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The Polar Jet Stream and Polar Vortex
The polar jet stream and polar vortex are two rings of fast-moving air around the Arcticwhich play a large role in world weather patterns. Many climate scientists believe that global warming is changing these ringsin ways that allow freezing air from the Arctic to intrude on the warmer mid-latitude regions. This means thateven as the Earth warms on averageclimate change may lead some places to see more extreme cold spells during winter.
These more frequent extreme weather events pose challenges to protect people and infrastructure from freezing.
The polar jet stream
The polar jet stream sits in the tropospherethe lowest layer of our atmosphere where most weather happens. It is characterized by a belt of wind that blows from west to east at speeds up to a couple of hundred miles per hour. While it shifts a bit north or south throughout the yearit usually stays between the 50th and 60th parallels: roughly between the Great Lakes and Canada’s northern territoriesor between France and Norway.1 The jet stream forms at the boundary where cold polar air and warm mid-latitude air try to mixcreating fast-moving waves and eddies.
Thanks to the rotation of the Earththese eddies don’t just move north and southbut bend in a bow-like shape to the east. The result is a strong system of eastward air currentskeeping cold air confined to the north. And the larger the temperature difference between the Arctic and mid-latitudesthe stronger the jet stream is—which is why it is usually fastest in winterwhen the Arctic is coldest.
Extreme cold snaps
Climate change affects the jet stream because different parts of the planet are warming at different rates. In particularthe Arctic is warming fastest. This means thatas the Earth warmsthe temperature difference between the Arctic and mid-latitudes is getting smaller.
This makes the polar jet stream slower and weaker. That slower jet stream has less eastward momentum and is more likely to bend north and south as it encounters small variations in temperature and pressure.
If it bends far enoughthe barrier between Arctic and mid-latitude air can plunge as far south as Mexicobringing Arctic temperatures with it. These wavy jet streams span the Earthso strange weather may be seen all around the northern hemispherewith unusually warm temperatures in parts of the Arctic at the same time extreme cold spells reach far south. In February 2021for instanceTexas endured over a week of freezing temperatures the state was not prepared forcausing power outages and killing hundredswhile much of northern Eurasia also saw extreme cold. And because a weak jet stream moves slowlythese weather conditions can last for days at a stretch.
Click here to see information from the infographic above in a table.
< type="text/css">table { border-collapse: collapse; } tabletdth { border: 1px solid black; }| Image | Description |
|---|---|
| A circle centered on the North polewith cold (blue) air inside and warm (red) air outside | When the polar jet stream is strongpowered by large temperature differences between the Arctic and mid-latitudesit moves relatively straight. This keeps the coldest winter temperatures in a predictable ring: the Arctic environments of Alaskanorthern CanadaGreenlandSiberia and Scandinavia. |
| A blob-like shape centered roughly on the North Polewith cold (blue) air inside and warm (red) air outside | A warming Arctic produces a weaker jetmore likely to bend off course. This wavy pattern can shift the boundary between cold Arctic air and warm mid-latitude air to strange places. Southerly regions used to mild winters may be exposed to freezing temperatures at the same time that parts of the Arctic deal with unusual warm spells. |
The polar vortex
The polar vortex is higher than the polar jet streamin the stratosphere up to 30 miles above the Earth. It is also further northsitting over the North Pole.2 Scientists are still working to fully understand the polar vortex: continuous long-term observations are only available after the satellite eraand many climate models struggle to simulate it. But some climate scientists believe that ittoois changing in ways that affect weather further south.
Like the polar jet streama strong polar vortex helps contain the cold air at its core. Sometimesnatural turbulence from below moves upward to disturb the vortexand it can break down. This is a natural (but rare) eventwhich has been happening since long before human-caused climate changebut a warmer Arctic with less predictable weather may make this kind of turbulence more likely.
If the polar vortex is disturbedit can slip off the North Poleor even break into two or three separate rings. Either distortion tends to weaken the jet stream to the south.
There have been recent spectacular examples of polar vortex breakdowns leading to extreme winter storms—like an event in January and February 2019 that broke cold temperature records across the eastern U.S. and Canada. But scientists still debate whether climate change will make these events more common in the future.
Click here to see information from the infographic above in a table.
< type="text/css">table { border-collapse: collapse; } tabletdth { border: 1px solid black; }| Status of the polar vortex | Description |
|---|---|
| Typical state | In its typical statethe lowest-pressure area of the polar vortex sits almost directly above the North Pole. |
| Displacement | In a “displacement” eventthat coldlow-pressure core can slip off the pole and travel south. |
| Split | In a “split” eventthe polar vortex breaks in two or threewith low-pressure areas forming in multiple places far from the pole. |
Published May 212024.
1 There is also a second polar jet stream in the southern hemispherecircling Antarctica roughly at the southern tip of South America. Both jets are affected by climate changebut the northern polar jet stream has a much greater effect on humanity simply because many more people live in the mid-latitudes of the northern hemisphere.
2 As with the polar jet streamthere are actually two polar vortexesone on the North and one on the South Pole.
More Resources for Learning
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Listen to this episode of MIT's "Today I Learned: Climate" podcast about climate change and extreme winter weather.
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