Download general circulation of air

GENERAL CIRCULATION OF AIR
Vicki Drake
Santa Monica College
I.
Circulation
A. The underlying causes of air circulation on the Earth surface
1. Differential heating/cooling of land masses and water
B. Models used to describe global circulation of Air
1. One-Cell – (George Hadley – British – 18th century)
a. Assumptions:
i. No rotation and no Coriolis Force in effect
ii. Sun only energy source
iii. No differential in heating/cooling of land and water
iv. Equal distribution of land/water on surface
v. Only Force in effect: Pressure Gradient Force
b. Excess solar energy at equatorial regions; deficit solar energy
near polar latitudes
c. Warm air rises from equatorial regions – creating a belt of Low
Pressure cells
d. Cold air sinks at Polar regions – creating Polar High Pressure
cells
e. Horizontal pressure difference between Polar High and
Equatorial Low begins air flow
2. Three-cell (William Ferrel, American, 20th century)
a. Assumptions:
i. Rotation and Coriolis in effect
ii. Pressure Force one of forces at work
b. Rising warm air columns create belt of surface Low Pressure
cells near equatorial regions
c. Sinking cold air columns create region of High Pressure cells at
Polar latitudes
d. Converging Upper Level air at ~300N and 300S create subsiding
air columns – not thermal in nature…
i. These are warm core cells with compressional heating of
air
ii. High Pressure Cells created at Earth’s surface (Dynamic)
e. Converging Surface air at ~600N and 600S creates rising air
columns – not thermal in nature
i. These are cold core cells with expansion cooling of air
ii. Low Pressure cells created at Earth’s surface (Dynamic)
II. Idealized Global Pressure Systems
1. Equatorial waters – warm air rising, weak winds
a. Belt of Low Pressure found in this region along InterTopical
Convergence Zone (ITCZ)
2. Middle latitudes (~30-450N/S)
a. Convergence of air aloft – falling air –
b. Compressional heating – warm core High Pressure Cells
3.
Mid-to-High latitudes (~600-750 N/S)
a. Collision zone between warm, moist subtropical air and cold, dry
polar air
b. Region of Rising Air – Subpolar Lows (Cold Core)
4. High Latitudes (~75-900N/S)
a. Cold sinking air – cold core High Pressure Systems (Thermal)
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
1
b. Polar Highs
III. Global Winds
A. Winds labeled by direction of origin
B .Air moves from Subtropical Highs toward Equatorial Lows (Trade Winds)
C. Air moves from Subtropical Highs toward Subpolar Lows (Westerlies)
C. Air move from Polar Highs toward Subpolar Lows (Polar Easterlies)
D. Coriolis Force affects movement of air in motion
1. Wind pulled to left in Southern Hemisphere
2. Wind pulled to right in Northern Hemisphere
IV. Semi-Permanent Pressure Systems
A. Northern Hemisphere
1. Bermuda-Azores High – Atlantic Ocean
3. Icelandic Low – Atlantic Ocean
4. Pacific (Hawaiian/California) High – Pacific Ocean
5. Aleutian Low – Pacific Ocean
B. These are both dynamic and thermal Pressure systems
V. Jet Streams
A. Thin superfast “rivers” of air – traveling around the world at 100 of
km/hour (Average ~200 km/hour)
B. Formed by a difference in air temperature and density, creating strong
pressure gradient force
C. Polar Jet Stream
D. Subtropical Jet Streams
VI. Global Winds and Surface Ocean Currents
A. Atlantic Ocean currents
1. Gulf Stream - warm
2. Labrador Current – cool-cold
3. North Atlantic Drift - warm
4. Canary Current – cool-cold
6. Equatorial Current - warm
B. Pacific Ocean currents
1. Kuroshio Current - warm
2. Pacific Drift - cool-warm
3. California Current – cool-cold
5. Equatorial Current – warm
VII. Upwelling and Coastal Currents
A. Found along West Coasts of large continental land masses (N and S
America, Africa)
B. All have a cold water current flowing parallel to coast
C. Coriolis Force tends to pull surface water (and water to greater depth)
away from the coast (to the right along the N. American coast, and to the
left along the South American and African coastlines)
D. As a result, colder, nutrient-rich water from depth is pulled up to the
surface, creating perfect environment for fishing, etc.
E. Upwelling an important process of providing stability in some of S.
America’s economies (i.e., Peru)
VIII. El Niño and the Southern Oscillation
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
2
A. El Niño is an atmospheric and oceanic disturbance in which a pool of warm
surface ocean water is transported across the Pacific from the Western Pacific (near
Australia) to the Eastern Pacific (South America).
B. This disturbance occurs generally toward the end of each calendar year
and was named by the Peruvian fisherman (“The Christchild”) because of the time of
appearance.
C. Most El Niños are mild and last only a few weeks. Long term disturbances,
have a global impact
D. During a strong El Niño, the fishing industry is disrupted as the warm
water suppresses upwelling and the cold,nutrient-rich water is not available at
the surface
E. The atmospheric disturbance include a reversal of Surface Air Pressure
systems – which also includes a weakening and even reversal of Surface
Winds (i.e. Trades) – called Southern Oscillation
F. Combined together this ENSO event can bring about devasating rains to
parts of Peru, and southern California.
G. Devastating droughts can occur in Australia, Indonesia, etc.
H. Other regions of the world are being investigated for possible interactions
– i.e. drought in Africa, forest fires in Arizona, etc,
© Vicki Drake
Santa Monica College
Fall 2000 Geography Lectures
3