Report: Severe Weather In Northeast Tied To Warming Arctic
March 16--This winter started with bone-chilling cold around Christmas that lasted into January; then came record warmth in February, including 60s on Cape Cod, and over 70 farther inland.
The third nor'easter in a two-week span may finally be in the rearview mirror for most people, but another storm is possible next week. And don't forget the big one in January that caused widespread flooding and erosion.
"It's pretty clear something is going on," said Jennifer Francis, a research professor at the Institute of Marine and Coastal Sciences at Rutgers University, who has investigated changes in Arctic climate and its influence on weather worldwide since 1994.
Francis is the co-author of a report published Tuesday in the online journal Nature Communications that found a rapidly warming Arctic may be behind what appears to be a shift towards a pattern of severe winter weather in the Northeast.
The Arctic is warming at two to three times faster than the global average and that warming air rising over the poles creates instability in the winds that race around the North Pole, corralling the coldest air in our hemisphere. That instability, a slowing of the jet stream, the fast moving winds at the outer edge of the polar vortex, can cause a sloshing effect. The cold air follows the jet stream south, dipping low over North America, causing extreme cold and triggering storms.
The report found the strongest correlation for this pattern existed in the Northeast. The authors also found that the incursion of cold air, once established, tended to remain in place, potentially creating a string of storms as has occurred over the past two weeks.
The article's lead author is Judah Cohen, a research affiliate at MIT's Parsons Lab and director of seasonal forecasting at Atmospheric and Environmental Research, a company that works with government agencies as well as large insurance, investment and energy companies "to anticipate, manage, react to and profit from weather and climate related risk," according to its website. Francis, AER principal engineer Karl Pfeiffer, and Patrick Taylor, a research scientist at NASA's Langley Research Center, are co-authors of the report.
The researchers used an index that measures the height of a low or high pressure mass above sea level and the polar cap temperature index, and compared it to a data base that tracks extreme snowfall and temperature recorded at 12 weather stations around the country. They used data from 1950 to 2016, and found that changes in the polar cap temperatures and in the height of the rising air amass over the poles as it warmed were related to increased frequency of severe winter weather, particularly in the Northeast. They also posited that continued warming of the Arctic could lead to more frequent episodes of severe winter weather.
"These types of storms happen always," Cohen said. "We've had more strong storms than usual this year, and ...this study is a piece of the puzzle."
The theory that a warming Arctic, creating instability in the jet stream and polar vortex, directly affects our winter weather is controversial. Critics say correlation is not causation, which the researchers don't necessarily dispute.
"There are naysayers and skeptics of this work, but we certainly established that there is a relationship," said Cohen, adding that disturbances in the polar vortex can be used as a way to forecast the onset of severe weather.
The polar vortex is like a spinning bowl filled with extremely cold air. If there is lower-than-average air pressure over the Arctic and higher-than-average pressure over North America, the jet stream remains far north, steering storms to the north as well, and the Northeast experiences fewer cold spells. But if air pressure is higher-than-average over the Arctic and lower to the south, the jet stream shifts southward and can develop waves, steering frigid, polar air south like water sloshing out of the bowl.
Francis compares it to water running downhill. Arctic air is cold, dense and forms a thinner layer than the warmer air to the south. From Cape Cod looking north it is essentially like looking down the hill. If the temperature and pressure difference is large, the hill is steep, and the water, or in this case wind, runs downhill fast and straight. The earth's rotational pull grabs it and it becomes that river of wind circling the pole as the polar vortex.
But if the Arctic air is warmer and the temperature differential with southern latitudes is not as significant it's more like a river that has hit a coastal plain and begins to meander.
It is what is known as the "Wavy Jet Stream" theory. When the jet stream dips into the Northeast, the temperature and pressure differentials along the edge spawn winds that are set to spinning by the earth's rotation and head offshore where they encounter the warm waters of the Gulf Stream. The heat and evaporation of that warm ocean river flowing north from the equator fuels the storms and become the winter hurricanes we call nor'easters.
Warmer ocean and air temperatures due to climate change are pumping more fuel into those storms.
"The oceans are much warmer now and we get a lot more evaporation and warmer air can hold more water vapor," Francis said.
The more water vapor, the more storm energy and higher levels of snowfall and rain.
"The heaviest snowstorms on record have increased in their frequency in the last 20 years," Francis said. "They used to be rare."
-- Follow Doug Fraser on Twitter:dougfrasercct.
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