Understanding Weather
and Climate
3rd Edition
Edward Aguado and James E. Burt
Anthony J. Vega
Part 3. Distribution and
Movement of Air
Chapter 8
Atmospheric Circulation and Pressure
Distributions
Introduction
Well-defined pressure patterns exist across the globe
These define the general circulation of the planet
In describing wind motions:
Zonal winds are those which blow parallel to lines of latitude
Meridional winds move along lines of longitude
The general circulation of the atmosphere may be
examined through a single-cell or three-cell model
Single-Cell Model
This model, proposed by George Hadley, assumed an ocean-only
planet with a fixed solar declination
A single convection cell per hemisphere would redistribute heat
from the equator to the poles under such conditions
Coriolis deflection would cause surface winds to be primarily
easterly
Although incomplete, Hadley’s single-cell model was essential in
identifying consequences of a thermally direct circulation
Single-Cell Model
Three-Cell Model
Each hemisphere is divided into three pressure cells
The first is the thermally driven:
Hadley cell
• In the tropics air is heated through high solar angles and constant day
length
• Air becomes heated, expands, and diverges toward higher latitudes
• The equatorward boundary of the Hadley cell is characterized by
expanding and ascending surface air that forms the equatorial low, or
intertropical convergence zone (ITCZ)
• The ITCZ (or doldrums) is usually found near the vertical solar ray
• It is usually characterized by clouds and heavy precipitation
– Reflects some of the wettest areas on Earth
• Ascending air diverges poleward aloft
• Air gains considerable westward momentum and descends in the
subtropics
– The zonal component far exceeds the meridional component
The ITCZ is observable
as a band of clouds
extending from northern
South America into the
Pacific
• Between 20 and 30o latitude, air descends forming the subtropical
highs, or horse latitudes
• Compressional warming creates clear, dry conditions near the centers
of the highs
• Surface air flow is primarily from the subtropical highs towards the
ITCZ
• The addition of Coriolis deflection results in the northeast
(southeast) trade winds in the northern (southern) hemisphere
• Hadley cell strength increases during the cool season when thermal
contrasts are maximized
Ferrel and Polar Cells
• Constitute the remaining hemispheric cells
• Ferrel cells lie poleward of each Hadley cell
• They circulate air between the subtropical highs and the subpolar
lows
• The subpolar lows result from surface air converging from the
equatorward subtropical high and the poleward polar high
• The Ferrel cell is an indirect cell as it is formed from air motions
initiated by adjacent cells
• Air moving from the subtropical highs towards the subpolar lows
undergoes Coriolis deflection causing the westerlies in both
hemispheres
Polar Highs
• Thermally direct cells formed by very cold temperatures near the
poles
• Air in these locations becomes very dense resulting in sinking
motions indicative of high pressure
• Air moving equatorward is deflected by Coriolis creating the polar
easterlies in both hemispheres
The Three-Cell Model
The Three-Celled Model vs. 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 poleward of about 30o do not show the persistence of
the trade winds, however, long-term averages do show a prevalence
indicative of the westerlies and polar easterlies
• For upper air motions, the three-cell model is unrepresentative
• The Ferrel cell implies easterlies in the upper atmosphere where
westerlies dominate
• Overturning implied by the model is false
• The model does give a good, simplistic approximation of an earth
system devoid of continents and topographic irregularities
Semipermanent Pressure Cells
Instead of cohesive pressure belts circling the Earth,
semipermanent cells exist
Cells are either dynamically or thermally produced
Fluctuate in strength and position on a seasonal basis
For the northern hemisphere they include:
• The Aleutian, Icelandic, and Tibetan lows
– The oceanic (continental) lows achieve maximum strength during winter
(summer) months
• Siberian, Hawaiian, and Bermuda-Azores highs
– The oceanic (continental) highs achieve maximum strength during
summer (winter) months
• The summertime Tibetan low is important to the east-Asia monsoon
• Sinking motions associated with the subtropical highs promotes desert
conditions across the affected latitudes
• Seasonal fluxes in the pressure belts relate to the migrating vertical
ray of the Sun
• The ITCZ lags slightly behind the vertical solar ray into the summer
hemisphere which causes a poleward migration of the subtropical
highs and a weakening of the higher latitude oceanic lows
• In the winter hemisphere, opposite conditions result as the oceanic
lows strengthen and the subtropical highs weaken and migrate
equatorward
• Such migrations greatly influence temperature and precipitation
regimes across the globe
– Especially evident concerning the Sahel region of Africa
• The situation is best exemplified in the tropics where seasonal
precipitation is closely tied to variations of the subtropical highs and
ITCZ
January
July
The Sahel, a region bordering the
southern Sahara Desert
Shifts in the ITCZ bring rain to
the Sahel in summer.
During most of the year the ITCZ
and the rain is located south of
the region
The Upper Troposphere
Upper tropospheric heights decrease poleward from lower latitudes
due to the increased density of colder air
• The arrangement of heights details stronger pressure gradients for the
winter hemisphere
• Heights are also higher during summer as density decreases with
heating
Decreasing heights
with latitude
Westerly Winds in the Upper Atmosphere
• Thermal differences corresponding to upper air height differences
ensure lowered heights from equator to pole
• Upper air motions are directed towards the poles but are redirected to
an eastward trajectory due to Coriolis deflection
• Westerly winds, therefore, dominate the upper troposphere
– 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
The Polar Front and Jet Streams
• Strong boundaries occur between warm and cold air
• In the mid-latitudes, the polar front marks this thermal discontinuity at
the surface
• The polar jet stream, a fast stream of air, exists in the upper
troposphere above the polar front
– Winds are about 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 poleward
Profile of the polar jet
The subtropical jet is seen as a
band of clouds extending from
Mexico on an infrared satellite
image
Troughs and Ridges
• Height contours meander considerably across the globe
• High heights extending poleward are called ridges
• Equatorward dipping lower heights are known as troughs
Rossby Waves
•
•
•
•
Ridges and troughs comprise waves of air flow
Rossby, or long-waves, are the largest of these configurations
Each has a particular 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
Profile of ridges and troughs
A trough over the mid-continent is
depicted in b
Sequence of Rossby Wave Migration
Rossby waves in the upper air
advect cold or warm air (left),
evident by surface temperatures
(below)
The Oceans
Ocean Currents
• Represent horizontal water motions
• Profound impact on the atmosphere which is influenced by the ocean
temperatures
• Currents are created by wind stress but water moves at a 45o angle to
the right (N.H.) from the wind flow
• Current speeds decrease and the direction turns increasingly towards
the right (N.H.) with depth
• This Ekman Spiral, initiated by Coriolis force, becomes negligible at a
depth of about 100 m
• The North and South Equatorial Currents pile water westward and help
create the Equatorial Countercurrent
• Western basin edges are dominated by warm poleward directed
currents (example: Gulf Stream) while cold currents, directed
equatorward, occupy the eastern basins
• Overlying air temperatures reflect these surface temperatures
Infrared Satellite Image of the Gulf Stream
Ekman Spiral
Global Ocean Circulation
Upwelling
• Strong offshore winds drag surface waters away from coastal
locations
• Colder waters from the deep ocean rise, or upwell, to replace these
waters
• Most pronounced off the western coast of South America where cold
water upwelling helps ensure the driest desert on Earth, the Atacama
Major Wind Systems
Monsoons
• The largest synoptic scale winds on Earth
• A seasonal reversal of wind due to seasonal thermal differences
between landmasses and large water bodies
• Best exemplified by the east-Asian monsoon which is characterized
by dry (wet), offshore (onshore) flow conditions during cool (warm)
months
• Orographic lifting assures large precipitation amounts for locations in
the Himalayas which record some of the greatest precipitation
amounts on Earth
The winter monsoon
The summer monsoon
Foehn, Chinook, and Santa Ana Winds
• Winds which flow down the lee side of mountain ranges
• Air undergoes compressional warming
• Foehn winds are initiated when mid-latitude cyclones pass to the
southwest of the Alps
• Chinooks are similar winds on the eastern side of the Rocky Mtns.
– 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
• Occur when high pressure is located to the east
• Often spread wildfires
Katabatic Winds
• Cold, dense air flows sinking down mountain sides from high
elevations
• Boras winds of the Balkan Mountains and the Mistral winds of
France are examples
Sea and Land Breeze
• Sharp interfaces between land and sea may produce a land and sea
breeze circulation
• Occur in relation to the differential surface heating on a diurnal scale
• During the day (night) land (water) surfaces are hotter (cooler) than
large water (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, land breezes blow out to sea
Valley and Mountain Breeze
• Diurnal variation similar to a land/sea breeze occur in mountainous
areas
• Solar facing mountain slopes heat more intensely than shaded valley
areas developing a thermal low during the day which produces a valley
breeze
• At night the situation reverses producing a mountain breeze
Sea breeze development
Sea breezes initiate clouds
and precipitation on the
island of Hawaii
Development of a Valley and Mountain Breeze
Air-Sea Interactions
El Niño, La Niña, and the Walker Circulation
• El Niño events are characterized by unusually warm waters in the
eastern equatorial Pacific Ocean
• These events have been linked to anomalous global weather events
• 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
• Western waters, piled previously by strong trade wind flow migrates
eastward
• Cooler waters in the east are replaced by warmer waters causing a
reversal of the Walker Circulation
• Under normal conditions, high atmospheric pressure dominates the
east while low pressure dominates the west
• As the warm water pool migrates eastward, the pressures reverse
• The Southern Oscillation is inherently linked to the oceanic
variations such 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
• Anomalous cooling occurs in the eastern Pacific during these events
• Distinct, but different, global teleconnection patterns result
• Individual ENSO and La Niña events produce different regional
weather anomalies
Pacific Decadal Oscillation
• A large, long-lived oscillation pattern exists across the Pacific Ocean
• The Pacific Decadal Oscillation (PDO) involves two modes of sea
surface temperatures (SST)
• One exists in the northern and westerly part of the basin, the other in
the eastern tropical Pacific
• The PDO describes a 20-30 yr SST oscillation
• The PDO affects climatic impacts associated with El Niño events
• When the PDO is in a warm phase (high temperatures in the eastern
tropical Pacific), El Niño impacts on weather are more pronounced
than when the PDO is in a cool phase
The “Normal” Walker Circulation
SSTs during an ENSO event
SSTs associated with the PDO
End of Chapter 8
Understanding Weather and
Climate
3rd Edition
Edward Aguado and James E. Burt