Geography Reference
In-Depth Information
Table 12.1
Characteristics of the Dominant Observed Planetary-Scale Waves in the Equatorial Lower
Stratosphere
a
Theoretical description
Kelvin wave
Rossby-gravity wave
Discovered by
Wallace and Kousky
Yanai and Maruyama
(1968)
(1966)
Period (ground-based) 2πω
−
1
15 days
4-5 days
Zonal wave number
s
= ka cos φ
1-2
4
Vertical wavelength 2 π m
−
1
6-10 km
4-8 km
+25 m s
−
1
-23ms
−
1
Average phase speed relative to
ground
Observed when mean zonal flow
Easterly
Westerly
(maximum
≈−
25ms
−
1
)
(maximum
≈+
7ms
−
1
)
is
Average phase speed relative to
maximum zonal flow
+50 m s
−
1
-30ms
−
1
Approximate observed amplitudes
u
8ms
−
1
2-3ms
−
1
v
2-3ms
−
1
0
T
2-3 K
1 K
Approximate inferred amplitudes
/g
30 m
4 m
w
1.5
×
10
−
3
ms
−
1
1.5
×
10
−
3
ms
−
1
Approximate meridional scales
2N
β
|
1/2
|
m
1300-1700 km
1000-1500 km
a
(After Andrews et al., 1987).
12.6
THE QUASI-BIENNIAL OSCILLATION
The search for periodic oscillations in the atmosphere has a long history. Aside,
however, from the externally forced diurnal and annual components and their har-
monics, no compelling evidence exists for truly periodic atmospheric oscillations.
The phenomenon that perhaps comes closest to exhibiting periodic behavior not
associated with a periodic forcing function is the quasi-biennial oscillation (QBO)
in the mean zonal winds of the equatorial stratosphere. This oscillation has the
following observed features.
1. Zonally symmetric easterly and westerly wind regimes alternate regularly
with periods varying from about 24 to 30 months.
2. Successive regimes first appear above 30 km, but propagate downward at a
rate of 1 km month
−
1
.
3. The downward propagation occurs without loss of amplitude between 30
and 23 km, but there is rapid attenuation below 23 km.
4. The oscillation is symmetric about the equator with a maximum amplitude
of about 20 m s
−
1
, and an approximately Gaussian distribution in latitude with a
half-width of about 12
◦
.
