The Weather is WACC, but Science Still Unsettled

February 17, 2021

Above, Warm Arctic, Cold Continents, or WACC is what scientists are calling disruptions like we’ve seen for the past month.
Scientists have made the connection between arctic warming and wild weather, but clear, “pound on the table, we’re done” understanding has been elusive.
In recent months, there has been some pushback on the whole idea that arctic changes have as much impact as had been suggested. (see below)

And yet, here we are.

Something is happening.

Seth Borenstein for AP:

This particular polar vortex breakdown has been a whopper. Meteorologists call it one of the biggest, nastiest and longest-lasting ones they’ve seen, and they’ve been watching since at least the 1950 s. This week’s weather is part of a pattern stretching back to January.

“It’s been a major breakdown,” said Jennifer Francis, a climate scientist at the Woodwell Climate Research Center on Cape Cod. “It really is the cause of all of these crazy weather events in the Northern Hemisphere.”

“It’s been unusual for a few weeks now — very, very crazy,” Francis said. “Totally topsy-turvy.”


Record subzero temperatures in Texas and Oklahoma knocked millions off the power grid and into deep freezes. A deadly tornado hit North Carolina. Other parts of the South saw thunder snow and reports of something that seemed like a snow tornado but wasn’t. Snow fell hard not just in Chicago, but in Greece and Turkey, where it’s far less normal. Record cold also hit Europe this winter, earning the name the “Beast from the East.”

“We’ve had everything you could possibly think of in the past week,” said Northern Illinois University meteorology professor Victor Gensini, noting that parts of the U.S. have been 50 degrees (28 degrees Celsius) colder than normal. “It’s been a wild ride.”

It was warmer Tuesday in parts of Greenland, Alaska, Norway and Sweden than in Texas and Oklahoma. And somehow people in South Florida have been complaining about record warmth that is causing plants to bloom early.

In the eastern Greenland town of Tasiilaq, it’s been about 18 degrees (10 degrees Celsius) warmer than normal, which “is a bit of a nuisance,” said Lars Rasmussen, a museum curator at the local cultural center. “The warm weather makes dog sledding and driving on snow scooters a bit of a hassle.”

Several meteorologists squarely blamed the polar vortex breakdown or disruption.

These used to happen once every other year or so, but research shows they are now close to happening yearly, if not more, said Judah Cohen, a winter storm expert for Atmospheric Environmental Research, a commercial firm outside of Boston.


The polar vortex spends winter in its normal place until an atmospheric wave — the type that brings weather patterns here and there — slams into it. Normally such waves don’t do much to the strong vortex, but occasionally the wave has enough energy to push the spinning top over, and that’s when the frigid air breaks loose, Gensini said.

Sometimes, the cold air mass splits into chunks — an event that usually is connected to big snowstorms in the U.S. East, like a few weeks ago. Other times, it just moves to a new place, which often means bitter cold in parts of Europe. This time it did both, Cohen said.

There was a split of the vortex in early January and another in mid-January. Then at the end of January came the displacement that caused cold air to spill into Europe and much of the United States, Cohen said.

Both Cohen and Francis said this should be considered not one but three polar vortex disruptions, though some scientists lump it all together. 

While both the vortex and the wave that bumped it are natural, and polar vortex breakdowns happen naturally, there is likely an element of climate change at work, but it is not a sure thing that science agrees on, Cohen, Gensini and Francis said.

Warming in the Arctic, with shrinking sea ice, is goosing the atmospheric wave in two places, giving it more energy when it strikes the polar vortex, making it more likely to disrupt the vortex, Cohen said.

“There is evidence that climate change can weaken the polar vortex, which allows more chances for frigid Arctic air to ooze into the Lower 48,” said University of Georgia meteorology professor Marshall Shepherd.

There were strong polar vortex disruptions and cold outbreaks like this in the 1980s, Cohen said.

“I think it’s historic and generational,” Cohen said. “I don’t think it’s unprecedented. This Arctic outbreak has to be thought of in context. The globe is much warmer than it used to be.”

It also feels colder because just before the outbreak, much of the United States was experiencing a milder-than-normal winter, with the ground not even frozen on Christmas Day in Chicago, Gensini said.

The globe as a whole is about the same temperature as the average was from 1979 to 2000 for this time of year, according to the University of Maine’s Climate Reanalyzer. That’s still warmer than the 20th century average, and scientists don’t think that this month has much of a chance to be colder than the 20th century average for the globe, something that hasn’t happened since the early 1980s.

One reason is that it will soon warm back up to normal when the polar vortex returns to its regular home, Cohen said.

As for people thinking this cold outbreak disproves global warming, scientists say that’s definitely not so.

Even with climate change, “we’ll still have winter,” said North Carolina state climatologist Kathie Dello. “What we’re seeing here is we’re pretty unprepared for almost every type of extreme weather. It’s pretty sad.”

Inside Climate News:

Nobody is arguing that Arctic warming has no effect, said University of Exeter climate scientist James Screen, who has worked on several studies focusing on how the linkage manifests in different regions. But the climate models don’t show the connection as strongly recorded observations, and it’s important to remember that, overall, the entire planet is warming.

“One of the most robust effects of Arctic warming is the reduction of cold extremes,” Screen said. “You get cold weather when wind is blowing from the north. Now those cold air outbreaks are less severe. You can see that effect in the observations and models. The bigger picture is, everything is going to get warmer, but the warming effects over continents might be slightly damped.” 

In a Nov. 16 correspondence in the journal Nature Climate Change, Screen suggested that evidence for mid-latitude impacts of Arctic warming has weakened since 2016.

“The short-term tendencies from the late 1980s through to early 2010s that fueled the initial speculation of Arctic influence have not continued over the past decade,” Screen and his co-author, Russell Blackport, with the University of Exeter, wrote. “Arctic amplification and sea-ice loss have indeed continued. But predictions of a more negative Arctic Oscillation, wavier jet stream, colder winters in mid-latitudes or, more specifically, in Eurasia, and more frequent and/or widespread cold extremes have not become reality.”

Screen added that “what we’re saying, really, is if you take the trends from the 1970s to the present, the trend lines are flat and the models show a muted response.To reconcile that, you have to say the models are wrong. And there is research suggesting that models are underestimating the predictability of things like changes to atmospheric circulation.”

The variability of the past few decades, however, doesn’t wipe out the previously observed correlation, Screen said. Low sea ice in the Chukchi Sea, north of Alaska, seems to match up with severe winter conditions in Eastern North America, and low sea ice in the Barents and Kara Seas, north of Siberia, match with cold winters over Asia.

The evidence is strongest for a link between sea ice loss and East Asian winter cold, but less robust for North America. And it’s also a chicken-and-egg question at some level, with question marks about whether the warming Arctic causes extreme cold; if it does, what the mechanisms are; and whether there are other overriding factors that cause both the Arctic warming and cold outbreaks over the continents.

Screen’s research in Nature.

Weakened Evidence for Mid-Latitude Impact of Arctic Warming

The idea that rapid Arctic warming might be changing weather patterns at lower latitudes rose to prominence in 2012. At that time, amidst rising global temperatures and record low Arctic sea-ice cover, parts of the mid-latitudes had just experienced a run of extremely cold winters1. Some scientists speculated that these cold snaps were driven by Arctic-induced changes in the atmospheric circulation, pointing to an unexpected 25-year winter cooling trend over Eurasia, an ostensible shift in the Arctic Oscillation and increased meandering of the jet stream as evidence2,3. These tendencies would continue as the Arctic warmed further, they predicted. Such ideas were controversial from the outset. Very quickly, other scientists questioned the idea, arguing that the cooling and circulation trends were not robust and unlikely to continue in the longer term4,5. Jennifer Francis, whose seminal work proposed that Arctic warming was leading to a wavier jet stream, predicted in 2014 that “within a few years, as Arctic amplification continues, we will have enough data to know whether or not we’re right”6.

So, six years on, what has changed? Arctic amplification and sea-ice loss have indeed continued (Fig. 1). But predictions of a more negative Arctic Oscillation, wavier jet stream, colder winters in mid-latitudes or, more specifically, in Eurasia, and more frequent and/or widespread cold extremes have not become reality (Fig. 1). The short-term tendencies from the late 1980s through to early 2010s that fuelled the initial speculation of Arctic influence have not continued over the past decade (Fig. 1). Long-term trends in the Arctic Oscillation and waviness, updated to winter 2019/20, are small and indistinguishable from internal variability (Fig. 1). Temperature-related metrics all indicate warming in the longer term, with fewer and milder cold extremes (Fig. 1). The multidecadal warming of minimum daily temperature is larger than that of average winter temperature (Fig. 1), implying a detectable reduction in mid-latitudes of subseasonal temperature variability7.

Time series of Arctic sea-ice extent (SIE; light blue), Arctic amplification (dark blue), Arctic Oscillation (AO; light green), atmospheric waviness (dark green), midlatitude (30–60° N) land surface air temperature (SAT; light red), central Eurasia (40–60° N; 60–120° E) SAT (dark red), midlatitude land minimum daily SAT (light purple) and the percentage area of mid-latitude land experiencing at least one cold (1 s.d. below average) winter month (dark purple). The SIE and Arctic amplification indices are averages for autumn and winter (September–February), whereas all other indices are for winter (December–February). Linear trends are shown for two time-periods: 1979/80 to 2019/20 and 1988/89 to 2011/12, the latter highlighted by grey shading. Thicker lines demark trends significant at the 95% confidence level. Arctic amplification is defined as the difference between Arctic (65–90° N) and Northern Hemisphere (0–90° N) SAT. Waviness is defined by the local wave activity11 (LWA) averaged over mid-latitudes (40–60° N). The Arctic sea-ice index was provided by the National Snow and Ice Data Center21. Atmospheric indices were calculated from the European Centre for Medium-Range Weather Forecasts ERA-5 reanalysis23, except for the Arctic Oscillation that was provided by the NOAA Climate Prediction Center.

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