Class #7: Thursday, July 15
Global wind systems
Chapter 10
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Chapter 10
Wind: Global Systems
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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
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General Circulation of the Atmosphere
•
Single Cell Model
–
Assume
1. uniform water surface
2. Sun always directly overhead the Equator
3. 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
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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
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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.
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Fig. 1, p. 267
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Fig. 2, p. 267
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.
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Jet Streams
• Polar and Subtropical Jet
– Established by steep temperature and pressure gradients
between circulation cells.
– Between tropical-mid-latitude cell (subtropical) and midlatitude-polar cell (polar)
– Gradients greatest at polar jet
• Topic: Momentum
– Low-latitudes: atmosphere gains momentum
– High-latitudes: atmosphere losses momentum
– Conservation of Momentum
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Jet Streams
• Other Jet Streams
– Tropical easterly jet stream
– Low-level jet (nocturnal)
– Polar night jet streams
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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
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Stepped Art
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Fig. 10-14, p. 273
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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
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Fig. 10-16, p. 275
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Fig. 10-17, p. 275
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Fig. 10-18, p. 275
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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
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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
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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
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Chapter 10
Wind: Global Systems
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