A massive winter storm has swept across large parts of the US this week, leaving more than a million people without power at its peak, killing at least 50 people and cancelling tens of thousands of flights. Economic losses from the storm are expected to surpass $100bn.
In New York City, 29cm (11.4 inches) of snow fell in a single day, the city’s biggest one-day snowfall since February 2021. Philadelphia recorded 24cm (9.3 inches), more than the city saw during the entirety of last winter. By Monday morning, 56 per cent of the contiguous United States was covered in snow, the second-highest coverage in the past 20 years, according to AccuWeather.
And the disruption is not over. Another powerful system, known as a bomb cyclone, is forecast to strengthen along the East Coast later this week, bringing further snow and wintry weather.
But to many, such scenes – whether they be gridlocked airports or Shaun White snowboarding in Central Park – can seem baffling. How can extreme cold still strike in a warming world?
The answer lies high above the ground, in a part of the atmosphere most of us never think about.
What is the polar vortex, and why does it matter?
The polar vortex is a ring of strong winds that circles the Arctic around 16 to 50km (10 to 30 miles) above the Earth’s surface, in the stratosphere. These winds normally form a tight barrier that keeps the coldest Arctic air locked near the North Pole.
When the vortex is strong and stable, that cold air tends to stay put. But when it weakens, shifts or stretches out of shape, the barrier becomes leaky. Instead of forming a neat circle, the vortex can become distorted and ‘wavy’, allowing lobes of cold air to spill southwards into North America, Europe or Asia.
The polar vortex is often confused with the polar jet stream, but they sit in completely different layers of the atmosphere. The jet stream flows much lower down, between about five and nine miles above the surface, and marks the boundary between cold polar air and warmer mid-latitude air. It plays a major role in shaping our day-to-day winter weather.
The two systems are connected, however. When the polar vortex high in the stratosphere is strong, it tends to keep the jet stream relatively smooth and pushed northwards, helping to confine cold air to the Arctic.
When the vortex weakens or becomes disrupted, the jet stream below often becomes more distorted and wavy. That allows warm air to surge north into the Arctic, while cold polar air can plunge south into regions that are not used to dealing with it.
The polar jet stream exists all year round and can cause cold snaps on its own. But in winter, when it is influenced by what is happening higher up in the stratosphere, the chances of large, persistent cold outbreaks increase.
Is climate change influencing the polar vortex?
One of the clearest signals of climate change is that the Arctic is warming faster than the rest of the planet, a phenomenon known as Arctic amplification.
As sea ice retreats, darker ocean and land surfaces are exposed, which absorb more sunlight and heat up even more quickly. That extra heat can alter atmospheric circulation patterns over the polar regions.
Paul Pastelok, a senior meteorologist at AccuWeather, said this warming and ice loss can encourage the formation of so-called ‘blocking patterns’ in the atmosphere.
These are persistent high-pressure systems that can stall weather patterns and make the jet stream more distorted.
“There is more warming occurring over the polar region than any other region,” Pastelok told BBC Science Focus.
“The lack of ice currently compared to past years has increased more blocking patterns across the northern latitudes, which has led to more wavy or amplified jet stream patterns across the globe and more extreme weather events.”
In some cases, these atmospheric changes can contribute to sudden stratospheric warming events, where temperatures in the stratosphere rise rapidly and disrupt the normal circulation around the polar vortex.
When that happens, the vortex can weaken or split, making it easier for cold air to escape southwards in the weeks that follow.
This winter, Pastelok said, the polar vortex has been relatively weak, with frequent blocking over the North Atlantic and northern Canada contributing to the cold outbreaks.
But the exact relationship between climate change and the polar vortex is still up for debate among scientists.
One study, published in the journal Science last year, found that stretching of the polar vortex was associated with more severe winter weather in the US over the past decade. The authors pointed out that climate change could be playing a role, but that the link is not yet established.
Observations of the stratosphere are relatively limited, and climate models do not agree on whether polar vortex disruptions will become more common as the planet continues to warm.
Are cold snaps becoming more common?
Despite how dramatic this storm has been, cold snaps in North America are, on average, becoming less frequent and less intense as the planet warms. Winters overall are milder than they were in the middle of the 20th century, and record-breaking cold events are becoming rarer.
But that does not mean severe cold spells have disappeared. And when they do occur, they can be especially disruptive, partly because societies and infrastructure are less adapted to prolonged cold than they once were.
Cold outbreaks that reach far into the southern US, for example, can overwhelm power grids, disrupt transport and affect populations that are not equipped for sustained freezing conditions, as we’ve seen this week.
What is clear is that climate change does not simply mean a smooth, steady rise in temperatures everywhere at all times. Storms like the one now affecting the US are not contradictions of climate change, but part of the complicated way a warming planet expresses itself through the atmosphere.
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