Chapter 7
Winds and the Global Circulation
System
OVERVIEW
This chapter considers winds and ocean currents. It examines how unequal surface
heating and the rotation of the Earth generate global circulation systems in the
atmosphere and oceans.
 The weight of air and the force of gravity pulling air towards the Earth create air
pressure. Air pressure is greatest at the Earth's surface and decreases with altitude.
 Differences in pressure cause air to move horizontally. This air in motion is called
wind. Winds move from areas of high pressure to areas of low pressure.
 Pressure differences between two places create pressure gradients and the resulting
pressure gradient force causes air to move from high pressure areas to low pressure
areas.
 Land and sea breezes are examples of winds caused by pressure differences that
result from temperature differences over land and water surfaces.
 Wind direction is measured by a wind vane, and wind speed is measured by an
anemometer.
 The Coriolis effect is due to the Earth's rotation and causes objects in motion to
appear to be deflected off course. This apparent deflection is to the right in the
northern hemisphere and to the left in the southern hemisphere. The effect is absent at
the equator and increases as you move towards the poles.
 Another force affecting the direction of wind is that of friction.
 Air flow spirals into a low-pressure center and rises while the air descends and flows
out of a high pressure center.
 The inward spiral at a low-pressure center is counterclockwise in the northern
hemisphere and clockwise in the southern hemisphere.
 The outward spiral at a high-pressure center is clockwise in the northern
hemisphere and counterclockwise in the southern hemisphere.
 Cyclones (low pressure centers) are associated with cloudy or rainy weather.
Anticyclones (high pressure centers) are associated with clear, dry weather.
 At the equator, heating causes air to rise creating an area of low pressure called the
Intertropical Convergence zone (ITCZ).
 At 30° latitude, air descends creating areas of high pressure in the subtropical high
pressure belt. Air moves out of these high pressure areas toward the equator creating
the Trade Winds. Winds also move toward the midlatitudes creating the Westerlies.
 The monsoon is a seasonally reversing wind pattern that brings heavy rains onto the
Asian subcontinent in summer and hot, dry conditions in the winter.
 Winds at an altitude of five to seven kilometers above the Earth’s surface are
influenced by pressure gradient force and Coriolis force but not by the force of
friction. These winds are the geostrophic winds that flow parallel to isobars.
 Rossby waves are large undulations in the flow of the upper air Westerlies along the
zone of contact between cold and warm air. They allow warm air to penetrate
northward and cold air to penetrate southward.
 Jet streams are narrow bands of high velocity air that form primarily along the polar
front and above the Hadley cell in the subtropics.
 The uppermost layer of ocean water is the warmest. Below this warm layer,
temperatures decline rapidly to around 0° and remains cold in a layer extending to the
ocean floor.
 Ocean currents are persistent, mainly horizontal flows of ocean water set in motion
by the prevailing surface winds. Coreolis force causes the flows to be deflected about
45 ° from the direction of the wind.
 Gyres are circular movements of water that are driven by the subtropical high pressure
cells.
 El Niño occurs when warm water replaces the usual upwelling cold water that flows
along the South American coast. El Niño affects climate in other parts of the world.
 Thermohaline circulation refers to slowly moving, deep ocean currents driven by the
sinking of cold, salty water in the northern Atlantic. This circulation is thought to play
an important role in the storage and release of CO2.
KEY TERMS
Intertropical convergence
zone
pressure gradient isobars
wind vane
anemometer
Coriolis effect [ALREADY
HAD THIS AS KEY
TERM IN CHAPTER 3]
cyclones
anticyclones
STUDY QUESTIONS
Hadley cell
doldrums
Subtropical high pressure
cell
monsoon
geostrophic wind
Rossby waves
jet stream
thermocline
ocean currents
gyres
El Niño
La Niña
Thermohaline circulation
1. What is the normal value of atmospheric pressure at sea level? How and why does it
change with altitude?
2. Use the example of land and sea breezes to illustrate how differential surface heating
produces pressure gradients and winds.
3. Define the Corliolis effect and describe how it influences winds and ocean currents at
the Earth’s surface.
4. Describe the three-dimensional pattern of air flow in cyclones and anticyclones in the
northern and southern hemispheres. What forces drive these air flow patterns?
5. Sketch a diagram to show the following features of the global atmospheric circulation
system: the doldrums, equatorial trough, Hadley cell, ITC, trade winds, subtropical
high pressure belts, westerlies, polar front, and polar easterlies.
6. Describe the seasonal pressure and circulation patterns of the Asian monsoon.
7. What are local winds and what factors lead to their development?
8. What are Rossby waves and what role do they play in the process of poleward heat
transfer?
9. What are jet streams? How do they form and where are they found?
10. Describe the thermal structure of the oceans.
11. Sketch a diagram to show the general pattern of ocean currents in the Pacific ocean.
How are these current patterns related to global wind patterns?
12. What are El Niño and La Niña?
13. What is Thermohaline circulation and why is it important to understanding the impact
of greenhouse gases on global climate?
14. What are some of the advantages and disadvantages of using wind as a power source?
CHAPTER QUIZ
Multiple Choice Questions
1. Atmospheric pressure will be approximately one half that of sea level at an altitude of:
a) One kilometer
b) Six kilometers
c) Ten kilometers
d) Sixty kilometers
2. The relative deflective force due to the Coriolis effect
a) is negligible at the poles
b) increases toward the equator
c) is negligible at the equator
d) both a and b
3. Air flow out of the subtropical high pressure cells drives the:
a) westerlies
b) trade winds
c) both a and b
d) neither a nor b
4. Large circulation cells that develop in the oceans at latitudes of 20° to 30° are called:
a) thermoclines
b) Rossby waves
c) gyres
d) Hadley cells
5. Deep ocean currents driven by the sinking of cold, salty water in the north Atlantic are
called:
a) El Niño
b) La Niña
c) Thermohaline circulation
d) both a and b
True/False Questions
1. High pressure cells develop over northern hemisphere continents in the summer.
(T/F)
2. Anticyclones in the northern hemisphere have a clockwise-outward spiral air flow
pattern. (T/F)
3. Pressure gradients develop because of unequal heating of the atmosphere. (T/F)
4. Most of the world’s great deserts are found in the Intertropical Convergence Zone.
(T/F)
5. Wind speed is measured by a wind vane. (T/F)
Short Answer Questions
1. What is a pressure gradient? How is pressure gradient calculated?
2. How are the trade winds, the westerlies, and the subtropical belts of high pressure
related?
3. How do geostrophic winds differ from surface winds?
4. What drives ocean currents?
Short Essay Questions (1 - 2 paragraphs)
1. Based on your knowledge of ocean currents and global winds, what routes would you
follow between Europe and North America if you were traveling by sailing ship?
2. Describe the El Niño/La Niña phenomenon and discuss how it could affect weather
patterns around the world.
Internet resources
1. El Niño/La Niña and the Pacific Decadal Oscilation: <http://topexwww.jpl.nasa.gov/science/el-nino.html>
2. US Department of Energy wind energy program:
<http://www.eren.doe.gov/wind/>
3. Wind Energy Resource Atlas of the United States:
<http://rredc.nrel.gov/wind/pubs/atlas/>
4. Misconceptions about the Coriolis Effect:
<http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html>
5. Simulation of the Los Angeles sea/land breeze:
<http://www.atmos.ucla.edu/~fovell/ASother/mm5/LA_seabreeze.html>
6. The thermohaline circulation including animated illustration of currents:
<http://www.cru.uea.ac.uk/cru/info/thc/>