Met 61

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MET 61 Introduction to Meteorology - Lecture 11

• Global Circulation
• Dr. Eugene Cordero
• Reading WH Pg. 297-298
• Ahrens Chapter 11 Wind Global Systems
• Class Outline
• Circulation models
• Jet stream
• El Nino

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Review

• Answer the following question based on material
  covered in Chapter 11 of Ahrens.
• Explain how and why the average surface pressure
  features shift from summer to winter
• Why it is impossible on the earth for the Hadley
  cell to extend from the pole to the equator?
• Why is the jet stream stronger in winter than in
  summer?
• Explain physically what an El Nino event is.
• Explain how the southern oscillation is related
  to El Nino.
• Explain why the trade winds are NE and SE in the
  northern and southern hemispheres respectively.

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Atmospheric Scales of Motion

• Scale Time Scale Distance Scale Examples
• Macroscale
• -Planetary Weeks to years 1000-40,000km Westerli
  es, trade winds
• -Synoptic Days to weeks 100-5000km Cyclones,
  anticyclones and hurricanes
• Mesoscale Minutes to days 1-100km Land-sea
  breeze,
• thunderstorms and tornadoes
• Microscale Seconds to minutes lt1km Turbulence,
  dust devils and gusts

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General Circulation of the Atmosphere

• Large scale flow of the atmosphere
• Focus on both upper level and lower level winds
• Definitions
• Westerly winds comes from the west
• Southwest winds, comes from the south west

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General Circulation of the Atmosphere

• Large scale flow of the atmosphere
• Focus on both upper level and lower level winds
• Definitions
• Zonal winds (East-West)
• Meridional winds (North-South).
• Westerly winds comes from the west
• Southwest winds, comes from the south west

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View of the atmosphere in motion

• http//www.atmos.washington.edu/gfd_exp/exp_e/doc/
  bc/images/bc01.gif

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Single Cell Model

• Early description of general circulation
• George Hadley (1685-1768) developed this model
• Assumptions
• Earth is primarily heated in the tropics
• Surface heat imbalance produces air movement to
  balance.

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Single Cell Model

• Early description of general circulation
• George Hadley (1685-1768) developed this model
• Assumptions
• Earth is primarily heated in the tropics
• Thermally direct circulation results from heating
  differences
• Surface heat imbalance produces air movement to
  balance.

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Three Cell Model

• Proposed to explain how the Earths heat balance
  is maintained
• Good simple model of global circulation
• Terms
• Hadley Cell The tropical circulation
• Horse Latitudes

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Three Cell Model

• Proposed to explain how the Earths heat balance
  is maintained
• Good simple model of global circulation
• Terms
• Hadley Cell The tropical circulation
• ITCZ - intertropical convergence zone
• Horse Latitudes
• Trade Winds

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Three Cell Model (2)

• Explains much of the observed surface pressure
  distributions

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Monsoon circulation

• The monsoon circulation involves a seasonal
  change in the wind direction.
• Monsoon circulations have a distinct wet and dry
  season.
• Monsoon circulations in various places around the
  globe.
• Southern Asia
• Northern Australia
• Southwest USA

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Cold and clear skies
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Warm and cloudy
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What is El Niño?

• El Niño is
• El Niño occurs
• The name, El Niño (means boy child in
  Spanish) as it often occurs around Christmas time.

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What is El Niño?

• El Niño is
• El Niño occurs
• The name, El Niño (means boy child in
  Spanish) as it often occurs around Christmas time.

• a warming of the equatorial eastern Pacific Ocean
  

• about about once every 2 to 7 years

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Normal conditions (sea surface temps)
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El Niño (sea surface temps)
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ENSO (El Niño-Southern Oscillation)

• The Southern Oscillation
• is the atmospheric part of El Niño
• The Southern Oscillation Index is the pressure
  difference between Tahiti and Darwin,
  Pressure(Tahiti) Pressure(Darwin)
• During El Niño, the pressure is _______ in the
  west and _______ in the east
• During El Nino, the Southern Oscillation Index is
  (positive/negative).

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ENSO (El Niño-Southern Oscillation)

• The Southern Oscillation
• is the atmospheric part of El Niño
• The Southern Oscillation Index is the pressure
  difference between Tahiti and Darwin,
  Pressure(Tahiti) Pressure(Darwin)
• During El Niño, the pressure is _______ in the
  west and _______ in the east
• During El Nino, the Southern Oscillation Index is
  (positive/negative).

higher
lower
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NAO Index is difference between the polar low and
the subtropical high during the winter season
(December through March)
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What is the NAO?

• The North Atlantic Oscillation (NAO) represents
• winter climate variability in the North Atlantic
  region
• ranging from central North America to Europe and
  much into Northern Asia
• The NAO is a large scale seesaw in atmospheric
  mass

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Phases of the NAO

• A positive index
• stronger than usual subtropical high pressure
• deeper than normal Icelandic low.
• The increased pressure gradients produces
• stronger winter storms crossing the Atlantic
  Ocean
• warm and wet winters in Europe
• cold and dry winters in northern Canada and
  Greenland
• mild and wet winter conditions in Eastern US

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Phases of the NAO

• A negative index
• weaker than usual subtropical high pressure
• weaker than normal Icelandic low.
• The increased pressure gradients produces
• fewer winter storms crossing the Atlantic Ocean
• Cold air to northern Europe
• Milder winters in Greenland
• More cold outbreaks and snowy conditions to the
  Eastern US

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Activity 12 (Due May 9th)

• Derive each component (horizontal and vertical)
  of the vorticity.
• For incompressible flow, show how the meridional
  winds would change if there exists a positive
  zonal wind gradient and no vertical wind shear.
• Chapter 11 (Ahrens Meteorology Today)
  Questions for Review 5,7,15,
• Questions for thought 2,6