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 Subtropical High ~ 30N Hadley Cell 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 HighandAzores 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