An unprecedented disruption of the atmospheric quasi-biennial oscillation

In the late 1950s Met Office and US scientists independently discovered that winds high up in the atmosphere over the equator alternate between easterlies and westerlies roughly every 28 to 29 months. This so-called quasi-biennial oscillation (QBO) is one of the most repeatable atmospheric phenomena and its regularity 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. However, in 2016 the regularity of the QBO broke down.

The QBO's influence on weather patterns over the UK and Northern Europe has led to its continuous monitoring by the Met Office's seasonal forecast team.  In early March 2016 this monitoring revealed that, in February, the usual regular QBO behaviour was unexpectedly disrupted for the first time in the 27 QBO cycles observed since its discovery. The World Climate Research Programme ( WCRP) SPARC QBO initiative was alerted of the unprecedented disruption. Scientists from the Met Office Hadley Centre joined forces with colleagues from the Universities of Oxford and Hawai`i and the Canadian Centre for Climate Modelling and Analysis to investigate. The team's findings have now been published in Science ( Osprey et al., 2016). They show that the disruption , which involved an unexpected reversal in wind direction below 30hPa, was caused by atmospheric waves in the Northern Hemisphere. This is identified by the red circle in the figure above. Significantly, these waves are not represented in the simplest theoretical models of the QBO which have been well established since the 1960s.

Dr Scott Osprey, an  NCAS scientist at the University of Oxford, 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."

Prof 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, though scientists believe that the quasi-biennial oscillation could become more susceptible to similar disruptions as the climate warms.

Reference

Osprey, S. M., N. Butchart, J. R. Knight, A. A. Scaife, K. Hamilton, J. A. Anstey, V. Schenzinger and C. Zhang (2016), An unexpected disruption of the atmospheric quasi-biennial oscillation, Science, 08 Sep 2016, DOI: 10.1126/science.aah4156

Last updated: 22 September 2016

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