Chapter 7: Chapter 7: Atmospheric Atmospheric CirculationsCirculations

By the end of this chapter you should:By the end of this chapter you should:

Understand the scales of motionUnderstand the scales of motion

Be able to give an example of an eddyBe able to give an example of an eddy

Have an understanding of the general circulationHave an understanding of the general circulationof the atmosphereof the atmosphere

Scales of Atmospheric Scales of Atmospheric MotionsMotions

Scales of motionScales of motion – different sizes of circulation – different sizes of circulation MicroscaleMicroscale – smallest scale of motion. 2 meters or – smallest scale of motion. 2 meters or less and last seconds to minutesless and last seconds to minutes MesoscaleMesoscale – circulation around a city (20 km). Can – circulation around a city (20 km). Can last minutes to hourslast minutes to hours Synoptic scaleSynoptic scale – Circulations around highs and lows – Circulations around highs and lows (2,000 km). Can last many days(2,000 km). Can last many days

Planetary scalePlanetary scale – Circulations around the entire – Circulations around the entire Earth.Earth.

Scales of Atmospheric Scales of Atmospheric MotionsMotions

Scales of Atmospheric Scales of Atmospheric MotionsMotions

Eddies - Big and SmallEddies - Big and Small EddyEddy – a circulation formed downwind from an object – a circulation formed downwind from an object

(examples?)(examples?) RotorRotor – rotation formed downwind from a mountain wave – rotation formed downwind from a mountain wave Wind shearWind shear – –

change of windchange of wind

speed orspeed or

direction withdirection with

heightheight

Thermal CirculationsThermal Circulations Thermal circulationsThermal circulations

Circulations broughtCirculations brought

on by changes in airon by changes in air

Temperature.Temperature.

Sea and Land BreezesSea and Land Breezes Types of thermal circulationsTypes of thermal circulations

Sea breezeSea breeze (scale of motion?) (scale of motion?) Land breezeLand breeze

Sea breeze front – leadingSea breeze front – leading

edge of the sea breezeedge of the sea breeze Florida sea breezes can Florida sea breezes can have huge walls of clouds. have huge walls of clouds. Make for great gliding due Make for great gliding due to vertical movementto vertical movement

Sea and Land BreezesSea and Land Breezes

Seasonally Changing Seasonally Changing Winds - the MonsoonWinds - the Monsoon

Monsoon wind system – “monsoon” means seasonalMonsoon wind system – “monsoon” means seasonal Asian monsoon – in winter the land is much colder than Asian monsoon – in winter the land is much colder than

ocean. In summer, opposite is trueocean. In summer, opposite is true

Seasonally Changing Seasonally Changing Winds - the MonsoonWinds - the Monsoon

Mountain and Valley Mountain and Valley BreezesBreezes

Valley breeze – explain thisValley breeze – explain this Mountain breeze – explain thisMountain breeze – explain this

Katabatic WindsKatabatic Winds Katabatic windsKatabatic winds – in general, any wind that flows – in general, any wind that flows

downhilldownhill Perfect scenario is elevated plateau with mountains Perfect scenario is elevated plateau with mountains

aroundaround BoraBora – katabatic – katabatic

wind in the Adriaticwind in the Adriatic

SeaSea

Chinook (Foehn) WindsChinook (Foehn) Winds Chinook windsChinook winds – fierce wind that flows on the eastern – fierce wind that flows on the eastern

slope of the Rockies.slope of the Rockies. Compressional heatingCompressional heating – main source of the heat – main source of the heat

from the Chinook. Air descends very rapidly and from the Chinook. Air descends very rapidly and warmswarms

Chinook wall cloud – a bank of clouds forming over the Chinook wall cloud – a bank of clouds forming over the mountainsmountains

Santa Ana WindsSanta Ana Winds Santa Ana windSanta Ana wind

Warm, dry wind thatWarm, dry wind that

originates from theoriginates from the

Desert. Usually needsDesert. Usually needs

a high in the Greata high in the Great

BasinBasin Compressional heatingCompressional heating Fans huge wildfiresFans huge wildfires

Desert WindsDesert Winds Dust stormsDust storms – formed by surface winds that form from – formed by surface winds that form from

surface heating. surface heating. Wind picks up dustWind picks up dust Dust devilsDust devils – Formed from combination of surface – Formed from combination of surface

heating, unstable atmosphere, and an obstacle in the heating, unstable atmosphere, and an obstacle in the wayway

General Circulation of General Circulation of the Atmospherethe Atmosphere

General circulation only represents the average General circulation only represents the average circulationcirculation Why are the prevailing winds in Hawaii northeasterly Why are the prevailing winds in Hawaii northeasterly

and in san Jose northwesterly?and in san Jose northwesterly?

Cause: Cause: unequal heating of the earth’s surfaceunequal heating of the earth’s surface

Effect: Effect: atmospheric heat transportatmospheric heat transport

Single-cell ModelSingle-cell Model Basic assumptionsBasic assumptions

Covered with waterCovered with water Sun is always over equatorSun is always over equator Earth does not rotateEarth does not rotate

Hadley cellHadley cell

Heat at the equatorHeat at the equatorcreates a low thatcreates a low thatbrings heat to thebrings heat to thepoles aloftpoles aloft

Single-cell ModelSingle-cell Model Why is the single-cell model wrong?Why is the single-cell model wrong?

Earth has the Coriolis force. In this model, all winds Earth has the Coriolis force. In this model, all winds would be easterly. We know that is wrongwould be easterly. We know that is wrong

So let’s spinSo let’s spin

the planet.the planet.

Three-cell ModelThree-cell Model Model for a rotating earth - keep first two assumptionsModel for a rotating earth - keep first two assumptions Hadley cell is still apparentHadley cell is still apparent DoldrumsDoldrums – equatorial region where winds are – equatorial region where winds are light. Warm air rises, condenses and moves light. Warm air rises, condenses and moves laterally towards the poleslaterally towards the poles Subtropical highsSubtropical highs – convergence of air aloft – convergence of air aloft creates highs at 30 degrees. Major deserts and creates highs at 30 degrees. Major deserts and horse latitudes.horse latitudes. Trade windsTrade winds – some of the surface air from the – some of the surface air from the Subtropical highs move back to the equator and Subtropical highs move back to the equator and deflect due to the Coriolis force.deflect due to the Coriolis force. Intertropical convergence zoneIntertropical convergence zone – converge of the – converge of the trade winds from both hemisphere to get rising air trade winds from both hemisphere to get rising air and thunderstormsand thunderstorms

Three-cell ModelThree-cell Model WesterliesWesterlies – some of the air from the Subtropical highs – some of the air from the Subtropical highs

move towards the poles and deflect towards the eastmove towards the poles and deflect towards the east

Polar frontPolar front – the boundary between the mild air towards – the boundary between the mild air towards the equator and the cold air near the pole (Subpolar lows)the equator and the cold air near the pole (Subpolar lows)

Polar easterliesPolar easterlies – air behind the polar front that is – air behind the polar front that is deflected to the west by the Coriolis forcedeflected to the west by the Coriolis force

Fig. 7-21, p. 185

Average Surface Winds Average Surface Winds and Pressure: The Real and Pressure: The Real

WorldWorld Semipermanent highs and lowsSemipermanent highs and lows – two areas of highs and – two areas of highs and

lows per hemispherelows per hemisphere Bermuda high & Pacific highBermuda high & Pacific high Icelandic low & Aleutian lowIcelandic low & Aleutian low Siberian highSiberian high

Fig. 7-22a, p. 188

Fig. 7-22b, p. 189

The General Circulation The General Circulation and Precipitation and Precipitation

PatternsPatterns Where would you expect regions of high and low Where would you expect regions of high and low

precipitation?precipitation?

Heavy precipitationHeavy precipitation Regions close to ITCZ, Regions close to ITCZ,

polar fronts, subpolar polar fronts, subpolar lowslows

Westerly Winds and the Westerly Winds and the Jet StreamJet Stream

Jet streamsJet streams – regions of very fast-moving air in the upper – regions of very fast-moving air in the upper atmosphere due to high pressure at the equator and low atmosphere due to high pressure at the equator and low pressure near the polespressure near the poles

Winds and UpwellingWinds and Upwelling UpwellingUpwelling – rising of cold water from below – rising of cold water from below Wind flow parallel to the coastline is deflected to the right Wind flow parallel to the coastline is deflected to the right

by what force? Cold water replaces thisby what force? Cold water replaces this

El Niño and the Southern El Niño and the Southern OscillationOscillation

El NiñoEl Niño – a natural event that occurs around Christmas. – a natural event that occurs around Christmas. Upwelling weakens along the west coast of South Upwelling weakens along the west coast of South America. Major events can cause great hardshipAmerica. Major events can cause great hardship

Southern OscillationSouthern Oscillation – seasaw pattern of high and low – seasaw pattern of high and low pressures switching on opposite sides of the Pacific pressures switching on opposite sides of the Pacific OceanOcean

La NiñaLa Niña – cold water episodes opposite El Niño – cold water episodes opposite El Niño

El Niño and the Southern El Niño and the Southern OscillationOscillation

Other Atmosphere-Ocean Other Atmosphere-Ocean InteractionsInteractions

North Atlantic OscillationNorth Atlantic Oscillation Arctic OscillationArctic Oscillation Pacific Decadal OscillationPacific Decadal Oscillation