Page 191 - Geosystems An Introduction to Physical Geography 4th Canadian Edition
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TABLE 6.1 Four Hemispheric Pressure Areas
Chapter 6 atmospheric and Oceanic Circulations 155 70°N
Name
Polar high Subpolar low Subtropical high equatorial low
Cause
Thermal Dynamic Dynamic Thermal
Location
90° n, 90° S
60° n, 60° S 20°–35° n, 20°–35° S 10° n to 10° S
60°N
50°N
40°N
1012
m
a
e
r
t
f
l
uS
G
Within the ITCZ, winds are calm or mildly vari 30°N Moist, able because of the weak pressure gradient and the ver unstable tical ascent of air. These equatorial calms are called the 20°N doldrums (from an older English word meaning “foolish”)
because of the difficulty sailing ships encountered when 10°N 1014 attempting to move through this zone. The rising air from
the equatorial lowpressure area spirals upward into a
geostrophic flow running north and south. These upper 10°S
1018 H 1022 1024
1020
Northeast trade winds
Dry, stable
Canaries Current
air winds turn eastward, flowing from west to east, begin ning at about 20° N and 20° S, and then descend into the highpressure systems of the subtropical latitudes.
Subtropical Highs: Hot and Dry Between 20° and 35° latitude in both hemispheres, a broad highpressure zone of hot, dry air brings clear, frequently cloudless skies over the Sahara and the Arabian Deserts and portions of the Indian Ocean (see Figures 6.10 and Figure GIA 6.2 and the world physical map on the inside back cover of the book).
These subtropical anticyclones generally form as air above the subtropics is mechanically pushed downward and heats by compression on its descent to the surface. Warmer air has a greater capacity to absorb water vapour than does cooler air, making this descending warm air relatively dry (discussed in Chapter 7). The air is also dry because moisture is removed as heavy precipitation along the equatorial portion of the circulation. Recent research indicates that these highpressure areas may intensify with climate change, with impacts on regional climates and extreme weather events such as tropical cyclones (discussed in Chapters 8 and 10).
Several highpressure areas are dominant in the subtropics (Figure 6.10). In the Northern Hemisphere, the Atlantic subtropical highpressure cell is the Bermuda High (in the western Atlantic) or the Azores High (when it migrates to the eastern Atlantic in winter). The Atlantic subtropical highpressure area features clear, warm waters and large quantities of Sargassum (a seaweed) that gives the area its name—the Sargasso Sea. The Pacific High, or Hawaiian High, dominates the Pacific in July, retreating southward in January. In the Southern Hemisphere, three large highpressure centres dominate the Pacific, Atlantic, and Indian Oceans, es pecially in January, and tend to move along parallels of latitude in shifting zonal positions.
Because the subtropical belts are near 25° N and 25° S latitude, these areas sometimes are known as the “calms of Cancer” and the “calms of Capricorn.” These zones of
90°W 80°W 70°W 60°W 50°W 40°W 30°W 20°W 10°W 0°
Air Temperature/ Moisture
Cold/dry Cool/wet Hot/dry Warm/wet
1012
1016
1010
1010 1012
Westerlies
1014
0°
Winds Warm current Cool current
▲Figure 6.11 Subtropical high-pressure system in the Atlantic. Characteristic circulation in the northern Hemisphere. note
deserts extending to the shores of africa with offshore cool currents, whereas the southeastern United States is moist and humid, with offshore warm currents.
windless, hot, dry desert air, so deadly in the era of sail ing ships, earned the name horse latitudes. Although the term’s true origin is uncertain, it is popularly attributed to becalmed and stranded sailing crews of past centuries who destroyed the horses on board, not wanting to share food or water with the livestock.
The entire highpressure system migrates with the summer high Sun, fluctuating about 5°–10° in latitude. The eastern sides of these anticyclonic systems are drier and more stable (exhibit less convective activity) and are associated with cooler ocean currents. These drier eastern sides influence climate along subtropical and midlatitude west coasts (discussed in Chapter 10 and shown in Figure 6.11). In fact, Earth’s major deserts generally occur within the subtropical belt and extend to the west coast of each continent except Antarctica. In Figures 6.11 and 6.18, note that the desert regions of Africa come right to the shore in both hemispheres, with the cool, southwardflowing Canaries Current offshore in the north and the cool, northwardflowing Benguela Current offshore in the south.
Westerlies Surface air diverging within the subtropical highpressure cells generates Earth’s principal surface winds: the trade winds that flow toward the equator, and the westerlies, which are the dominant winds flowing from the subtropics toward higher latitudes. The wester lies diminish somewhat in summer and are stronger in winter in both hemispheres. These winds are less consis tent than the trade winds, with variability resulting from
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0 500 1000 KILOMETRES
