Wind Systems
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Ch. 8: Global Scale Winds
• Size Scales of Weather and Winds
• Circulation Cells—The General Circulation
• Surface Wind Patterns
• Semi-permanent Highs, Lows
• Upper-air Westerlies
• Jet Streams, Rossby Waves
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Ch. 8: Wind Systems
• Monsoon Circulations
• Chinook and Santa Ana Winds
• Sea and Land Breezes
• Mountain and Valley Breezes
• El Niño/La Niña
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Size Scales• Microscale
Less than about 1 km
• Mesoscale
Up to about 100 km
• Synoptic-scale
Up to about 10,000 km
• Global-scale
Planetary-size
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The General Circulation
Non-rotating Earth
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The General Circulation
Rotating Earth
Hadley Cell
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GC Surface Winds90°N
90°S
60°N
30°N
0°
30°S
60°S
Polar Easterlies
Prevailing Westerlies
Trade Winds
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Surface Pressure Distribution
L
HL
H
HH
Aleutian Low
EasternPacific High
Icelandic Low
Bermuda-Azores High
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Upper-Level Westerlies
z
10
Upper-Level Westerlies
z
Cold Pole Warm Tropics
Poleward PGF aloft
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North Pole
Equator
Vg
PGF
Fcor
500-mb heights
N
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Equator
South Pole
Vg
Fcor
PGF
500-mb heights
Upper-level winds are generally west-to-east, both hemispheres
N
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Jet Stream
z
To pole 60°N To Equator
Steep pressure gradient in narrow band
Discontinuity at convergence zone
Convergence of warm and cold air near the surface
Jet stream
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Rossby Waves
• “Long waves”
Global scale: large wavelength waves in upper-level westerlies, going around either pole
Propagate very slowly
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Monsoon Circulation
• Synoptic-scale thermally driven circulation
• Large size scale — longer cycle time — flips seasonally instead of daily
Winter Summer
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Chinook Wind
In Europe/Asia: Foehn Wind
Latent heat release warms the rising air
Condensed water precipitates out on windward side
Dry adiabatic compression
Hot, dry!
Compression heat+
Condensation heat
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Santa Ana Wind
So. Cal.
High Deserts
San Gabriel/ Tehachapi
Mts. Hover Great Basin
Adiabatic compression during descent warms the air
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Thermally Driven Winds:Sea and Land Breezes
Ocean
Ocean
Land
Land
Cool Warm
Cool Cold
H L
L H
Thermal High Thermal Low
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Mountain and Valley Breezes
Valley Breeze
Mountain Breeze
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El Niño/Southern Oscillation
“Normal” Tradewind
Upwelling
z
N
E
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El Niño/Southern Oscillationz
N
E
Reversed El Niño wind
Oscillation period: 3–7 years
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La Niña: The Pendulum Swings Back
z
N
E
Upwelling
Strong Easterly Tradewind
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