Understanding Weather and Climate 3rd Edition Edward Aguado and James E. Burt

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Understanding Weather and Climate 3rd
EditionEdward Aguado and James E. Burt

• Anthony J. Vega

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Part 3. Distribution and Movement of Air

• Chapter 8
• Atmospheric Circulation and Pressure Distributions

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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

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• 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, Hadleys single-cell model
  was essential in identifying consequences of a
  thermally direct circulation

Single-Cell Model
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• 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

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The ITCZ is observable as a band of clouds
extending from northern South America into the
Pacific
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• 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

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• 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

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The Three-Cell Model
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• 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

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• 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

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• 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

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January
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July
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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
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• 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
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• 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

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Profile of the polar jet
The subtropical jet is seen as a band of clouds
extending from Mexico on an infrared
satellite image
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• 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

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Profile of ridges and troughs
A trough over the mid-continent is depicted in b
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Sequence of Rossby Wave Migration
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Rossby waves in the upper air advect cold or warm
air (left), evident by surface temperatures (below
)
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• 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

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Infrared Satellite Image of the Gulf Stream
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Ekman Spiral
Global Ocean Circulation
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• 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

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The winter monsoon
The summer monsoon
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• 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

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• 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

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Sea breeze development
Sea breezes initiate clouds and precipitation on
the island of Hawaii
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Development of a Valley and Mountain Breeze
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• 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

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• 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

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• 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
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SSTs during an ENSO event
SSTs associated with the PDO
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End of Chapter 8 Understanding Weather and
Climate 3rd EditionEdward Aguado and James E.
Burt