High in the stratosphere, winds whip around Earth's equator, switching from westerlies to easterlies and back again roughly every 28 months. Although the length of the cycle varies from year to year, the quasi-biennial oscillation (QBO), as it is known, is as dependable as they come: It had not skipped a beat since scientists first reported its existence in 1960.
Earlier this year, westerly winds did not yield smoothly to easterlies, as expected, sending scientists scrambling to understand both cause and consequences. One repercussion is already clear: The accuracy of seasonal forecasting could take a hit. But scientists don't know whether the break in the pattern was a one-off thing, or a more ominous sign of the things to come in a warming world. One thing is for sure: One of atmospheric science's most durable precepts has cracked. "All the textbooks will have to be rewritten," says Kevin Hamilton, an atmospheric scientist at the University of Hawaii's International Pacific Research Center in Honolulu, and a co-author of a study published online this week in Science that describes the unprecedented disturbance.
You can't encounter any new phenomenon without wondering if there's some impact of climate change.
The QBO consists of bands of strong stratospheric winds that appear in the midstratosphere and slowly descend, weakening until they dissipate near the base of the stratosphere around an altitude of 16 kilometers. As each band of wind moves down, a new band heading in the opposite direction begins forming above it, with each cycle lasting between 22 and 36 months.
But in February, scientists began to notice something strange in the data from a weather balloon above Singapore. A band of easterly winds began to form above the westerlies, as expected, at the end of 2015, but it was cut off by a new band of westerly winds that appeared below it, keeping the QBO from completing a normal cycle. "This is really, really unexpected," says Steven Pawson, an earth scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and a co-author of a study published last week in Geophysical Research Letters that also describes the disruption.
A capricious QBO would deprive scientists of a seasonal forecasting tool. Although it occurs in the tropics, the QBO affects global weather through teleconnections: interactions with large-scale waves in the stratosphere that project its influence to higher latitudes. This reach, coupled with its regularity, has made the QBO a stabilizing force on global weather patterns. So the QBO's unexpected deviation is "kind of a shock" to forecasters' ability to predict weather, Hamilton says.
For example, there's a strong statistical suggestion that the QBO influences the North Atlantic Oscillation, a pattern of seesawing atmospheric pressures that dominates European weather. When the QBO winds are in a westerly phase, pressure differences over the North Atlantic tend to be more extreme. That strengthens the jet stream and boosts the chances that northern Europe will experience warmer, stormier winters like this past one, which brought floods to the United Kingdom. The expected easterly phase at the end of this year would have given northern Europe a good shot at a colder, drier winter. Instead, the return to westerly winds means that Europeans are more likely to see another stormy winter. "It's not a sure thing that that would be the forecast, but it loads the dice toward those sorts of conditions," says Scott Osprey, a climate scientist at the University of Oxford in the United Kingdom and lead author of the Science study.
Scientists have identified several possible causes for the break. The QBO is thought to be driven by tropical waves—generated by warm, circulating air—that propagate up from the troposphere into the stratosphere. But when Osprey's team plugged the anomalous QBO data into a climate model, the disruption appeared to originate outside of the tropics. One possible culprit is this past winter's strong El Niño, which not only brought unusually warm waters to the eastern equatorial Pacific Ocean, but also shook up atmospheric waves and weather patterns well beyond the tropics. A "blob" of warm water that has been growing in the northern Pacific Ocean since 2013 is another possible cause, as is a sudden stratospheric warming event that occurred this past winter in the high latitudes of the Northern Hemisphere.
The two teams are not yet ready to indict climate change as the more fundamental culprit, but they are concerned. "You can't encounter any new phenomenon without wondering if there's some impact of climate change," says Anne Smith, an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. Osprey's team suspects that global warming will slow down the QBO and make it more vulnerable to future disruptions.
And indeed, they found hints of this, in one of the three climate models they studied. Under an extreme climate change scenario that forecasts warming of about 3.7°C by the end of the century, the model suggested QBO disruptions could occur up to three times every 100 years. If warming is playing a role, this first break in the QBO might not be the last.
Betsy Mason is a freelance writer in the San Francisco Bay Area in California.