IN LOW LATITUDES
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.
QBO DOWN TO THE EAST
QBO DOWN TO THE WEST
Schematic illustration of the QBO
which shows a model of the mean :
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.
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
here to see this animation on the evolution of the QBO
from 1952 to 1996
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
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