We're talking about a huge sprawling area of circulating cold air originating from the North Pole. It's a low-pressure center, and typically during the winter months it resides up there. At times, some tentacles of it will slip southward and bring cold air outbreaks into the U.S., but this year, we're seeing a huge chunk of it, most of it descending into the U.S. High-pressure systems over Alaska and Greenland are allowing the jet stream to dive south over the U.S. and also for this polar vortex to drop south with the jet stream.
Why??
More and more Arctic sea ice is melting during summer months. The more ice that melts, the more the Arctic Ocean warms. The ocean radiates much of that excess heat back to the atmosphere in winter, which disrupts the polar vortex. Data taken over the past decade indicate that when a lot of Arctic sea ice disappears in the summer, the vortex has a tendency to weaken over the subsequent winter.
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