Part 3. Distribution and Movement of Air

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Part 3. Distribution and Movement of Air

• Chapter 8
• Atmospheric Circulation and Pressure Distributions

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Planetary Winds

• Well-defined pressure patterns exist across the
  Earth that induce the global wind patterns on the
  planet

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• Idealized Single-Cell Convection Model for a
  Planet

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The ITCZ is a band of clouds across the tropics
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The Three-Cell Model
Polar cell -- northeasterly winds at surface
Ferrel cell -- southwesterly winds at surface
Subtropical high -- Air subsides (dry climate)
Hadley cell -- tropical convection cell
Intertropical convergence zone (ITCZ) -- surface
low pressure with clouds and rain
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• The three-celled model vs. reality
• Hadley cells are close approximations of real
  world equatorial winds
• Ferrel and polar cells do not approximate the
  real world winds very well at all
• Model is unrepresentative of westerly flow aloft
• Continents and topographic irregularities cause
  significant variations in real world wind
  patterns compared to the model

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• Semi-Permanent Pressure Cells are large areas of
  higher or lower atmospheric pressure than the
  surface average
• They may be thermally induced (rising warm air or
  subsiding cold air) or they may be caused
  dynamically by converging or diverging wind
  patterns)
• They fluctuate seasonally
• Northern hemisphere semi-permanent cells
• The Aleutian, Icelandic, and Tibetan lows
• Siberian, Hawaiian, and Bermuda-Azores highs
• ITCZ (low)

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Average atmospheric air pressure and wind
patterns in January
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Average atmospheric air pressure and wind
patterns in July
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The Sahel reflects seasonal migration of the ITCZ
The Sahel is rainy during northern hemisphere
summer and dry during northern hemisphere winter
as the ITCZ shifts north and south through the
year. It is the reason for annual flooding of
the Nile River.
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Shifts in the ITCZ affect the Sahel
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• Mid-latitude winds in the middle and upper
  troposphere are controlled b the pressure
  gradient force and the Coriolis force, giving the
  westerly winds
• Stronger pressure gradients in winter give
  stronger westerlies

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• Westerly winds in the upper atmosphere
• Air motions directed towards poles
• Redirected by Coriolis deflection
• Westerly winds aloft result
• The polar front and jet streams
• Fast stream of air in upper troposphere
• Above polar front
• Stronger in winter

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Polar jet profile
Strongest pgf here
300 mb
500 mb
800 mb
The fastest middle and upper troposphere
mid-latitude winds (the jet stream) are at polar
front. There is a sharp temperature contrast and
horizontal pgf to the south and north of the
polar front.
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The subtropical jet transports low latitude
moisture and energy
Subtropical front and jet
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Ridges and trough profile
Ridge
Ridge
Trough
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A trough over the mid-continent in (b)
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• Rossby waves are the wave-like pattern of
  ridges and troughs in the upper troposphere
  winds. Ridges and troughs (Rossby waves) will
  migrate either east or west with time

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Sequence of Rossby wave migration -- the dashed
line shows the migration and evolution of a
trough with time across the country
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• Ocean currents are driven by wind stresses and
  are deflected by the Coriolis force. Thus, the
  water moves at about a 45 angle to the winds.
  The diection of movement of the ocean changes
  with depth following a pattern called the Ekman
  spiral.
• Western basins usually experience warm ocean
  currents
• Eastern basins usually experience cold ocean
  currents

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• Upwelling in along-shore winds
• Cold waters rise due to Ekman transport

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Global Ocean Circulation
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Infrared Satellite Image of the Gulf Stream
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• Major Wind Systems of the Earth
• Monsoons
• Thermal induced seasonal wind patterns associated
  with shifts of the ITCZ
• Monsoons are characterized by dry offshore winter
  flow and wet onshore summer flow
• The monsoon in East Asia experiences orographic
  enhancement

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Winter monsoon
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Summer monsoon
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Topography enhances monsoonal effects
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• Foehn winds are strong, downslope winds that
  adiabiatically compress, raising the air
  temperature. Foehn winds are associated with
  hot, dry, clear weather
• Chinook winds are foehn winds along the east
  slope of the Rockies (snow eaters)
• Santa Ana winds are foehn winds that blow from
  the deserts and over the mountains into the
  valleys of southern and central California
• Katabatic winds are cold, dense winds that flow
  down mountain slope. They warm as they descend,
  but they are still colder than the surrounding
  air.
• Boras and mistral winds are forms of katabatic
  winds in Europe

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Southern California Santa Ana induced fires
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• Sea and land breezes form due to temperature
  differences over land and sea. Sea breezes form
  during the day, and land breezes form at night.
• Valley and mountain breezes form due to heating
  and cooling on mountain sides. Valley breezes
  form during the day, and mountain breezes form at
  night (similar to katabatic winds)

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Sea breeze development
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Sea breeze-initiated clouds over Hawaii
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Valley and mountain breeze development
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• Air-Sea Interactions in the Equatorial Pacific
• El Niño, La Niña, and the Walker circulation
• El Niño events
• Unusually warm water in the eastern equatorial
  Pacific Ocean
• Linked to global weather anomalies
• 2 to 5 year recurrence
• La Niña events -- wind and temperature patterns
  reversed of El Niño patterns
• Walker circulation
• Vertical and horizontal tropospheric flow in the
  equatorial Pacific that controls areas of heavy
  rainfall

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The Normal Walker Circulation (no El Niño
conditions)
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• ENSO (El Niño/Southern Oscillation) events --
  oscillations between El Niño and La Niña
  conditions
• ENSO results in global teleconnection patterns
  (weather effects far from the equatorial Pacific)

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El Niño SST Anomalies
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La Niña SST Anomalies
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The Southern Oscillation Index
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The PDO Index
Warmer waters in western tropical Pacific
Cooler waters in western tropical Pacific