THE STRATOSPHERE

THE COMPOSITION OF STRATOSPHERE - The ozone layer - The polar ozone layer depletion - The stratosphere in hight latitudes - The polar vortex - The stratosphere in low latitudes - The Q.B.O (Quasi Bisuannuel Oscillation )

THE STRATOSPHERE
 IN LOW LATITUDES

THE Q.B.O (Quasi Bisannuel Oscillation )

Above the equator, between 12,42 and 31,06 miles altitudes (20 and 50 km altitude), the stratospheric winds around the globe eastbound or westbound. Every 15 to 20 months the direction is reversed. This phenomenon is known as the Quasi Biennial Oscillation (QBO). From a fluid in dynamics perspective, the QBO is a coherent stream, the mean oscillation is carried out vertically propagating waves free periods compared to the result of the QBO cycle. We don't understand fully the cause, but we know that influence various parameters of the atmosphere, the ozone levels at mid and high latitudes.

When stratospheric winds blows to the East, QBO West phase (weasterly), an acceleration of wind is felt, while wind to West, QBO East phase (Esterly), cause deceleration of the winds. Alternative accelerations and decelerations of wind are related if there are the eastward or westward winds in the lower stratosphere. The easterlies are generally stronger (30-35 m/s) than the westerlies (15-20 m/s).

Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. So the QBO affects the polar vortex. When there is a westerly there are three times more likely to have an important polar vortex and when the Q.B.O is East phase it is twice as likely that the polar vortex be weak. This has an effect on the ozone levels in the atmosphere and should have links to A.O. index and therefore on winters. In 1993, 1995 and 1997 the Q.B.O. was in the West phase and there were large ozone loss in the Arctic.

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QBO DOWN TO THE EAST tropic-------------- equator -------------- tropic QBO DOWN TO THE WEST tropic-------------- equator -------------- tropic Schematic illustration of the QBO which shows a model of the mean : temperature, flow, temperature and winds.

 

The QBO has links on tropical cyclone activity. Generally, a cyclone activity is 50 to 70% stronger when the QBO is West phase and vice versa when the Q.B.O. is East phase.

Several researchers have considered the effects of remote ENSO may influence the extratropical stratosphere. Such an influence could hide the QBO signal. WALLACE and CHANG were unable to separate the effects of ENSO and QBO on tropical stratosphere out of 21 winters in the Northern Hemisphere in 30-hPa geopotential.VAN LOON and LABITZE also found that the phases of the QBO and ENSO tended to coincide. Removing years of warm ENSO (El Niño) there is only cold years with the weak ENSO (La Niña) have shown similar results in Holton and Tan. Subsequent observational studies (HAMILTON ...), and the exposure model (HAMILTON ...) have a logical picture in the ENSO influence on zonal structure of the vortex that is largely trapped in the stratosphere. In the lower stratosphere, ENSO seems modulate the amplitudes of stationary waves on a large scale.

This graph shows the direction and magnitude of the wind in the equatorial stratosphere. When
the index is negative the winds blow from East to West with a speed depending on the value.
This graphic is produced and made days constructed according to the data NOAA

 

Click here to see this animation on the evolution of the QBO from 1952 to 1996

 

CClick here to see a video showing the evolution of the QBO
from 1991 to 1996 following the altitude (Source NASA)

Elisabeth and Jean-Claude Ribes, Philippe Merlin and Istvan Vince showed that the QBO could be triggered by the "small" cycle of the Sun, one thousand days.

In 1950 Van Loon and Karin Labitzke made links between QBO and the solar cycle of 11 years.

The variability of 10 years, may be shown by a solar cycle by a 11-year solar cycle, which exists in data registers which clearly began in the 1950s. Represented in 29 abstracts, their results show complete dispersion with an average height of 30 hPa geopotential in January and February above North Pole depending on the Solar Radio Flux (10,7 cm wavelength). The data can be grouped into four categories based on the phase of the QBO and the level of solar activity. In a year with low solar activity, the winter polar vortex tends to be disturbed and small when the QBO is easterly, but more importantly and less disturbed when the QBO is in the West. In years with high solar activity, however, the QBO West phase are associated with disturbed winters, while the QBO East phase are accompanied by a stronger and less disturbed polar vortex. Hence, the QBO acts as predicted by Holton and Tan (1980) in the years with low solar activity, but seems overthrow his behavior during the year with important solar activity. Only two cases are not exactly in the plan : 1989 and 1997.

This is the subject of active debate whether or not the 10-year variability is caused by the solar cycle of 11 years, but there is growing evidence through the model that the solar cycle has a considerable influence on the winds and the temperatures in the upper stratosphere. On the 11-year solar cycle, the solar constant varies by less than 0,10%. From minimum to maximum of the solar cycle the UV is the one that varies the most (1%). Which may indirectly affect the ozone chemistry nevertheless enhanced by the production of oxygen which in turn could decrease, which affects the rate of warming of the upper atmosphere.