I.
GENERAL CIRCULATION OF AIR
Vicki Drake
Santa Monica College
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 – 18 th 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
Only Force in effect: Pressure Gradient Force v.
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
Three-cell (William Ferrel, American, 20 th century) 2.
a.
Assumptions: i.
Rotation and Coriolis in effect b.
ii.
Pressure Force one of forces at work
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
Converging Upper Level air at ~30 0 N and 30 0 S create subsiding d.
air columns – not thermal in nature… i.
These are warm core cells with compressional heating of ii.
air
High Pressure Cells created at Earth’s surface (Dynamic)
Converging Surface air at ~60 0 N and 60 0 S creates rising air e.
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 (~30-45 0 N/S)
3.
a.
Convergence of air aloft – falling air – b.
Compressional heating – warm core High Pressure Cells
Mid-to-High latitudes (~60 0 -75 0 N/S)
Collision zone between warm, moist subtropical air and cold, dry a.
polar air b.
Region of Rising Air – Subpolar Lows (Cold Core)
4.
High Latitudes (~75-90 0 N/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.
D.
Polar Jet Stream
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
F.
Winds (i.e. Trades) – called Southern Oscillation
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