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Chapter 8 Lecture
Understanding
Weather and
Climate
Seventh Edition
Atmospheric
Circulation
and Pressure
Distributions
Frode Stordal, University of Oslo
Redina L. Herman
Western Illinois University
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The Concept of Scale
• Some atmospheric features cover large portions of
Earth and are maintained over extensive time
period, referred to as global scale.
• High and low pressure patterns over large parts of
continents (hundreds or thousands of square km)
occur at what is called synoptic scale.
– Mesocale covers just a few square km to hundreds of
square km.
– Microscale refers to a very small scale, like ripples that
form on snow or a sandy beach.
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Single-Cell and Three-Cell Models
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Single-Cell Model
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Single-Cell Model
• The single-cell model describes the
general movement of the atmosphere
and was proposed by George
Hadley.
• Zonal winds move in an east/west or
west/east direction, while meridional
winds move in north/south or
south/north direction.
• Hadley thought heating at the equator
caused a circulation pattern in which
air expands upwards and diverges
toward the poles, sinks to the
surface, and returns to the equator.
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Single-Cell Model
• Coriolis deflection would cause
surface winds to be primarily
easterly.
• Although incomplete, Hadley’s
single-cell model was essential in
identifying the consequences of a
thermally direct circulation.
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The Three-Cell Model
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The Three-Cell Model
• The three-cell model
was proposed by William
Ferrel.
• This model divides each
hemisphere into three
cells.
– Hadley cell: circulates air
between the tropics and
subtropics
– Ferrel cell: circulates air in
the middle latitudes
– Polar cell: circulates air at
the poles
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The Three-Cell Model
• Each cell consists of
rising air with low surface
pressure, a zone of
sinking air with surface
high pressure, a surface
wind zone with air flowing
from high to low
pressure, and an airflow
in the upper atmosphere
from the rising and
sinking air.
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The Three-Cell Model
• The Hadley Cell
– Intense heating at the
equator creates a zone of
low pressure called the
equatorial low, or the
Intertropical Convergence
Zone (ITCZ).
– The ITCZ is the rainiest
latitude zone in the world.
– The Hadley cell sinks
toward the surface about
20–30° latitude to form the
subtropical highs (large
band of high pressure).
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The Three-Cell Model
• The Hadley Cell
– The NE trade winds in
the Northern Hemisphere
and the SE trade winds in
the Southern Hemisphere
are deflected to the right
and left.
– The Hadley cell is
strongest in the winter
season, when temperature
gradients are the
strongest.
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The Three-Cell Model
• The Ferrel Cell
• Ferrel cell circulates air
between the subtropical
highs and the subpolar
lows (areas of low
pressure).
• Air moving from the
subtropical highs toward
the subpolar lows is
deflected by Coriolis,
causing the westerlies in
both hemispheres.
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The Three-Cell Model
• The Polar Cell
• Polar cell circulates surface
air from the polar highs
(areas of high pressure) to
the subpolar lows.
• Thermally direct circulations
are formed by very cold
temperatures near the
poles.
• Air moving toward the
equator is deflected by
Coriolis, creating the polar
easterlies in both
hemispheres.
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The Three-Cell Model
• The Three-Cell Model versus
Reality: The Bottom Line
– Pressure and winds associated
with Hadley cells are close
approximations of real-world
conditions.
– Ferrel and Polar cells do not
approximate the real world as well.
– Surface winds of about 30
degrees and above do not show
the persistence of the trade winds;
however, long-term averages do
show a prevalence indicative of
the westerlies and polar easterlies.
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The Three-Cell Model
• The Three-Cell Model
versus Reality: The
Bottom Line
– For upper-air motions, the
three-cell model is
unrepresentative.
– The model does give a
good, simplistic
approximation of an earth
system devoid of
continents and topographic
irregularities.
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Semipermanent Pressure Cells
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Semipermanent Pressure Cells
• Instead of cohesive pressure belts
circling Earth, semipermanent
cells of high and low pressure
exist, fluctuating in strength and
position on a seasonal basis.
• These cells are either dynamically
or thermally created.
• For the Northern Hemisphere they
include:
– The Aleutian, Icelandic, and Tibetan
lows
– Siberian, Hawaiian, and BermudaAzores highs
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Semipermanent Pressure Cells
• Sinking motions associated
with the subtropical highs
promote desert conditions
across specific latitudes.
• Seasonal fluxes in the
pressure belts relate to the
migrating Sun (solar
declination).
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Semipermanent Pressure Cells
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Semipermanent Pressure Cells
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The Upper Troposphere
• Westerly Winds in the
Upper Atmosphere
– Thermal differences
correspond to upper-air
height differences.
– Upper-air motions are
directed toward the poles but
are redirected to an eastward
trajectory due to Coriolis
deflection.
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The Upper Troposphere
• Westerly Winds in the
Upper Atmosphere
– Westerly winds dominate the
upper troposphere and are
strongest during winter when
latitudinal thermal gradients
are maximized.
– Speeds also increase with
altitude as contours slope
more steeply with height due
to latitudinal thermal
differences.
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The Upper Troposphere
• Westerly Winds in the Upper Atmosphere
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The Upper Troposphere
• The Polar Front and Jet Streams
– Polar fronts are strong boundaries that occur between
warm and cold air.
– In the midlatitudes, the polar front marks this thermal
discontinuity at the surface.
– The polar jet stream, a fast stream of air sometimes called
“rivers,” exists in the upper troposphere.
• Winds are twice as strong in winter as summer.
– Near the equator, the subtropical jet stream exists as a
mechanism to transport moisture and energy from the
tropics toward the poles.
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The Upper Troposphere
• The Polar Front and Jet Streams
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The Upper Troposphere
• Troughs and Ridges
– Height contours meander considerably across the globe.
– The bulges of heights extending toward the poles are
called ridges.
– The valley of low heights extending toward the equator is
known as troughs.
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The Upper Troposphere
• Troughs and Ridges
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The Upper Troposphere
• Rossby Waves
– The largest of the atmospheric long waves is called the
Rossby wave.
– Three to seven Rossby waves circle the globe at any one
time, and each has its own wavelength and amplitude.
– Although they have preferred anchoring positions, they do
migrate eastward.
– The number of Rossby waves is maximized in winter and
decreases in summer.
– They are instrumental to meridional transport of energy
and also play an important role in determining areas of
divergence and convergence important to storm
development.
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The Upper Troposphere
• Rossby Waves
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The Upper Troposphere
• Atmospheric Rivers
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Major Wind Systems
• Monsoons
– Monsoon indicates a seasonal reversal in surface winds.
– Monsoon occur due to seasonal thermal differences
between landmasses and large water bodies.
– The East Asian monsoon is characterized by dry, offshore
flow conditions during cool months and wet, onshore flow
conditions during warm months.
– Orographic lifting brings larger precipitation amounts for
locations in the Himalayas, which record some of the
highest precipitation amounts on Earth.
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Major Wind Systems
• Monsoons
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Major Wind Systems
• Foehn, Chinook, and Santa Ana Winds
– Foehn winds flow down the side of mountain slopes. Air
undergoes compressional warming. They are initiated
when midlatitude cyclones pass to the southwest of the
Alps.
– Chinooks are similar winds on the eastern side of the
Rocky Mountains and form when low pressure systems
occur east of the mountains.
– Both Foehn and Chinook winds are most common in
winter.
– Santa Ana winds occur in California during the transitional
seasons, especially autumn, when high pressure is located
to the east. The Santa Ana winds often contribute to the
spread of wildfires.
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Major Wind Systems
• Katabatic Winds
– Katabatic winds warm by compression but originate when
air is locally chilled over high elevations. The air becomes
dense (with low temperature) and flows downslope.
– Common along Antarctica and Greenland ice sheets.
– Also referred to as Boras winds of the Balkan Mountains
and the Mistral winds of France.
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Major Wind Systems
• Sea and Land Breezes
– Temperature differences between land and sea produce a
land and sea breeze circulation.
– During the day, land surfaces are hotter than large water
surfaces. During the night, water surfaces are hotter than
land surfaces.
– A thermal low develops over the warmest region.
– Air converges into the low, ascends, and produces clouds
and possibly precipitation.
– Sea breezes blow from the sea to land, while land breezes
blow out to sea from the land.
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Major Wind Systems
• Sea and Land Breezes
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Major Wind Systems
• Valley and Mountain Breezes
– Diurnal variation similar to a land–sea breeze occurs in
mountainous areas and are called valley and mountain
breezes.
– Mountains facing the Sun heat more intensely than shaded
valley areas. This develops a thermal low during the day
which produces a valley breeze.
– At night, the situation reverses producing a mountain
breeze.
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Major Wind Systems
• Valley and Mountain Breezes
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Ocean–Atmosphere Interactions: ENSO
• El Niño, La Niña, and the Walker Circulation
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Ocean–Atmosphere Interactions: ENSO
• El Niño, La Niña, and the Walker Circulation
– El Niño events are characterized by unusually warm
waters in the eastern equatorial Pacific Ocean.
• Higher water temperatures lead to increased evaporation
rates and reduced air pressure.
• Occur every two to five years when trade winds, pushing
equatorial waters westward, reduce in strength.
– Cooler waters in the east are replaced by warmer waters
causing a reversal of the Walker Circulation.
– As the warm water pool migrates eastward, the pressures
reverse.
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Ocean–Atmosphere Interactions: ENSO
• El Niño, La Niña, and the Walker Circulation
– The Southern Oscillation is inherently linked to the
oceanic variations that most El Niño events are dubbed
ENSO (El Niño/Southern Oscillation) events.
– The offsetting of atmospheric pressures contributes to
worldwide unusual weather events.
– After an ENSO event, the equatorial Pacific returns to a
normal phase, or a strengthened normal phase, La Niña.
– Individual El Niño and La Niña events produce different
regional weather anomalies.
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
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Ocean–Atmosphere Interactions: ENSO
A→O
O→A
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Ocean–Atmosphere Interactions: ENSO
A→O
O→A
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Normal / LaNina situation
El Nino situation partly reversed
Ocean–Atmosphere Interactions: ENSO
• Positive and negative feedbacks (Box 8.5)
– Internal processes
– Positive feedback explains appearance of
both El Niño and La Niña events
• Trade winds push ocean currents
• Ocean currents impact the SST, surface pressure,
Walker circulation and trade winds
• Well understood, first by Jacob Bjerknes
– Negative feedbacks break down both El
Niño and La Niña events
• Lag behind the positive feedbacks
• Not well understood
• Many unanswered questions, e.g. why irregular?
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Ocean–Atmosphere Interactions: NAO
NAO
Based on pressure
difference
Azores - Reykjavik
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Positive NAO Index
• The Positive NAO index phase shows a stronger than usual
subtropical high pressure center and a deeper than normal Icelandic
low.
• The increased pressure difference results in more and stronger
winter storms crossing the Atlantic Ocean on a more northerly track.
• This results in warm and wet winters in Europe and in cold and dry
winters in northern Canada and Greenland
• The eastern US experiences mild and wet winter conditions
http://www.ldeo.columbia.edu/res/pi/NAO/
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NAO
+
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http://www.ldeo.colum
bia.edu/res/pi/NAO/
Negative NAO Index
• The negative NAO index phase shows a weak subtropical high and
a weak Icelandic low.
• The reduced pressure gradient results in fewer and weaker winter
storms crossing on a more west-east pathway.
• They bring moist air into the Mediterranean and cold air to northern
Europe
• The US east coast experiences more cold air outbreaks and hence
snowy weather conditions.
http://www.ldeo.columbia.edu/res/pi/NAO/
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NAO
-
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http://www.ldeo.colum
bia.edu/res/pi/NAO/
Ocean–Atmosphere Interactions: NAO
NAO
+
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NAO
-
Ocean–Atmosphere Interactions: NAO
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Ocean–Atmosphere Interactions: NAO
• Arctic Oscillation and North Atlantic Oscillation
– The oscillations of the Atlantic Ocean are known as the Arctic
Oscillation (AO) and the North Atlantic Oscillation (NAO).
– The NAO is in a positive phase when the pressure gradient is
greater than normal and negative when it is less than normal.
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Ocean–Atmosphere Interactions: NAO
NAO+
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NAO-
Ocean–Atmosphere Interactions: NAO
NAO: Atmospheric variability vs trend
under global warming (IPCC/AR5)
 In the Northern Hemisphere, the NAO exhibit considerable
variability comparable in magnitude to anthropogenically
forced trends.
 Hence, while the NAO is likely to exhibit a small trend towards
its positive polarity, there will continue to be considerable
variability on all time scales.
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The Oceans STARTS HERE Not shown
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The Oceans
• Causes of Ocean Currents
– Ocean currents are horizontal water motions of surface
water that are often found along the rims of the major
basins.
– Ocean currents greatly impact the atmosphere.
– Currents are created by wind stress but water moves at a
45° angle to the right (N.H.) from the wind flow.
– Current speeds decrease and the direction turns
increasingly toward the right (N.H.) with depth.
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The Oceans
• Causes of Ocean Currents
– The Ekman Spiral, initiated by Coriolis force, becomes
negligible at a depth of about 100 m.
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The Oceans
• Causes of Ocean Currents
– The North and South Equatorial Currents turn water
westward and help to create the Equatorial
Countercurrent.
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The Oceans
• Causes of Ocean Currents
– Western basin edges are dominated by warm polewarddirected currents (for example, Gulf Stream), while cold
currents, directed equatorward, occupy the eastern basins.
– Overlying air temperatures reflect these surface temperatures.
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The Oceans
• Upwelling and Downwelling
– Upwelling occurs when strong offshore winds along a
coastal region drag warmer surface waters seaward.
– Upwelling draws up cooler waters from below.
– Upwelling is most pronounced off the western coast of
South America, where cold water upwelling helps to create
the driest desert on Earth, the Atacama.
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The Oceans
• Upwelling and Downwelling
– Downwelling occurs when surface waters cool, and also
when they lose moisture through evaporation. Salt is left
behind, which makes the water denser than the previously
fresher water.
– A good example of downwelling is in the North Atlantic,
where the warm North Atlantic Drift loses huge quantities
of heat and moisture to the atmosphere above.
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