Dangerously cold temperatures have been forecast across the Central and Eastern US this week as an arctic air mass spreads across the nation.
The winter storm is expected to bring blizzard conditions to much of the Midwest and Great Lakes, with the potential to develop into a powerful “bomb cyclone.” This follows record low temperatures in the South last February.
But if the planet is supposed to be getting warmer, why is it so cold?
How a wobbly polar vortex brings cold weather
Judah Cohen, a climatologist at MIT and the director of seasonal forecasting at Atmospheric and Environmental Research, told Newsweek that, as average global temperatures rise, winter temperatures are increasing at a slower rate than expected.
“Over the past 30 to even 40 years, winter has warmed the least of all four seasons in the mid-latitudes, especially in the center of the major Northern Hemisphere continents—Asia and North America—where temperatures have actually cooled,” Cohen said .
One explanation for this discrepancy is that warming in the Arctic has disrupted the movement of air around the North Pole. Usually, cold air in the Arctic is contained within the Arctic circle by a ring of fast-moving air that circles the North Pole, called the stratospheric polar vortex.
“When the polar vortex is in its normal or strong state, there is a strong ribbon or river of air that flows rapidly from west to east…that acts like a barrier that separates cold air to the north over the Arctic and milder air to the south across the mid-latitudes,” Cohen said.
However, as temperatures rise in the Arctic, Cohen said that this vortex could become less stable—in other words, it starts to wobble like a slowing down spinning top.
“When the circulation around the polar vortex becomes less and less circular in shape, the cold air normally confined to the Arctic can expand southward to the mid-latitudes, including the US, Europe and East Asia,” he said.
Miles below the stratospheric polar vortex, less than 10 miles above the surface, a narrow band of strong wind called the polar jet stream follows the boundary between these hot and cold winds.
“The jet stream tends to follow the polar vortex and when you get the disruption to the polar vortex that translates down through the layers of the atmosphere,” Grahame Madge, a spokesperson for the Met Office in the UK, told Newsweek.
“When it gets to our level, it can cause disruption to the flow of the jet stream. The jet stream in a normal state is what drives our weather, and it is these interruptions to the jet stream which bring us different weather patterns.”
It is still uncertain whether this vortex destabilization is a direct consequence of climate change, but it has been recorded a lot more frequently over the last few decades, in correlation with rising global temperatures. The Arctic has also warmed nearly four times faster than the rest of the world over the last 40 years, and the impacts of this warming are having global consequences.
“The elements of the climate system are all—in one way or another—interlinked, and you can get an effect from one affecting the others,” Madge said. “The fact that we had a relatively cold spell for a couple of weeks does not take anything away from the fact that climate change is happening,”
Polar vortex and bomb cyclones—what is the difference?
The destabilization of the polar vortex, combined with increased evaporation of water from our warming oceans, has created the perfect storm, literally.
“The formation of any mid-latitude storm system requires a contrast of warm air from the lower latitudes and cold, polar air masses from the high latitudes,” Jason Furtado, a professor of meteorology at the University of Oklahoma, told Newsweek. “As such, middle latitude low-pressure centers feed on this temperature difference.
“The larger the difference in temperature, the more energy available to lower the central pressure and hence the more rapid the intensification.”
If the central pressure drops fast enough, the storm can develop into a bomb cyclone.
“A ‘bomb cyclone’ really refers to a rapidly-intensifying mid-latitude storm,” Furtado said. “The low pressure center of the storm needs to drop 24 millibars in 24 hours to be characterized as a ‘bomb cyclone.'”
The winter storm passing over the US has the potential to become a bomb cyclone, particularly over regions of the Midwest. Combined with the cold Arctic winds, produced by destabilization of the polar vortex, this storm could usher in heavy snow and dangerously cold conditions.
“The biggest threat will be the absolutely frigid air descending into the Plains, the Midwest, and eventually the East behind the storm system,” Furtado said. “Life-threatening cold air is expected in parts of the Northern Plains and Front Range of the Rockies, where wind-chill values could reach -70 F in spots, which is dangerous. Plus, with all of this happening during Christmas week, there will be major headaches for those looking to travel for the holidays.”
The National Weather Service has issued storm alerts for much of the Midwest, and significant disruption is expected for people wishing to travel over the Christmas weekend.
“The best way to stay safe is to prepare now and look for updates from your local meteorologists and National Weather Service personnel,” Furtado said. “Avoid unnecessary travel and, with the severe cold, be sure to bundle up and stay warm.”
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Cohen J, et al., Linking Arctic variability and change with extreme winter weather in the United States, Science, September 1 2021, DOI: 10.1126/science.abi9167
Kretschmer et al., More-Persistent Weak Stratospheric Polar Vortex States Linked to Cold Extremes, Bulletin of the American Meteorological Society, January 1 2018, https://doi.org/10.1175/BAMS-D-16-0259.1
Rantanen, M., Karpechko, A Y, Lipponen, A. et al. The Arctic has warmed nearly four times faster than the globe since 1979. Commun Earth Environ, August 11 2022, https://doi.org/10.1038/s43247-022-00498-3