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

2. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University 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 Myneni Lecture 17: Atmospheric Circulation Feb-28-07 (2 of 11)

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

4. Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University 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 30 o 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 Surface Winds on an Ideal Earth Myneni Lecture 17: Atmospheric Circulation Feb-28-07 (4 of 11)

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

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

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

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

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

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

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