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Congratulation to Al Gore Congratulation to Al Gore and IPCC for winning and IPCC for winning

2007 Nobel Peace Prize2007 Nobel Peace Prize

An Inconvenient Truth

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Chapter 7: Atmospheric Chapter 7: Atmospheric Circulation and Climate Circulation and Climate

Objectives:Objectives:• Driving forceDriving force• Meridional circulation cellsMeridional circulation cells

–Hadley, Ferrel & Polar cells• Surface winds & sea-level pressureSurface winds & sea-level pressure• wind, precipitation and temperature wind, precipitation and temperature

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• 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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Primary High-Pressure and Primary High-Pressure and Low-Pressure AreasLow-Pressure Areas

• Equatorial low-pressure trough• Subtropical high-pressure cells• Subpolar low-pressure cells• Polar high-pressure cells

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Driving forceDriving force

• At higher lat., solar energy flux spreads over a wider area => less solar energy per unit area.

• Earth emits outgoing radiation.

•Net radiation = (incoming solar rad.) - (outgoing rad.) .

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• 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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Single Convection cell in a Single Convection cell in a non-rotating Earth 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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Farrell CellFarrell Cellpolar Cellpolar Cell

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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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Polar cell and Farrell cellPolar 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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A idealized pattern of surface wind without A idealized pattern of surface wind without rotation of the earthrotation of the earth

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The pattern of surface wind with the rotation of EarthThe pattern of surface wind with the rotation of Earth

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• 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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• 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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– 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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A) Idealized winds generated by pressure gradient A) Idealized winds generated by pressure gradient and Coriolis Force. B) Actual wind patterns and Coriolis Force. B) Actual wind patterns

owing to land mass distribution.. owing to land mass distribution..

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• 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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Surface winds & sea-level pressure Surface winds & sea-level pressure (SLP)(SLP)

January

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Surface winds & sea-level pressure Surface winds & sea-level pressure (SLP)(SLP)

July

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• 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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Meridional cells & precip.Meridional cells & precip.

• Northward shift of cells during summer, & southward shift during winter => precip. changes.

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Shifts in the ITCZ affect the Sahel

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• 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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The Walker The Walker circulation is caused circulation is caused by the by the pressure gradient forcepressure gradient force that results from a that results from a high pressure systemhigh pressure system over the eastern pacific over the eastern pacific ocean, and a ocean, and a low pressure systemlow pressure system over over IndonesiaIndonesia. When . When the Walker circulation the Walker circulation weakens or reverses, weakens or reverses,

an an El NiñoEl Niño results. results.

The Southern Oscillation Index (SOI) is calculated from the The Southern Oscillation Index (SOI) is calculated from the monthly or seasonal fluctuations in the air pressure difference monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin.between Tahiti and Darwin.

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• 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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• 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

The Polar Front and Jet Streams

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Jet streamsJet streams• 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.

Jet stream turning south=> cold air moves south.

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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.

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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.

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2 mechanisms for jet streams

1) 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).

2211 rmvrmv

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Sea breezeSea breeze

• Daytime: land warms more than sea => rising air & low p on land. Air flows from sea to land.

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Land breezeLand breeze

• Night: Land cools more than sea. => Sinking air & high p over land. Air flows from land to sea.

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Valley breeze & mountain breezeValley breeze & mountain breeze

• 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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Chinook windChinook wind

• 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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• 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

Monsoons

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MonsoonsMonsoons

• 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.

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Winter monsoon

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Summermonsoon

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Temperature & Temperature & Precipitation Precipitation

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Mean air temp. at sea level Mean air temp. at sea level (Jan.)(Jan.)

• Greater T range over land than over oc.• Oc. currents affect land T (e.g. Gulf Stream warms western

Europe).

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Mean air temp. at sea level (July)Mean air temp. at sea level (July)

• 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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Annual T rangeAnnual T range

• Largest T range over land.

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Temp. recordsTemp. 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.

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Principal Controls on Temperature   Principal Controls on Temperature   

1. Latitude2. Altitude3. Atmospheric Circulation4. Land-Water Contrasts5. Ocean Currents6. Local Effects

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Marine v. Marine v. Continental Continental

ClimatesClimates

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Principal Controls on Temperature   Principal Controls on Temperature   

1. Latitude2. Altitude3. Atmospheric Circulation 4. Land-Water Contrasts5. Ocean Currents6. Local Effects

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Ocean Ocean

CirculationCirculation

• 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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Hydrological cycleHydrological cycle

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Mean annual precip.Mean annual precip.

•Driest regions near 30° and poles: high p, subsiding air.