How kinks in the jet stream can push our weather to extremes
Large-scale weather events happen when things in the atmosphere get off-kilter
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You see it in the winter when we hear about cold, Siberian air moving through Canada, or the polar vortex creating weather havoc in the United States. You'll also see it in the summer during heat waves that last for days on end.
You even see it in movies, when a giant killer storm takes over the entire continent of North America. (I'm looking at you, The Day After Tomorrow).
Sure, that last one is a bit Hollywood fantastical but these big weather pattern realities are far from imaginary in our world.
What we're still learning, is the impact climate change will have on large-scale weather events and the forces that create them.
Jet stream a big player
A discussion about large-scale weather events needs to start at the beginning: what causes them?
The jet stream is often a major force.
In Canada, the polar jet stream is responsible for many of our weather patterns. It is a narrow band of fast-moving air in the upper atmosphere that can help direct our weather systems.
It also divides warm and cold air — cold air remains to the north of it and warm air to the south.
Though it generally travels from west to east, the jet stream can weaken and become kinked and wavy. These waves can lead to colder than normal — or, conversely, warmer than normal — weather, by allowing polar air to spill southward or warm air to push north.
What you get depends on where you are in relation to the jet stream's position. If it's well to the north, your weather will be on the warmer side. If it's to the south — like we saw in much of Western Canada at the end of December — that's when you'll be reaching for parkas and hand warmers.
In these wavy instances, when one area of the world is seeing colder than normal conditions — like we saw this holiday season on the Prairies — regions on the other side of the jet stream are often seeing warmer than normal conditions. It's kind of like a teeter-totter. One area of the jet stream is far south, another area is far to the north.
Sometimes, it feels like those winter cold snaps or summer heat waves will last forever, with day after day of abnormal weather and no end in sight.
There is a reason for that. And it's called a blocking pattern.
Large-scale systems can clog up our atmosphere, creating a sort of traffic jam that can disrupt the jet stream and bring prolonged stretches of abnormal weather to large regions. The deadly heat dome this past summer was an extreme example of one of these blocking events, affecting much of Western Canada.
Other reasons for large-scale weather phenomena
We can see patterns when we are looking at large-scale weather phenomena.
A strong and stable polar vortex — that is, the large area of low pressure in the upper atmosphere near the North Pole — generally means the jet stream will stay put further north. That means more "normal" winter weather, and fewer extremes to either end of the spectrum.
Sometimes the polar vortex will migrate or split, leading to cold snaps.
The jet stream can also be buffeted out of its usual position by El Niño or La Niña, which can lead to unseasonable conditions right across Canada.
The future is unclear
Will we still see large-scale weather events with climate change? The short answer is yes. But the frequency is unclear.
We do believe that we will see more heat waves as our climate warms, and in general we should also see shorter, warmer winters, but when you look at extreme cold snaps, it's not clear if we'll see them more or less.
Large atmospheric blocks can be hard to forecast or to build into long-range models.
Some experts believe that the warming of polar regions at a faster rate than the rest of the world will lead to a weaker and more consistently wavy jet stream.
As we know when the jet stream has those large waves, things can get blocked. However there is not complete agreement that polar warming will in fact lead to more blocking events.
In other words, while we know that with climate change we can expect more extreme weather events — like stronger hurricanes or longer, more active wildfire seasons — we are still learning about what effect our changing climate has on the jet stream and large-scale weather events.
Our planet is changing. So is our journalism. This story is part of a CBC News initiative entitled Our Changing Planet to show and explain the effects of climate change and what is being done about it.