A stretching of the polar vortex helped to push frigid air into the state. Climate change may be increasing such stretching.
By Bob HensonSeptember 3, 2021
In a study released Thursday in the journal Science, the devastating Texas cold wave in February is linked to a stretching of the polar vortex in the stratosphere miles above ground level. This stretching mode, only recently categorized, has become more common over the last 40 years, the paper finds, and the increase may be related to human-caused climate change.
Led by Judah Cohen, a climate scientist at Atmospheric and Environment Research, the new study is the latest salvo in a decade-long debate over how Arctic warming may be driving some winter extremes in the mid-latitudes, paradoxically leading to intense cold spells in a warming climate.
The stratospheric polar vortex is a semi-permanent pool of cold air over the poles about 10 to 30 miles high, encircled by strong winds. When the vortex is split in two, or stretched out, associated shifts in the jet stream at lower altitudes can push frigid surface air into the mid-latitudes, including the United States.
The stretched-polar-vortex concept was first brought into the dialogue with a 2018 paper in the Bulletin of the American Meteorological Society by Marlene Kretschmer of the University of Potsdam; Cohen was a co-author. A stretched vortex can pull a frigid air mass from high latitudes and drive it toward low latitudes even more effectively than when the vortex is weakened and split and a piece of it moves to lower latitudes, the mode that has been studied more closely.
The new study uses observations to identify the stretched vortex as an increasingly prevalent mode since 1980, and one that is especially likely to be related to intense mid-latitude cold outbreaks. The authors then use climate modeling to show that changes in Arctic sea ice and Eurasian snow cover may be fostering the stretched mode and contributing to high-impact winter weather extremes.
This year’s Texas cold wave, which the study identifies as related to vortex stretching, caused close to 150 deaths and at least $20 billion in damages. Houston was below freezing for nearly 48 hours, and millions across the state lost power. The impacts were widely considered to be a result of the state’s main power grid operator, ERCOT, being unprepared for such high-end events in a fast-growing region.
“Last winter following the Texas cold wave, many debated for and against the contribution of climate change to the event,” Cohen said in an email. “However, there were no studies supporting or refuting the link between climate change and the dynamical mechanism behind the Texas cold wave until our study.”
Overall, the most extreme winter cold is on the decrease globally and nationally, consistent with a warming climate. Yet some winter extremes, such as the Texas cold blast, have been particularly impactful. It’s been argued that rising temperatures and reduced sea ice in the Arctic are driving a chain of events behind some of the worst recent mid-latitude cold waves in North America and Eurasia.