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Thegeneralcirculationoftheatmosphere

SectionI:Overview&tools

Historyandoverviewofthegeneralcirculation

Generalinformation

• Lecturer• MartinSingh[Rm213,9RainforestWalk]

• Lecturetime• Thurs1pm-3pm[CoE Boardroom,9RainforestWalk]

Courseaims• Characterisethelarge-scalecirculationswithinEarth’satmosphereandtheirassociatedenergy,momentumandwaterbudgets

• Developatheoreticalframeworkandasetofmathematicaltoolstoanalyseandunderstandthephysicalanddynamicalprocessesthatmaintainthelarge-scalecirculation

• Engagewiththescientificliteratureunderpinningourunderstandingofthegeneralcirculationandhowitmaychangeinresponsetochangesinclimate

Structureofthecourse1. Overviewandtools

1. Overviewofthegeneralcirculation2. Governingequationsandtheirdecompositionintoeddiesandmean3. Stateestimation:radiosonde-based&dataassimilation

2. Theangular-momentumbudget1. Theatmosphericangularmomentumbudget2. Maintenanceofabarotropicjet3. Radiative-convectiveequilibrium&Hide’stheorem4. AxisymmetrictheoriesoftheHadleycirculation5. Eddies,theHadleycellandmonsoons

3. Forcingofthezonal-meancirculation1. ThetransformedEulerianmean2. Thecirculationonisentropes

4. Theenergybudget1. Theatmosphericenergybudget2. Energytransportandtropicalprecipitation

5. Thewatercycle1. Water-vapourbudget2. Thewatercycleunderclimatechange

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References

• Thereisnospecifiedtextforthiscourse• Iborrowheavilyformthefollowingsources

• Held,I.M.,Thegeneralcirculationoftheatmosphere,Proc.GeophysicalFluidDynamicsProgram,2000,1-54.Available:https://www.whoi.edu/fileserver.do?id=21464&pt=10&p=17332

• Vallis,G.K.,Atmosphericandoceanicfluiddynamics:fundamentalsandlarge-scalecirculation,CambridgeUniversityPress,2006,745p.

• Peixoto,J.P.&Oort,A.H.,PhysicsofClimate,J.Climate,AIPPress,1992,520p.

• Stone,P.,Generalcirculationoftheatmospherelectures,course12.812,Mass.Inst.Tech.,2005.Available:https://ocw.mit.edu/courses/earth-atmospheric-and-planetary-sciences/12-812-general-circulation-of-the-earths-atmosphere-fall-2005/

• Iwillprovideadditionalreferencesthroughoutthecourse

Assessment

• Honoursstudents:

• 2Assignments(15%each)

• Report&oralpresentationonapeer-reviewedpaper(20%)

• Writtenexam(50%)

• PhDstudents(MonashDoctoralProgram)

• Report&oralpresentationonapeer-reviewedpaper

• Forthoseauditingtheclass

• Presentapapertotheclassinthefinalweek

Paperreport

• Pickapaperfromthelistontheweb(orofyourownchoosing)andwriteasummary

• 1500wordreport

• 10-minuteoralpresentationtotheclass

Introductiontothegeneralcirculation

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Introductiontothegeneralcirculation

• Ahistoricalperspective

• Theobservedgeneralcirculation

• Thermalstructureoftheatmosphere• Meancirculation• Eddies• Precipitationandthehydrologicalcycle

Thegeneralcirculation:ahistoricalperspective

“Duringthepastthreecenturies,theprevailingideasaboutthegeneralcirculationoftheearth'satmospherehaveevolvedinastepwisemanner.Earlyineachstep,anewtheoreticalideaisformulated.Lateineachstep,theideagainsgeneralacceptance,but,moreorlessconcurrently,newobservationsshowthattheideaiswrong.”

- Lorenz(1983),Bull.Amer.Met.Soc.,64,730-734.

Hadley(1735):Singlecell

• Coriolisforceonmeridionalmotionsinducessurfaceeasterliesandwesterlies

• Angular-momentumbalancebetweenthesurfaceandatmosphere

• Butatmidlatitudes meansurfacewindshavepolewardcomponent!

Lorenz(1983),Bull.Amer.Met.Soc.,64,730-734.

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Thompson(1857)&Ferrel(1859):Asecondcell

• Coriolisforceonzonalmotions

• Maximuminpressureatboundarybetweensurfaceeasterlies&westerlies

• Shallow,thermallyindirectcellatmidlatitudes

• Butupperlevelwindsatmidlatitudes driftequatorwards!

Lorenz(1983),Bull.Amer.Met.Soc.,64,730-734.

Bigelow(1902),Defant (1921)&Jefferys (1926):Theimportanceofeddies

• Bigelow(1902)realisedthatheattransportcouldbeeffectedbyzonallyasymmetricmotions

• Jefferys (1926)appliedtheseideastoangular-momentumtransports

• Keyconceptualadvance:

Thezonal-meancirculationmaynotbeasolutiontothezonally-symmetricequationsofmotion.

Sourceoftheeddies:Baroclinicinstability

• Bjerknes (1919):“thekineticenergy[ofthecyclones,is]furnishedbythepotentialenergyofthesystemofwarmandcoldairlyingbesideeachother.”

• Charney (1947)&Eady (1949):Theoryofbaroclinicinstability

• Theoryofavailablepotentialenergy(Lorenz,1955)

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Themeridionaloverturningcirculation(1980-2001ERA40)The general circulation: 1980-2001 (ERA40)

Latitude

Sigm

a

100

−60 −30 0 30 60

0.2

0.8

Units:109 kg/s

“Perhapsneartheendofthe20thcenturyweshallsuddenlydiscoverthatwearebeginningthefifthstep.”

- Lorenz(1983),Bull.Amer.Met.Soc.,64,730-734.

Theobservedgeneralcirculation

Solarinsolationforcing

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Question

• Atwhattimeofyearisthemaximumglobally-averagedsolarinsolation?

Question

• Atwhattimeofyearisthemaximumglobally-averagedsolarinsolation?

4th ofJan

PerihelionoccursinAustralsummer

• AtthecurrenttimeinEarth’shistory,Earth’sclosestapproachoccursinoursummer.

• Inthemid-Holocene(6ka)PerihelionoccurredintheBorealsummer

• Thisisthoughttoaccountforthe“GreenSahara”

Question

• Whereisthemaximumdaily-meansolarinsolationatthesolstice?

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Question

• Whereisthemaximumdaily-meansolarinsolationatthesolstice?

Thepole!

Daily-meanSolarinsolationmaximisesatthepoles!

2.7 Distribution of Insolation 31

90N

60N

30N

1 EQ

2 30s

60s

JAN FEE MAR APR MAY JUh' JUL AUG SEP OCT NOV DEC

Fig. 2.6 Contour graph of the daily average insolation at the top of the atmosphere as a function of season and latitude. The contour interval is 50 W m-*. The heavy dashed line indicates the latitude of the subsolar point at noon.

-YO -60 -30 0 30 60 90 Latitude

Fig. 2.7 Annual-mean and solstice insolation as functions of latitude.

AtmosphericcirculationisdrivenbyunevendistributionofsolarradiationincidentontheEarth’s

surface2.7 Distribution of Insolation 31

90N

60N

30N

1 EQ

2 30s

60s

JAN FEE MAR APR MAY JUh' JUL AUG SEP OCT NOV DEC

Fig. 2.6 Contour graph of the daily average insolation at the top of the atmosphere as a function of season and latitude. The contour interval is 50 W m-*. The heavy dashed line indicates the latitude of the subsolar point at noon.

-YO -60 -30 0 30 60 90 Latitude

Fig. 2.7 Annual-mean and solstice insolation as functions of latitude.

“Topofatmosphere”insolation

Thermalstructureoftheatmosphere

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Maximumzonal-meantemperaturestaysneartheequator

-50 0 50latitude (deg)

-60

-40

-20

0

20

40

surfa

ce a

ir te

mp.

(deg

C)

JanJul

(CRUTEM41961– 1990)

Meansurfaceairtemperature(CRUTEM41961– 1990)

Seasonalrangeofsurfaceairtemperature

(CRUTEM41961– 1990)

Whatdeterminesthestrengthoftheseasonalcycleoftemperature?

Zonal-meantemperature(K)

(ERA40 reanalysis data 1980-2001)

Zonal and time mean temperature (K) (annual average)

Latitude

Sigm

a290

250 260

200

220

−60 −30 0 30 60

0.2

0.8

courtesyofPaulO’Gorman

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Zonal-meanpotentialtemperature(K)

courtesyofPaulO’Gorman

(ERA40 reanalysis data 1980-2001)

Potential temperature (K)

Latitude

Sigm

a

300

340

270270

−60 −30 0 30 60

0.2

0.8

Zonal-meanequivalentpotentialtemperature(K)

courtesyofPaulO’Gorman

(ERA40 reanalysis data 1980-2001)

Equivalent potential temperature (K)

Latitude

Sigm

a

340

340

280

310

270

−60 −30 0 30 60

0.2

0.8

Zonal-meansaturationequivalentpotentialtemperature(K)

courtesyofPaulO’Gorman

(ERA40 reanalysis data 1980-2001)

Saturation equivalent potential temperature (K)

Latitude

Sigm

a

350

350

340

290

280

280

300

−60 −30 0 30 60

0.2

0.8

Whyisthetropicalatmosphereclosetoconstantsaturationequivalentpotentialtemperature? (ERA40 reanalysis data 1980-2001)

Latitude

Sigm

a

350

350

340

290

280

280

300

−60 −30 0 30 60

0.2

0.8

LatitudeSigm

a

340

340

280

310

270

−60 −30 0 30 60

0.2

0.8

Latitude

Sigm

a

300

340

270270

−60 −30 0 30 60

0.2

0.8

Latitude

Sigm

a

290

250 260

200

220

−60 −30 0 30 60

0.2

0.8

Saturation equivalent potential temperature (K)Equivalent potential temperature (K)

Potential temperature (K)Temperature (K)

courtesyofPaulO’Gorman

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Themeancirculation

Zonal-meanzonalwind(colours)andzonal-meantemperature(redcontours)

(NCEPreanalysis1979-2016)

Upper-levelwinds

Jul

Jan

Zonal-meansurfacewinds

• Upper-levelwindsconsistentwiththermalwindbalance

• Butthisdoesnotdeterminesurfacewinds

• Whatdrivesthesurfacewindpattern?

ANRV273-EA34-21 ARI 17 April 2006 23:56

1. INTRODUCTIONIn the mean, zonal surface winds on Earth are easterly (westward) in low latitudes,westerly (eastward) in midlatitudes, and easterly or nearly vanishing in high latitudes.The strength of the mean zonal surface wind varies seasonally, but the pattern ofalternating easterlies and westerlies is present throughout the year, with slight seasonalshifts of the latitudes at which the mean zonal surface wind changes sign (Figure 1bshows January as an example). The mean meridional surface wind is weaker than themean zonal surface wind. It is directed poleward in regions of surface westerlies andequatorward in regions of surface easterlies. In boreal summer, the monsoons of theNorthern Hemisphere lead to a mean northward surface wind across the equator,which typically has a westerly component in monsoon regions.

That mean surface winds have definite directions has been exploited in centuriespast by navigators, who called winds with a prevalent direction trade winds, a term wenow use more restrictively to denote the tropical easterly winds. “The Causes of the

Sig

ma

300

350

270

280

32 440.2

0.8

40

Latitude

b

0˚ 50˚

0

8

Eas

twar

d s

tres

s (P

a)

Latitude

d

0˚ 50˚

0

0.2

Hadley cellsFerrel cells

Easterlies

WesterliesWesterliesEddies

Mean

a c

Figure 1Temporal and zonal mean circulation statistics for January according to reanalysis data for theyears 1980–2001 (Kallberg et al. 2004). (a) Zonal wind (magenta) and potential temperature(light blue). Contour intervals are 4 m s−1 for zonal wind and 10 K for potential temperature.The thick magenta line is the zero zonal wind contour. (b) Zonal wind at the surface.(c) Eulerian mass flux streamfunction (magenta) and angular momentum (light blue). Contourintervals are 20 × 109 kg s−1 for streamfunction and 0.1!a2 for angular momentum, withangular momentum decreasing monotonically from the equator to the poles. Negativestreamfunction values (dashed contours) correspond to clockwise rotation, positive values (solidcontours) to counterclockwise rotation. (d ) Vertically integrated momentum flux convergence(eastward stress) due to mean circulations (light blue) and due to eddies (magenta), with eddiesdefined as fluctuations about the temporal and zonal mean. The vertical coordinate in (a) and(c) is σ = p/ps (pressure p normalized by surface pressure ps).

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Meansurfacewinds

(NCEPRenalysis 1981– 2010)

Jul

Jan

instantaneousupper-troposphericwinds

Whyarethejetssosharp?

Jetsintheocean

FIG. 6. Instantaneous surface speed in 1° and 1⁄6° models after 40 yr. Note that the large-scale structure of the 1° model is quite similar to the 1⁄6° model (the currents have similarlocations and have similar horizontal extents). The main difference is in the presence of intense jets and eddies in the 1⁄6° model.

DE

CE

MB

ER

2006H

AL

LB

ER

GA

ND

GN

AN

AD

ES

IKA

N2239

Fig6

live4/C

Jetsonotherplanets

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Themeridionaloverturningcirculation

(ERA40 reanalysis 1980-2001)

Mean meridional streamfunction (1010 kg s-1)

Latitude

Sigm

a

10

2

2

−8

−4

−60 −30 0 30 60

0.2

0.8

Latitude

Sigm

a 0.5

1

−60 −30 0 30 60

0.2

0.8

Contour interval 2

Contour interval 0.5

courtesyofPaulO’Gorman

contourinterval0.5x1010 kgs-1

WhatsetsthestrengthoftheHadleycell?

WhatsetsthewidthoftheHadleycell?

Seasonaloverturningcirculation

(ERA40 reanalysis 1980-2001)

Eulerian mean meridional streamfunction (1010 kg s-1)

Latitude

Sigm

a

4

2−20−4

−60 −30 0 30 60

0.2

0.8

Latitude

Sigm

a

24

−4

−2

−60 −30 0 30 60

0.2

0.8

JJA

DJF

courtesyofPaulO’Gorman

contourinterval2x1010 kgs-1

WhydotheFerrelcellsexist?

• Whatdrivesathermallyindirectcirculation?• Whyaretherethreecellsineachhemisphereandnot1?or5?or23?Whatdoesthisdependon?

Streamfunction calculatedonisentropes

(ERA40 reanalysis 1980-2001)

Dry-isentropic mean meridional streamfunction (1010 kg s-1)

LatitudePo

tent

ial t

empe

ratu

re [K

]

10

2 −8

−2

−50 0 50

250

300

350

Red: TropopauseMagenta: 10, 50, 90 percentiles of surface potential temperature distribution

courtesyofPaulO’Gorman

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Eddies

• Eddiesaredefinedasdeviationsfromthetimeandzonalmean

• Wewillcategorisethemintotransientandstationaryeddies

Kineticenergyperunitmass(m2 s-2)

(Peixoto and Oort, fig 7.22)

Kinetic energy (m s-2)

Transient

Stationary

Mean

Total

latitude

Turbulenceintheatmosphere

© Nature Publishing Group1984

Nastrom et al, Nature, 1984: Fig. 1commercial aircraft data near the tropopause (meridional data is shifted one decade to the right)

Whatsetstheturbulencekineticenergyspectrumintheatmosphere?

Thehydrologicalcycle

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Annual-meanprecipitation

(CMAPmergedprecipitation1981-2010)

SeasonalcycleofprecipitationSource:NASA

time- andzonal-meanrelativehumidityTime and zonal mean relative humidity

ERA40, 1980-2001

Latitude

Sigm

a

0.3

0.4

0.7

0.80.8

0.7

0.8

0.1

0.1

0.5

−60 −30 0 30 60

0.2

0.8

courtesyofPaulO’Gorman

Questions

• Whyisthetropicalatmosphereclosetoconstantsaturationequivalentpotentialtemperature?• Whatdrivesthepatternofmeansurfacewinds?• Whyareatmosphericjetssosharp?• WhatsetsthestrengthandwidthoftheHadleycell?• Whatdeterminesthestrengthandpositionofthemonsoon?• WhydotheFerrelcellexist?WhydoesEarthhavethreecells?• Whatsetstherelativehumidityoftheatmosphere?

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Howwilltheatmosphericgeneralcirculationchangeunderglobal

warming?