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Chapter 7
Circulation of the Atmosphere
Gale force winds create
impressive waves.
Atmospheric Circulation
1.
2.
Global atmospheric circulation can be
thought of as a series of deep rivers that
encircle the planet.
Imbedded in the main current are vortices
of varying sizes including hurricanes,
tornadoes and mid-latitude cyclones
Scales of Atmospheric Motion
1.
We sort weather events by size
2.
The larger the event, the longer it lasts
3.
Three major categories of atmospheric
circulation - macroscale, mesoscale and
microscale
Swirling wind pattern
on lee side of island
Scales of Atmospheric Motion
1.
2.
3.
Macroscale Winds - planetary scale,
extending around the entire globe.
Westerlies, trade winds
Smaller macroscale - synoptic scale or
weather-map scale. Travelling cyclones and
anticyclones.
Still smaller macroscale -tropical cyclones
& hurricanes
Scales of Atmospheric Motion
1.
2.
Mesoscale Winds - last from minutes to
hours. Usually < 100 km across.
Thunderstorms & tornadoes. Embedded
within macroscale circulation patterns.
Thunderstorms provide most of the vertical
movement found in mesoscale systems.
Microscale Winds - small & chaotic, last
for seconds to minutes. Gusts, dust devils.
Scales of Atmospheric Motion

Real global winds are a mixture of all
scale sizes . e.g., hurricanes rotate, but also
move east to west. Also have small scale
structures such as thunderstorms.
Local Winds
1.
2.
3.
4.
Local winds are medium-scale winds produced
by a locally produced pressure gradient.
Winds are named for the direction from which
they blow. e.g., sea breezes flow from the sea.
Cause of Sea & Land breezes discussed in
Chapter 6. Have moderating effect. Reach only
50 to 100 km inland.
Smaller-scale sea breezes can develop along
shores of large lakes.
Local Winds
1.
2.
3.
4.
Sea breezes also affect amount of cloud cover.
e.g., Florida. Convergence of sea breezes
from two coasts causes convergence and
leads to a summer maximum in rainfall.
Tropical regions experience stronger sea
breezes than mid-latitude regions.
Cool ocean currents enhance the effect in
tropical regions.
Mid-latitudes - sea breezes dominated by
migration of low & high pressure systems.
Local Winds
1.
2.
Mountain & Valley Breezes - slopes are heated
more than valley air, at same elevation, during
the day. Heated air glides up mountain slopes valley breeze. Can cause clouds & thunders
showers. More common in summer than
mountain breezes.
After sunset, slopes cool more than valley
floor. Cooler air flows down into the valley mountain breeze. More common than valley
breezes in winter.
Local Winds - 4
1.
2.
Dust devils - few meters in diameter, < 100m
tall. Last only minutes. Form on clear days.
Occur mostly in the afternoon. Surface heating
causes surface air to rise, and air is drawn in.
Conservation of Angular Momentum leads to
rotation.
Chinook (Foehn) winds - warm, dry winds
that sometimes move down the east slope of
the Rockies. (Called Foehn in the Swiss Alps)
7-A Dust Devils start at the hot surface
Local Winds - 5
1.
2.
Chinook winds often created when a
pressure system like a cyclone pulls air over
the mountains. As the air descends on the
leeward side, it is heated adiabatically by
compression. Descending air will be warmer &
drier than on the windward side.
Winds are relatively warm. Both good and bad
side effects. Chinook = snow eater.
Local Winds - 6
1.
2.
Santa Ana - clockwise flow of air in
anticyclone over the great basin (centered
on Nevada) draws hot & dry desert air to
the sea. Air becomes even warmer & drier
as it descends.
Country breeze - Hot urban air rises, and
cooler air flows from country into the city.
Santa Ana
winds
& Wild fires
Katabatic (fall) Winds
Very cold air "falls off" a highland area, is
heated adiabatically, but is still colder than the
air at lower altitudes (else it would not fall).
 Such winds originate in places like the
Greenland and Antarctica ice shelves.
 If the air is channeled into valleys, high wind
speeds are generated,
 Examples: Mistral blows from French Alps to
the Mediterranean Sea; Bora blows from
mountains of Yugoslavia to the Adriatic Sea.

photo by Jim Laatsch - US ITASE
Hannes Grobe, Alfred Wegener Institute for Polar and Marine
Research, Bremerhaven, Germany
Global Circulation
1.
2.
Our knowledge of global winds comes from
the pattern of pressure & winds observed
worldwide, and from theoretical studies of
fluid motion.
Have two models - Single Cell & Three
Cell
Global Circulation - 2
1.
2.
3.
Single cell model is Hadley model. Proposed
one single large convection cell. Tropical air
rises into tropopause and spreads towards the
poles. There cooling causes it to sink, and to
spread out (diverge) at the surface as cool
equator-ward winds that close the circulation
loop.
Hadley model ignores the Earth's rotation.
Three-Cell model - splits at latitudes of 30⁰
and 60⁰.
Global Circulation - 3
1.
2.
Equator to 30⁰ latitude similar to Hadley
model. Called the Hadley cell. As the
airflow aloft moves pole-wards, it begins to
subside between 20⁰ to 35⁰ latitude.
Subsidence between 20⁰ to 35⁰ latitude is
large because radiation cooling increases
the density of the air.
Global Circulation - 4
1.
Subsidence is also greater because
the Coriolis force becomes stronger with
increasing distance from the equator, so the
pole-ward moving air is deflected into a nearly
west-east flow by the time it reaches 25⁰
latitude. There is a general pileup of air
(convergence) aloft, so general subsidence
occurs between 20⁰ and 35⁰ latitude.
Global Circulation - 5
1.
2.
Subsiding air is relatively dry (it has
released most of its moisture near the
equator), and adiabatic heating during
descent decreases RH. Thus, you will find
deserts at these latitudes.
Winds are weak & variable near the center
of the zone of descending air - Horse
Latitudes.
Global Circulation - 6
1.
2.
3.
4.
From center of Horse Latitudes, surface flow
splits into a poleward and an equatorward
branch.
Equatorwards flow is deflected by Coriolis
force, and become the Trade Winds.
Trade winds are NE in NH, SE in SH (both
from the east)
Trade winds from two hemispheres meet at the
equator. Light winds and humid conditions.
Doldrums.
Global Circulation - 7
1.
2.
Circulation between 30⁰ and 60⁰ latitude - net
surface flow is pole-ward, but Coriolis force
leads to winds becoming prevailing
westerlies.
Cell from 60⁰ to 90⁰ latitude is not well
understood. Subsidence near the poles
produces an equator-ward surface flow, that is
deflected and becomes the polar easterlies.
When these encounter the warmer westerly
flow, get a polar front.
Global Distribution
Idealized Zonal Pressure Belts
Equatorial low - ascending hot air, low pressure,
precipitation. ITCZ due to converging easterly
trade winds
Sub-tropical highs - 20-35 degrees, high
pressure. Zones caused largely by Coriolis
force that stops the poleward movement of
upper level winds in Hadley cells. Pile-up of air
gives high pressure (upper level convergence
exceeds low level divergence). Subsiding air
column and diverging winds at surface result in
warm, clear weather. Semi-permanent
features.
Global Distribution of Pressure & Winds
1.
2.
Sub-polar lows - 50 to 60 degrees, polar
front, easterly & westerly trade winds clash
and cause convergence. Stormy weather (mid
latitudes), especially in winter.
Polar highs - polar easterlies clash with
subpolar westerlies. High pressure caused by
surface cooling (not same as in sub-tropical
highs)
Global Distribution of Pressure & Winds
1.
2.
3.
4.
Real pressure belts - semi-permanent
pressure systems
Real pattern broken up into elongated cells
by land masses, except for southern sub-polar
low (all water).
Equatorial low is more or less continuous
Pressure cells strengthen & weaken, and
move in latitude with the Sun. Position of ITCZ
very dependent on position of Sun.
Global Distribution of Pressure & Winds
1.
2.
3.
4.
Sub-tropical highs last most of year.
January - Siberian high , cold & dense air,
subsidence causes clear skies & divergent
surface air flow. Polar easterlies.
Others - Azores high. Aleutian & Icelandic
lows composed of multiple cyclones. Cyclones
have low-level convergence & upward air flow.
Cloudy skies & abundant precipitation.
Aleutian-low cyclones arise from frigid air,
directed by Siberian high they flow to the
Pacific, where they overrun warm air.
Global Distribution of Pressure & Winds
1.
2.
3.
July - warm land generates lows. Warm
ascending air produces surface convergence.
Strongest low develops over S.E. Asia.
Sub-tropical highs - migrate westwards in NH,
stronger than in winter. Dominate summer
circulation over the ocean. Pump warm moist
air onto land to their west.
North subtropical high is near Bermuda
(Bermuda high). In winter, this high is west of
Spain, called the Azores high.
ITCZ
Inter-Tropical Convergence Zone
 Region where trade winds converge (SE
trades in S.H., NE trades in N.H.)
 Region of ascending moist hot air, marked by
abundant precipitation.

ITCZ marked by rainfall band
Monsoons
1.
2.
3.
Term refers to a wind system that has a
pronounced seasonal reversal of direction.
Winds are (as usual) caused by pressure
differences due to uneven heating of Earth's
surface.
In general, winter monsoons blow off
continents, and are dry. In summer, winds blow
very moist air from the sea to the land.
Monsoons - 2
1.
2.
3.
Asian monsoon. Caused by large solar
heating & northward movement of ITCZ.
Summer temperatures in S.E. Asia get very
high (>40°C). This solar heating generates
low-pressure area. Air rises, diverges aloft, and
converges at the surface. Resulting wind
brings in very moist air from Indian Ocean.
Orographic lifting at slopes of Himalayas
accentuates rainfall there.
Monsoons - 3
1.
2.
3.
In winter, air flow from Siberian high causes
flow of dry air towards the sea.
ITCZ also moves northward in summer,
bringing warm, moist air low pressure &
convergence.
Himalayas serve to keep apart cold
continental air from milder coastal air.
Monsoons - 4
1.
2.
3.
4.
North American Monsoon
Affects large areas of S.W. USA, and northern
Mexico.
Summer heating gives low-pressure center
over Arizona that draws in warm moist air from
Gulf of California (and some from Gulf of
Mexico). Increased precipitation.
Most rain in Tuscon falls in July & August.
The Westerlies
1.
2.
3.
4.
Air flow aloft in mid-latitudes is approx W-E.
Air pressure decreases more rapidly with
altitude in a cold air column than in a warm
column. Pressure at a given altitude is
therefore greater at equator than at poles.
Get pressure gradient from equator to pole.
Poleward movement of airflow aloft is
deflected to the east by Coriolis force.
Final balance between pressure gradient force
and Coriolis force causes the wind to flow
basically west to east.
The Westerlies - 2
1.
2.
3.
Jet Streams - narrow streams of high
speed winds embedded in westerly flow
aloft. Best known is mid-latitude jet stream.
Flow at 7.5 to 12 km altitudes. Widths 100
to 500 km. Thickness a few km. Speeds
often > 200 km/hour.
Used/avoided by commercial aircraft.
The Westerlies - 3
1.
2.
Located in regions of the atmosphere
where large horizontal temperature
differences exist over short distances.
In particular along polar fronts (meeting of
cool polar easterlies and warm mid-latitudes
westerlies). (Westerly flow because of
Coriolis force)
The Westerlies - 4
1.
2.
3.
Jet stream is further south in winter than in
summer. If jet stream is equatorwards of a
location, expect cold & storm weather. If
polewards, expect warmer & drier conditions.
As jet stream retreats northward, get severe
thunderstorms & tornadoes.
Subtropical jet stream - winter only, not
sufficient temperature gradients in summer.
Centered at about 25 degrees latitude, 13 km
altitude.
Waves in the Westerlies
1.
2.
Upper level westerlies follow wavy paths.
Long wavelength Rossby waves (4,000 to
6,000 km).
Shorter wavelength waves exist in middle &
upper tropospheres. Often associated with
surface cyclones.
Westerlies & Earth's Heat Budget
1.
Temperature/pressure gradients give NorthSouth meridional winds. Coriolis force
changes them to E-W zonal winds. How do
E-W winds transfer heat from south to north?
2.
Jet stream is basically E-W, but often has
wavy structure with N-S flow.
Westerlies & Earth's Heat Budget - 2
1.
2.
3.
Strong temperature gradients arise, causing
strong pressure gradients that organize into
cyclonic systems. The N-S component of this
wavy flow transfers cold polar air equatorwards, and visa-versa.
This transfer of heat decreases temperature
gradients, and jet stream returns to normal.
Cycles in jet stream behavior can last 1 to 6
weeks.
Global Winds & Ocean Currents
1.
2.
3.
Winds are primary source of ocean currents
(friction & drag). Ocean currents are therefore
closely related to general atmospheric
circulation.
Near the equator, N.E. and S.E. trade winds
cause a westward ocean current. Coriolis force
deflects these currents polewards (to the right)
in N.H., to form clockwise spirals.
Water from the Caribbean is deflected
northwards, giving warm Gulf Stream.
Global Winds & Ocean Currents - 2
1.
2.
3.
Gulf Stream becomes North Atlantic Drift &
the cool Canaries current.
Ocean Currents affect climate, such as
temperature of adjacent land. Great Britain &
N.W. Europe warmed by North Atlantic Drift.
Cold currents affect tropical deserts along west
coasts of continents. Aridity along coast is
intensified (cold air is stable & will not rise up
mountain ranges).
Global Winds & Ocean Currents - 3
1.
Cold currents also increase R.H. and fog. Such
deserts are cool & damp, often with fog.
2.
Winds can also cause vertical water movement
- upwelling - cold water from deeper layers
rises to replace the warmer surface water.
Upwelling most common along eastern shores
of the ocean (e.g., California)
3.
Global Winds & Ocean Currents - 4
1.
2.
3.
Upwelling occurs when winds blow towards the
equator parallel to the coast. Coriolis force
directs (surface) water away from the coasts.
Rising cold water creates zone of low
temperature near the coast.
Rising cold water also causes upwelling of
dissolved nutrients such as nitrates &
phosphates, promoting growth of plankton.
Monitoring Winds from Space
Traditionally, winds that drive ocean currents
were measured at the surface
 “SeaWinds” is a scatterometer
 Rough water surfaces return stronger
echoes than smooth
 Can deduce the surface winds causing the
roughness
 Carried on satellites (Midori 2)

Western Pacific wind patterns
Hurricane
Ivan
El Nino & La Nina
1.
2.
3.
Usually, the cold Peruvian current flows
equatorwards, causing upwelling & increased
nutrients for fish.
Near the end of the year, a warm current flows
southwards along the coasts of Ecuador &
Peru for a few weeks. El Nino.
At some times, this warm current is much
stronger, and can last for years. This current
blocks the upwelling of nutrients, and cause
wetter weather than normal in inland areas.
“Normal”
El Nino & La Nina - 2
1.
2.
3.
East Pacific higher pressure than west.
In the US in 1982/3, California had ferocious
storms that caused beach erosion, landslides
& floods. Texas & the Gulf states had floods.
The number of hurricanes was below usual
average.
During El Nino years, equatorial currents flow
eastward, pressure increases over PNG,
easterly trade winds weaken, and the western
side of the Pacific gets warmer than average.
Get drastic weather changes.
El Nino & La Nina - 3
1.
2.
Oscillation of high pressure between east &
west Pacific is called the Southern
Oscillation. ENSO. Occurs ~ 3 to 7 years.
Changes of winds in lower equatorial
atmosphere cause El Nino. (what causes the
wind changes?) Usually, trade winds converge
near the equator, and flow westward. Winds
cause warm surface current that flows E to W,
piling up a thick layer of warm water. Eastern
Pacific is colder and has lower sea levels.
During El Nino years, reverse occurs.
El Nino & La Nina - 4
1.
2.
During El Nino years, the subtropical and
midlatitude jets are displaced, and cause
changes in weather. Winters in northern US
and in Canada are warmer than normal.
La Nina is opposite of El Nino. Surface
temperatures in eastern Pacific are lower than
average (not just average). Also causes major
climate changes, such as increase in number
of hurricanes.
Global Distribution of Precipitation
1.
2.
3.
General features of pattern of precipitation can
be explained in terms of known winds and
pressure systems.
Generally, high pressure regions are
associated with subsidence and divergent
winds, and dry conditions.
In low pressure regions, have converging
winds and ascending air, and ample
precipitation.
Global Distribution of Precipitation - 2
1.
2.
Also have latitudinal differences because
warm air can hold more water than cold air.
Equatorial regions have large rainfall. Polar
regions are much drier.
Distribution of land & water also affects
precipitation pattern. Centers of continents are
drier, windward sides of mountain ranges
wetter than leeward.
Global Distribution of Precipitation - 3
1.
Zonal Distribution of Precipitation
2.
Four major pressure zones (idealized) equatorial low (ITCZ), subtropical high,
subpolar low, polar high.
Global Distribution of Precipitation - 4
1.
2.
3.
4.
ITCZ has heavy rain in all seasons, but ITCZ
moves with seasons.
Sub-tropical zone tends to be dry all year,
because of subsidence.
Transition region has dry winter and wet
summer (then under the influence of ITCZ).
Midlatitudes (between subtropical high &
subpolar low) receive most of their
precipitation from travelling cyclonic storms.
Global Distribution of Precipitation - 5
1.
2.
3.
Cyclones frequently generated along polar
front (convergent zone between cold polar air
and warm westerlies).
Polar front migrates between 30 and 70
degrees, so midlatitudes receive ample rain.
Moves equatorwards in winter. In summer,
midlatitudes dominated by subsidence
associated with dry subtropical high.
Polar regions have cold air that contains little
moisture. In summer, high pressure blocks
polar movement of cyclones.
Distribution of Precipitation over the
Continents
1.
2.
3.
Simple zonal mode of precipitation has many
exceptions
Model says: Abundant precipitation in
equatorial & midlatitude, substantial portions of
subtropical & polar are relatively dry
Example - have midlatitude deserts. Patagonia
(southern South America) is dry because of the
Andes to the west.
Distribution of Precipitation over the
Continents – 2
1.
Most notable anomaly occurs in the zonal
distribution of precipitation in the subtropics.
Expect low rainfall/deserts. But get regions of
abundant rainfall.
2.
Pattern of subtropical rainfall is controlled by
the subtropical highs (anticyclones).
Distribution of Precipitation over the
Continents - 3
1.
2.
3.
Subsidence is greater on eastern side of
oceanic highs.
A strong temperature inversion is encountered
very near the surface and results in stable
atmospheric conditions.
The upwelling of cold water along the west
coasts of the adjacent continents cools the air
from below and adds to the stability.
Distribution of Precipitation over the
Continents - 4
1.
2.
3.
Anticyclones tend to cluster in the eastern side
of the oceans, particularly in winter, and
deserts are found on the western sides of the
continents.
For example, Sahara, Namib, Atacama, Baja
Peninsula, Great Desert of Oz.
On the western sides of the ocean,
convergence with associated rising of air is
more common. This air also comes off the
oceans, and is moist (as well as unstable), so
eastern sides of continents get ample
precipitation. e.g., Florida)
End of Chapter 7
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