Cold snaps more likely during El Niño winters
FRISCO —El Niños don’t just affect anchovy fishermen in Peru and the ski resorts of the Sierra Nevada. The somewhat cyclical variation in equatorial Pacific sea surface temps can shift weather patterns worldwide, including in Europe, which may be more susceptible to extreme cold outbreaks in El Niño years, according to a new study led by a University of Colorado, Boulder researcher.
Other research has hinted at the connection, but the new paper is the first to show that El Niños might be linked with Sudden Stratospheric Warming events, when temperatures high in the atmosphere change radically, affect the polar vortex, a belt of winds that form a boundary between the cold Arctic and the temperate mid-latitudes. Sudden Stratospheric Warming weakens those winds, often leading to outbreaks of bitter cold Arctic air across Europe and possibly the eastern U.S.
Although it’s still not clear exactly how El Niño may influence the SSW events, the researchers said El Niño conditions could help forecasters know when to look for an Arctic blast.
When an El Niño event arrives every few years, it shifts the location of warm sea-surface temperatures and persistent showers and thunderstorms eastward across the tropical Pacific Ocean. The rising air forces rearrangements nearby, and the signals reverberate outward through the troposphere (the lowest few miles of the atmosphere).
Depending on location, this shifts the seasonal odds toward warm, cool, wet, or dry in many parts of the Americas, Asia, and Australia. The status of the El Niño–Southern Oscillation (ENSO) is one of the main tools used by U.S. seasonal forecasters in looking ahead to what a given winter might bring.
The new study, led by Amy Butler (University of Colorado Boulder/CIRES, is published in Environmental Research Letters, finds clear evidence for the link over the last half-century.
“Ours is the first study to examine the observations and really pull apart how El Niño can bring impacts all the way to the other side of the world,” said coauthor Clara Deser (with the National Center for Atmospheric Research. “It’s an interesting story, separating these stratospheric and tropospheric pathways.”
The combination of an El Niño event and an SSW appears to be especially conducive to outbreaks of Arctic air. Looking at the period November through March, the scientists found that seasonal temperatures across northern Eurasia plunged more than twice as far below average during an El Nino winter with at least one SSW as compared to an El Nino winter with no SSWs.
Looking at the period 1958–2013, the scientists also found that SSWs were more than twice as likely to occur during El Niño years as during neutral years.
“When you focus on those years, the stratospheric pathway emerges with great clarity,” said Butler.
SSWs can’t be predicted more than a few days in advance, and it’s not yet clear exactly how they form and how the influence of El Niño makes their way through the stratosphere. However, the new work suggests that forecasters could use the presence of El Niño to alert people that an SSW’s cold-inducing effects could be stronger than usual in northern Eurasia (and perhaps in eastern North America, although the current study doesn’t examine that problem directly).
According to Deser, “We can’t predict whether the sudden stratospheric warmings are going to occur, but we can say: If they do, this is what to expect.”