WASHINGTON — Below-normal temperatures have swept across much of the
continental United States, and the early arctic invasion could be the first of
many. Research links autumn snow cover on the opposite side of the planet to
colder, harsher winters in the Northern Hemisphere.
“We found a relationship between Siberian snow cover in October and our
winters,” says Judah Cohen, director of seasonal forecasting at Atmospheric
and Environmental Research. AER and its parent company, Verisk, work with
government agencies to better anticipate and manage climate- and
weather-related risks.
Cohen says the snow cover per square mile last month was the highest since
1976. He notes that last year, a particularly cold and snowy winter in
Washington, October’s snow pack was the third-highest in reliable records
dating to 1972.
“This October was the second-highest snow cover that we observed using
satellites across Siberia,” he says.
Throughout Eurasia, 14.14 million square kilometers (8.8 square miles) of snow
cover was observed last month. Roughly 12.85 million square kilometers (8
million square miles) were seen by the end of October 2013.
The snow back for October last year, a particularly cold and snowy winter in
Washington, was the third highest in reliable records
dating to 1972. (WTOP/Dave Dildine)
“The most direct relationship between the snow cover and the climate is
through the polar vortex. When you have high snow cover, there tends to be a
weaker polar vortex. As we learned from last winter, when the polar vortex
weakens, that’s when you have a much higher probability of these Arctic
outbreaks in these latitudes.”
The polar vortex is a normal fixture of the Arctic and frequently dips south
during wintertime. Cohen says frigid air above the snow pack in Eurasia
contributes to large ripples that can course through the jet stream over long
periods of time. The waves can break down the polar vortex, causing sections
of it to slide southward, allowing cold Arctic air along for the ride.
“I would say 2009-10 is the best example of what happens when the polar vortex
breaks down,” says Cohen.
He says the force behind that blockbuster winter was tied
to a weakened vortex and a sizable deviation in the jet stream’s path through
the northern Atlantic Ocean.
“When the vortex is strong, it acts like a dam. It keeps all the cold air
locked up in the arctic. When the vortex weakens it’s like the dam is
breaking. All that cold air that’s locked up the arctic will start flooding…
into the mid-latitudes,” says Cohen.
The shape of the polar vortex reflects a large-scale weather pattern that
camps out near Greenland during the winter months called the North Atlantic
Oscillation. The magnitude of the NAO directly influences winter weather in
the eastern U.S. and western Europe. To this end, Cohen believes widespread
snow cover in Siberia encourages the polar vortex to sculpt the winds above
the northern Atlantic into a shape that can lead to colder weather on the
opposite side of the globe.
“The snow cover is the best predictor we have of the North Atlantic
Oscillation. I think everybody would agree if you could accurately predict the
upcoming winter’s North Atlantic Oscillation value, that that’s the biggest
piece in the prediction puzzle.”
Along with his peers at AER, Cohen partially attributes the vaster expanse of
early snow cover in Siberia in recent years to the poleward movement of
moisture brought by by warmer-than-usual summer temperatures in lower
latitudes.
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