Around the world and particularly across much of the United States, a question has emerged: What happened to winter?
In Japan, record low snowfall has forced ski resorts to close prematurely. In Finland, forests that would normally be stark and bare at this time of year appear lush after what was an "exceptionally" warm January. And in one county in Michigan, municipal workers who usually spend winter plowing snow have taken to trimming trees. The lack of snow in Russia led Moscow to bring in fake snow for its New Year festivities and pushed one lawmaker to accuse the United States of using a "climate weapon" against his country.
If the mild conditions in the U.S. persist through February, this could be the country’s warmest winter in recorded history, according to the National Oceanic and Atmospheric Administration (NOAA).
Though it may seem like another symptom of global warming, the warmer-than-usual conditions are more directly caused by an Arctic weather pattern that is trapping cold air in the polar region. Scientists are watching this system closely to try to understand whether this winter is an outlier or a preview of what could become more common for the Northern Hemisphere.
Karin Gleason, a climate scientist at NOAA’s National Centers for Environmental Information, said this winter is on pace to edge out the 2015-16 season, which holds the record for warmest winter. At that time, the average temperature across the U.S. from December through February was 36.7 degrees Fahrenheit (2.6 degrees Celsius).
NOAA recently announced that last month was the fifth warmest January in the agency’s 126-year climate record. All 48 of the contiguous states experienced a warmer-than-usual month, with the average temperature more than 5 degrees above the 20th century average.
Alaska had its coldest January since 2012, but much of the rest of the country had a temperate start to the year, with states in the Northeast and the Great Lakes region seeing more rain than snow last month.
"The first part of February has been fairly warm, so barring any large cold period for the rest of the month, we could approach that record or get to near-record status," she said.
But climate change is not necessarily to blame. Though greater social awareness of global warming has led some people to regard the warm winter with suspicion, it’s more immediately due to a weather phenomenon known as Arctic Oscillation, which reached record-setting strength Monday. It is currently trapping frigid air in the polar region and preventing it from flowing south.
The mild conditions this winter are driven by a "positive phase" of the Arctic Oscillation, which strengthens the jet stream, a ribbon of fast-moving air that flows west to east over the Northern Hemisphere. Changes in the jet stream’s behavior can have a profound impact on weather systems around the world.
The jet stream is powered by temperature differences between the cooler Arctic region and warmer air masses to the south. As the jet stream circles the planet, it can turn wavy — characteristics of a "negative phase" of the Arctic Oscillation. When these ripples form, polar air rushes to fill in the troughs, bringing rain and colder temperatures further south. Conversely, warmer air will fill the ridges that bulge northward, bringing with it hotter and drier conditions.
Right now, the jet stream appears flatter, meaning, "the cold, Arctic air is being kept at bay," said Mark Serreze, director of the National Snow and Ice Data Center in Boulder, Colorado. This has resulted in above-average winter temperatures for most of the U.S., Europe and Russia.
Serreze said the Arctic Oscillation has been mostly in a positive phase since early December, and though this weather pattern is highly variable, flipping frequently between positive and negative phases, it’s unusual for it to stay stuck in place for longer than a month or two at a time.
Still, the pattern can change quickly and its behavior can be difficult to predict.
"This is actually an area of active research because if we could forecast it, we could really enhance longer-term predictability of weather conditions," Serreze said. "If you could say in November that the coming winter is going to be preferentially in a positive phase, that would be valuable for aviation or cities planning snowplow budgets, for example."
Computer models are used to try to forecast shorter-term changes in the Arctic Oscillation. Carl Schreck, a meteorologist at the North Carolina Institute for Climate Studies, said there are indications that the current positive phase could last for several more weeks.
"Some models are showing it well into March," he added.
This strong Arctic Oscillation also explains the dearth of extreme winter weather across much of the U.S.
"Storms tend to develop along the jet stream and are guided by it," Serreze said. "What we’re seeing is the polar front jet stream shifted north and that Arctic air is just trapped at high latitudes."
Scientists are studying whether climate change is affecting the Arctic Oscillation, but for now, any concrete links are tough to pin down, according to Serreze.
Studies have suggested that as global warming minimizes temperature differences between the Arctic and regions further south, the jet stream could become wavier than usual.
"We’re already seeing the Arctic warming at an outsize rate compared to the rest of the planet," he said. "If we’re changing the temperature gradient between higher and lower latitudes, the jet stream is going to respond to that."
And while scientists can’t draw conclusions based on one season or the climate patterns of one year, Serreze said this past winter demonstrates how conditions in the Arctic — a region especially sensitive to climate change — can have global implications.
"In other words, what happens in the Arctic doesn’t stay in the Arctic," he said.