Geoscience Reference
In-Depth Information
influence on 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 land
masses (in summer). It is noteworthy that land
surfaces occupy over 50 percent of the Northern
Hemisphere between latitudes 40
2
⎛
⎜
L
2
⎞
⎟
c
=
U
- β
π
where
c
is the phase speed (or propagation)
of the wave relative to the surface
, U
is the
background zonal current,
y is the beta
plane effect, and
L
is the wavelength (the distance
between successive troughs or ridges defining the
wave). Immediately apparent is the critical role of
the wavelength. For a given zonal current and beta
plane value, a longer (shorter) wavelength leads
to a smaller (larger) phase speed with respect to
the surface. It should also be clear that if the
wavelength is sufficiently long for the given zonal
current, the Rossby wave may remain stationary
(c = 0) or even move westward with respect to the
surface (c < 0). Conversely, for two waves of the
same wavelength and beta plane value, the one
associated with the larger background zonal wind
will propagate faster. The general observation is
that long Rossby waves tend to be quasi-stationary
or move slowly eastward, although westward-
moving waves with respect to the surface are
indeed observed. Shorter waves (often simply
termed shortwaves) tend to move eastward. It is
instructive to calculate the stationary wavelength,
where c = 0 and
L
= 2
β
=
δ
f/
δ
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.
As mentioned, the flow pattern is much more
symmetric in the Southern Hemisphere. This
aligns with the fact that oceans comprise 81
percent of the surface. 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, particu-
larly the Humboldt and Benguela currents (see
Figure 7.31
), and the associated cold coastal
upwellings.
°
and 70
°
latitude,
this stationary wavelength is 3120km for a
zonal velocity of 4m s
-1
, increasing to 5400km at
12m s
-1
. The wavelengths, at 60° latitude for zonal
currents of 4 and 12m s
-1
are, respectively, 3170
and 6430km. The pattern of waves seen in a mid-
tropospheric contour chart can be quite complex,
with shorter waves tending to be embedded within
longwaves. An important concept is that the
shorter waves (associated with transitory cyclones
and anticyclones) tend to migrate along and
be steered by the quasi-stationary longwaves.
Knowing the pattern of the longwaves hence
provides information on the path of the shorter
waves.
Turning back to
Figures 7.3
and
7.4
, the two
major Northern Hemisphere troughs at about
70
π
(U/
β
). At 45
°
3 Upper wind conditions
Imagine two sets of dinner plates, one set of plates
being thicker than the other. The thick and thin
plates are stacked into separate piles. As we add
more plates to each pile, the height of the pile of
thick plates becomes increasingly greater than
the height of the pile with thin plates. Similarly, as
the thickness between pressure levels is greater
at lower latitudes than at higher latitudes (recall
from section A.1 and
Figure 7.1
above that
thickness is proportional to the mean temperature
of the layer), the difference in height of a given
pressure surface between high and low latitudes
increases upward. This means that the geostrophic
wind also increases with height; that is, there is a
vertical wind shear. The zonal winds are strongest
where and when the meridional temperature
E, again best expressed in winter,
are thought to be induced by the combined
°
W and 150
°
