When winter sets in, “polar vortex” becomes one of the most dreaded phrases in the Northern Hemisphere. It’s enough to send shivers even before the first blast of bitter cold arrives.

New research shows that some northern regions have been getting hit with these extreme cold spells more frequently over the past four decades, even as the planet as a whole has warmed. While it may seem counterintuitive, the scientists believe these bitter cold snaps are connected to the warming of the Arctic and the effects that that warming is having on the winds of the stratospheric polar vortex, high above the Earth’s surface.

Here’s what scientists involved in the research think is happening: The evidence is clear that the Arctic has been warming faster than the rest of the planet. That warming is reducing the amount of Arctic sea ice, allowing more heat to escape from the ocean. The scientists think that the ocean energy that is being released is causing a weakening of the polar vortex winds over the Arctic, which normally keep cold air centered over the polar region. That weakening is then allowing cold polar air to slip southward more often.

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