by It’s been a wild winter.
The end of February marked the end of “meteorological winter”, which includes December, January and February and is a designation separate from the astronomical seasons, capping off a series of months in which the weather played out in unexpected ways.
In the Northern Hemisphere, many normally frigid spots experienced dry and warmer-than-usual conditions, while others were plagued by heavy snow and damaging ice storms. Some cities in the southeastern U.S. recorded temperatures above 80 degrees Fahrenheit last month — one place in Texas even hit triple digits.
While official numbers won’t be confirmed for a few weeks, experts say this winter will almost certainly be among the 10 warmest on record, another worrying milestone in a trend driven by climate change.
“It’s really been quite a rollercoaster,” Bob Henson, a meteorologist and writer for Yale Climate Connections, an online news service, said of the extreme winters.
And warmer winters have real consequences: Vegetation, including spring foliage on trees, is blooming days and weeks ahead of schedule in some parts of the country. In Washington, DC, for example, cherry blossoms are appearing earlier than expected and are expected to peak in about four weeks.
“The concern is that if it turns out that we get a cold in mid to late March, it could really negatively impact early blooming crops or flowers,” Henson said.
Henson pointed to “striking” temperature contrasts in the US as an example of the odd seasonal variability. Last week, temperatures dipped in California, producing rare snow even at low elevations, while much of the Northeast basked in milder-than-normal conditions and received little to no snow during the winter months.
The sharp swings were fueled in part by an unusually rippling jet stream and a natural climate pattern known as La Niña, Henson said.
La Niña is characterized by a cooling of the eastern tropical Pacific Ocean, which produces atmospheric reflections that can greatly influence weather around the world. La Niña conditions can also affect the movement of the jet stream, a ribbon of fast-moving air that flows from west to east and makes its way across the Northern Hemisphere.
The jet stream is driven by temperature differences between the colder Arctic in the north and warmer air masses in the south. Ripples in the jet stream produce day-to-day weather, but deeper valleys and ridges can form that contribute to extremes by blowing cold air into one region and warming others.
For most of February, the jet stream was kinked over North America in such a way that cold air along the west coast was channeled southward while tropical air pushed up to the northeast, Henson said.
“It’s not that different from what you might see in the summer that produces hot and humid weather,” he said. “Of course it doesn’t get hot and humid in January or February, but just as that high temperature often persists in summer, it is quite persistent in winter.”
The result was snow along the West Coast, including rare blizzard warnings in Southern California, while all-time heat records for February were set in some cities in Georgia, Florida and Tennessee, as well as further up the coast in North Carolina and Virginia.
Extremes were also felt outside of North America. Record high winter temperatures were recorded in parts of Europe in January, and France had 32 days without rain in any part of the country this winter, raising fears of increasing drought across the continent.
Seasonal variations are normal, but some scientists are actively investigating whether global warming is causing the jet stream to become more wavy than usual. Some studies have made links, but more research is needed. So far, there is no consensus within the scientific community.
“It’s still being worked on, but something to keep in mind is that the inability to find an exact relationship doesn’t mean the relationship isn’t there,” Henson said.
He added that what is well known is the general warming trend caused by climate change.
“There’s no question that the overall climate is warming,” Henson said. “When it rains, it rains harder. When it’s dry, droughts tend to be more intense because of warmer temperatures. Those things are good and firmly anchored.”
The overall shift to warmer winters doesn’t mean there won’t be violent storms in the next few years. While this winter was generally warm, a relentless winter storm toppled Buffalo, New York, in December, a severe ice storm swept across the South in early February, and the Midwest was hit by heavy snow later in the month that knocked out power to hundreds of thousands people.
“A warming climate doesn’t rule out extreme winter weather,” Henson said, “but the trend is less snow and less cold over time. We know that very well.”
This article was originally published on NBCNews.com
