Geoscience Reference
In-Depth Information
The basic westerly flow with its embedded
disturbances characterizes both hemispheres in
the regions poleward of the subtropical high
pressure cells (centered aloft at about 15
angular momentum is the product of the linear
velocity of a body rotating around an axis and its
perpendicular distance from the axis. Angular
momentum tends to be conserved, meaning that
if the radial distance of rotation of an air parcel
decreases (increases), then the speed of rotation
increases (decreases) Consider now a belt of
westerly winds at latitude 40°. If the winds shift
northward, the radial distance decreases, and so
the wind velocity increases. In the atmosphere,
angular momentum conservation is a major
contributor to the maintenance of westerly jet
streams.
Figure 7.8
shows a highly generalized north-
south cross-section with three westerly jet streams
in the Northern Hemisphere. The more northerly
ones, termed the
Polar Front
and
Arctic Front
Jet Streams
(Chapter 9E), are associated with the
steep temperature gradient where polar and
tropical air and polar and arctic air, respectively,
interact, but the
Subtropical Jet Stream
is related to
a temperature gradient confined to the upper
troposphere, 12-15km (~ 200mb). Wind speeds
over East Asia regularly exceed 100m s
-1
. The Polar
Front Jet Stream is very irregular in its latitudinal
latitude)
Between the subtropical high pressure cells and the
equator the winds are easterly. The dominant
westerly circulation reaches maximum speeds of
45-65m s
-1
, which even increase to 135m s
-1
in
winter. These maximum speeds are concentrated
in narrow bands, typically between 9000 and
15000m, called
jet streams
(see Note 2 and
Box 7.2
).
A jet stream is essentially a fast-moving ribbon
of air, which in turn coincides with the latitude of
maximum poleward temperature gradient, or
frontal zone, shown schematically in
Figure 7.7
.
The thickness effect , as described above, is a major
component of jet streams, but the basic reason for
the concentration of the meridional temperature
gradient in a narrow zone (or zones) is dynamical.
In essence, the temperature gradient becomes
accentuated when the upper wind pattern is
confluent (see Chapter 6B.1). It is helpful to
introduce the concept of angular momentum and
its conservation. The momentum of an air parcel
is the product of its mass times its velocity; the
°
200
12
9
6
500
3
O
P
IC
A
L
A
I
R
0
-
40
°
C
-2
0
°
C
0
°
C
1000
2
0
0
0
1
5
0
0
1
0
0
0
D
is
t
a
n
c
e
(
km
)
5
0
0
0
Figure 7.7
Structure of the mid-latitude frontal zone and associated jet stream showing generalized
distribution of temperature, pressure and wind velocity.
Source: After Riley and Spalton (1981). Courtesy of Cambridge University Press.
