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Chapter 10. Wind: Global Systems. General Circulation of the Atmosphere. General refers to the average air flow, actual winds will vary considerably. Average conditions help identify driving forces. The basic cause of the general circulation is unequal heating of the Earth’s surface

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general circulation of the atmosphere
General Circulation of the Atmosphere
  • General refers to the average air flow, actual winds will vary considerably.
  • Average conditions help identify driving forces.
  • The basic cause of the general circulation is unequal heating of the Earth’s surface
    • Warm air is transferred from the Tropics to the Poles
    • Cool air is transferred from the Poles to the Tropics
general circulation of the atmosphere1
General Circulation of the Atmosphere
  • Single Cell Model
    • Assume
      • uniform water surface
      • Sun always directly overhead the Equator
      • Earth does not rotate

Result: huge thermally direct convection cell (Hadley)

  • Three Cell Model
    • Allow earth to spin = three cells (Hadley, Ferrell, Polar)
    • Alternating belts of pressure starting with L at Equator
    • Alternating belts of wind with NE just North of Equator
general circulation of the atmosphere2
General Circulation of the Atmosphere
  • Average Surface Wind and Pressure: The Real World
    • Semi-permanent high and lows
    • Northern vs. Southern Hemisphere
    • Major features shift seasonally with the high sun
      • North in July
      • South in December
general circulation of the atmosphere3
General Circulation of the Atmosphere
  • General Circulation and Precipitation Patterns
    • Rain where air rises (low pressure)
    • Less rain where air sinks (high pressure)
  • Average Wind Flow and Pressure Patterns Aloft
    • North-South temperature and pressure gradient at high altitudes creates West-East winds, particularly at mid to high latitudes.
jet streams
Jet Streams
  • 100-200 kt winds at 10-15km, thousands of km long, several 100 km wide and a few km thick (polar and subtropical)
  • Observations: Dishpan Experiment
    • Illustrates waves, with trough and ridge, develops in a rotating pan with heat on the exterior and cold at the center.
jet streams1
Jet Streams
  • Polar and Subtropical Jet
    • Established by steep temperature and pressure gradients between circulation cells.
    • Between tropical-mid-latitude cell (subtropical) and mid-latitude-polar cell (polar)
    • Gradients greatest at polar jet
  • Topic: Momentum
    • Low-latitudes: atmosphere gains momentum
    • High-latitudes: atmosphere losses momentum
    • Conservation of Momentum
jet streams2
Jet Streams
  • Other Jet Streams
    • Tropical easterly jet stream
    • Low-level jet (nocturnal)
    • Polar night jet streams
atmosphere ocean interactions
Atmosphere Ocean Interactions
  • Global Winds and Surface Ocean Currents
    • Ocean surface dragged by wind, basins react to high pressure circulation forming gyres
    • Cold current, flowing north to south, on west side of continent
    • Warm current, flowing south to north, on east side of continent
    • Oceanic front

Stepped Art

Fig. 10-14, p. 273

atmosphere ocean interactions1
Atmosphere Ocean Interactions
  • Upwelling
    • Ekman spiral, Ekman transport
    • Water moving away from the coast causes upwelling
  • El Nino and the Southern Oscillation
    • El Nino: irregular warm episode off west coast of South America
    • Southern Oscillation: rise in pressure over W Pacific, fall in the E Pacific, equatorial countercurrent
    • ENSO
    • La Nina
    • teleconnection
atmosphere ocean interactions2
Atmosphere Ocean Interactions
  • Pacific Decadal Oscillation
    • Reversal in Pacific Ocean temperatures
    • Warm = more Pacific storms
    • Cool = cool, wet NW North America, wetter over the Great Lakes, salmon fisheries decline
atmosphere ocean interactions3
Atmosphere Ocean Interactions
  • North Atlantic Oscillation
    • Reversal of pressure in North Atlantic Ocean affecting weather in Europe and eastern coast of North America
    • Positive = strong Westerlies, storms in N Europe, wet and mild in eastern US
    • Negative = wet southern Europe and Mediterranean, cold and dry in eastern US
atmosphere ocean interaction
Atmosphere Ocean Interaction
  • Arctic Oscillation
    • Closely related to NAO
    • Pressure changes between Arctic and adjacent southern areas causes changes upper-level winds
    • Positive = mild winter in US and W Europe
    • Negative = cold US, cold dry Europe, wet Mediterranean