Mysterious Anomaly: Reversal of Stratospheric Wind Pattern Observed for the First Time
Earth’s stratosphere lies just above the red-orange troposphere in this photo snapped by International Space Station astronauts in 2011. Late last year, unusual wind behavior interrupted a reliable stratospheric wind pattern known as the quasi-biennial oscillation. Credit: NASA/JSC Gateway to Astronaut Photography of Earth
For the first time, scientists have observed a deviation from the typical alternating pattern of easterly and westerly winds in the equatorial stratosphere.
The weather we experience on Earth typically occurs in the troposphere, the lowest layer of the atmosphere. But the stratosphere, which envelops the planet just above the troposphere, is home to winds of its own. In a new study, Newman et al. report an anomalous interruption in an otherwise reliable stratospheric wind pattern known as the quasi-biennial oscillation.
Each cycle of the quasi-biennial oscillation begins with strong westerly winds that flow through the stratosphere in a belt around the equator. Over the course of about 1 year, these winds gradually weaken and descend in altitude to the lower stratosphere as easterly winds replace them. These easterly winds slowly sink and weaken, too, as westerly winds return. The cycle repeats roughly once every 28 months.
Since 1953, scientists have observed equatorial winds by instruments known as radiosondes, which are carried skyward by weather balloons. The quasi-biennial oscillation was discovered in the early 1960s. Although the timing of each cycle has sometimes varied by a few months, the pattern as a whole has remained uninterrupted—until now.
Using radiosonde data from several equatorial locations around the world, the scientists discovered that the quasi-biennial oscillation began to deviate from its usual pattern in late 2015. At that time, westerly winds were descending in altitude and should have continued to sink and weaken as easterlies replaced them.
Instead, the westerly winds shifted upward and seemed to cut off the descent of high-altitude easterlies before they could begin their usual dominance. Additional easterly winds developed at lower altitudes in the stratosphere, beneath the rising westerlies. However, by June, the westerlies appeared to have resumed their normal descent.
The researchers plan to continue analyzing wind and temperature data to determine what caused this anomaly and what its implications may be. Their investigation will include an exploration of possible connections with the 2015–2016 El Niño and climate change. (Geophysical Research Letters, doi:10.1002/2016GL070373, 2016)
Sarah Stanley, Freelance Writer