Geoscience Reference
In-Depth Information
Global Atmospheric Circulation
Now that you have investigated atmospheric circulation and the
seasonal migration of pressure systems, a good review is to ob-
serve the patterns in animated form. Go to the
Geo Media Library
and select
Global Atmospheric Circulation
. This animation
demonstrates the specific components of the global circulatory
system and how they move. Once you view this animation, be
sure to answer the questions at the end to test your understand-
ing of this process.
consists of a mass of descending air that rotates in a clockwise
fashion in the Northern Hemisphere and counterclockwise in
the Southern Hemisphere. These rotating systems direct cold,
dry air toward the polar front in the form of
polar easterlies
and are strongest in the Northern Hemisphere, where large
landmasses exist. If you want to review why this geographic
pattern exists, return to Figure 5.11 to view the annual range
of temperature on Earth and note the variation that exists be-
tween the hemispheres. As a result of this variability, the Polar
High located over northern continental regions of the Earth
makes northernmost Canada and Siberia bitterly cold and dry
during the winter.
A good reference point for studying the seasonal pres-
sure migration is the subsolar point and its relationship with
the ITCZ. Remember that the ITCZ migrates with the subsolar
point. For example, during the Spring and Fall Equinoxes, the
Sun is directly overhead at the Equator. This geographic posi-
tion results in the Equator receiving the most intense radiation.
Because the Equator is generally the warmest zone on Earth
during these months, the ITCZ is generally located there as well
(Figure 6.22a). During winter in the Northern Hemisphere, the
subsolar point is located at the Tropic of Capricorn because the
Sun is directly overhead at that latitude. Because this zone re-
ceives the most intense radiation, this is where the ITCZ is gen-
erally located (Figure 6.22b).
Similarly, the pattern is reversed with the approach of the
Northern Hemisphere spring and summer. With the coming of the
Northern Hemisphere Spring Equinox, the ITCZ follows the sub-
solar point back to the Equator. Subsequently, the ITCZ migrates
into the Northern Hemisphere during April and May, reaching the
Tropic of Cancer in the latter part of June. As the days shorten in
July and August, the ITCZ migrates back to the south.
Seasonal Migration
of Pressure Systems
So far in this discussion, the focus has been on the geography
of air pressure systems and how they transfer heat energy in
the atmosphere. Given your understanding of the Earth-Sun
geometric relationship, however, you might also suspect that a
distinct seasonal component occurs in association with global
atmospheric circulation. In fact, a powerful seasonal compo-
nent does come into play, one that is illustrated in Figure 6.22.
Band of easterly winds at high latitudes.
Polar easterlies
H
H
H
L
L
L
Sun's rays
Sun's rays
Sun's rays
H
H
H
Tropic of Cancer
Equator
L
L
L
Tropic of Capricorn
H
H
H
L
L
L
H
H
H
(a) Equinoxes, equatorial
low, ITCZ around Equator
(b) December 21, equatorial high,
ITCZ around Tropic of Capricorn
(c) June 21, equatorial high,
ITCZ around Tropic of Cancer
Figure 6.22 Seasonal migration of atmospheric pressure.
The positions of the major high- and low-pressure systems during
(a) Equinoxes, (b) Northern Hemisphere Winter Solstice, and (c) Northern Hemisphere Summer Solstice.
