Geoscience Reference
In-Depth Information
0
10
0.5
1.0
2.0
-5
10
5
20
5
15
10
15
0
0
0
5
0
0
0
-5
0
-10
-15
-20
-15
5.0
-5
-10
-5
-10
-10
0
-20
-30
0
10.0
20.0
5
-30
5
-20
10
10
50.0
100.0
200.0
5
0
0
0
0
-5
5
-5
0
-5
-10
-5
500.0
1000.0
Jan.
Jul.
Jan.
Jul.
Jan.
Jul.
Jan.
Jul.
Figure 3.7 Monthly mean zonal mean wind (m/s) at 1.25°N for November 1991-
November 1995. Easterly flow is shaded, and the contour interval is 5m/s.
Figure 3.7
shows the zonal wind speed as a function of time for 4 years
through the depth of the tropical stratosphere at 1.25°N. Above about 4 hPa,
in the upper stratosphere (see
Fig. 2.8
), the wind direction changes sign every
6 months. This is a seasonal signal, forced by insolation changes, called the
semiannual oscillation.
Much longer periodicity is evident below 4-5 hPa, where zonally uniform
easterly and westerly phases alternate every 24-30 months (26 months is the
average). This is the QBO. Successive phases of the QBO start above 10 hPa
and propagate downward, reaching the lower stratosphere about 26 months
later. Meanwhile, another phase of the QBO, with winds in the opposite direc-
tion, has begun in the upper stratosphere. As the signal propagates downward,
there is no loss in magnitude until the lower stratosphere where the wind speed
is strongly damped.
The QBO is thought to be caused by atmospheric wave breaking. The waves
are generated by convection in the upper tropical tropopause. They travel up-
ward through the stratosphere until conditions there cause them to break—
much like waves in the ocean break at the shore. On breaking, the waves
deposit energy and accelerate the flow.
3.4 DECADAL CLIMATE VARIABILITY
Although this may not seem like a very natural time scale for the climate sys-
tem, there is significant variability on decadal time scales. One example is
“inter-ENSO” variability, that is, the fact that one ENSO can be different from
another, or that there may be decades in which there are more ENSO events
than in others (see
Fig. 3.6)
. It is particularly important to understand natural
variability on this time scale because it is also the time scale of human-induced
changes in chemical composition of the atmosphere (see
chapter 10
).
