As Washington’s winter climate has warmed several degrees over the past 120 years, average snowfall has declined by about half a foot, from roughly 21 inches to 15 inches. Yet recent decades have also featured several of the biggest snowstorms the city has ever recorded.
Snowfall trends in Washington, as well as other East Coast cities, are leading scientists to this conclusion: Global warming, while eating away at some snow events, may paradoxically be contributing to an uptick in big East Coast snowstorms.
Several recent studies show that this trend toward more blockbuster storms will continue into the coming decades, although there are open questions about how climate change is skewing the odds toward particular winter weather scenarios.
Judah Cohen, a meteorologist at AER, a Verisk Analytics company, has published multiple studies that link changing snowfall trends in the eastern United States to change in the Arctic. His research shows that the loss of Arctic sea ice is contributing to an increase in fall snowfall in parts of Siberia. This is, in turn, having an influence on weather across the Arctic, extending high into the atmosphere above the vast region, favoring weather patterns that tend to direct Arctic air into the Lower 48 states.
Specifically, Cohen has shown that a chain of events beginning with global warming-driven sea ice melt can disrupt the polar vortex, a ribbon of rapidly flowing air at high altitudes above the Arctic circulating around an area of low pressure. When the polar vortex is strong, the coldest air tends to stay locked up in the far north. When it weakens, splits or wobbles, it can allow ultracold air to spill out to the south, including into the Mid-Atlantic.
“Arctic change favors more disruptions of the polar vortex,” Cohen said, noting that he is somewhat lonely in that view. He sees polar vortex splits as a prerequisite to blockbuster East Coast snowstorms.
Other researchers, such as Jennifer Francis of Woods Hole Research Center, have put forward the hypothesis that rapid Arctic warming — the region is warming at more than twice the rate of the rest of the world — is reducing the temperature difference between the equator and the Arctic. Because this temperature difference helps power the jet stream, Francis and others argue that the jet stream is slowing down and increasing the frequency of high-amplitude dips, or troughs, as well as ridges.
This can lock in weather patterns for longer-than-average periods, thereby contributing to extreme events. One historic storm event associated with such an unusual jet stream flow was Hurricane Sandy in 2012, and there have been many other examples since.
However, this is still an area of active scientific research.
“Larger snowfalls return more often. They happen more often now than they used to,” Cohen says. In Washington, he notes, the return period for snowstorms of over one foot before 1990 compared with since then shows that such heavy snowstorms are occurring twice as frequently as they used to.
“My argument, and I’m on a little bit of an island on this,” Cohen says, is that Arctic warming knocks the polar vortex off balance, which can bring intense periods of winter weather to places such as Boston, New York and Washington.
Another factor that favors heavier precipitation events, including snowstorms, is that as the climate warms, there’s an increasing availability of water vapor in a warmer climate, which can help to intensify storms more rapidly and churn out more precipitation.
Other researchers have published studies looking at other factors that may be contributing to changes in East Coast snowstorms.
For example, a computer modeling study published in Geophysical Research Letters last year found that smaller snowstorms will significantly diminish across a broad swath of the Northeast, including Washington, by late in the century. However, the bigger storms will get even more destructive and are unlikely to diminish in number, the study found.
Colin Zarzycki, the lead author of the study, says that as winters continue to warm, overall snowfall will decline in the Northeast, and the total number of snowfall events will also decline. However, when conditions align to produce snow, it will fall at more intense rates than typically occurs now. This will increase the odds of having a big snowstorm, he says.
D.C.’s winters are already warmer and shorter than they used to be.
A major federal climate assessment released in 2018 found that U.S. winters will warm sharply and skew precipitation more toward rain, rather than snowfall. Growing seasons will become longer, as the fall freeze-up occurs later and spring thaw creeps earlier. This could have an influence on when the cherry blossoms reach their peak, though earlier blooms will be more vulnerable to a sudden freeze, which would still be possible, especially earlier in the spring.
Already, the peak bloom date for cherry blossoms is arriving five days earlier than it did in the 1920s in Washington. The average date has advanced from around April 4 to March 31.
Both thermometers and snowfall trends are consistent with a warming winter climate in the Washington region.
Nationwide, winter is warming far faster than summer, and this can be seen in Washington’s temperature statistics. The average December-February temperature is now about 4 degrees higher than it was at the turn of the last century, for example. A warming climate will tend to produce fewer snow events, particularly considering that temperatures in D.C. snowstorms tend to be marginal, anyway, with the rain-snow line straddling the city and its immediate suburbs.
As Washington’s average snowfall has declined, the number of accumulating snow events per winter in the District has declined from an average of six at the beginning of historical records to three today. The sharpest decreases in snow have occurred in November and December, as well as March and April, as warming temperatures have flipped what might have fallen as snow to rain events. For example, all five of the District’s latest measurable snows on record (in April) occurred before 1960.
However, all hope for D.C. snow lovers is far from lost. Big snowstorms still occur, and they may become more routine as air and sea-surface temperatures rise, supercharging coastal storms. In addition, snow events have not dropped sharply yet in January and February, which tend to be the coldest winter months. But instead of expecting a winter full of small snow events to keep things looking wintry, it’s possible that we’ve already entered a climate characterized more by a feast-or-famine scenario, in which a major snowstorm or two accounts for the bulk of our seasonal snowfall.
Those events may be more impactful, and occur with more regularity, than they used to, but the disappearance of the nuisance-type snow event will also be evident.
Credit: Source link