Just in time for cooler weather, scientists are presenting new evidence that extreme winter temperatures in the Northern Hemisphere are linked to the polar vortex. And they say the new research may support a heavily debated scientific theory that the warming Arctic — where temperatures are rising faster than anywhere else on Earth — can influence extreme weather events elsewhere in the world.
The idea, itself, is not necessarily new. In the last few years, extreme winters in the northern United States, Europe and Asia have pushed the polar vortex to the forefront of public attention. It's a fast-moving, low-pressure band of air encircling the Arctic. Research suggests that changes in the polar vortex — if it shifts or dips, for instance — can send cold air streaming out of the Arctic into places like the northern United States, causing unusually frigid conditions.
Eurasia, in particular, has experienced an unexpected trend of colder winters over the last 25 years, despite the progression of global warming — a phenomenon that scientists are still working to explain. But some scientists suggest that an ongoing weakening in the polar vortex's flow may be influencing the lower temperatures.
Now, the new study, published today in the Bulletin of the American Meteorological Society, is among the first to demonstrate a link between the recent winter cooling trends in the Northern Hemisphere and high-altitude changes in the polar vortex, the authors say.
By analyzing daily data on wind speeds and other patterns in the stratosphere, collected during winters from 1979 to 2015, they suggest that the polar vortex has been progressively shifting to a weaker state over the last few decades. While the vortex fluctuates in terms of how strong or weak it is, the analysis suggests there's been a steady increase in the number of weak events it experiences each winter.
Furthermore, the researchers say that these weak events are associated with unusually low temperatures in northern Eurasia and the eastern United States. And they note that while other factors may also contribute to extreme winters, such as the climatic changes produced by El Niño events, the influence is comparatively weak.
In a statement about the research, climatologist Judah Cohen, director of seasonal forecasting at the analytics organization Atmospheric and Environmental Research and one of the study's co-authors, noted that the findings "not only confirm the link between a weak polar vortex and severe winter weather, but also calculated how much of the observed cooling in regions like Russia and Scandinavia is linked to the weakening vortex. It turns out to be most."
The researchers have also suggested that one of the causes behind the weakening of the polar vortex may involve climate change in the Arctic — a highly controversial idea that's been gaining attention among climate scientists within the last five years.
The analysis suggests that weak polar vortex events are often preceded by changes in pressure over Eurasia and North America — and while the new study provides no direct evidence on this front, the researchers note that these kinds of pressure changes have been linked by some studies to the loss of Arctic sea ice. And they speculate that Arctic warming may play a role in their recent findings, as well.
It's an idea that's recently inspired widespread debate among climate scientists. Over the last few years, some scientists have begun to suggest that rapid warming in the Arctic may influence extreme weather events elsewhere in the world, particularly in the north Atlantic. The idea is largely credited to Jennifer Francis, an Arctic climate expert at Rutgers University.
The basic idea is that a temperature gradient exists between the cooler poles and the warmer equator. And because warm air is thicker than cooler air, this means there's a kind of atmospheric slope between the high latitudes and the low latitudes, over which air flows.
Francis and colleagues have suggested that if the Arctic warms at a faster rate than the rest of the world, the temperature gradient changes, altering the atmospheric slope and changing the way air flows over it. And this could potentially alter the flow of winds and major air currents, like the polar vortex, influencing weather events in the process.
But many scientists have expressed extreme skepticism about the idea, and multiple studies have suggested that the link between the warming Arctic and extreme weather events — including frigid winter weather — may not actually exist.
One 2014 study, which specifically examined the link between climate change and winter weather, suggested that the theory is "an interesting idea, but alternative observational analyses and simulations with climate models have not confirmed the hypothesis, and we do not view the theoretical arguments underlying it as compelling."
The idea remains contested among climate scientists for now. But while the factors affecting the polar vortex may still be the subject of debate, the new study reinforces the idea that the vortex itself plays an important role in the planet's winter weather.
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