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Historic 2021 polar vortex disruption analyzed by NCAR in Boulder

Sudden stratospheric warming connected to historic February 2021 arctic intrusion but not directly responsible according to new research from NCAR in Boulder.

BOULDER, Colo. — Colorado is coming out of another long stretch of extremely cold weather. If the temperature in Denver rises above 55 degrees on Saturday, it will end a seven-day stretch of below-average temperatures. 

During that stretch, the high temperature stayed below freezing 4 out of the seven days, and the low of -7 degrees on Thursday morning was the coldest March temperature in Denver since 1960.

Another cold stretch in February kept the high temperature in Denver below freezing for 119 consecutive hours. 

But this winter’s cold snaps were minor compared to last year. In February of 2021, a historic blast of extreme cold swept through the country bringing Denver a 12-day stretch of below-freezing temperatures with a record-breaking low temperature of -14 on Valentine's Day, and a near-record -16 the next morning. 

That cold snap broke record cold temperatures from Huntsville, AL, all the way to Dallas while crippling the energy sector in Texas with snow and ice. 

Scientists from the National Center for Atmospheric Research (NCAR) in Boulder, just completed an analysis of the 2021 arctic intrusion, in an attempt to figure out what made that one so extreme.

On January 5, 2021, more than a month before the historic cold outbreak, something called “sudden stratospheric warming” was detected high above the North Pole.

Occasionally, the stratosphere can warm by nearly 100 degrees in just a few days. It happens about once every two years, and it is often followed by extreme outbreaks of cold air over the United States.

“Obviously the natural inclination is that you want to understand how did these interface? Is there any connection?” said atmospheric scientist Nick Davis with NCAR.

Davis centered his research project around exploring the relationship between what happens up in the stratosphere and the weather here on earth.

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He said waves of energy get sent down towards the earth during these sudden stratospheric warming events. It was previously thought that those waves disrupt the polar vortex, a swirling bubble of extremely cold air at the north pole, and that allows frigid arctic air to spill out over the United States.

But his study showed that those waves coming from stratospheric warming were not the direct cause of the polar outbreak in February 2021.

“I was very surprised at what the experiment showed, which was not quite the opposite but close to the opposite,” said Davis. "It showed that the stratosphere is still important, but maybe it's just not the mechanism we thought it was."

He programed NCAR's supercomputer to resemble the actual atmospheric conditions back in January 2021, but he used a technique he called scrambling to change the tropospheric conditions in one set of solutions to match a possible climatological setup instead of the actual state that existed at the time, and he changed stratospheric conditions in other solutions. Again replacing the actual stratospheric conditions with conditions that are likely to exist at that time of year.

"What's unique about this study, is that we really tried to experiment, like what would happen if we could just take the sudden warming out of the forecast," Said Davis. "We were trying to isolate which part of the atmosphere mattered the most at what times."

The experiment showed that the cold outbreaks were likely to happen even without the sudden stratospheric warming event, especially in the first few weeks of the forecast. But he said there was a stronger coupling between the stratosphere and the troposphere as time went on. 

The cold outbreak lasted about 6 weeks, ending about the third week of February.

“So essentially you still need the stratosphere to feedback on the troposphere and guide that behavior," he said. "But I think our experiments are decisive enough that no we don’t think this was a direct cause of the extreme cold.”

He said one of the keys to that conclusion was that the waves coming down from the stratosphere never really made it into the troposphere to impact the polar vortex. The data showed that they may have dissipated in the lower stratosphere. 

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Davis believes that weather conditions in the troposphere play more of a role in the disruption of the polar vortex and those same weather conditions have a role in the onset of sudden stratospheric warming. The wave movement from the troposphere up to the stratosphere and then the reflection of those waves back down in the troposphere are part of a balancing act that may help sustain arctic air intrusions into the middle latitudes, but just not the initial cause.

"It's important to know that sudden stratospheric warming is still a useful indicator of polar vortex splitting or stretching, our experiment doesn't change that," said Davis. "We are just trying to refine the actual mechanism that folks are trying to use to explain that connection."

And since there was not a major stratospheric warming event this winter, there is a high chance for one over the 2022-23 season. That means climatologists will be able to focus on the next event with this new information in mind. 


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