Major snowfalls in eastern US are becoming more frequent. The return period (y axis; 0 to 15 years) of varying snowfall events (x axis; 0 to 18 inches) for weather stations during two periods: cold Arctic (1950–1989; blue) and warm Arctic (1990–2016; green). Lower values indicate more frequent snowfalls (shorter return period). The time series that were found to be significantly different at the 95% confidence level are shown in bold lines and include Atlanta, Boston (Blue Hill), Des Moines, Detroit, Helena, New York, Salt Lake City, Seattle, and Washington. Graphic: Cohen, et al., 2018 / Nature Communications

By Ken Branson
13 March 2018
(Rutgers Today) – Scientists from Rutgers University-New Brunswick and Atmospheric and Environmental Research (AER) have linked the frequency of extreme winter weather in the United States to Arctic temperatures.Their research was published today in Nature Communications.“Basically, this confirms the story I’ve been telling for a couple of years now,” said study co-author Jennifer Francis, research professor of marine and coastal sciences in Rutgers’ School of Environmental and Biological Sciences. “Warm temperatures in the Arctic cause the jet stream to take these wild swings, and when it swings farther south, that causes cold air to reach farther south. These swings tend to hang around for awhile, so the weather we have in the eastern United States, whether it’s cold or warm, tends to stay with us longer.”The research is timely given the extreme winter of 2017-2018, including record warm Arctic and low sea ice, record-breaking polar vortex disruption, record-breaking cold and disruptive snowfalls in the United States and Europe, severe “bomb cyclones” and costly nor’easters, said Judah Cohen, director of seasonal forecasting at AER and lead author of the study.In their study, Cohen, Francis, and AER’s Karl Pfeiffer found that severe winter weather is two to four times more likely in the eastern United States when the Arctic is abnormally warm than when the Arctic is abnormally cold. Their findings also show that winters are colder in the northern latitudes of Europe and Asia when the Arctic is warm.Paradoxically, the study shows that severe winter weather in the western United States is more likely when the Arctic is colder than normal.The researchers found that when Arctic warming occurred near the surface, the connection to severe winter weather was weak. When the warming extended into the stratosphere, however, disruptions of the stratospheric polar vortex were likely. These disruptions usually cause severe winter weather in mid- to late winter and affect large metropolitan centers of the northeastern United States.“Five of the past six winters have brought persistent cold to the eastern U.S. and warm, dry conditions to the West, while the Arctic has been off-the-charts warm,” Francis said. “Our study suggests that this is no coincidence. Exactly how much the Arctic contributed to the severity or persistence of the pattern is still hard to pin down, but it’s becoming very difficult to believe they are unrelated.”

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Ken Branson,
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908-797-2590,
kbranson@rutgers.edu

Warm Arctic Means Colder, Snowier Winters in Northeastern U.S., Study Says

As the Arctic warms the continents become colder. Northern Hemisphere surface temperature anomalies plotted for 500 hPa PCH anomalies binned on the intervals a [−3.0, −0.5], b [0.5, 3.0] and 500 hPa PCT c [−3.0, −0.5], and d [0.5, 3.0] for all winters 1950–2016. Climatological averages computed over the period 1981–2010. Where difference was found to be statistically significant above 95% is hatched in light gray (e.g., [−3.0, −0.5] to [0.5, 3.0]). We also tested for field significance in all plots and the differences were found to be highly significant. Ocean mask was applied south of 60° N. Graphic: Cohen, et al., 2018 / Nature Communications

ABSTRACT: Recent boreal winters have exhibited a large-scale seesaw temperature pattern characterized by an unusually warm Arctic and cold continents. Whether there is any physical link between Arctic variability and Northern Hemisphere (NH) extreme weather is an active area of research. Using a recently developed index of severe winter weather, we show that the occurrence of severe winter weather in the United States is significantly related to anomalies in pan-Arctic geopotential heights and temperatures. As the Arctic transitions from a relatively cold state to a warmer one, the frequency of severe winter weather in mid-latitudes increases through the transition. However, this relationship is strongest in the eastern US and mixed to even opposite along the western US. We also show that during mid-winter to late-winter of recent decades, when the Arctic warming trend is greatest and extends into the upper troposphere and lower stratosphere, severe winter weather—including both cold spells and heavy snows—became more frequent in the eastern United States.

Warm Arctic episodes linked with increased frequency of extreme winter weather in the United States