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Wind Systems
Readings
A&B: Ch.8 (p. 213-247)
CD Tutorial: El Niño – Southern Oscillation
Topics
1. Concepts
4. Macroscale Winds
1. Scale
2. Wind Direction
3. Differential Heating
1. Global Circulation
1. Single-cell Model
2. Three-cell Model
3. Zonal Precipitation
Patterns
4. Semi-Permanent
Pressure Cells
2. Asian Monsoon
3. Jet Stream
4. Rossby Waves
2. Microscale Winds
3. Mesoscale Winds
1.
2.
3.
4.
5.
Land & sea breezes
Mountain or valley winds
Chinook
Santa Ana winds
Katabatic winds
5. El Niño – Southern
Oscillation
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Concepts
Scale
•
Three major divisions
Scale
Space
Time
Micro
Meters
Seconds – Minutes
Meso
Kilometers
Seconds – Hours
Macro Synoptic
100 – 1000 km
Days
Planetary >1000 km (global)
Days – Weeks
•
Wind Direction
Based on where the wind is
ƒ Sea breeze: air coming from the sea
ƒ Northwest wind: wind blowing from the northwest
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Concepts
Differential Heating
•
Spatially - get differences in surface heating
ƒ Some areas are warmer than others
ƒ Occurs across the range of scales
ƒ e.g. Micro: grass - concrete (Lab 5)
Meso: land - lake
Macro: equator - poles
•
Heating rate and T differences →
•
→ winds
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Microscale Winds
•
Examples
ƒ Turbulent eddies
• Small whirls of air
• Dust devils
• Gusts
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Mesoscale Winds: Land-Sea Breeze
•
Land-Sea (or Land-Lake) Breeze
ƒ Daily T differences between land and sea
• Daytime: land heated
more intensely than water
ƒ Air above land heats
more, expands vertically
ƒ Air aloft starts to flow
ƒ Near Surface:
•
•
ƒ Pressure Gradient Force
•
ƒ Cool air blown onto land
ƒ
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Mesoscale Winds: Land-Sea Breeze
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Nighttime: reverse
ƒ Land cooled more rapidly than water
ƒ Warmer over the water
ƒ Air blown from the land to the ocean
ƒ
•
Sea breeze – can have a significant
modifying effect on the temperature in
coastal areas
ƒ E.g., Chicago lake breeze
•
Size of breeze
ƒ
ƒ
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Mesoscale Winds: Mountain/Valley Wind
•
Daytime
ƒ Slopes of mountains get
more intense heating
than air at the same
elevation over the valley
floor
ƒ May see cumulus
clouds over peaks ⇒
thunderstorms in the
afternoons
→
ƒ Most common in
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Mesoscale Winds: Mountain/Valley Wind
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Sunset & Nighttime
ƒ Rapid cooling of slopes
ƒ Cool air drainage
→
ƒ Most common in
ƒ Lowest areas are first to
experience radiation
fog, frost damage
.
•
Note: seasonal preference
ƒ Valley breezes are most common in
ƒ Mountain breezes are most common in
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Mesoscale Winds: Chinook Winds
•
Chinook / Foehn
ƒ Different names in different places
• Chinook – Rockies (Montana, Wyoming, Alberta)
• Foehn - Alps, N.Z.
ƒ Low pressure system on the
of a
mountain barrier – pulls the air across
ƒ
as it comes down mountain
ƒ T can rise by 20oC
ƒ Usually occur
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Mesoscale Winds: Santa Ana Winds
•
Santa Ana Winds – California
ƒ High pressure system over the
Rocky Mountains
ƒ Air flows away from high,
down western slopes
ƒ
as it
comes down mountain
ƒ T can rise by 30oC
ƒ Usually occur
ƒ Often contributes to spread
of forest fires in CA
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Mesoscale Winds: Katabatic Winds
•
Katabatic Winds
ƒ Cold downslope wind –
ƒ Cold air sinks because more dense –
but still
than lower
elevation air it displaces
ƒ If channeled into narrow valleys → high velocities
ƒ Frequently occur at edges of Greenland and
Antarctic ice sheets
ƒ Different names
in different places
• Bora: Balkans
→ Adriatic sea
• Mistral: Alps
→ France
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Macroscale Winds: Global Circulation
•
Synoptic and planetary (macroscale) winds
influence the smaller scale (mesoscale and
microscale) winds
•
Global Circulation
ƒ Differential heating between equator and poles
→ Global scale pressure differences
→ Persistent large-scale motion
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Macroscale Winds: Global Circulation
•
Single Cell Model –
ƒ Differential heating
ƒ Assumptions:
• Earth is uniformly
covered with
water
• Sun is directly
over equator
→ Single-cell pattern
of flow – Hadley Cell
• Warm air rises at
• Cold air sinks at
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Macroscale Winds: Global Circulation
•
•
Single Cell Model – Hadley Cell
ƒ Earth’s rotation →
Coriolis force: winds
deflected to right in
Northern hemisphere,
to left in Southern
hemisphere
ƒ Winds:
winds from poles to
equator
Single-cell pattern is not
what we observe
ƒ Breaks down due to:
•
•
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Macroscale Winds: Global Circulation
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Three-Cell Model – more realistic model
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Macroscale Winds: Global Circulation
•
•
•
•
Three-Cell Model – more realistic model
Hadley Cell:
ƒ
Inter-Tropical Convergence Zone (ITCZ) (0o)
ƒ Very strong low pressure zone – rising air
ƒ Light winds: doldrums
Sub-tropical High (30o N/S)
ƒ Sinking air
ƒ Light winds: horse latitudes
Trade winds (0-30oN/S)
ƒ
ƒ
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Macroscale Winds: Global Circulation
Three-Cell Model – more realistic model
•
•
Ferrel cell –
ƒ Some of sinking air at subtropical high diverges
poleward
(mid-latitudes)
ƒ
ƒ
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Macroscale Winds: Global Circulation
•
•
•
•
Three-Cell Model – more realistic model
Polar cell: high latitudes
ƒ Thermally driven circulation
Polar High (90o)
ƒ Very cold conditions
ƒ Sinking, diverging air
Sub-polar Low (60o N/S)
ƒ Rising air
Polar
ƒ Flow from
ƒ Very strong deflection by Coriolis force
ƒ
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Macroscale Winds: Global Circulation
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•
Zonal Precipitation Patterns
Equa
Equatorial
Low
ƒ Rising air →
•
•
Sub-tropical High
ƒ Sinking air →
ƒ
ƒ Migrates N / S with seasons
Sub-polar Low
ƒ Rising air →
•
Polar High
ƒ Sinking air →
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Macroscale Winds: Global & Synoptic
•
•
•
•
Three-cell model not quite true: doesn’t
include land/water differences
Three-cell model breaks down in upper-level
winds – do not have the distinct structure of
Ferrel cell and polar cell, although surface
winds are correct there
But it was a very useful starting point for
considering global circulation
In the real atmosphere, we instead find a
number of semi-permanent High and Low
pressure cells
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Macroscale Winds: Global & Synoptic
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Semi-permanent Pressure Cells
ƒ January
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Macroscale Winds: Global & Synoptic
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Semi-permanent Pressure Cells
ƒ July
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Macroscale Winds: Asian Monsoon
•
•
Seasonal wind due to seasonal changes in
mean pressure
Winter:
ƒ Sinking air from jet stream →
ƒ
•
Summer:
ƒ Strong heating over continent →
ƒ Draw moisture from warm Indian Ocean toward
India and Asia
ƒ Himalayan Mountains cause strong orographic
uplift
ƒ
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Macroscale Winds: Jet stream
•
•
An area of increased wind speeds
ƒ Narrow band: 100 - 500 km wide
ƒ Speeds: 200 - 500 km h-1
ƒ Height: 9 - 12 km (
)
Typically found above the largest horizontal T
gradient – e.g., at polar front
Move north and south with
the seasons
Stronger in the
when
the T gradients are largest
Most powerful jet-stream:
•
Weaker jet-stream:
•
•
•
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Macroscale Winds: Rossby Waves
•
Recall: Upper air
(zones of low
pressure extending equator-ward) and
(zones of high pressure extending poleward)
→ Wavelike flow around earth at mid-latitudes
• Rossby waves: “long waves”
in flow
.
ƒ Usually 3-7 Rossby waves
encircling earth
ƒ Migrate west to east
ƒ Change in wavelength and
amplitude
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Macroscale Winds: Rossby Waves
•
•
•
Large amplitude Rossby waves (
flow) transport:
.
ƒ Warm air from subtropics to high latitudes
ƒ Cold polar air to low latitudes
Small amplitude Rossby waves (
flow)
ƒ Flow is more westerly, less equator-pole
exchange of heat
Changes in the flow along the wave lead to:
ƒ Divergence aloft
• Draws air
• Leads to
ƒ Convergence aloft
• Forces air
• Inhibits
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El Niño Southern Oscillation
•
El Niño – weak warm current occurring along
the west coast of South America (particularly
Peru)
ƒ
ƒ
ƒ
ƒ
•
Appears every 3-7 years around Christmas time
Lasts about 1 year
Warm current is not good for fishing industry
1997-98 was warmest event ever recorded
Occurs due to a reversal in “Walker
Circulation” – the interaction between
atmospheric circulation and ocean
circulation in the equatorial Pacific
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El Niño Southern Oscillation
•
During a normal (non-El Niño) year:
ƒ Easterly trade winds drag warm surface
water from East to West across Pacific
ƒ Upwelling of cold water along the west
coast of South America
ƒ Low pressure area:
ƒ High pressure:
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El Niño Southern Oscillation
•
A normal (non-El Niño) year
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El Niño Southern Oscillation
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During an El Niño year:
ƒ Weakening or reversal of trade winds drag warm
surface water from W to E across Pacific
ƒ No upwelling of cold ocean water
ƒ Sea Surface Temps (SST’s) in Eastern Pacific
become warmer than normal
ƒ Low pressure area shifts to Eastern Pacific →
along west coast of South America,
Central America and even California
ƒ High pressure shifts from to western Pacific
ƒ The reversal in surface pressure is called the
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El Niño Southern Oscillation
•
During El Nino year:
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El Niño Southern Oscillation
•
•
When El Niño dissipates:
ƒ Normal (non-El Niño) conditions
ƒ OR La Niña conditions
During a La Niña year:
ƒ Very strong easterly trade-winds in the Pacific
ƒ Very strong upwelling of cold water along the
west coast of South America
ƒ SST’s become colder than normal
ƒ In Western Pacific: warm water promotes uplift,
which intensifies surface low, and intensifies
easterly trade winds
ƒ Along west coast of America’s: very High
pressure →
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