologist David Phillips told the Globe. "I mean, you can count the number of nasty days on your hands, in various places."

Meteorologists earlier were warning Canadians to brace for a harsh winter based on the understanding that this would be another La Nina season.

La Nina's colder-than-normal surface temperatures in the eastern Pacific Ocean results in weather patterns that bring more precipitation to parts of Canada and the northern U.S. El Nino does the opposite.

Last year, Arctic oscillation was more negative than positive, resulting in a relatively colder, snowier winter, said Andre Viau, a University of Ottawa climatologist. This year it's been largely positive, pulling the west-east jet stream so taut, "it's been almost straight," he said.

The oscillation relaxed a little last week, allowing some cold Arctic air south, but that's not expected to last.

Phillips said this winter is shaping up to be short and mild.