Even as the planet warms, parts of the U.S. will still suffer from harsh winter weather, and scientists now have a clearer idea of why. A team of researchers has discovered two distinct patterns in the polar vortex, a mass of cold, swirling air high in the stratosphere. These patterns influence where the coldest air in the Arctic ends up. One pattern tends to send freezing temperatures into the northwest United States, while the other pushes cold air toward the central and eastern regions.
Although winter is generally getting warmer across the United States, severe cold snaps are still hitting much of the country with alarming intensity. A recent study provides new insights into this apparent paradox. The reason may lie in the evolution of the stratosphere, 10 miles above the ground.
The study was carried out by an international team of scientists. Their findings, published in the journal Science Advances, highlight how two different patterns in the stratospheric polar vortex, a band of extremely cold air flowing over the Arctic, can lead to severe cold snaps in different parts of the United States.
"The public often hears talk of a 'polar vortex' when winter brings severe cold, but we wanted to dig deeper and understand how changes in this vortex affect when and where extreme cold strikes," the researchers said.
The team found two different variations of the polar vortex, both of which are associated with what scientists call a "stretched" vortex - a twisted and shifted circulation pattern that causes unusual weather on the surface.
One of these changes pushed the vortex toward western Canada, setting the stage for severe cold in the northwestern United States.
Another pushes the vortex toward the North Atlantic, releasing frigid air across the central and eastern United States
Both cases are related to changes in the way atmospheric waves bounce around the globe—essentially changing the jet stream and dragging Arctic air far south.
Perhaps most strikingly, winter temperatures have continued to drop across much of the northwestern United States since 2015, bucking the overall warming trend. The researchers attribute this change to an increase in the frequency of westward-focused vortex patterns, which also coincides with an increase in the negative phase of the El Niño/Southern Oscillation (ENSO), a key driver of global climate.
"Climate change means not just global warming, but also more complex and sometimes counterintuitive shifts in where extreme weather occurs," the researchers explained.
The findings help explain recent cold snaps in places like Montana, the U.S. Plains and even Texas, such as the one that occurred in February 2021 (which resulted in costly deaths and insurance losses), while other areas may experience milder winters. Understanding the impact of the stratosphere on weather patterns can improve long-term forecasts, allowing cities, power grids and agriculture to better prepare for extreme winter weather, even as the climate warms overall.
Compiled from /scitechdaily