Global Circulation

1
Global Circulation

• Geog. 1, Week 4, 19 March 2002
• Chapter 6 of Christopherson
• Need to know
• Major Wind Patterns
• High and Low Pressure Systems

2
Start From Highly Simplified Model

1. Pressure differences
2. Equator/Pole air movement
3. Add in upper troposphere cooling
4. Add in rotation of Earth
5. Coriolis Force
6. Effects of land mass

3
Basic Air Movement Caused by Pressure Differences

• Air moves from high pressure to low pressure
  region
• If air is heated, density drops and so pressure
  drops, e.g. sea breeze

High 1025hPa
Average Pressure 1013hPa
Low 995hPa
Air movement
4
High to Low Air Flow Causes Vertical Movement as
Well

• Clouds WHY? Clear

Falling air above a high
Rising air above a low
Low air flow in at ground
High air flow out at ground
5
Equator to Pole Air Movement

• This model is too simple to be useful

Equator
South Pole
North Pole
Heating
6
Equator to Pole Air Movement

• More realistic model

Loses heat by radiation
Polar Front
Hadley cell
L
L
H
H
South Pole
Equator
30o S
60o S
Heating
7
30oN
Equator
30oS
From Christopherson, p. 149
8
Now Add In a Rotating Earth

• Apparent change in direction referred to as the
  Coriolis Effect

• Coriolis Force
• Northern Hemisphere
• Clockwise round high
• Anticlockwise round low
• Southern Hemisphere
• Anticlockwise round high
• Clockwise round low

9
Southern Hemisphere

• This clockwise/anticlockwise air movement around
  lows and highs defines the synoptic wind
  direction for Tasmania as these systems move past
  the State

H
L
10
Christopherson p.150
11
Friday 8 March 2002 (Mercury Newspaper)
Hobart cool, early drizzle, light to moderate
southerly winds
12
C
13
northeasterly winds developing
14
Strong northerlies, very warm
15
Ocean Currents

• Similar general pattern except, obviously,
  restricted to the oceans
• Surface currents driven by prevailing winds
• Deep currents driven by changes in water density
  (e.g. cold Arctic water sinking and moving south
  through Atlantic ocean)
• Movement slower than air (1000 years for full
  cycle)

16
Christopherson p.162
17
Christopherson p. 164
18
Summary

• Large scale atmospheric movement of air due to
  uneven heating and cooling plus effect of Earths
  rotation and position of continents. This moves
  heat (energy) from equator towards poles.
• Ocean currents driven by wind and uneven
  heating/cooling of oceans. Slower than wind but
  large amounts of heat carried from equator
  towards poles.

19
Equator movement of warm moist air into upper
troposphere
20
Subtropical Highs around 30oS e.g. Byron Bay
21
Storms at polar convergence about 60oS
22
Next Week

• The importance of evaporation and condensation of
  water in the atmosphere
• Chapter 7 of Christopherson