9 September 2016 (Oxford University) – The normal flow of air high up in the atmosphere over the equator, known as the quasi-biennial oscillation, was seen to break down earlier this year. These stratospheric winds are found high above the tropics, and their direction and strength changes in a regular two to three-year cycle, which provides forecasters with an indication of the weather to expect in northern Europe. Westerly winds are known to increase the chance of warm and wet conditions, while easterlies bring drier and colder weather.
Scientists from the National Centre for Atmospheric Science (NCAS) at the University of Oxford and the Met Office were part of an international team that observed the unusual behaviour in February, noticing a reversal of the expected pattern in the winds. This same team then identified the reason why.
The quasi-biennial oscillation is a regular feature of the climate system. On average, these equatorial eastward and westward winds alternate every 28 to 29 months, making them very predictable in the long term. The team's findings, published in the journal Science this week, show that this unexpected change in wind direction was caused by atmospheric waves in the northern hemisphere.
Dr Scott Osprey, an NCAS scientist in the University of Oxford's Department of Physics, said: “The recent disruption in the quasi-biennial oscillation was not predicted – not even one month ahead. If we can get to the bottom of why the normal pattern was affected in this way, we could develop more confidence in our future seasonal forecasts.”
Professor Adam Scaife, Head of Long-range Forecasting at the Met Office and Honorary Visiting Professor at the University of Exeter, said: “This unexpected disruption to the climate system switches the cycling of the quasi-biennial oscillation forever. And this is important, as it is one of the factors that will influence the coming winter.”
A return to more typical behaviour within the next year is forecast, although scientists believe that the quasi-biennial oscillation could become more susceptible to similar disruptions as the climate warms.
Later this month, international research groups will meet in Oxford to discuss the origins and implications of this event.
The paper, “An unexpected disruption of the atmospheric Quasi-Biennial Oscillation”, is published in Science.
12 September 2016 (UH News) – One of the most repeatable, predictable phenomena of atmospheric winds threw scientists for a loop February 2016 by breaking its long-standing routine. However, a study published last week in Science presents a mechanism to explain this unexpected and unprecedented disruption. The international group of atmospheric scientists was led by Scott Osprey of the National Centre for Atmospheric Science at the University of Oxford, and included Kevin Hamilton, recently retired professor of atmospheric sciences, and Chunxi Zhang, atmospheric modeling specialist, both with the International Pacific Research Center (IPRC) at the University of Hawaiʻi at Mānoa.
Changing of the routine
High altitude (16-50 km) winds above the equator typically oscillate between prevailing eastward and westward wind-jets, with a period of about two to three years. This pattern (Fig. 1) of descending and alternating directional wind-jets, called the Quasi-Biennial Oscillation (QBO), has held since weather balloons began taking the appropriate measurements in January 1956 (27 cycles). In February 2016, though, the pattern was unexpectedly disrupted when an anomalous westward wind-jet formed during an established eastward phase (red arrow). The presence of this jet could not be explained by the mechanism understood to drive the QBO: the vertical transport of momentum in the atmosphere.
The predominance of one wind direction over the other is important for forecasters, particularly in predicting the weather patterns that will dominate Northern Europe each winter.
“If we can get to the bottom of why the normal pattern was affected in this way, we could develop more confidence in our future seasonal forecasts,” explains Osprey.
Understanding the disturbance
In an effort to understand the disruption of the QBO pattern and find an underlying mechanism that may have generated the disturbance, the group analyzed four types of data—direct, in situ wind observations by balloons, global model assimilation of balloon and satellite observations, global model predictions of the atmosphere several months into the future and free-running global climate models.
“Dr. Hamilton and I, at IPRC, helped initiate this investigation by analyzing the winds directly measured by balloons at many locations near the equator. We then devised a quantitative measure of how extremely unusual the behavior this year has been,” said Zhang.
The primary cause of the QBO disruption determined by the study was atmospheric waves transporting momentum from the Northern Hemisphere southward to the equatorial region, thus causing the formation of a westward wind-jet, which disrupted the eastward flow. Analysis of very long term computer climate simulations illustrated that one model spontaneously produced similar disruptions of the QBO, but with a frequency of less than once per century.
“The development of the upper level winds in early 2016 caught all the experts by surprise,” explained Hamilton, “We cannot be completely sure of how the disruption in the QBO will be resolved. It seems mostly likely that there will be a return to typical QBO behavior through the rest of 2016 into 2017, although reversed in polarity: winter of 2016/2017 was expected to be westward dominated and may instead be eastward dominated again.”
For Europe, that means their winter is likely to have more storms and heavy rain.
ABSTRACT: One of the most repeatable phenomena seen in the atmosphere, the quasi-biennial oscillation (QBO) between prevailing eastward and westward wind-jets in the equatorial stratosphere (~16-50 km altitude), was unexpectedly disrupted in February 2016. 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. Seasonal forecasts did not predict the disruption but analogous QBO disruptions are seen very occasionally in some climate simulations. 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.