1. Congratulation to Al Gore and IPCC for winning
2007 Nobel Peace Prize
An Inconvenient Truth
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2. Chapter 7: Atmospheric Circulation and Climate
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Chapter 7: Atmospheric Circulation and Climate
Objectives:
Driving force
Meridional circulation cells
Hadley, Ferrel & Polar cells
Surface winds & sea-level pressure
wind, precipitation and temperature
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3. Atmospheric circulation is the large-scale movement of air.
The large-scale structure of the atmospheric circulation varies from year to year, but the basic structure remains fairly constant.
Individual weather systems – mid-latitude weather may occur "randomly“. However, the average of these systems - the climate - is quite stable.
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4. Primary High-Pressure and Low-Pressure Areas
Equatorial low-pressure trough
Subtropical high-pressure cells
Subpolar low-pressure cells
Polar high-pressure cells
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5. Driving force Net radiation = (incoming solar rad.)
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Driving force
At higher lat., solar energy flux spreads over a wider area => less solar energy per unit area.
Earth emits outgoing radiation.
[Ahrens 2003, Fig.3.6]
Net radiation = (incoming solar rad.)
- (outgoing rad.) .
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6. Net radiation has deficit poleward of 37°, & surplus equatorward of 37°.
This means poles should keep cooling while tropics keep warming. Since this is not happening, other processes must operate to maintain net energy balance at each lat.
Atm. & oc. circulation (climate & weather) due to unequal lat. distr. of energy.
Annual incoming solar rad.
Annual outgoing terrestrial rad.
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7. Single Convection cell in a non-rotating Earth
Imagine the earth as a non-rotating sphere with uniform smooth surface characteristics.
the sun heats the equatorial regions much more than the polar regions. In response to this, two huge convection cells develop.
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8. polar Cell Farrell Cell
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9. Hardly Cell
Atmosphere is heated in the equator => Air becomes less dense and rises => Rising air creates low pressure at the equator.
Air cools as it rises because of the lapse rate =>
Water vapor condenses (rains) as the air cools with increasing altitude => Creates high rainfall associated with the Intertropical Convergence Zone in the tropics (ITCZ).
As air mass cools it increases in
density and descends back to the
surface in the subtropics (30o N
and S), creating high pressure.
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10. Polar cell and Farrell cell
In the pole area, the surface is much cold, especially in winter. This results in increased air density near the surface => higher pressure. The higher density and pressure lead to divergence => surface air moves towards tropic. The cold air from pole will meet the warm air from Tropic around to form “Pole Front Zone.
For mass conservation, there are aloft circulations corresponding the surface circulations, which forms two cells, called Pole cell and Farrell cell.
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11. A idealized pattern of surface wind without rotation of the earth
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12. The pattern of surface wind with the rotation of Earth
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13. The sfc. winds converg. at the ITCZ (Intertropical Convergence Zone).
Sfc. winds converg. towards Eq., deflected by Coriolis => Easterlies (NE & SE Trades)
The sfc. winds converg. at the ITCZ (Intertropical Convergence Zone).
Rising air => clouds.
Rising air at ITCZ spreads poleward, sinking at 30° (high p belts = subtropical highs).
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14. High p at poles => sfc.air flows equatorward;
Deflected by Coriolis => Polar Easterlies converg. to the Subpolar Low (low p at 60°)
Coriolis deflects air flowing from subtropical high to subpolar low => Westerlies
Polar cell 60°-90°, Ferrel cell 30°-60°
Where mild air from Ferrel cell meets cold air from polar cell => polar front
Hadley & polar cells are thermally driven, but Ferrel cell is a thermally indirect cell.
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15. The three-celled model vs. reality: the bottom line
Hadley cells are close approximations of real world
Ferrel and polar cells do not approximate the real world
Model is unrepresentative of flow aloft
Continents and topographic irregularities cause model oversimplification
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16. A) Idealized winds generated by pressure gradient and Coriolis Force
A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns owing to land mass distribution..
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17. Horse Latitudes Around 30°N we see a region of subsiding (sinking) air
Horse Latitudes Around 30°N we see a region of subsiding (sinking) air.  Sinking air is typically dry and free of substantial precipitation. Many of the major desert regions of the northern hemisphere are found near 30° latitude.  E.g., Sahara, Middle East, SW United States.
Doldrums Located near the equator, the doldrums are where the trade winds meet and where the pressure gradient decreases creating very little winds.  That's why sailors find it difficult to cross the equator and why weather systems in the one hemisphere rarely cross into the other hemisphere.  The doldrums are also called the intertropical convergence zone (ITCZ).
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18. Surface winds & sea-level pressure (SLP)
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Surface winds & sea-level pressure (SLP)
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[Tarbuck & Lutgens, 2003, Fig.17.13]
January
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19. Surface winds & sea-level pressure (SLP)
July
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20. July: Lows over continents in N.Hem., highs over contin. in S. Hem.
N.Hem.: Bermuda (Azores) High, Pacific High, Icelandic Low, Aleutian Low.
July: Bermuda High & Pacific High stronger & further north. Icelandic Low & Aleutian Low weaken & shift northward.
ITCZ shifts northward
Jan.: Highs over continents in N.Hem., lows over contin. in S. Hem. (monsoon effect).
July: Lows over continents in N.Hem., highs over contin. in S. Hem.
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21. Meridional cells & precip.
Northward shift of cells during summer, & southward shift during winter => precip. changes.
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22. Shifts in the ITCZ affect the Sahel
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23. Hadley, Ferrell, and Polar cells are major players in global heat transport of the south-north. They are called Latitudinal circulation, caused by latitudinal difference of incident solar radiation.
Longitudinal circulation, on the other hand, comes about because water has a higher specific heat capacity than land and thereby absorbs and releases heat less readily than land.
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24. The Walker circulation is caused by the pressure gradient force that results from a high pressure system over the eastern pacific ocean, and a low pressure system over Indonesia. When the Walker circulation weakens or reverses, an El Niño results.
The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.
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26. The Walker circulation, which spans almost half the circumference of Earth, pushes the Pacific Ocean’s trade winds from east to west, generates massive rains near Indonesia, and nourishes marine life across the equatorial Pacific and off the South American coast. Changes in the circulation, which varies in tandem with El Niño and La Niña events, can have far-reaching effects.
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27. The Polar Front and Jet Streams
Gradual change in temperature with latitude does not always occur
Steep temperature gradients exist between cold and warm air masses
polar front - marks area of contact, steep pressure gradient  polar jet stream
polar jet stream - fast stream of air in upper troposphere above the polar front
stronger in winter, affect daily weather patterns
Low latitudes  subtropical jet stream
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28. Jet streams Jet stream turning south => cold air moves south.
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Jet streams
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Jet streams: air currents thousands of km long, hundreds of km wide, a few km thick (centred near tropopause).
Max speed > 200 km/hr.
Polar jet stream near polar front, separating cold air from mild air.
[Ahrens 2003, Fig.11.9]
Jet stream turning south
=> cold air moves south.
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29. EOSC 112
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So there is sinking air around 30 degree, which forms divergence region on surface and convergence region aloft. The convergence between cold air with warm air can cause a great temperature gradient in this region, and further causing a large gradient in pressure => speeding the air flow => cause the jet.
[Ahrens 2003, Fig.11.10]
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30. Subtropical jet stream at ~30°
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Subtropical jet stream at ~30°
Jet streams meander, polar jet may merge with subtropical jet.
Polar jet may also branch into 2.
[Ahrens 2003, Fig.11.10]
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31. 2 mechanisms for jet streams
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2 mechanisms for jet streams
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Where polar cell meets Ferrel cell, or Ferrel cell meets Hadley, airs of different T meet
=> large T gradient => large p gradient
=> geostrophic winds.
2) As air moves from low to
high lat., its circular orbit
shrinks. => orbiting speed incr.
(conservation of angular
momentum; e.g. spinning
skater moves arms
towards body).
[Ahrens 2003, Fig.11.14]
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32. Sea breeze Daytime: land warms more than sea
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Sea breeze
[Ahrens 2003, Fig.10.21]
Daytime: land warms more than sea
=> rising air & low p on land. Air flows from sea to land.
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33. Land breeze Night: Land cools more than sea.
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Land breeze
[Ahrens 2003, Fig.10.21]
Night: Land cools more than sea.
=> Sinking air & high p over land. Air flows from land to sea.
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34. Valley breeze & mountain breeze
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Valley breeze & mountain breeze
[Tarbuck & Lutgens 2003, Fig.17.15]
Daytime: At same elevation, air on mountain slope heated more than air over valley => low p over mountain slope => air flows upslope from valley (valley breeze).
Night: Air on mountain slope cooled more than air over valley => mountain breeze.
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35. Chinook wind Chinook: warm, dry wind on eastern slope of Rockies.
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Chinook wind
[Ahrens, 2003, Fig.10.30]
Chinook: warm, dry wind on eastern slope of Rockies.
Western slope: condensation => release of latent heat. Moisture lost from precip.
Descending wind on eastern slope => warming from compression.
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36. Monsoons largest synoptic scale winds on Earth
A seasonal reversal of wind
Asian monsoon which is characterized by dry (wet), offshore (onshore) flow conditions during cool (warm) months
Orographic lifting leads to high precipitation
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37. Monsoons Winter: continents cool more than oc.
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Monsoons
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Winter: continents cool more than oc.
=> sinking air & high p over continent
Summer: continents warm more than oc.
=> rising air & low p over continent
Most prominent with the massive Asian land mass.
[Ahrens 2003, Fig.10.25]
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38. Winter
monsoon
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39. Summer
monsoon
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40. Temperature & Precipitation
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Temperature & Precipitation
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41. Mean air temp. at sea level (Jan.)
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Mean air temp. at sea level (Jan.)
[Tarbuck & Lutgens2003, Fig.15.26]
Greater T range over land than over oc.
Oc. currents affect land T (e.g. Gulf Stream warms western Europe).
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42. Mean air temp. at sea level (July)
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Mean air temp. at sea level (July)
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[Tarbuck & Lutgens2003, Fig.15.27]
Cool Californian Current => cools adjacent land
Hottest regions at ~20°-30° (not at Eq.)
High p, subsiding air, clear sky, low humidity => hot deserts.
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43. Annual T range Largest T range over land. EOSC 112 4/16/2017
[KKC 1999, Fig.4-1C]
Largest T range over land.
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44. Temp. records High T records: Low T record:
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Temp. records
High T records:
World: El Azizia, Libya (32°N) 58°C, in 1922
Western Hem: Death Valley, CA(36°N) 57°C
Canada: Midale, Sas.(49°N) 45°C
Low T record:
World: Vostok, Antarc. (78°S) -89°C, 1983
N.Hem.: Verkhoyansk, Russia (67°N) -68°C
N.America: Snag, Yukon (62°N) -63°C.
Antarctica very cold: snow cover => high albedo; large land mass. Colder than Arctic.
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45. Principal Controls on Temperature
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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46. Marine v. Continental Climates
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47. Principal Controls on Temperature
Latitude
Altitude
Atmospheric Circulation
Land-Water Contrasts
Ocean Currents
Local Effects
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48. Ocean Circulation
Primary driving force – wind (generated by pressure differences)
Links atmosphere and ocean together
East coast of continents  northward moving currents
Transfer energy poleward
West coast of continents  southerly currents
Energy transferred to atmosphere overlying oceans
affects coastal areas
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49. Hydrological cycle
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50. EOSC 112
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Mean annual precip.
[Ahrens 2003, Fig.A-18]
Driest regions near 30° and poles: high p, subsiding air.
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