Here's what El Nino means for Canada's winter

CTV National News: How El Niño could affect Canada Heather Butts reports on how El Niño could potentially impact Canadians, as the world may have seen its hottest day ever.

El Niño has entered the chat.

After seven years characterized by La Niña conditions, the World Meteorological Organization announced in July that the surface temperature of the tropical Pacific Ocean has warmed again, signalling the switch to a global El Niño event.

Although an El Niño event can last up to 12 months, its effects in Canada are more pronounced in the winter, according to Jen Smith, national warning preparedness meteorologist with the Meteorological Service of Canada.

"Most of the impacts of El Niño occur during the winter time because that’s when the jet streams are more active, but it may linger into spring," Smith told CTVNews.ca in a phone interview on Monday.

With autumn well underway and winter on the horizon, here is what Canadians can generally expect in the coming months.

WARMER AND DRIER

In the parts of the country that feel the effects of El Niño the most — western and central Canada as well as the Great Lakes — temperatures over the winter and possibly into spring will be warmer than average.

How many degrees winter temperatures will rise is hard to predict, Smith said. Since El Niño is a global phenomenon measured in averages, its small-scale effects can be unpredictable.

"The general warming or potential for low precipitation for the winter for Canada represents a seasonal average, so there still is variation month to month, week to week, day to day, that's going to happen within the winter season itself," Smith said. "So we can still expect winter snow events, we can still expect cold spells. It's just that the seasonal average is warmer."

Another hallmark of El Niño in Canada is lower precipitation, which can spell bad news for ski resorts and snow sports enthusiasts. As with the cold, Smith said Canadians in the path of El Niño can still expect precipitation throughout the winter. They might just receive less of it and what falls could resemble freezing rain more often than snow.

"Just because warmer temperatures may be occurring, it doesn't necessarily lead to an easier winter," Smith said. "You could have a change in the type of precipitation that may fall or it can have an effect on the winter season in a different way."

'DIVIDING LINE'

To better understand why El Niño's impacts are less concentrated in some parts of the country — such as Eastern Canada and the Maritimes — than others, Smith said it's important to know how the phenomenon works.

El Niño occurs when the surface temperature in the tropical Pacific Ocean rises. Warm air just above the surface of the ocean is carried around the planet by global atmospheric circulation patterns, driving a surge in global temperatures and disrupting weather and climate patterns.

In an El Niño event, the Pacific jet stream shifts south of its neutral position, causing areas in the northern U.S. and Canada to become warmer and drier than usual while other parts of the world, such as southern South America, the southern United States, the Horn of Africa and central Asia, experience more rainfall.

The pacific jet stream flows west to east across the country, carving a line during El Niño that curves roughly from the Yukon in the northwest to the Great Lakes region to the southeast.

"It's almost like a dividing line," Smith said. "Everything west of that line is probably going to be on the warmer side and then everything that's on the north and east side is likely to see less of an impact."

Other factors like the polar vortex can also affect how winter plays out across Canada, since a weak polar vortex can deliver frigid Arctic air to lower latitudes.

"This is only setting the stage for what is likely to occur," Smith said. "There is obviously room for variability."

– With files from CTVNews.ca Writer-Producer Noushin Ziafati