Instructor's Manual and Test Bank to accompany Meteorology Today, 10th Edition
Jonathan D. W. Kahl
University of Wisconsin-Milwaukee
Chapter 10
Wind: Global Systems
Summary
The study of winds continues in this chapter with a discussion of global scale circulation patterns.
The three-cell model of the general circulation is presented and is compared with real world
observations of pressures and winds. The seasonal movement of these features and their effect on
regional climate is studied.
The chapter examines the major jet streams found in the earth's atmosphere. The polar jet
stream and the subtropical jet form in regions with strong horizontal temperature gradients. Conservation
of angular momentum also plays a role in the formation of these fast moving currents of air. Focus
sections on “The Dishpan Experiment” and “Momentum - A Case of Give and Take” elucidate the
physical reasons for the occurrence of global wind patterns and jet streams.
Interactions between global scale wind patterns and the earth's oceans are examined, and the
positions and directions of the major ocean currents are shown. Finally, the El Niño/Southern Oscillation,
the North Atlantic Oscillation, the Pacific Decadal Oscillation and the Arctic Oscillation are described,
with considerable attention given to the possible climatological effects associated with both El Niño and
La Niña events. The Ocean Niño Index (ONI) is introduced as a way of quantifying the occurrence and
strength of El Niño and La Niña events. A final focus section on “El Niño and the Wiinter Olympics of
2010” presents an interesting example of the ways that global wind systems and their variations affect the
public.
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Teaching Suggestions
1.
Clouds produced by some of the mesoscale and global scale wind circulation patterns discussed
in this chapter can be seen on satellite images. The ITCZ is often very clearly defined on a full or half
disk image from a geostationary satellite.
2.
During the winter, examine a surface weather map in class and compare temperatures to the north
and the south of the polar front. There is often a very sharp temperature gradient. Locate the polar jet
stream on an upper-level chart and determine the region where the maximum winds are found.
3.
Using realtime weather imagery, have students locate the prevailing westerlies and the polar
easterlies.
4.
Have students search weather charts for different regions of the world to locate polar easterlies,
middle latitude westerlies and trade winds. A good resource for this is the World Meteorological
Organization’s website:
http://www.wmo.int/pages/members/index_en.html
5.
Locate the ITCZ using satellite imagery, then examine a sounding along the ITCZ using the
University of Wyoming’s Radiosonde data website. Examine the atmospheric stability revealed by the
sounding:
http://weather.uwyo.edu/upperair/sounding.html
6.
Examine a surface weather map to determine which of the semi-permanent highs or lows are
affecting your current weather.
7.
Discuss the effects of the jet stream on your local weather. Which jet stream influences your
weather more, the subtropical or polar front jets? How does the jet stream affect the local weather?
8.
If you are close to an ocean, discuss the influence the ocean has on your local weather.
9.
During periods of El Niño or La Niña, discuss any effects these large-scale features have on your
local weather.
10.
Compare and contrast an upper-level weather chart showing your location with a similar chart for
the same latitudes in the southern hemisphere. Relate any differences to concepts presented in this
chapter.
Student Projects
1.
Have students research and plot the path of some of the early voyages of discovery made aboard
sailing ships (Columbus or Magellan, for example). Do the routes appear reasonable in light of what the
students know about the earth's general circulation pattern?
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2.
Have students collect climatological data for their location (or another region) corresponding to
one or two strong El Niño events. Select two or three periods when there was not a strong El Niño to act
as a control. What effects might the El Niño have on local weather or climate? Do the students see any
evidence of this in their climate data?
3.
Have students present actual weather data for different areas of the world that demonstrate the
main characteristics of the following features: Hadley cell, subtropical jet, trade winds, westerlies, polar
front, and polar front jet.
4.
Have students record 500 hPa winds every day for a week from the nearest radiosonde station. Is
the wind direction consistent with what one might expect from the atmospheric general circulation
presented in class? A good resource for this is:
http://weather.uwyo.edu/upperair/sounding.html
5.
Have students read and summarize or present Prof. Alistair Fraser’s “Bad Coriolis” article:
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html
Answers to Questions for Review
1.
Answer not provided.
2.
When we compare the January and July maps, we can see several changes in the semipermanent
pressure systems. The strong subpolar lows so well developed in January over the Northern Hemisphere
are hardly discernible on the July map. The subtropical highs, however, remain dominant in both seasons.
Because the sun is overhead in the Northern Hemisphere in July and overhead in the Southern
Hemisphere in January, the zone of maximum surface heating shifts seasonally. In response to this shift,
the major pressure systems, wind belts, and ITCZ shift toward the north in July and toward the south in
January.
3.
Because the earth rotates, creating a Coriolis force, which would deflect the wind from purely
north-south motion.
4.
Precipitation occurs at the boundaries of the general circulation cells where air rises. Rising air
occurs at the equator along the ITCZ, and at 60o N and S.
5.
Prevailing westerlies.
6.
During the summer, the Pacific high drifts northward to its eastern side produces a strong upperlevel subsidence inversion, which tends to keep summer weather along the West Coast relatively dry.
Along the east Coast, the clockwise circulation of winds around the Bermuda high brings warm tropical
air northward into the United States and southern Canada from the Gulf of Mexico and the Atlantic
Ocean. Because sinking air is not as well developed on this side of the high, the humid air can rise and
condense into towering cumulus clouds and thunderstorms.
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7.
In both hemispheres, the air is warmer over low latitudes and colder over high latitudes. This
horizontal temperature gradient establishes a horizontal pressure (contour) gradient that causes the winds
to blow from the west, especially in middle and high latitudes.
8.
Since the polar front is a boundary separating the cold polar air to the north from the warm
subtropical air to the south, the greatest contrast in air temperature occurs along the frontal zone.
9.
Angular momentum is always conserved, so the product of the quantity mvr at one time will
equal the numerical quantity mvr at some later time. Hence, a decrease in radius must produce an increase
in speed and vice versa. When heated air parcels rise from the equatorial surface and approach the
tropopause, they spread laterally and begin to move poleward. Because of the curvature of the earth,
poleward-moving air constantly moves closer to its axis of rotation (r decreases). Because angular
momentum is conserved (and since the mass of air is unchanged), the decrease in radius must be
compensated for by an increase in speed.
10.
The equator-to-pole temperature gradient is stronger in winter than in summer.
11.
As the wind blows over the oceans, it causes the surface water to drift along with it. The moving
water gradually piles up, creating pressure differences within the water itself. This leads to further motion
several hundreds of meters down into the water. In this manner, the general wind flow around the globe
starts the major surface ocean currents moving. Major ocean currents do not follow the wind pattern
exactly; rather, they spiral in semiclosed circular whirls called gyres. In the North Atlantic, the prevailing
winds blow clockwise and outward from the subtropical highs, while the ocean currents move in a more
or less circular, but still clockwise, pattern. As the water moves beneath the wind, the Coriolis force
deflects the water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
This deflection causes the surface water to move at an angle between 20° and 45° to the direction of the
wind. Hence, surface water tends to move in a circular pattern as winds blow outward, away from the
center of the subtropical high.
12.
Winds and ocean currents; upwelling; Ekman spiral; El Nino/Southern Oscillation.
13.
As the wind blows over an open stretch of ocean, the surface water beneath it is set in motion.
The Coriolis force bends the moving water to the right. If we imagine the top layer of ocean water to be
broken into a series of layers, then each layer will exert a frictional drag on the layer below. Each
successive layer will not only move a little slower than the one above, but (because of the Coriolis effect)
each layer will also rotate slightly to the right of the layer above. Consequently, descending from the
surface, we would find water slowing and turning to the right until, at some depth (usually about 100 m),
the water actually moves in a direction opposite to the flow of water at the surface. This turning of water
with depth is known as the Ekman Spiral.
14.
West coast: northerly winds. East coast: southerly winds.
15.
(a) A situation in which El Niño conditions last for many months, and a more extensive ocean
warming occurs. This extremely warm episode occurs at irregular intervals of two to seven years and
covers a large area of the tropical Pacific Ocean.
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(b) They reverse in a seesaw pattern: high at one end and low at the other.
(c) The easterly trades may actually become westerly winds. As these winds push eastward, they
drag surface water with them. This dragging raises sea level in the eastern Pacific and lowers sea level in
the western Pacific. The eastward-moving water gradually warms under the tropical sun, becoming much
warmer than normal in the eastern equatorial Pacific. Gradually, a thick layer of warm water pushes into
coastal areas of Ecuador and Peru, choking off the upwelling that supplies cold, nutrient-rich water to
South America’s coastal region.
16.
Unusually cold surface temperatures.
17.
El Niño: persistent trough of low pressure over the north Pacific, wet weather and storms in
California and the southern United States. A weak polar jet stream over eastern Canada allowing warmer
than normal weather to prevail over a large part of North America. La Niña: persistent high-pressure area
south of Alaska forcing the polar jet stream into Alaska, then southward into Canada and the western
United States. The southern branch of the polar jet, which forms south of the high, directs moist air from
the ocean into the Pacific northwest, producing a wet winter for that region. Meanwhile, winter months in
the southern part of the United States tend to be warmer and drier than normal.
18.
During the warm (or positive) phase, unusually warm surface water exists along the west coast of
North America, while over the central North Pacific, cooler than normal surface water prevails. At the
same time, the Aleutian low in the Gulf of Alaska strengthens, which causes more Pacific storms to move
into Alaska and California. This situation causes winters, as a whole, to be warmer and drier over
northwestern North America. Elsewhere, winters tend to be drier over the Great Lakes, and cooler and
wetter in the southern United States. The cool (or negative) phase finds cooler-than-average surface water
along the west coast of North America and an area of warmer-than-normal surface water extending from
Japan into the central North Pacific. Winters in the cool phase tend to be cooler and wetter than average
over northwestern North America, wetter over the Great Lakes, and warmer and drier in the southern
United States.
19.
During the positive phase, winters in the eastern United States tend to be wet and relatively mild,
while northern Canada and Greenland are usually cold and dry. The negative phase occurs when pressure
near the Icelandic Low rises, while dropping near the Bermuda High. This reduces the pressure gradient
across the North Atlantic Ocean, weakens the westerlies and produces fewer and weaker storms. The
storms that do occur bring wet weather to southern Europe and the Mediterranean. Winters in northern
Europe and the east coast of North America are usually cold and dry. Greenland and northern Canada
usually experience mild winters.
20.
Cold ones.
Answers to Questions for Thought
1.
Continents would disrupt the flow by creating several smaller circulations.
2.
The position of the major features of the general circulation would change. In summer, the
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subtropical highs and ITCZ would be displaced farther north than now; in the winter, the subtropical
highs and polar front would move farther south than they now do.
3.
The surface low is a shallow, thermal low due to hot surface air; it disappears at upper levels.
The subtropical high shows up clearly on upper-level charts such as those at 850 mb, 700 mb, and 500
mb.
4.
Because of the Ekman Spiral, the average movements of surface water down to a depth of about
100 m is at right angles to the surface wind direction. Icebergs, which may extend downward to depths
greater than 100 m, move with this surface water at nearly right angles to the surface wind direction.
5.
The fastest winds are found in the jet stream core. Clear air turbulence is found above and below
the jet stream core.
6.
This is due mainly to the reversal of winds associated with the summer and winter monsoon.
7.
Upwelling is strongest in summer and when the winds blow parallel to the coast. Strong
upwelling conditions bring cold water to the surface. In winter when upwelling is not as strong, the
surface water is not as cold.
8.
The subtropical jet stream forms to the south of the polar-front jet.
9.
Large subtropical highs tend to be centered over the oceans off the western margin of continents
in both hemispheres. Winds around the highs blow clockwise (from the north along the western margin
of the continents) in the Northern Hemisphere, and counterclockwise (from the south along the western
margin of continents) in the Southern Hemisphere.
Answers to Critical Thinking Questions
Figure 10.14. The variations in the 500 mb surface will magnify: it will be higher south of the front and
lower north of the front. This will increase the strength of the jet stream.
Figure 10.19.
It would spiral towards the left rather than towards the right.
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Multiple Choice Exam Questions
1.
A westerly wind means that the atmosphere
a. would act to decrease the rotation of the earth.
b. is moving faster than the earth spins.
c. is moving slower than the earth spins.
d. is moving as fast as the earth rotates.
ANSWER: B
2.
Which below is not an assumption of the single-cell model of the general circulation of the
atmosphere?
a. The earth's surface is covered with water.
b. The earth rotates once in 24 hours.
c. The sun is always overhead at the equator.
d. none of the above
ANSWER: B
3.
The large thermally driven convection cell that is driven by convective "hot" towers along the
equator is the
a. Ferrel cell.
b. Hadley cell.
c. Ekman spiral.
d. El Niño cell.
ANSWER: B
4.
Air moving eastward more slowly than the earth rotates would appear to be ____ wind to an
observer on the earth.
a. a counterclockwise
b. an east
c. an upward
d. a west
ANSWER: B
5.
Chicago, Illinois (latitude 42N) is located in the
a. northeast trades.
b. southeast trades.
c. westerlies.
d. doldrums.
ANSWER: C
6.
The intertropical convergence zone (ITCZ) is a region where
a. the polar front meets the subtropical high.
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b.
c.
d.
e.
northeast trades meet the southeast trades.
northeast trades converge with the subtropical high.
the Ferrel cell converges with the Hadley cell.
polar easterlies converge with the air at the doldrums.
ANSWER: B
7.
In terms of the three-cell model of the general circulation, areas of surface low pressure should be
found at
a. the equator and the poles.
b. the equator and 30 latitude.
c. the equator and 60 latitude.
d. 30 latitude and 60 latitude.
e. 30 latitude and the poles.
ANSWER: C
8.
In Honolulu, Hawaii (latitude 21N), you would most likely experience winds blowing from the
a. northeast.
b. south.
c. southwest.
d. northwest.
ANSWER: A
9.
At Barrow, Alaska (latitude 70N), you would expect the prevailing wind to be
a. northerly.
b. easterly.
c. southerly.
d. westerly.
ANSWER: B
10.
Generally, along the polar front, one would not expect to observe
a. temperatures on one side lower than on the other side.
b. an elongated region of lower pressure.
c. clouds and precipitation.
d. converging surface air.
e. sinking air aloft.
ANSWER: E
11.
According to the three-cell general circulation model, at the equator we would not expect to find
a. the ITCZ.
b. a ridge of high pressure.
c. cumuliform clouds.
d. light winds.
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e. heavy showers.
ANSWER: B
12.
The wind belt observed on the poleward side of the polar front is called the
a. polar easterlies.
b. prevailing westerlies.
c. northeast trades.
d. doldrums.
ANSWER: A
13.
On a weather map of the Northern Hemisphere, one would observe the westerlies
a. north of the subpolar lows.
b. south of the tropical highs.
c. between the doldrums and the horse latitudes.
d. between the subpolar lows and the subtropical highs.
ANSWER: D
14.
The majority of the United States lies within this wind belt:
a. westerlies
b. easterlies
c. northerlies
d. trades
e. southerlies
ANSWER: A
15.
On a weather map of the Northern Hemisphere, the trade winds would be observed
a. north of the polar front.
b. between the polar front and the subtropical highs.
c. south of the subtropical highs.
d. between the subpolar lows and the subtropical highs.
ANSWER: C
16.
In the general circulation of the atmosphere, one would find the region called the doldrums
a. near 30 latitude.
b. at the equator.
c. at the poles.
d. near 60 latitude.
ANSWER: B
17.
In terms of the three-cell general circulation model, the driest regions of the earth should be near
a. the equator and the polar regions.
b. the equator and 30 latitude.
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c. the equator and 60 latitude.
d. 30 latitude and 60 latitude.
e. 30 latitude and the polar regions.
ANSWER: E
18.
The world's deserts are found at 30 latitude because
a. the intertropical convergence zone is located there.
b. of the sinking air of the polar front.
c. of the convergence of the prevailing westerlies and the Northeast Trades.
d. of the sinking air of the subtropical highs.
e. of the doldrums.
ANSWER: D
19.
On a surface weather map during the month of July, one would expect to find what type of
pressure system over the desert southwest of the United States?
a. monsoon low
b. Pacific high
c. thermal low
d. Aleutian low
ANSWER: C
20.
The position of the Pacific high over the North Pacific Ocean shifts ____ in winter and ____ in
summer.
a. northward, southward
b. southward, northward
c. eastward, westward
d. westward, eastward
ANSWER: B
21.
A thermally direct cell is one that
a. is found in both the northern and southern hemispheres.
b. involves heat.
c. occurs on both sides of a large lake or ocean.
d. is driven by energy from the sun.
ANSWER: D
22.
Many small-scale processes are ____ in General Circulation Models.
a. ignored
b. approximated
c. parameterized
d. both approximated and parameterized
e. none of the above
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ANSWER: D
23.
Which of the following does not describe the subtropical jet stream?
a. forms along the polar front
b. generally blows from west to east
c. is found at the tropopause
d. is normally equatorward of the polar front jet stream
ANSWER: A
24.
Which below does not describe the polar front jet stream?
a. is strongest in winter
b. moves farther south in winter
c. forms near the boundary called the polar front
d. is normally found at a higher elevation than the subtropical jet
ANSWER: D
25.
In the Northern Hemisphere, the polar jet stream is strongest when
a. air north of the polar front is much colder than air south of the polar front.
b. air north of the polar front is much warmer than air south of the polar front.
c. air temperatures on opposite sides of the polar front are about equal.
d. air temperatures on the East Coast of the US are much colder than on the West Coast of
the U.S.
ANSWER: A
26.
The jet stream flows
a. directly from west to east.
b. directly from east to west.
c. from the equator towards the poles.
d. in a wavy pattern from west to east.
ANSWER: D
27.
As an air parcel aloft moves northward from the equator, it moves closer to the earth's axis of
rotation. Because of the conservation of angular momentum, the air parcel's motion should
a. remain constant.
b. slow and eventually reverse direction.
c. slow but continue in the same direction.
d. increase in speed.
ANSWER: D
28.
In the Northern Hemisphere, air found to the north of the polar front is ____, while air further
south is ____.
a. cold, warm
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b. cold, cold
c. warm, cold
d. warm, warm
ANSWER: A
29.
The average winds aloft are strongest in
a. summer.
b. winter.
c. fall.
d. spring.
ANSWER: B
30.
for
The low-level jet that forms over the Central Plains of the United States appears to be responsible
a.
b.
c.
d.
e.
the influx of cold, polar air.
the development of El Niño.
the onset of a chinook.
the formation of a subtropical high pressure area.
nighttime thunderstorms.
ANSWER: E
31.
Generally, on an upper-level (100 mb) chart in the Northern Hemisphere during July, we would
expect to find the tropical easterly jet stream
a. north of the polar front.
b. north of the upper-level subtropical high.
c. directly over the polar front.
d. south of the upper-level subtropical high.
e. in the upper stratosphere.
ANSWER: D
32.
At any given time, only one jet stream can be found in the atmosphere.
a. true
b. false
ANSWER: B
33.
The cold water observed along the northern California coast in summer is due mainly to
a. the California current.
b. oceanic fronts.
c. upwelling.
d. cold air moving over the water.
e. evaporation.
ANSWER: C
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34.
The name given to the current of warm water that replaces cold surface water along the coast of
Peru and Ecuador during December is
a. Brazil current.
b. Humbolt current.
c. Benguela current.
d. El Niño.
ANSWER: D
35.
The two ocean currents, warm and cold, that produce fog off the coast of Newfoundland are the
a. Gulf stream and Canary current.
b. Labrador current and Canary current.
c. Gulf stream and Labrador current.
d. North Atlantic Drift and Canary current.
e. North Atlantic Drift and Gulf stream.
ANSWER: C
36.
The Ekman Spiral describes
a. the turning of water with depth.
b. the air flow into a center of low pressure.
c. the circulation of surface water around a gyre.
d. the air flow out of a region of high pressure.
e. the wind-flow pattern in a jet stream.
ANSWER: A
37.
Upwelling is
a. the lifting of air along the polar front.
b. the rising of cold water from below.
c. increasing heights in an upper-level ridge.
d. the rising air motion found in a low pressure center.
ANSWER: B
38.
During a major El Niño event,
a. Peruvian fishermen harvest a record amount of fish near Christmas time.
b. extensive ocean warming occurs over the tropical Pacific.
c. the Northeast trade winds increase in strength.
d. California experiences severe drought conditions.
ANSWER: B
39.
At jet streak is a place where ____ is (are) often found.
a. clear air turbulence
b. strong vertical wind speed shear
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c. very strong winds
d. all of the above
ANSWER: D
40.
Upwelling occurs along the northern California coast because
a. winds cause surface waters to move away from the coast.
b. of seismic activity on the ocean bottom.
c. of gravitational attraction between the earth and the moon.
d. water flows from the Atlantic ocean into the Pacific because they are at different levels.
ANSWER: A
41.
A condition where the central and eastern tropical Pacific Ocean turns cooler than normal is
called
a. El Niño.
b. La Niña.
c. the Southern Oscillation.
d. the Ekman Spiral.
ANSWER: B
42.
The reversal of the positions of surface high and low pressure at opposite ends of the Pacific
Ocean is called
a. El Niño.
b. the Southern Oscillation.
c. upwelling.
d. La Niña.
ANSWER: B
43.
If the earth's surface was homogeneous (either all land or all water),
a. the semipermanent highs and lows would be stronger.
b. the semipermanent highs and lows would be weaker.
c. the semipermanent highs and lows would disappear altogether.
d. the semipermanent highs and lows wouldn't change in intensity.
ANSWER: C
44.
Because clouds often block their view, instruments onboard satellite cannot regularly make
observations of surface winds.
a. true
b. false
ANSWER: B
45.
Satellites provide a global picture of winds by converting ____ into wind speed and direction.
a. wind shear
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b. thermal turbulence
c. sea roughness
d. both wind shear and thermal turbulence
ANSWER: C
46.
The southern hemisphere has ____ semi-permanent lows than the northern hemisphere.
a. fewer
b. more
ANSWER: A
47.
Areas of rising air tend to be associated with ____ precipitation.
a. abundant
b. little
ANSWER: A
48.
Jet streams sometimes split into two branches.
a. true
b. false
ANSWER: A
49.
A jet stream transfers more heat across the globe when it is
a. zonal.
b. meridional.
ANSWER: B
50.
The ultimate cause of jet streams is
a. the fact that water can exist in all three phases at once.
b. ultraviolet energy from the sun.
c. the energy imbalance between high and low latitudes.
d. all of the above
ANSWER: C
Essay Exam Questions
1.
Why is the single cell model of the general circulation not an accurate description of the actual
circulation of the atmosphere?
2.
On a large circle, show where the major pressure and wind belts would be found according to the
3-cell model of the earth's general circulation.
3.
What changes might you expect to see in the earth's general circulation if the earth's rotation were
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in the opposite direction?
4.
Briefly describe where each of the following features is found in the earth's general circulation.
horse latitudes
trade winds
ITCZ
polar front
doldrums
5.
What meteorological conditions are typically associated with the ITCZ?
6.
If you had to build a bridge linking South America to Africa, at what latitude would you build it?
Why?
7.
What occurs during a major El Niño event? How might this affect global wind and precipitation
patterns?
8.
Explain why the semipermanent lows in southern hemisphere are weaker than those in the
northern hemisphere.
9.
How do jet streams form?
10.
What is the polar jet stream and where is it found?
11.
Does the position of the polar jet stream change during the year? If so, how and why?
12.
What features in the earth's general circulation help determine where the driest and wettest places
on earth are found?
13.
Explain why large high and low pressure systems don’t move from east-to-west across the
continental United States.
14.
Describe at least two important ways that the ocean interacts with large-scale atmospheric wind
systems.
15.
How does El Niño change the weather where you live?