Geoscience Reference
In-Depth Information
Figure 6.7a shows an elevated map corresponding to
the surface map in Figure 6.7b. The elevated map shows
height contours on a surface of constant pressure (500
hPa) and winds traveling around centers of low and high
heights. Height contours on a constant pressure graph
are analogous to isobars on a constant altitude graph;
thus, high (low) heights in Figure 6.7a correspond to
high (low) pressures on a constant height graph. The
lows aloft lie slightly to the west of the surface lows.
The figure indicates that winds traveling around the
highs and lows aloft connect, resulting in sinusoidal
west-to-east flow around the globe.
Thus, high- and low-
pressure systems aloft are responsible for accelerating
winds aloft in the Ferrel cell from west to east (in the
westerly direction)
.
80
5600
5600
5500
L
5550
70
5450 54
00
5600
60
5
550
L
54
50
50
H
5800
40
H
5900
30
5900
5850
20
10
-180
-170
-160
-150
-140
-130
-120
-110
Longitude (degrees)
= 1.767e+01
(a)
80
1
022
6.3.7. Polar Easterlies
Air moving poleward aloft in the polar cells is turned
toward the east in both hemispheres by the ACoF, caus-
ing elevated winds in the polar cells to be westerly. At
the poles, cold air aloft descends, increasing surface air
pressure. The surface high-pressure regions are called
polar highs
.Air at the polar surface diverges equator-
ward. The ACoF turns this air toward the west. Friction
is weak over the Arctic because polar surfaces are either
snow or ice, and sea ice is relatively flat; therefore, sur-
face winds are relatively easterly. The Antarctic is a
continent with high mountains and rough surfaces, so
the surface winds are turned more toward low pressure
by friction and thus are more southeasterly (coming
from the southeast). Nevertheless, in both hemispheres,
the resulting surface winds in the polar cells are called
polar easterlies
(Figure 6.5).
1018
70
L
10
06
60
1018
1006
L
50
1010
1018
H
H
40
1026
1022
1022
30
1018
1014
20
10
-180
-170
-160
-150
-140
-130
-120
-110
Longitude (degrees)
= 3.215e+01
(b)
Figure 6.7.
Maps of (a) 500-hPa height contours (m)
and wind vectors (m s
−1
)(b)sealevel pressure
contours (hPa) and near-surface wind vectors
(m s
−1
)obtained from National Centers for
Environmental Prediction (NCEP, 2000) for August 3,
1990, at 12 GMT for the northern Pacific Ocean.
Height contours on the constant pressure map (a) are
analogous to isobars on a constant height map; thus,
high (low) heights in map (a) correspond to high (low)
pressures on a constant height map. The surface
low-pressure system at
6.4. Semipermanent Pressure Systems
The subtropical high-pressure belts in the Northern and
Southern Hemispheres are dominated by surface high-
pressure centers over the oceans. These high-pressure
centers are called
semipermanent surface high-
pressure centers
because they are usually visible on a
sea-level map most of the year. These pressure systems
tend to move northward in the Northern Hemisphere
summer and southward in the winter. On average,
they are centered near 30
◦
Nor30
◦
S. In the North-
ern Hemisphere, the two semipermanent surface high-
pressure systems are the
Pacific high
(in the Pacific
Ocean) and the
Bermuda-Azores high
(in the Atlantic
Ocean). In Figure 6.7b, the Pacific high is the surface
high-pressure center at
148
◦
W, 53
◦
Nin(b)isthe
Aleutian low. The surface high-pressure system at
−
−
134
◦
W, 42
◦
Nin(b)isthePacific high. The arrow
below each map gives the scale of the wind speed
arrow in m s
−1
.
in Figure 6.7b create a near-surface west-to-east flow
that meanders sinusoidally around the globe. The flow
created by these highs and lows is consistent with the
expectation that near-surface winds in the Ferrel cell
are predominantly westerly.
134
◦
W, 4 2
◦
N. In the Southern
Hemisphere, semipermanent high-pressure systems are
−
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