Jet stream slows on Prairies

Residents of eastern Saskatchewan and western Manitoba witnessed a rare event on the Prairies in late June of 2014: a three-day summer rain.

The skies opened on a Friday and the rain fell until Sunday, dumping 125 to 200 millimetres of moisture and swamping cropland throughout the region.

A 72-hour rainfall may be nothing in Seattle, Washington, or Prince Rupert, B.C., but it is peculiar in North America’s Great Plains.

There is no simple explanation for rain that won’t go away, but changes in the jet stream may be a factor.

“We know, in the Northern Hemisphere, the jet stream since the 1990s is about 15 percent less speedy,” said David Phillips, senior climatologist with Environment Canada.

“That means the weather systems tend to move across the country at a slower pace.”

Phillips said the jet stream once resembled a skipping rope, pulled tight, with cold air to the north and warmer air to the south.

These days, the jet stream often looks like a roller coaster, dipping southward into the continental United States.

“The jet stream is more loopy.… Instead of looking like a straight line … it goes up and down,” Phillips said.

“That also creates the issue of (weather) blocking. You get a weather system that just hangs out for too long … and some places get stuck in a particular weather system.”

It’s not an established fact, but a few climate specialists now believe that warming temperatures in the Arctic are affecting the jet stream and causing periods of unseasonably warm, cold, wet or dry weather across North America.

Jennifer Francis, a Rutgers University climate scientist, said the wavy jet stream contributed to the extremely cold winter of 2015 in the eastern U.S.

“We think with the warming Arctic, these types of very wavy patterns, although probably not in the same locations, will happen more often in the future,” she said.

“When the jet stream has large northward bulges (called ridges), strong Arctic warming intensifies the ridge, causing it to be more persistent.”

Phillips said the relationship between a warmer Arctic and the jet stream is only a theory, but temperature differences between the Arctic and the south do provide energy for the jet stream.

“The greater that difference (in temperature), the stronger the jet stream.”

Conversely, the jet stream slows and becomes wavy if the temperature gradient between north and south is less. In turn, the jet stream may cause weird weather to stick around for days or weeks, such as seven days of above zero temperatures in Winnipeg in January.

There is some evidence to support the notion that weather is lasting longer in Western Canada. John Pomeroy, a University of Saskatchewan Canada research chair in water resources and climate change, found there’s been a 50 percent increase in multi-day rains on the Prairies.

“It starts raining on a Thursday and it’s still raining on a Saturday, from the same system.”

That type of weather pattern, where rain or heat persists, is unusual in Canada.

“I’ve always said the best thing about Canadian weather … is that it hits and runs,” Phillips said.

“It doesn’t stand around and clobber you, like in other parts of the world.”

A number of experts question the jet stream theory because strange and persistent weather could be explained by year to year variability. Francis admitted the data set is too small to make definite conclusions.

“Rapid Arctic warming started very recently, so detecting a clear atmospheric response and linking it to a particular cause may take another decade. In the meantime, Mother Nature seems to be acting out.”

About the author



Stories from our other publications