Outline

1
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (1 of 11)
Further Reading Chapter 07 of the text book
Outline
- Surface Winds on an Ideal Earth
- Subtropical High-Pressure Belts
- The ITCZ and Monsoon Circulation
- Wind and Pressure Features at Higher Latitudes
2
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (2 of 11)
Surface Winds on an Ideal Earth-1

• Ideal Earth
• No pattern of land and water
• No seasonal changes
• Equatorial regions receive the most insolation
• Surface has the warmest temperature
• Air rises
• Poles recieve the least insolation
• Surface has the coolest temperature
• Air descends
• Pressure gradients exist between the equator and
  poles

3
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (3 of 11)
Surface Winds on an Ideal Earth-2
Polar High
H
Polar Easterlies
Polar front 60N
L
Mid-latitude Westerlies
Hadley Cell
Subtropical High 30N
H
Trade winds
L
Inner-tropical Convergence Zone
Equatorial Trough
4
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (4 of 11)
Surface Winds on an Ideal Earth

• Equatorial Region
• Air rises at the equator
• Produces the equatorial trough at the surface
• Surface winds blow into this low pressure,
  forming the inter-tropical convergence zone
  (ITCZ)
• Winds veer to the right/left in the
  northern/southern hemisphere under the influence
  of the Coriolis force
• Produce the Northest/Southeast Trades at the
  surface
• Aloft air moves towards the poles, then cools and
  sinks around 30o N/S
• This closed circulation is called the Hadley
  cell
• Sub-tropical Region
• Sinking air creates subtropical high
• Surface winds blow both towards the equator and
  towards the pole
• Winds veer to the right/left in the
  northern/southern hemisphere under the influence
  of the Coriolis force
• Produces the Westerlies at the surface

5
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (5 of 11)
Surface Winds on an Ideal Earth

• The Poles
• Cold air descends over the poles
• Produces the polar high at the surface
• Surface winds blow out of this high pressure
• Winds veer to the right/left in the
  northern/southern hemisphere under the influence
  of the Coriolis force
• Produces the polar easterlies
• Midlatitudes
• Polar easterlies converge with the Westerlies
  
• Convergence produces a low, air rises and
  subsides over the poles and the mid-latitudes
• The circulation in this region is very noisy and
  is only seen in the average patterns
• How do seasons affect this ideal circulation?
• change the latitude of most intense solar heating
• change the temperature contrast between land and
  ocean, and hence the surface pressure patterns

6
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (6 of 11)
Subtropical High-Pressure Belts
July
7
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (7 of 11)
Subtropical High-Pressure Belts

• Southern Hemisphere
• Confirms well to the pattern of the ideal
  circulation
• Three large high pressure-cells persist year long
• A fourth forms in July due to the cooling of
  Australia (southern hemisphere winter)
• Northern Hemisphere
• Hawaiian High in the Pacific
• Azores High in the Atlantic
• Intensify in summer and move northward
• Have effects on east and west coasts
• Rainless summer in west coast due to Hawaiian
  High
• Hot and humid summer in central and eastern US
  due to Azores High

8
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (8 of 11)
The ITCZ and the Monsoon Circulation
9
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (9 of 11)
The ITCZ and the Monsoon Circulation

• The ITCZ
• As the seasons change, the region with the most
  insolation changes, hence the position of the
  ITCZ changes
• Over the ocean the shift is moderate 10 degrees
• Over land, because of large seasonal changes in
  temperature, large shift occurs particularly over
  Asia
• Monsoon in Asia
• In the summer, high insolation warms the
  continent and produces low-pressure
• Winds blow from the ocean to the land and then
  rise
• These warm and moist air brings heavy
  precipitation
• In the winter, the continent cools quickly,
  producing high-pressure
• Winds blow from the continent to the ocean
• Dry conditions prevail

10
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (10 of 11)
Wind and Pressure Features of Higher Latitudes
11
Natural Environments The Atmosphere GE 101
Spring 2007 Boston University
Myneni Lecture 17 Atmospheric Circulation Feb-28-
07 (11 of 11)
Wind and Pressure Features of Higher Latitudes

• Difference in land-water patterns
• Northern Hemisphere large continental masses
• Southern Hemisphere large ocean area with
  glacial ice sheet in the center
• Northern Hemisphere
• In the winter
• Siberian High and Canadian High over continents
• Icelandic Low and Aleutian Low over the oceans
• Brings cold air to the south
• In the summer
• Low pressure over continents (Asiatic Low)
• High pressure over the oceans (Hawaiian High and
  Azores High)
• Warm and dry conditions in west coasts
• Warm and moist conditions in east coasts
• Southern Hemisphere
• South Polar High persists all year long due to
  the glacial ice sheet
• Surrounding low pressure
• Strong prevailing westerlies over higher
  latitudes