Last fall, a massive storm hammered Windsor, inundating streets and houses across the city. The record rainfall flooded more than 1,700 homes, with damages estimated at $108 million. The storm had gotten stuck in a stalled-out current, whirling above the city for two days as it dumped water on a small area.
The event was characteristic of a changing climate. Normally high-altitude currents — which form when hot air from the equator clashes with cold air from the poles — would have blown the storm away. But high above Windsor, a slow spot in the jet stream allowed the rainclouds to linger.
More of the same is in our future: with the Arctic heating up, the temperature difference between equatorial air and polar air is getting smaller — and it’s doing weird things to the jet stream.
That may seem like a too-vague term to use, but the fact is, scientific research into jet stream changes is being thrown for a loop by the rapid pace of global warming. It’s still not clear how drastic changes to the jet stream will be.
“We like to focus on things that are high-impact and affect our lives, but it tends to be something of a paradox that those things are less understood,” says Chris Fletcher, an assistant professor of geography and environmental management at the University of Waterloo.
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Imagine the global weather system as a car: scientists have been focused on driving, navigating potholes and pedestrians, and keeping their eyes on the road. But then the rules of the road change, the steering stops working right, and the engine sputters unpredictably.
“With the new jet stream behaviour — moving slower, becoming wavier in the north-south direction — the frequency of occurrence and severity and duration of extreme weather events is increasing rapidly,” says Paul Beckwith, a University of Ottawa professor and climate system scientist.
What kind of storm are you?
Rain and snow storms fit into two broad categories, and climate change is affecting both profoundly. Thunderstorms, which occur in warmer months and stay close to the ground, generally dump water from the local area that’s been drawn into the air via the hydrologic cycle. Conversely, cyclonic storms — the kind that hit Windsor last year — float higher in the sky and travel hundreds of kilometres. Those that pass through Ontario are, for the most part, carried by the jet stream.
“The physics involved in those two types of storms are quite different,” Fletcher explains. “Climate change, this wild card that's coming on-stream, may affect those two types of storms differently.”
But there’s one thing that affects both types, and it’s well understood: hot air is capable of holding more water than cold air. An increase of just 1 C raises air’s water capacity by 7 per cent, according to a report from the National Centre for Atmospheric Research. The effect is known as the Clausius-Clapeyron relationship, and it could change the way storms function.
“All other things being equal, if all you did was turn a dial and increase the water vapour, the intensity of precipitation events would increase,” Fletcher says. “There's more fuel on the fire.”
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The changing jet stream
Cyclonic storms are moved by air currents called jet streams. The currents in Ontario are created when cold air from the north collides with warm air from the south.
“The jet stream or winds in the mid-levels of the atmosphere help guide low pressure systems as they move across North America,” says Geoff Coulson, an Environment Canada climatologist. “Every once in a while the winds in the mid-level of atmosphere will slacken off and will be in an area where there isn't much push. This low-pressure system basically sat in that area to the south of Windsor and kept pumping moisture.”
Both poles are heating up faster than the equator is, and that’s contorting the jet stream, lowering its speed and increasing its waviness — it typically travels east-west, but waviness makes it travel north-south sometimes.
The Arctic is warming faster, Beckwith explains, because as the sea ice melts, it exposes more ocean. Since the ocean is darker than ice, it absorbs more heat — which causes more ice to melt, which exposes still more dark surfaces. Similarly, less snow cover on land exposes more dark ground.
“There are more and more examples occurring all around the world of these torrential rain events where a region or a city will get four to five months of rainfall and they get it in a night or they get it in a couple days,” Beckwith says. “And that leads to flooding because the infrastructure just can't handle it.”
The changing jet streams have influenced not just storms but other catastrophic weather events too, Beckwith says. The European heat wave of 2003, the 2010 Russian drought, and the recent California drought were all caused by jet-stream oddities.
Beckwith says storms like that in Windsor are going to keep happening as the temperature difference between the poles and the equator continues to shrink. “We're heading rapidly to an arctic with less sea ice and much less snow cover,” he explains. “The extreme weather events that we're seeing will get much worse.”