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. There are actually two different polar vortexes: a stratospheric polar vortex, about 65,000 feet above the earth’s surface, and a tropospheric polar vortex about 18,000 to 30,000 feet up.
Weather forecasters most frequently refer to the tropospheric vortex, which sometimes breaks apart in winter and causes cold snaps when the broken fragments plunge towards the mid-latitudes. This study, however, focuses on the effects of sea ice changes on the stratospheric polar vortex, whose behavior can have important effects on mid-latitude weather, but more indirectly.
The study published in Nature Climate Change last week found the stratospheric polar 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, and cold air outbreaks from the Arctic southward toward the mid-latitudes become more likely. (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.
Kevin Trenberth, climate scientist at the National Center for Atmospheric research, said he did not find the study convincing. “The problem with most if not all of the Arctic/jet stream studies has been the lack of a clear physical cause and effect relationship, with correlations found but mechanisms as yet uncovered,” he said.
But James Screen, a climate scientist at the University of Exeter, found it to be a valuable contribution to the growing body of literature that suggests Arctic climate change produces far away consequences. “I thought the paper presented adequate evidence to support its conclusions, but obviously one paper is not going to settle an issue,” he said, noting connections between Arctic sea ice loss and mid-latitude weather remain “highly uncertain.”