The Quasi-Biennial zonal wind Oscillation (QBO)

Andrew Heaps , Willam Lahoz and Alan O'Neill

Centre for Global Atmospheric Modelling,
Department of Meteorology,
University of Reading

The discovery of the QBO

The eruption of the Krakatau volcano (6 ° S 105 ° E) on August 27th 1883 led people to believe that the stratospheric wind above the equator blew in a westward direction. Dust from the eruption took 13 days to circle the equator and this upper air wind became known as the Krakatau easterlies.

In 1908 Berson launched observational balloons above Lake Victoria in Africa and found westerly winds at about 15km (120mb). These westerly winds are called Berson's westerlies.

These conflicting results were resolved through the work of Reed (1961) and Veryard and Edbon (1961), who showed that the wind above the equator oscillates in direction. It was shown that the wind in the stratosphere changed direction on average every 26 months and that the alternating easterly and westerly wind regimes descend with time.

The QBO as seen in the UKMO Assimilated dataset

The UKMO assimilated dataset, Swinbank and O'Neill (1994), was one of the principle contributions to the Upper Atmosphere Research Satellite (UARS) mission. Atmospheric data such as winds, temperature and irradiance from satellite and balloon measurements are combined with a global atmospheric weather model to provide one of the best representations of the atmosphere that is currently available. The top level in the dataset is at 0.316mb (57 km) making it ideal for looking at stratospheric phenomena. It is worthwhile noting that the model doesn't produce a QBO on its own, so the assimilation system must use the ingested atmospheric data to produce one in the output data.

Fig 1. The monthly zonal mean wind in m/s against pressure in mb as seen the the UKMO assimilated dataset at 1.25 ° north of the equator. Easterlies are coloured yellow to blue and westerlies orange to red. The zero line is in thick black and every 5m/s is delineated in thin black. The QBO is roughly between 10mb and 100mb in height extent. Above 3mb the Semi-Annual Oscillation (SAO), a harmonic of the seasonal cycle can be seen, being westerly near the equinox and easterly near the solstice.


From Fig 1., and data from longer period datasets (Naujokat 1986) the defining characteristics of the QBO are:

1) The wind regimes propagate down as time progresses.

2) They move downwards at roughly 1km/month and decrease in magnitude as the height decreases.

3) The period of the oscillation is 20 to 36 months with a mean of around 28 months.

4) They start at 10mb and descend to 100mb.

5) The maximum amplitude of 40 to 50m/s is seen at 20mb.

6) Easterlies are generally stronger than westerlies.

7) Westerly winds last longer than easterly winds at higher levels while the converse is true at lower levels.

8) The westerlies move down faster than the easterlies as shown by the steepness of the zero line.

9) The transition between westerly and easterly regimes is often delayed between 30 and 50mb.

10) There is considerable variability of the QBO in period and amplitude.


Latitudinal extent of the QBO and a movie


When looking at the extent of the QBO in latitude on a pressure surface plot, Figs 2(a) and 2(b), the strong seasonal cycle around 60 degrees north and south is readily seen. Focussing on the equator , the alternating easterly and westerly wind regimes that make up the QBO change from being strong at 10mb (Fig 2(a)) to being very weak at 100mb (Fig 2(b)).

Fig 2(a) Zonal mean wind as a function of time and latitude at 10mb. Note the alternating easterlies and westerlies along the equator.


Fig 2(b) Zonal mean wind as a function of time and latitude at 100mb.



Latitude-pressure plots of the monthly mean wind show that the annual cycle again dominates in the extra-tropics in the plots shown in Fig 3(a) and Fig 3(b). Selecting April 1993 and April 1994 the descent of the easterlies and westerlies can be seen. The whole picture is, however, not as simple as these two plots may suggest.

Fig (3a) April 1993 monthly zonal mean wind as a function of latitude and height

Fig 3(b) April 1994.


To see the complexity of the situation in full, it is necessary to look at a movie of the QBO as a function of latitude and pressure.

This file is a 3.5Mb gif animation. The movie shows monthly latitude height sections of zonal mean wind from November 1991 to July 1998. Note the annual cycle in the northern and southern hemispheres and their interaction with the more time persistent QBO in the equatorial region.

(You may wish to download this image to your local disk for later viewing.)

The movie shows that the QBO as seen in the zonal mean wind field looks quite complicated with considerable interaction with the northern and southern seasonal cycle. Dunkerton and Delisi (1984) used twenty years of radiosonde data and with the winds deseasonalised to show that the QBO has an approximate Gaussian distribution about the equator with a latitudinal half-width of about 12 degrees.


The theory of the QBO

Holton and Lindzen (1972) were the first to propose a model of the QBO based on vertically propagating waves. Originally it was thought that the Semi Annual Oscillation (SAO) in the upper stratosphere played an important role in the QBO. More recently they showed that while the SAO was important, it was not necessary for the formation of the QBO.The mechanism was further explained by Plumb (1977) who showed that the maximum acceleration occurs just below the maximum phase speed, leading to descent of the maximum with time.

It is now thought that equatorially trapped Kelvin waves provide the westerly momentum and Rossby-gravity waves provide easterly momentum to produce the QBO oscillation.


Fig 4. Plumb's analog of the QBO in six stages. Wavy blue and red lines indicate penetration of easterly and westerly waves.

In Fig. 4(a) both easterly and westerly maxima are descending as the upward propagating waves deposit momentum just below the maxima. When the westerly shear zone is sufficiently narrow, viscous diffusion destroys the westerlies and the westerly waves can propagate to high levels through the easterly mean flow, Fig. 4(b).The more freely propagating westerlies are dissipated at higher altitudes and produce a westerly acceleration leading to a new westerly regime, Fig. 4(c). Fig. 4(d) shows both regimes descending downwards until the easterly shear zone becomes vulnerable to penetration and the easterlies can then propagate to high altitudes, Fig. 4(e), and so onto the formation of a new easterly regime in Fig. 4(f).


Why the QBO is important?

1) The phase of the QBO affects hurricanes in the Atlantic and is widely used as a prognostic in hurricane forecasts.
Increased hurricane activity occurs for westerly (or positive) zonal wind anomalies; reduced hurricane activity for easterly or negative zonal wind anomalies.

2) The QBO along with sea surface temperatures and El Niño Southern Oscillation are thought to affect the monsoon.

3) Tropical cyclone frequency in the northwest Pacific increases during the westerly phase of the QBO. Activity in the southwest Indian basin, however, increases with the easterly phase of the QBO.

4) Major winter stratospheric warmings preferentially occur during the easterly phase of the QBO, Holton and Tan (1980).

5) Predictions of ENSO use the expected wind anomalies at 30mb and 50mb to forecast the strength and timing of the event.

6) The QBO is thought to affect the Sahel rainfall pattern and is used in forecasts for the region.

7) The decay of aerosol loading following volcanic eruptions such as El Chichon and Pinatubo depends on the phase of the QBO.


Some of the latest QBO research

1) A number of interesting laboratory experiments concerning waves and the QBO by Satoshi Sakai at Kyoto University.

2) Kinnersley and Pawson (1996) showed that extratropic planetary waves have an effect on the QBO.

3) Takahashi et al. (1997) have demonstrated a QBO like oscillation in the lower equatorial stratophere of a T21, 60 layer version of the Center for Climate System Research / National Institute for Environmental Studies (CCSR/NIES) atmospheric general circulation model.

4) Untch, A. (1997) Observed a QBO like oscillation in a 50 level version of the ECMWF model with a semi-Lagrangian scheme. A similar model run with Eulerian advection did not produce a QBO.

5) Lahoz, W. et al. , report improved stratospheric representation with the UKMO Unified Model with 74 levels and ~0.8km resolution in the stratosphere. No QBO is observed, however.

6) Scaife,A.A. et al have simulated the quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO) in the Met Ofice Unified Model.




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