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Regular equatorial wind patterns high up in the Earth's stratosphere broke down earlier this year when eastward winds suddenly reversed direction. Now, a report published in Science on Sept. 8 showed that atmospheric waves in the Northern Hemisphere prompted the event.
Scientists expect that more disruptions are possible as the climate warms, and say that these changes can influence weather as well as the amount of ozone in the air near the equator.
The main character in this story is the Quasi-Biennial Oscillation (QBO), stratospheric winds above the tropics that change direction every 28 to 29 months like clockwork. The stratosphere is typically where you'll fly on a flight from Los Angeles to New York, but near the equator, this layer of the atmosphere begins at about 59,000 feet, far above the flight levels for commercial airliners. Air in the stratosphere is very dry and stable, and this where the Earth's ozone layer is found.
In February, scientists from the National Centre for Atmospheric Science (NCAS) at the University of Oxford in the U.K., along with the Met Office, observed the unusual and unpredicted reversal of wind patterns. The team went on to probe the possible cause. Dr Scott Osprey, an NCAS scientist, said, "The recent disruption in the Quasi-Biennial Oscillation was not predicted, not even one month ahead."
The report stated, "An unprecedented westward jet formed within the eastward phase in the lower stratosphere and cannot be accounted for by the standard QBO paradigm based on vertical momentum transport. Instead, the primary cause was waves transporting momentum from the Northern Hemisphere."
Possible influences include the past winter's El Nino or the "blob" of warm water in the north Pacific Ocean.
Another possibility is the sudden stratospheric warming event that occurred this spring. We all heard about the polar vortex during the brutal winter of 2014-15, but the vortex is an annual occurrence. When a polar vortex breaks down, it leads to a sudden stratospheric warming event.
Dr. Amy Butler, a research scientist at the University of Colorado Cooperative Institute for Research in Environmental Sciences, explained:
"When the vortex breaks down, there is an event called a major "sudden stratospheric warming" (SSW). The 'warming' refers to the dramatic increase in temperature (~50-70 degrees F in a few days!) over the polar stratosphere during these events. These events occur roughly every other year in the Northern Hemisphere."
Weather forecasters look to the QBO to help predict long-term patterns, particularly for Europe. Westerly winds are associated with warmer and wetter conditions, while easterly winds are accompanied by drier and cooler weather. But as a result of this disruption, seasonal forecasts may be off. Because the Quasi-Biennial Oscillation has operated on a steady schedule and has been documented since 1953, "This unexpected disruption to the climate system switches the cycling of the Quasi-Biennial Oscillation forever," Prof. Adam Scaife, head of long-range forecasting at the Met Office, said.
Studies have indicated that higher carbon concentrations in the atmosphere will impact the QBO.
"In the future climate, the period of the QBO becomes longer by about 1–5 months, and the amplitude becomes smaller, especially in the lower stratosphere, despite the stronger mean precipitation in the equatorial region," concludes a study published by the American Meteorological Society.
In the just-released study announcing the findings of this year's event, researchers concluded, "A return to more typical QBO behavior within the next year is forecast, though the possibility of more frequent occurrences of similar disruptions is projected for a warming climate."
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