The Quasi-Biennial Oscillation is a regular variation of the winds that blow high above the equator. Strong winds in the stratosphere travel in a belt around the planet, and every 14 months or so, these winds completely change direction. This means a full cycle takes roughly 28 months, making it the most regular slow variation in the atmosphere after the cycle of the seasons.
The QBO is driven by atmospheric waves that rise from the troposphere and are produced by intense tropical weather systems. These waves break in the tenuous stratosphere and provide a force to 'push' the wind and make it descend with time. Once these high-level winds reach the tropopause, the opposite phase of the oscillation descends from above. It takes roughly 14 months for each reversal to occur.
The Quasi-Biennial Oscillation can affect the Atlantic jet stream. The speed of the winds in the jet stream weaken and strengthen with the direction of the QBO. The jet stream is an important atmospheric feature that brings us our weather here in the UK, and the risk of winter conditions in Northern Europe can differ depending on the phase of the QBO:
A change in wind direction in the equatorial stratosphere was first spotted by Met Office meteorologists analysing weather balloon data in the 1950s. The mechanism behind the QBO was only understood many years later thanks to American scientists James Holton and Richard Lindzen. These days the physics of the QBO is generally well understood and it is included in the computer models that help forecast our weather.
Last updated: 7 September 2016