Here and across much of the Eastern United States, dozens of cities experienced a “meteorological winter” — the three months from December through February — that ranked among their 10 warmest on record.

Deepti Singh.

Deepti Singh, a climate scientist in the School of the Environment at Washington State University Vancouver, said the Arctic Oscillation has been in an unusually strong positive phase this winter, which has resulted in a strong polar vortex that’s kept Arctic air trapped up north.

That’s a contrast to some recent winters, when a weaker polar vortex has allowed frigid air to descend, resulting in extremely cold days and strong snowstorms across portions of the United States, she said.

Even when there are cold air outbreaks associated with a weak polar vortex, the cold air coming from the Arctic regions over the Eastern U.S. is warmer than it was a couple of decades ago, Singh said.

Overall, with climate change, she said, spring is generally coming sooner, colder seasons are warming faster “and we are getting fewer extreme cold events.”

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