Unprecedented atmospheric behaviour disrupts one of Earth's most regular climate cycles

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.