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Republished from thewashingtonpost.com
By Jason Samenow
Global warming could be making winters in eastern North America even longer, according to a new study.
The study finds that, because of sea ice loss in the Arctic, the polar vortex is shifting and temperatures are turning colder during March.
The polar vortex is the zone of frigid air that encircles the Arctic and is most pronounced during winter. Occasionally, it fragments and pieces of it plunge into eastern North America carrying bitter cold air. The winters of 2013-2014 and 2014-2015 witnessed several such polar vortex disintegrations and resulted in record-setting cold snaps.
The study published in Nature Climate Change last week found the vortex has changed position while weakening over the past three decades. The net result has been to increase delivery of cold air into parts of Eurasia and North America, particularly in late winter and early spring.
It’s counterintuitive, but when the polar vortex is weak, it’s more unstable, likely to break apart and then transport chilled air from the Arctic southward toward the mid-latitudes. (When it’s strong, the cold air is more contained in the Arctic and the mid-latitudes are milder.)
Multiple studies have shown the vortex weakening over time but this new study shows a marked shift in the vortex from North America toward Europe and Asia during February. The meteorology is complicated, but the study says this shift tends to result in more of a dip in the jet stream over the East Coast during March, which leads to colder temperatures.
Cooler March temperatures due to a shifted polar vortex could offset some climate warming from rising greenhouse gases, the study says.
The study says the vortex shift can be linked to declining sea ice in the Arctic, focused over the Barents-Kara seas, triggered by warming temperatures.
Sea ice loss in this region may also be responsible for the polar vortex weakening as well, according to independently published work by climate researcher Judah Cohen of the private firm Atmospheric Environmental Research. When the ice melts in this region and leaves behind open water, more autumn snow falls downwind over the Eurasian continent during fall, Cohen has found. The enhanced snow cover then promotes atmospheric waves that destabilize and weaken the polar vortex, Cohen hypothesizes.
Sea ice levels this fall have been at or near record lows, and Eurasian snow cover has accumulated quickly. These factors have led Cohen to predict a weak polar vortex and colder-than-normal winter for the Eastern U.S.
Despite this new study and Cohen’s research, the idea that climate warming and Arctic sea ice loss is destabilizing the polar vortex remains controversial. Some studies have found sea ice loss could actually strengthen the polar vortex.
But the study adds another piece of evidence to suggest climate changes in the Arctic can have consequences that reverberate around the Northern Hemisphere.