Nature has regularly occuring phenomena that you can count on, repeating over days, months, and years. But sometimes, a spanner is thrown in the works and things start getting interesting.
For 60 years, an atmospheric phenomenon has repeated regularly in the tropical stratosphere. Since 1953, scientists have detected this quasi-biennial oscillation (QBO) in which westerly winds gradually descended and are replaced by easterlies.
The pattern has been repeating every 28 months without change, until late 2015. As reported in a paper in Geophysical Research Letters, the westerly winds began to fall as usual but then they suddenly rose, blocking the onset of the winds from the east. This new configuration held out for about six months until the regular pattern resumed in July 2016.
"The quasi-biennial oscillation is the stratosphere's Old Faithful," said lead author Paul Newman, Chief Scientist for Earth Sciences at NASA's Goddard Space Flight Center, in a statement.
"If Old Faithful stopped for a day, you'd begin to wonder about what was happening under the ground."
The delayed onset of the oscillation has important consequences for the atmosphere. For example, the amount of ozone in the tropics changes by about 10 percent depending on the direction the winds are going, and the oscillation could also contribute to the distribution of ozone in the polar regions.
The QBO has also been linked with the winter weather in the northern hemisphere. When western winds dominate, cold winters happen in northern Europe and the eastern US, while a mild winter in the US and a wet stormy one in Europe happen if the easterlies are on top.
Newman and his colleagues are now busy figuring out what caused the change in the QBO. They have two hypotheses: either the QBO was affected by the particularly strong El Niño of last winter or this might be a consequence of the long-term increase in global temperature caused by anthropogenic climate change.
The scientists are trying to understand if this was a one-off event or more of a “canary in the coal mine”, indicating another change produced by global warming.
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