For the past 60 years, scientists have been watching how strong winds blow in a continuous circuit around Earth’s tropical stratosphere - a layer up to 48 km (30 miles) above the equator.
This atmospheric phenomenon, known as quasi-biennial oscillation, has been repeating the same pattern every two years since the 1960s, but late last year, everything changed.
"The quasi-biennial oscillation is the stratosphere's Old Faithful," says Paul Newman from NASA's Goddard Space Flight Centre. "If Old Faithful stopped for a day, you'd begin to wonder about what was happening under the ground."
Up in the tropical stratosphere, which occupies the space 16 to 48 km (10 to 30 miles) above the equator, winds circulate the planet in alternating easterly and westerly directions.
This pattern occurs over a two-year period that’s been incredibly predictable up until this point.
It works like this: up the very top of the stratosphere, westerly winds start to develop and gradually sink to the bottom. As they approach the lower edge of the stratosphere, they are replaced by a weak layer of easterly winds from above.
As the European Centre for Medium-Range Weather Forecasts explains, at any one time, there is one region of easterlies and one region of westerlies, and the whole cycle progresses at a fairly uniform rate, taking roughly 26 to 28 months to return to the start.
This pattern has been repeating since 1953, according to measurements taken by weather balloons floating above the surface at various points above the tropics, and no one expected these winds to change their course.
But in late 2015, they noticed something strange.
The westerly winds appeared at the top of the tropical stratosphere as usual, but as they approached the bottom, the easterly winds weren’t there to replace them.
Instead, the westerly winds started to move back up again towards the top of the stratosphere, and in doing so, they blocked the downward movement of the easterlies.
This strange reversal of the six-decade-long pattern ran for almost six months, but then suddenly returned to normal in July 2016.
What’s perhaps even stranger than the fact that this steadfast pattern suddenly changed is that scientists have no idea how it happened, or what effect, if any, it had on Earth.
So far, the reversal hasn’t had any noticeable impact on any other weather patterns, but that doesn’t mean subtle changes haven't occurred that we haven’t picked up on just yet.
The NASA team found that the amount of ozone at the equator changes about 10 percent between the peak phases of the easterlies and westerlies, and the oscillation effect also appears to have an impact on the ozone levels in the polar regions. But whether the reversal disrupted this is still unclear.
"We know, too, that large volcanic eruptions or meteorite impacts can fling particles up into the stratosphere where they may linger for months, affecting the Earth's global climate, " Karen Graham explains for Digital Journal.
The team is now focussing on figuring out the cause of the six-month reversal, suggesting that it could have been caused by the recent extreme El Nino event of 2015 to 2016.
Or was the reversal a side effect of the unprecedented levels of global warming we’ve been experiencing of late?
So the question now is was this just an anomaly, a once-in-a-generation event, or are we looking at a 'canary in the coal mine' - a warning of things to come?
"It's really interesting when nature throws us a curveball," says Newman.
The study has been published in Geophysical Research Letters.