The Quasi-Biennial Oscillation was the subject of a workshop in the Canadian city of Victoria from 16 to 18 March. Tim Stockdale, who represented ECMWF at the meeting, explains what the QBO is all about.
The Quasi-Biennial Oscillation is one of the most remarkable phenomena in the Earth's atmosphere. High above the equator, in the stratosphere, strong zonal winds blow in a continuous circuit around the Earth. At a given altitude, the winds might start as westerlies, but over time they weaken and eventually reverse, becoming strong easterlies.
Looking at different heights, we see that the peak amplitude of the westerly winds migrates slowly downwards, with the zone of easterly winds coming behind it also migrating downwards. When the winds approach the base of the stratosphere, at around 80 hPa, they dissipate. New bands of zonal winds appear in the mid-stratosphere, at around 10 hPa, to replace the bands which migrate downwards. At any one time, there is one region of easterlies and one region of westerlies. The whole cycle progresses at a fairly (but not entirely) uniform rate, taking on average 26 months to return to the starting state.
When the QBO was first discovered in the late 1950s, it took scientists some time to figure out what was causing it, but by the early 1970s the basic physics was established. Waves propagate up from the troposphere into the stratosphere and, by a process known as critical layer absorption, some of these waves break and deposit momentum at just the right altitude to reinforce the zonal winds and cause them to migrate downwards. The waves involved are both large-scale equatorial waves, of the kind that ECMWF’s Integrated Forecasting System (IFS) can resolve, and small-scale gravity waves which must be parametrized. The waves carry both eastward and westward momentum, but the absorption process extracts the momentum at different levels, giving the alternative easterly and westerly winds.
Although the basic physics of the QBO is well known, the quantitative details and balances of the different processes are still rather unclear. Worse, many of the models used for numerical weather prediction (NWP) or climate modelling are unable to produce a QBO, or they produce a QBO which looks very different from observations. For example, only 4 of more than 30 models used for the last IPCC report have any sort of QBO.
At ECMWF, the IFS does have sufficient vertical resolution and physics to allow a reasonable simulation of the QBO, for example as seen in ECMWF’s ERA-20CM, a set of extended model runs covering the 20th century. Our seasonal forecasts also have skill in predicting the future evolution of the QBO signal. However, we would like to improve the accuracy and skill of the IFS, so we are working with other members of the international scientific community to better understand and model the processes driving the QBO.
A major outcome of the meeting was agreement to work together on a set of experiments and analyses to improve the QBO in models. ECMWF is playing a major role in helping to design the ‘initialized forecast’ experiments, where we have much experience, and other groups are contributing expertise on longer-term simulations and diagnostics.
Why is the QBO important? It is certainly relevant for seasonal prediction, where the state of stratospheric winds affects interactions between the tropics and the mid-latitudes, and may also affect the tropical troposphere directly and possibly how the solar cycle interacts with the atmosphere. For those groups working on climate, the QBO has a role in modulating transport out of the tropical stratosphere to higher altitudes, thus influencing the concentration of gases in the stratosphere, which in turn might lead to further climate feedbacks.
The poor representation of the QBO in climate change models means that no-one knows what will happen to the QBO in the decades ahead – will it remain largely unchanged, will its period lengthen, or will it change more radically? There is also increasing interest in stratospheric wind forecasts in their own right: Google has a programme, dubbed Project Loon, to provide global internet coverage using actively controlled stratospheric balloons. This both requires and contributes to data on stratospheric winds, with forecasts being needed on various timescales to control the system.