Geoscience Reference
In-Depth Information
upper-air circulation of large orographic barriers, such
as the Rocky Mountains and the Tibetan Plateau, and
heat sources such as warm ocean currents (in winter)
or landmasses (in summer). It is noteworthy that land
surfaces occupy over 50 per cent of the northern hemi-
sphere between latitudes 40° and 70°N. The subtropical
high-pressure belt has only one clearly distinct cell in
January over the eastern Caribbean, whereas in July
cells are well developed over the North Atlantic and
North Pacific. In addition, the July map shows greater
prominence of the subtropical high over the Sahara and
southern North America. The northern hemisphere
shows a marked summer to winter intensification of the
mean circulation, which is explained below.
In the southern hemisphere, the fact that oceans com-
prise 81 per cent of the surface makes for a more zonal
pattern of westerly flow. Nevertheless, asymmetries are
initiated by the effects on the atmosphere of features
such as the Andes, the high dome of eastern Antarctica,
and ocean currents, particularly the Humboldt and
Benguela currents (see Figure 7.29), and the associated
cold coastal upwellings.
Figure 7.5
A schematic illustration of the mechanism of long-
wave development in the tropospheric westerlies.
The symbol d/d
t
denotes a rate of change following the
motion (a total differential). Consequently, if air moves
poleward so that
f
increases, the cyclonic vorticity tends
to decrease. The curvature thus becomes anticyclonic
and the current returns towards lower latitudes. If the
air moves equatorward of its original latitude,
f
tends
to decrease (Figure 7.5), requiring ζ to increase, and the
resulting cyclonic curvature again deflects the current
polewards. In this manner, large-scale flow tends to
oscillate in a wave pattern.
C-G. Rossby related the motion of these waves to
their wavelength (
L
) and the speed of the zonal current
(
U
). The speed of the wave (or phase speed,
c
), is
3 Upper wind conditions
It is often observed that clouds at different levels move
in different directions. The wind speeds at these levels
may also differ markedly, although this is not so evident
to the casual observer. The gradient of wind velocity
with height is referred to as the (vertical)
wind shear
,
and in the free air, above the friction level, the amount
of shear depends upon the vertical temperature profile.
This important relationship is illustrated in Figure 7.6.
The diagram shows hypothetical contours of the 1000
and 500 mb pressure surfaces. As discussed in A.1
above, the
thickness
of the 1000 to 500 mb layer is
proportional to its mean temperature: low thickness
values correspond to cold air, high thickness values
to warm air. This relationship is shown in Figure 7.1.
The theoretical wind vector (
V
T
) blowing parallel to
the thickness lines, with a velocity proportional to their
gradient, is termed the
thermal wind
. The geostrophic
wind velocity at 500 mb (
G
500
) is the vector sum of the
1000 mb geostrophic wind (
G
1000
) and the thermal wind
(
V
T
), as shown in Figure 7.6.
The thermal wind component blows with cold air
(low thickness) to the left in the northern hemisphere
when viewed downwind; hence the poleward decrease
of temperature in the troposphere is associated with
c
=
U
- ß
(
L
)
2
2
π
where ß =
y (i.e. the variation of the Coriolis
parameter with latitude) (a local, partial differential).
For stationary waves, where
c
= 0,
L
= 2
∂
f/
∂
(
U
/ß). At
45° latitude, this stationary wavelength is 3120 km for
a zonal velocity of 4 m s
-1
, increasing to 5400 km at 12
m s
-1
. The wavelengths at 60° latitude for zonal currents
of 4 and 12 m s
-1
are, respectively, 3170 and 6430 km.
Long waves tend to remain stationary, or even to move
westward against the current, so that
c
≤0. Shorter waves
travel eastward with a speed close to that of the zonal
current and tend to be steered by the quasi-stationary
long waves.
The two major troughs at about 70°W and 150°E are
thought to be induced by the combined influence on
π√
