Download Tropics net gain

Chapter 11
Atmospheric Circulation:
Local & Global Wind Systems
General Circulation – the
average flow of the atmosphere
across the entire planet over long
periods of time.
Q: Why do we have global
circulations?
A.:Unequal heating of the earth’s
surface.
Tropics
 net gain
Poles  net loss
To achieve balance between these
regions, heat is transported from
Tropics to poles.
The first person to attempt a
description of this redistribution:
George Hadley
– 1700’s
Hadley Cell
H
equator
L
L
L
One-cell model of the atmosphere
 Original Hadley Cell – entirely
convective (like a thunderstorm)
This would actually occur if we
assume:
1. Uniform water surface (no
land-water discontinuities)
2. Sun is always over equator
3. Earth does not rotate.
Thermally direct circulation = warm
air moves up and cold, down
Thermally indirect circulation =
warm moves down and cold, up
60
30
Eq
30
60
3 – Cell Model
1. Still uniform water surface
2. Still sun always over equator
BUT
3. ALLOW IT TO ROTATE
Quite similar to actual earth
circulation process:
1. Ascent in ITCZ
2. Turn poleward at tropics
3. Cool by radiation
4. Sink at ~30N (subtropical high)
5. Split poleward and equatorward
(subtropical high)
6. Sink at poles, turn equatorward.
7. Converge with southerly flow to
south (polar front low)
Real World:
1. Land – Water contrasts
2. “Moving” sun across face of
planet as well as rotation
Pressure Patterns:
 January (North Hemisphere)
 Bermuda - Azores High and
Pacific High (25-35N)
 Icelandic Low and Aleution Low
(40-65N)
 Siberian High
 ITCZ farther south
 July (North Hemisphere)
 Bermuda High and Pacific High
are stronger
 Icelandic Low is weaker and
Aleution Low is GONE
 Siberian High is gone
 ITCZ is farther north
Precipitation Patterns
ITCZ
Average Wind Flow
 Variance of speeds greater in NH
 SH mostly water
 Slower to warm and cool
 Stronger winds in the winter
hemisphere
Jet Streams
 A switfly flowing current of air
thousands of kilometers long, a
few hundred kilometers wide, and
1-2 kilometers deep. Usually at
10-15 km altitude.
Subtropical Jet Stream - ~13 km
Polar Front Jet Stream - ~ 10km
Jet Streak – a region of locally
strong wind speeds.
Formation of Jet Streams
1. Polar Front Jet
 Sharp change in temperature
across polar front jet
 Sharp change in pressure across
polar front jet (height of
pressure surface)
 Strong pressure / height
gradient intensifies wind speed
2. Subtropical Jet Stream
 There is some evidence of a
“subtropical front” aloft due to
heat transport by Hadley Cell
 Governed somewhat by angular
momentum
 Objects in motion possess
MOMENTUM
Linear Momentum
massobject * velocityobject
Angular Momentum
Mass*velocity*radiual
distance from axis
 At the equator, the Earth
rotates eastward at > 1,000 mph
 On a calm day at the equator,
air above the surface also moves
eastward at > 1,000 mph
 Since parcels in motion are on
the rotating planet earth, they
possess ANGULAR
MOMENTUM.
If there are no net external
twisting forces acting on a
rotating system, then the angular
momentum of the system does
not change.
Ice Skater
 Arms extended  slow rotation
 Arms tucked-in  fast rotation
rs > rn
for m=constant:
 r decreases, v increases
 r increases, v decreases
1.
2.
3.
4.
5.
Parcels rise at equator
Parcels turn poleward
r decreases
v increases at turn to the right
Generates westerlies aloft