Title Photo Page
“Who has seen the wind? Neither you nor I but
when the trees bow down their heads, the wind
is passing by.”
by.” —Christina Rossetti
(Brainyquote.com)
Chapter Five Vocabulary
anticyclone (p. 112)
antitrade winds (p. 125)
atmospheric pressure (p.
107)
barometer (p. 109)
chinook (p. 131)
cyclone (p. 112)
doldrums (p. 121)
dynamic high (p. 109)
dynamic low (p. 109)
El Niño (p. 131)
ENSO (El Niño–Southern
Oscillation) (p. 133)
foehn (p. 131)
friction layer (p. 111)
geostrophic wind (p. 111)
Hadley cells (p. 115)
high (p. 109)
horse latitudes (p. 117)
intertropical convergence
zone
(ITCZ) (p. 121)
isobar (p. 109)
jet stream (p. 122)
katabatic winds (p. 130)
land breeze (p. 128)
La Niña (p. 134)
low (p. 109)
millibar (p. 109)
monsoon (p. 126)
mountain breeze (p. 130)
offshore flow (p. 126)
onshore flow (p. 126)
polar easterlies (p. 124)
polar front (p. 124)
polar high (p. 124)
pressure gradient (p. 109)
ridge (p. 109)
Rossby waves (p. 122)
Santa Ana winds (p. 131)
sea breeze (p. 128)
Southern Oscillation (p.
133)
subpolar lows (p. 124)
subtropical high (STH) (p.
116)
teleconnection (p. 136)
thermal high (p. 109)
thermal low (p. 109)
thermocline (p. 134)
trade winds (p. 117)
trough (p. 109)
valley breeze (p. 130)
Walker Circulation (p. 133)
westerlies (p. 121)
wind (p. 110)
The Impact of Pressure and Wind
on the Landscape
• Humans not as sensitive to air pressure as
they are to other three climate elements
(heat, air movement, and humidity).
• Air pressure acts and responds to other
three climate elements, but most intimately
with wind.
• Spatial variations in pressure create air
movements.
1
The Nature of Atmospheric
Pressure
• Pressure—the force a
gas exerts on some
specified area of the
container walls.
• Atmospheric pressure—
the force exerted by gas
molecules in the
atmosphere.
• Affects Earth’s surface as
well as any other body on
Earth.
The Nature of Atmospheric
Pressure
• Omnidirectional
force—exerted
equally in all
directions.
– Force drops with
increasing altitude
because actual
number of gas
molecules also drops.
Factors Influencing Atmospheric
Pressure
• The pressure of a gas is proportional to its
density and temperature.
– This cause-and-effect relationship between
these three variables is explained by the ideal
gas law.
– Variations in any one—pressure, density, and
temperature of atmosphere—affect the other
two.
• Relationship is very complex, so difficult to make
exact predictions of how change in one changes
the others.
2
Density and Pressure
• Density is the mass of matter in a unit of
volume. Density of gas changes easily
because gas expands as far as the
environmental pressure will allow.
– The denser the gas, the greater the pressure
it exerts.
Temperature and Pressure
• If air is heated the molecules become
more agitated and they exert greater
pressure.
– An increase in temperature equals an
increase in pressure and a decrease in
temperature equals a decrease in pressure.
– If a mass of air is not confined, it will expand
when heated and may actually lead to a
decrease in pressure as temperature
increases.
Dynamic Influences on Air
Pressure
• Surface air pressure may also be
influenced by dynamic factors such as the
movement of air.
– i.e., upper atmosphere convergence can
cause higher pressure at the surface.
3
Dynamic Influences on Air
Pressure
• Generalizations regarding high and low pressure
at the surface:
– Strongly descending air is associated with high
pressure at the surface—a dynamic high.
– Very cold surface conditions are often associated with
high pressure at the surface—a thermal high.
– String rising air is often associated with low pressure
at the surface—a dynamic low.
– Very warm surface conditions are often associated
with relatively low pressure at the surface—a thermal
low.
Mapping Pressure with Isobars
• Barometer—instrument for measuring
atmospheric pressure.
• Millibar—an “absolute” measure of pressure,
consisting of one-thousandth part of a bar, or
1000 dynes per square centimeter; equals
0.0147 pound per square inch.
– Average sea-level pressure is 1013.25 millibars.
• Isobar—a line joining points of equal
atmospheric pressure.
– “High” and “low” pressures are relative conditions,
with the distinction depending on the pressure of the
adjoining areas.
• Mapping Pressure with Isobars
– Fig. 5-4
4
Mapping Pressure with Isobars
• Ridge—an elongated
area of relatively high
pressure.
• Trough—an elongated
area of relatively low
pressure.
• Pressure gradient—the
horizontal rate of
pressure change,
representing the
“steepness” of the
pressure slope; has a
direct effect on the speed
of wind.
Wind Speed
Determined by the pressure gradient
Closer spacing of isobars, steeper the
pressure gradient, faster the wind
blows
The Nature of Wind
• Wind—horizontal movements of air;
involve more area than do vertical
motions.
– Updrafts and downdrafts—small-scale vertical
motions.
– Ascents and subsidences—large-scale
vertical motions.
5
Direction of Movement
•
Depends on the
interaction of three
factors:
1. pressure gradient
2. friction
3. Coriolis effect
(Earth’s rotation)
Direction of Movement
•
•
Coriolis effect—the apparent
deflection of free moving objects
to the right in the Northern
Hemisphere and to the left in the
Southern Hemisphere, in
response to the rotation of Earth.
Geostrophic wind—a wind that
moves parallel to the isobars as a
result of the balance between the
pressure gradient force and the
Coriolis effect.
– Fig. 5-7: Friction slows the wind and weakens
the Coriolis Force
6
Cyclones and Anticyclones
• Cyclone—low-pressure cell.
• Anticyclone—high-pressure cell.
Cyclones and Anticyclones
• Eight circulation patterns are possible
because of the interaction of the pressure
gradient, Coriolis effect, and friction.
– Four involve anticyclones.
– Four involve cyclones.
• Each is dependent on the cell’s location
(hemisphere and altitude [whether surface-layer or
upper air]).
7
Cyclones and Anticyclones
• Vertical Movement
Within Cyclones and
Anti cyclones
– Anticyclone pattern is
upper air sinking down
into the center of the
high and diverging
near the surface.
– Cyclonic pattern is
converging air at the
surface and then
rising,
Wind Speed
• Is determined by pressure gradient.
– The steeper its slope, the faster the wind.
– Most persistent winds are usually in coastal
areas or high mountains.
Vertical Variations in Pressure and
Wind
• Atmospheric pressure
usually decreases
rapidly with height.
• Wind speed usually
increases with height;
winds tend to move
faster above friction
layer.
8
The General Circulation of the
Atmosphere
• Rotation of Earth and its variable
surfaces is key in creating a
complex circulation pattern for
atmosphere.
– Only the tropical regions have a
complete vertical cell.
– Hadley cell—complete vertical
circulation cells in which warm air
rises to elevations of about 50,000
feet (15 km), where it cools and
moves poleward, then subsides. The
cell’s air rises at the equator and
descends at about 30° of latitude
(either north or south, depending on
cell).
• There are two.
Idealized Pattern
-Uniform surface
-No Earth rotation
The General Circulation of the
Atmosphere
•
At midlatitudes and high latitudes, vertical cells
do not exist or are weakly and sporadically
developed.
– The general circulation of the atmosphere has seven
surface components:
1.
2.
3.
4.
5.
6.
7.
Polar high
Polar easterlies
Subpolar low
Westerlies
Subtropical high
Trade winds
Intertropical convergence zone
• The seven components are a focus of this
presentation: The order of presentation is:
5.
6
7
4.
1.
2.
3.
– Fig. 5-27
9
Subtropical Highs
• Subtropical latitudes serve as the “source’ of the major
surface winds of the planet.
• Subtropical highs— (STHs) large semipermanent highpressure (anticyclone) cells centered at about 30°
latitude over the oceans; have average diameters of
3,200 kilometers (2,000 miles) and are usually elongated
east–west. Develop from the descending air of the
Hadley cells.
Subtropical Highs
• Horse latitudes—areas in the subtropical highs characterized by
warm, tropical sunshine and an absence of wind; created because
weather within an STH is nearly always clear, warm, and calm.
– STHs also coincide with most of the world’s major deserts.
– STHs serve as source for two of the world’s three major surface
systems:
• Trade winds
• Westerlies
Trade Winds
• Trade winds—the major
wind system of the
tropics, issuing from the
equatorward sides of the
subtropical highs and
diverging toward the west
and toward the equator.
– Most reliable of all winds,
being extremely consistent
in both direction and
speed.
– Winds are named for the
direction they blow from.
– Trade winds’ origin
depends on which
hemisphere they are in.
10
Trade Winds
• In Northern Hemisphere,
originate in northeast, so are
sometimes called northeast
trades.
• In Southern Hemisphere,
originate in southeast, so are
sometimes called southeast
trades.
– Warming, drying winds
capable of holding enormous
amounts of moisture.
– Do not release moisture
unless forced by a topographic
barrier or pressure
disturbance.
– Pass over low-lying islands,
which thus are desert islands.
– Windward slopes in trade
winds, as in Hawaii, are some
of the wettest places on Earth.
– Tropical coastal areas are typically breezy
• Fig. 5-20
– Trades do not produce rain unless forced to rise.
• Fig. 5-21
11
Intertropical Convergence Zone
• Intertropical convergence zone— (ITCZ) a belt of calm
air where northeast trades and southeast trades
converge, generally in the vicinity of the equator.
• Also called equatorial front, intertropical front, and
doldrums.
– Front—a zone of discontinuity between unlike air masses.
Intertropical Convergence Zone
• ITCZ zone’s
thunderstorms
provide the updrafts
where all the rising air
in of the tropics
ascends.
– Often appears as a
narrow band of clouds
over oceans, but it is
less distinct over
continents.
The Westerlies
• Westerlies—the great
wind system of the
midlatitudes that flows
basically from west to
east around the world
in the latitudinal zone
between about 30°
and 60˚ both north
and south of the
equator.
12
Jet Streams
• Two cores of highspeed winds at high
altitudes in the
westerlies:
– Polar front jet stream
– Subtropical front jet
stream
• Fig. 5-26: Relative position of two jet streams
Rossby Waves
• Rossby waves—sweeping north–south undulations that
westerlies frequently develop in upper air.
– Rossby waves and the migratory pressure systems and storms
associated with westerly flow make midlatitudes have more
short-run variability of weather than any other place on Earth.
13
Polar Highs
• Polar high—a highpressure cell situated
over either polar region.
– Because it forms over an
extensive, high-elevation,
very cold continent,
Antarctic high differs
greatly from Arctic high.
– Antarctic high is strong,
persistent, and almost a
permanent feature, while
Arctic high is much less
pronounced and more
transitory.
Polar Easterlies
• Polar easterlies—a
global wind system
that occupies most of
the area between the
polar highs and about
60° of latitude.
• The winds move
generally from east to
west and are typically
cold and dry.
Polar Front
•
•
•
•
Polar front is sometimes clearly
visible by the semi-permanent
zones of low pressure called the
subpolar low
A zone of low pressure that is
situated at about 50° to 60° of
latitude in both Northern and
Southern hemispheres and which
often contains the polar front.
Characteristics vary in either
hemisphere because the
continents interrupt Northern
subpolar system, while Southern
is virtually continuous over the
oceans.
Polar front—the meeting ground of
the polar easterlies’ cold winds
and the westerlies’ warm winds.
14
Vertical Patterns of the General
Circulation
• Winds in upper elevations of troposphere are different from surface
winds.
• Most dramatic difference occurs between surface trade winds and
the upper-elevation antitrade winds.
• Antitrade winds—tropical upper air winds that blow toward the
northeast in the Northern Hemisphere and toward the southeast in
the Southern Hemisphere.
Modifications of the General
Circulation
• The general
circulation varies
because of many
factors, but the two
principal modifications
are seasonal
variations in location
and monsoons.
Seasonal Variations in Location
Mumbai (Bombay) is slowly limping
back to life after days of being
under water.
• The seven surface
components of the
general circulation shift
latitudinally with the
changing seasons.
• Affect weather only
minimally in equator and
polar regions, but
significantly alter weather
in midlatitudes and their
fringes.
15
Monsoons
• Monsoon—a seasonal reversal of winds; a general onshore
movement in summer and a general offshore flow in winter, with a
very distinctive seasonal precipitation regime.
• Most significant disturbance to the pattern of general circulation.
• Offshore flow—wind movement from land to water.
• Onshore flow—wind movement from water to land.
Monsoons
• Control the climates of regions with more than
half of the world’s population.
• Origin of monsoons is still not understood,
though there is increasing evidence that it is
associated with upper-air phenomena,
particularly jet stream behavior.
• Monsoons have an essential impact—their
failure or even late arrival of monsoonal
moisture inevitably causes widespread
starvation and economic disaster.
• Monsoons
– The most significant disturbance of the general
circulation
– Seasonal Winds
• Wet summer
• Dry winter
• Fig. 5-32
16
– Principal Monsoon Areas
• Fig. 5-31
– Two Major Monsoon Systems
• Fig. 5-33
– Two Minor Monsoon Systems
• Fig. 5-34
17
Localized Wind Systems
• Lesser winds have a considerable effect
on weather and climate on a localized
scale.
Sea and Land Breezes
• Cycle of sea breezes and land breezes is a common local wind
system along tropical coastlines and somewhat in summer in
midlatitude coastal areas.
• Essentially a convectional circulation caused by differential heating
of land and water surfaces.
• Land breeze—local wind blowing from land to water, usually at night
(and normally considerably weaker flow than that of sea breeze).
• Sea breeze—local wind blowing from sea toward the land, usually
during the day.
Valley and Mountain Breezes
• Daily cycle of airflow occurs with
valley and mountain breezes.
• Convectional circulation caused by
differential heating of higher versus
lower elevations.
• Mountain air cools quickly at night,
allowing cooler air to drain down the
slope in the evening. Conversely,
valley air heats more rapidly during
the day, allowing warm air to move
upslope during the day.
• Valley breeze—an upslope flow,
during day.
• Mountain breeze—a downslope
flow, during night.
• Air drainage—the sliding of cold air
downslope to collect in the lowest
spots, usually at night; a modified
form of mountain breeze common in
winter.
18
Katabatic Winds
• Katabatic wind—a wind that
originates in cold upland areas
and cascades toward lower
elevations under the influence
of gravity.
• Air is cold and dense, and
usually colder than the air it
displaces in its downslope
flow.
• Mistral—a cold, high-velocity
wind that sometimes surges
down France’s Rhone Valley,
from the Alps to the
Mediterranean Sea. Has
considerable destructive
power.
• Similar winds are called bora
in Adriatic region and taku in
Alaska.
Foehn/Chinook Winds
• Chinook—a localized
downslope wind of relatively
dry and warm air, which is
further warmed adiabatically
as it moves down the leeward
slope of the Rocky Mountains.
• Called foehn when it occurs in
Europe.
• Santa Ana Winds—similar to
chinook/foehn. Have high
speed, high temperature, and
extreme dryness and prompt
wildfires.
El Niño-Southern Oscillation
• El Niño—an anomalous
oceanographic-weather
phenomenon of the eastern
equatorial Pacific, particularly
along coast of South America.
• Occurs when southeastern
trades abnormally slacken or
reverse direction, which
triggers a warm surface flow,
which displaces the cold,
nutrient-rich upwellings that
usually prevail on the surface.
Disrupts productive offshore
fisheries.
19
El Niño-Southern Oscillation
• Was once believed to be a local
phenomenon, but is now understood to be
associated with changes in global
pressure, wind, and precipitation.
• Occurs every few years around Christmas
time (El Niño is Spanish for “the Christ
child”).
El Niño-Southern Oscillation
• Even though archeological and
paleoclimatological records have indicated past
El Niño phenomenon, it was not until 1982–1983
that great attention was drawn to it.
• In 1982–1983, the El Niño event brought on a
series of weather events called “the most
disastrous in recorded history” (1500 human
deaths, about $9 billion in damage, and vast
ecological havoc):
El Niño-Southern Oscillation
• Crippling droughts (in Australia, India, Indonesia,
the Philippines, Mexico, Central America, and
southern Africa);
• Devastating floods (in western and southeastern
United States, Cuba, and northwestern South
America);
• Destructive tropical cyclones (in Tahiti and
Hawaii);
• Die offs of fish, seabirds, and coral (from
abnormally warm water over a 12,888-kilometer
[8000-mile] stretch of equatorial Pacific).
20
El Niño-Southern Oscillation
• In 1997-1998 another strong El Niño cycle
occurred.
• Property damage exceeded $30 billion
and at least 2100 people died.
– Forecasting El Niño
• Array of 70 buoys – Galapagos Is. to New Guinea
Source: NOAA (http://www.pmel.noaa.gov/tao/)
- Fig. 5-B
• Cruise ship: Ocean Seeker (Ka’im-im-o-ana)
Source: NOAA
(http://www.pmel.noaa.gov/tao/proj_over/diagrams/buoy.html)
21
• Other Multi-Year Atmospheric and Oceanic Cycles
– Pacific Decadal Oscillation (PDO)
– North Atlantic Oscillation (NAO)
– Arctic Oscillation (AO)
Fig. 5.42
Pacific Decadal Oscillation
Normal Pattern
• In order to understand El Niño phenomenon, it is critical
to understand normal pressure, wind, and ocean current
patterns in the Pacific.
• Dominance of subtropical high in the eastern Pacific
causes westward movement of the trade winds toward
low-pressure cell in the western Pacific.
• Trade winds create frictional drag on the Pacific Ocean
and create westward moving warm equatorial current.
• The removal of surface water near the western coast of
South America allows cold water to upwell.
• This phenomenon is known as the Walker Circulation.
• This is a closed convection cell, but in reality is probably
more complex.
El Niño Pattern
• Every few years normal pressure pattern
changes.
• High pressure develops over northern Australia,
and low pressure develops to the east over
Tahiti.
• This reversal is known as the Southern
Oscillation, which is a large-scale fluctuation in
sea-level atmospheric pressure that occasionally
occurs in the eastern and western tropical
Pacific; caused by differences in water
temperature.
22
El Niño Pattern
• When El Niño and Southern Oscillation coincide (called
ENSO), unusual atmospheric and oceanic conditions are
more frequent and more intense than when either event
occurs alone.
• Pressure reversal causes the trade winds to reverse
direction, and this allows warm water from the western
Pacific to “backwash” toward the eastern Pacific.
• For many months before the onset of El Niño, the trade
winds pile up warm water in the western Pacific, and
then a bulge of warm equatorial water about 25 cm high
moves eastward in a series of bulges known as Kelvin
waves.
• These waves can take 2 to 3 months to arrive off the
coast of South America.
El Niño Pattern
• This causes the sea level to rise off the coast of South
America as the warm water pools.
• The thermocline boundary between near-surface and
cold deep ocean waters lowers.
• This impedes the upwelling of cold water off of the coast,
and thus causes temperatures off the coast of South
America to rise.
• This shift in the normal pressure pattern in the Pacific
can cause increased precipitation in the deserts of Peru,
droughts in northern Australia and Indonesia, decreased
monsoon activity in South Asia, and more powerful
winter storms in the southwestern United States.
La Niña
• La Niña—unusually cold temperatures in
the eastern Equatorial Pacific.
• Both El Niño and La Niña are extreme
cases of a naturally occurring climate
cycle that involves large-scale changes in
sea-surface temperatures across the
eastern tropical Pacific.
• La Niña is not as predictable as El Niño.
23
Causes of ENSO
• There is no clear “trigger” of ENSO. It is
not clear whether the changes in the
ocean temperature or the changes in the
pressure and wind occur first.
• The effects of ENSO are also not
completely predictable.
Teleconnections
• Some generalizations, however, can be
made (i.e., floods are more likely to occur
in California during El Niño years).
• There is increasing recognition of El Niño
connections with atmospheric and oceanic
conditions outside of the Pacific.
• These connections are known as
teleconnections.
Other Multi-Year Atmospheric and
Oceanic Cycles
• Pacific Decadal Oscillation
• The North Atlantic Oscillation and the
Arctic Oscillation
24
Pacific Decadal Oscillation
•
•
•
•
•
•
•
•
The Pacific Decadal Oscillation (PDO) is a long-term pattern of sea-surface
temperature change between the northern/west tropical and eastern tropical
Pacific Ocean.
About every 20 to 30 years the sea-surface temperatures in these zones
abruptly shift.
From the late 1940s to late 1970s, the northern/west tropical Pacific was
relatively warm while the eastern tropical Pacific was relatively cool
This is the PDO “negative” or “cool” phase.
From the late 1970s through the mid-1990s, this pattern switched, with
cooler sea surface temperatures in the northern/west tropical Pacific and
warmer conditions in the east tropical Pacific.
This is the PDO “positive” or “warm” phase.
In the late 1990s the switch back to the negative phase was underway, but
by the early 2000s it wasn’t clear if this negative phase was going to be
short-lived.
Although the PDO is not well understood, it seems to influence the location
of the jet stream.
The North Atlantic Oscillation and
the Arctic Oscillation
• In the North Atlantic Ocean basin, two related but somewhat
irregular multi-year cycles of pressure, wind patterns, and
temperature exist:
• The North Atlantic Oscillation—an irregular “seesaw” of pressure
differences between two regional components of the general
atmospheric circulation in the North Atlantic Ocean basin: the
Icelandic Low and the subtropical high (the Azores High).
• “Positive” phase of the NAO equals a greater pressure gradient
between the Icelandic Low and the Azores High.
• During such a positive phase, winter storms tend to take a more
northerly track across the Atlantic.
• Bring mild, wet winters to Europe and the eastern United States but
colder, drier conditions in Greenland.
The North Atlantic Oscillation and
the Arctic Oscillation
• During a “negative” phase of the NAO, both the Icelandic Low and
the Azores High are weaker.
• Winter storms tend to bring higher than average precipitation to the
Mediterranean and colder winters in northern Europe and the
eastern United States, while Greenland experiences milder
conditions.
• The Arctic Oscillation alternates between warm and cold phases that
are closely associated with the NAO.
• During Arctic Oscillation “warm” phase (associated with the NAO
positive phase), the polar high is weaker.
• Cold air masses don’t move as far south and sea-surface
temperatures tend to be warmer in Arctic waters.
• During the Arctic Oscillation “cold” phase (associated with the NAO
negative phase), the polar high is strengthened, bringing cold air
masses farther south.
25
People and the Environment:
Forecasting El Niño
• Since the powerful El Niño of 1982–1983 there have
been international efforts to understand El Niño and
teleconnections.
• Deployment of some 70 instrument buoys in the tropical
eastern Pacific Ocean in the Tropical Atmosphere/Ocean
Array (TAO/TRITON array) to monitor ocean and
atmospheric conditions
• Especially sea-surface temperature and wind direction.
• By 1994, sufficient data had been gathered to begin to
develop computer models to predict the onset of an El
Niño event several months in advance.
• Analysis of data from this array successfully predicted
the 1997–1998 El Niño event.
• Basis f or Global Wind System
90º N
Heat Loss
60º N
30º N
Heat Gain
0º S
30º S
60º S
Heat Loss
90º S
• Pressure Belts and Circulation Cells
*Intertropical Convergence Zone
Also area of the doldrums
**Horse latitudes
90º N
H
Pressure Belts
3
Polar High
Polar Front Zone
Subtropical High (STH)**
1
Equatorial Low (ITCZ*)
1
Subtropical High (STH)**
Polar Front Zone
Polar High
60º N
L
2
1 – Hadley cell
2 – Ferrel cell
3 – Polar cell
H
30º N
L
0º S
H
30º S
2
60º S
L
3
H
90º S
26
• Global Winds
90º N
Expected path (PGF)
Coriolis effect
Global Winds
H
Polar Easterlies
L
60º N
Westerlies
H
30º N
L
0º S
H
30º S
60º S
L
NE Trades
SE Trades
Westerlies
Polar Easterlies
H
90º S
27