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Global Circulation

Global Circulation. GEOG 1112- Lecture 11. Chapter 5: Atmospheric Pressure and Wind. McKnight’s Physical Geography : A Landscape Appreciation, Tenth Edition, Hess. Atmospheric Pressure and Wind. The General Circulation of the Atmosphere Modifications of the General Circulation

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Global Circulation

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  1. Global Circulation GEOG 1112- Lecture 11

  2. Chapter 5: Atmospheric Pressure and Wind McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess

  3. Atmospheric Pressure and Wind • The General Circulation of the Atmosphere • Modifications of the General Circulation • El Niño-Southern Oscillation • Other Multiyear Atmospheric and Oceanic Cycles

  4. Global Pressure Pattern- January

  5. January • Siberian and Canadian Highs • Siberian High: intensively developed center of high atmospheric pressure located in northern central Asia in winter • Canadian High: high pressure area that tends to develop over the central North American continent in winter • Aleutian and Icelandic Lows • Aleutian Low: center of low pressure in the Aleutian Islands, especially persistent in winter • Icelandic Low: center of low pressure located in the north Atlantic, persistent in winter

  6. Global Pressure Pattern- July

  7. July • Pacific High and Bermuda/Azores High • Bermuda High: persistent, high pressure center located in the subtropics of the north Atlantic Ocean • Pacific High: persistent cell of high atmospheric pressure located in the subtropics of the North Pacific Ocean

  8. Idealized Global Pressure Belts • Single-Cell circulation model • George Hadley (1735) • Convection cells

  9. The General Circulation of the Atmosphere • Simple example: A non-rotating Earth • Strong solar heating at equator • Little heating at poles • Thermal low pressure forms over equator • Thermal high forms over poles • Ascending air over equator • Descending air over poles • Winds blow equatorward at surface, poleward aloft Figure 5-12

  10. Three-Cell Circulation Model

  11. Global Wind and Pressure Patterns Subtropical high-pressure belts: belts of persistent high atmospheric pressure centered at about lat. 30º N and S Polar Front: front lying between cold polar air masses and warm tropical air masses Intertropical Convergence Zone (ITCZ): zone of convergence of air masses along the equatorial trough Hadley Cell: low-latitude atmospheric circulation cell with rising air over the equatorial trough and sinking air over the subtropical high-pressure belts

  12. The General Circulation of the Atmosphere • Observed general circulation • Addition of Earth’s rotation increases complexity of circulation • One semipermanent convective cell near the equator • Three latitudinal wind belts per hemisphere • Hadley cells Figure 5-14

  13. Global Circulation Patterns

  14. Global Circulation Patterns • Intertropical Convergence Zone (ITCZ) • Equatorial Low Pressure (Uplift) • Doldrums • Tropical Rain Forests • Trade Winds • Subtropical High Pressure (STHP) • Upper air subsidence • Horse Latitudes • Deserts • Westerlies • Subpolar Low • Uplift • Polar Front • Mid-Latitude Cyclone • Polar Winds • Polar High • Subsidence • Polar Desert

  15. The BEST Model of the GCM was produced by ______. • Team 1 • Team 2 • Team 3 • Team 4 • Team 5

  16. The General Circulation of the Atmosphere • Intertropical Convergence Zone (ITCZ) • Region of convergence of the trade winds • Constant rising motion and storminess in this region • Position seasonally shifts (more over land than water) • Doldrums Figure 5-21

  17. Global Wind and Pressure Patterns • ITCZ and Monsoon Circulation • ITCZ and Hadley cells shift with the seasons • Shift is very large in Asia • Cold temperaturesSiberian High intense in January • Warm temperaturesAsiatic Low intense in July

  18. The General Circulation of the Atmosphere • Trade winds • Diverge from subtropical highs • Exist between 25°N and 25°S latitude • Easterly winds: southeasterly in Southern Hemisphere, northeasterly in Northern Hemisphere • Most reliable of winds • “Winds of commerce” Figure 5-17

  19. The General Circulation of the Atmosphere • Trade winds (cont.) • Heavily laden with moisture • Do not produce rain unless forced to rise • If they rise, they produce tremendous precipitation and storm conditions Figure 5-20

  20. The General Circulation of the Atmosphere • Components of the general circulation • Subtropical highs • Persistent zones of high pressure near 30° latitude in both hemispheres • Result from descending air in Hadley cells • Subsidence is common over these regions • Regions of world’s major deserts • No wind, horse latitudes Figure 5-16

  21. The General Circulation of the Atmosphere • Westerlies • Form on poleward sides of subtropical highs • Wind system of the midlatitudes • Two cores of high winds – jet streams • Rossby waves Figure 5-22 Figure 5-24

  22. The General Circulation of the Atmosphere • Polar highs • Thermal highs that develop over poles due to extensive cold conditions • Winds are anticyclonic; strong subsidence • Arctic desert • Polar easterlies • Regions north of 60°N and south of 60°S • Winds blow easterly • Cold and dry

  23. The General Circulation of the Atmosphere • Polar front • Low pressure area between polar high and westerlies • Air mass conflict between warm westerlies and cold polar easterlies • Rising motion and precipitation • Polar jet stream position typically coincident with the polar front Figure 5-25

  24. The General Circulation of the Atmosphere • The seven components of the general circulation Figure 5-26

  25. The General Circulation of the Atmosphere • Vertical wind patterns of the general circulation • Most dramatic differences in surface and aloft winds is in tropics • Antitrade winds Figure 5-28

  26. The Global Pattern of Atmospheric Pressure • Seasonal Variations in the Pattern • Thermally induced, increase w/ latitude b/c of increasing contrast in length of daylight and incidence angle • Differential heating • Friction • Landforms

  27. Winds Aloft • Pressure decreases less rapidly with height in warmer air than in colder air • There is a temperature gradient from the equator to the poles • So, at high altitudes there is a pressure gradient from the equator to the polesstrong winds at high altitudes

  28. The General Circulation of the Atmosphere • Seasonal differences in the general circulation Figure 5-15

  29. Global Wind and Pressure Patterns • Wind and Pressure Features of Higher Latitudes • The north pole is on water, the south pole is on land. • Land-water differences influence pressure patterns. • January: • Northern Hemisphere • Highest pressure in Siberia and Canada • Southern Hemisphere • Highest pressure in Antarctica • July: • Northern Hemisphere • Low pressure over continents • Southern Hemisphere • High pressure in Antarctica

  30. The General Circulation of the Atmosphere • Atmosphere is in constant motion • Major semipermanent conditions of wind and pressure—general circulation • Principal mechanism for longitudinal and latitudinal heat transfer • Second only to insolation as a determination for global climate

  31. Modifications of the General Circulation • Seasonal modifications • Seven general circulation components shift seasonally • Components shift northward during Northern Hemisphere summer • Components shift southward during Southern Hemisphere summer Figure 5-29

  32. Modifications of the General Circulation • Monsoons • Seasonal wind shift of up to 180° • Winds onshore during summer • Winds offshore during winter • Develop due to shifts in positions of ITCZ and unequal heating of land and water Figure 5-30

  33. Global Wind and Pressure Patterns • ITCZ and Monsoon Circulation • Monsoon: seasonal reversal of winds • Summer monsoon: warm, humid air moves onshore: heavy rains • Winter monsoon: cold dry air moves offshore; dry conditions

  34. Modifications of the General Circulation • Major monsoon systems Figure 5-32

  35. Modifications of the General Circulation • Minor monsoon systems Figure 5-33

  36. The Effects of Seasonal Migration • North-South shift in global pressure belts • Causes variable weather conditions

  37. Longitudinal Differences in Winds • STHP causes varying weather patterns on different sides of the high pressure zone

  38. Bermuda High

  39. Pacific High

  40. The air that descends and spirals out of the subtropical high pressure belt is the source of polar easterlies. trade winds and the westerlies. Chinooks. land and sea breeze. the monsoons.

  41. The air that descends and spirals out of the subtropical high pressure belt is the source of • polar easterlies. • trade winds and the westerlies. • Chinooks. • land and sea breeze. • the monsoons.

  42. The air that descends and spirals out of the subtropical high pressure belt is the source of polar easterlies. trade winds and the westerlies. Chinooks. land and sea breeze. the monsoons. Figure 5-14 Explanation: As seen in Figure 5-14, the subtropical high is the source location for both the mid-latitude westerlies and the trade winds.

  43. Winds Aloft • The Geostrophic Wind • Forces acting on an upper air parcel: • 1. Pressure gradient force: • Moves from high pressure to low pressure • 2. Coriolis force • Deflects to right in NH, to left in SH Pressure gradient force balances coriolis force • Wind blows parallel to isobars Geostrophic Wind: wind at high levels above the Earth’s surface blowing parallel with a system of straight parallel isobars

  44. Winds Aloft • Global Circulation at Upper Levels: • Weak easterlies at equator • Tropical high-pressure belts • Westerlies in the mid- and high- latitudes • Low pressure at poles

  45. Winds Aloft • Rossby Waves, Jet Streams, and the Polar Front • Polar-front jet stream • Westerly; associated with Rossby waves • Subtropical jet stream • Westerly; above subtropical high-pressure cells • Tropical easterly jet stream • Runs east to west • Summer only • Develops in Asia Jet streams: high-speed air flow in narrow bands within the upper-air westerlies and along certain other global latitude zones at high levels

  46. Jet Streams Polar Jet Stream Subtropical Jet Stream Flow of warm tropical air aloft toward poles Strongest in winter Upper Air Winds

  47. Poleward Flow of Energy

  48. Horizontal undulations in the flow of the upper air winds of the middle and upper latitudes Troughs of low pressure from cold polar air Ridges of high pressure from warm tropical air Net poleward flow of energy Rossby Waves

  49. Winds Aloft Rossby Waves, Jet Streams, and the Polar Front • Rossby waves: • Arise in the polar front • Contribute to variable weather in mid-latitudes Rossby waves: horizontal undulations in the flow path of the upper-westerlies; upper-air waves Polar front: zone where cold polar air meets warm tropical air

  50. El Niño-Southern Oscillation • Warming of waters in the eastern equatorial Pacific • Associated with numerous changes in weather patterns worldwide • Typically occurs on time scales of 3 to 7 years for about 18 months Figure 5-37

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