Download Monsoons: an introduc^on

Monsoons:anintroduc.on
JosephineBrown-BureauofMeteorology,Research&Development
WithmaterialfromMa.Wheeler
Mainreference:Introduc.ontoTropicalMeteorology,Chapter3.5
Outline
1.  Theglobalmonsoonsystem&physicalmechanisms
2.  Regionalmonsoons
–  SouthAsian&EastAsian
–  WestAfrican,American
–  Australian-MariDmeConDnent
3.  TheAustralianmonsoon:amoredetailedview
4.  Variabilityofthemonsoon
–  Interannualvariability:TBO,ENSOandIOD
–  Intraseasonalvariability:bursts&breaks,MJO
5.  Monsoonsandclimatechange
1.  Theglobalmonsoonsystem
&physicalmechanisms
Earlyobserva.onsofthemonsoon
§  Theword“monsoon”comesfromtheArabicword“mausim”meaning
season.Itwasfirstusedtorefertotheseasonalreversalofwindsover
SouthernAsiaandtheIndianOcean.
§  Halley(1686)proposedthatthemonsoonwindsweredrivenby
differenDalheaDngoverlandandocean.
§  Hadley(1735)suggestedthatthemonsoonwindsweredeflectedbythe
Earth’srotaDon,leadingtotheirobserveddirecDons.
Mapofthetradewinds,EdmondHalley,1686.
Observedaustralsummercircula.on
JANUARY
Figure 3.14(a)
§  InJanuary,westerlywindsextendovernorthernAustralia.North-easttradewindsflow
fromSouthAsiatowardstheequatorialIndianOcean.
§  HeatlowsdevelopovernorthernAustralia,helpingtodriveonshoreflow.
§  TheITCZisdisplacedsouthwards.
Observedaustralwintercircula.on
JULY
Figure 3.14(b)
§  InJuly,therearesouth-westerlywindstowardsIndia,andsouth-easterlytradestowards
northernAustralia.
§  TheNHconDnentswarm,generaDnglowsurfacepressures,whichdrawthelowlevelwinds
towardsthem.
§  TheITCZisdisplacednorthwards.
Observedseasonalcycleofprecipita.on
JANUARY
JULY
§  DeepconvecDonshiasacrosstheequatortoIndonesiaandnorthern
AustraliainJanuary,andtoSouthAsiainJuly
§  SimilarseasonalshiasindeepconvecDonoccuroverAfricaandAmerica
Theglobalmonsoonsystem
TheclassicalcriteriaforamonsoonasspecifiedbyRamage(1971)are:
§  Prevailingwindshias120°betweenJanuaryandJuly
§  Averagefrequencyofprevailingwind>40%
§  Speedofmeanwindexceeds3ms-1
§  PressurepaiernssaDsfyasteadinesscriterion
Figure3.29
Theglobalmonsoonsystem
§  ThedefiniDonoftheglobalmonsoonnowincludesregionsoftheAmericas
whosesummerprecipitaDonandwindcharacterisDcsaresimilartothe
Indianmonsoon.
§  However,thoseregionsdonothaveaseasonalwindreversalsodonot
matchtheclassicalcriteriaforamonsoon.
Figure3.30
Theplanetary-scalemonsooncanbeconsideredtohavethesefundamental
mechanisms:
§  TheseasonaloscillaDonofsolarheaDngwithnetheaDnginthesummer
hemisphere,whichleadstomigraDonoftheequatorialtroughandthe
tropicalconvergencezones
§  ThedifferenDalheaDngbetweenthelandandoceanandtheresulDng
pressuregradient(Halley1686)
Figure3.31.1
Theplanetary-scalemonsooncanbeconsideredtohavethesefundamental
mechanisms:
§  ThecurvatureofthewindsduetotherotaDonoftheEarth(Hadley1735)
§  MoistureprocessesandconvecDon(Webster1987)
Figure3.31.2
PhysicalMechanisms
1.The“giantseabreeze”
Pressure
surfaces
Cool denser air
Height
P1
P2
Warm continent
P3
P4
P5
Cool ocean
Direction of the horizontal
pressure gradient force (PGF)
Resulting air circulation
§  Heatedairoverthewarm
surfaceexpandsverDcally.
Surfacesofequalpressureat
heightslopedownwardtowards
thecoolersurface,withthePGF
acceleraDngairfromthewarm
zonetothecoolatthisheight.
§  Assoonasthishappens,thereis
asmallermassofairoverthe
warmsurfaceandsurface
pressurestheredrop.NowaPGF
actsonairnearthesurfaceto
accelerateairfromthecoolzone
tothewarm.
§  TomaintainconDnuity,afull
circulaDondevelopswithascent
overthewarmsurfaceand
descentoverthecoolone.
PhysicalMechanisms
2.TheCoriolisforce
Airmovingacrosstheequatorfromhightolow
pressureisdeflectedbytheCoriolisforce.
Equator
TheCoriolisforceisanapparentforce,arising
fromtherotaDonoftheplanet,andactstothe
leaformoDonintheSouthernHemisphere,and
totherightintheNorthernHemisphere.
3.Theavailabilityofmoist
convec.onprocesses
Equator
MoistconvecDonprovideslatentheaDngofthe
atmosphere,whichfurtherpromotesthelargescale“giantseabreeze”circulaDon.
2.Regionalmonsoons
AsianSummerMonsoon
TheevoluDonoftheregionalmonsoonsdependsonthedistribuDonof
landandoceanaswellasSSTgradientsandtopography.
ThelargestregionalmonsoonsystemistheAsianmonsoon,whichis
madeupoftheIndian/SouthAsianmonsoonandtheEastAsian
monsoon.TheWesternNorthPacificmonsoonisasub-systemoftheEast
Asianmonsoon.
Figure3.34(c)
AsianSummerMonsoon
§  TheSouthAsianandEastAsianmonsoonsdifferduetodifferentland-ocean
distribuDonandtopography.
§  TheSouthAsian(Indian)monsoonisstronglyforcedbynorth-south
temperaturegradientandenhancedheaDngovertheTibetanPlateau
§  TheEastAsianmonsoonisforcedbybothmeridionalandzonaltemperature
gradients,andhenceisweaker,withaddiDonalrainfallalongsubtropical
frontalzones.
Figure3.37
AsianWinterMonsoon
§  ThewintermonsoonisstrongeroverEastAsiathanovertheIndianSubconDnent
becauseofthecontrastbetweentheverycoldAsianlandmassandthewarmNorth
PacificOcean.
§  Cross-equatorialflowtotheAustralian-Indonesiamonsoonisenhancedbythe
differencebetweenhotAustraliaandtherelaDvelycoolnorthPacific.
§  Thenorth-southcontrastisreducedovertheIndiansubconDnentbecausetheTibetan
PlateaublocksthecoldSiberianairmass.
Figure3.43
WestAfricanMonsoon
Figure3.45
§  TheWestAfricanMonsooncloselymatchestheclassicalmonsooncriteria,
withmoist,coolsouthwesterlywindsduringthesummermonsoonanddry,
warm,dustynortheasterly“Harmaian”windsduringthewinter.
§  TheIntertropicalFront(ITF)orIntertropicaldisconDnuity(ITD)isthe
boundarybetweenthemoistsouthwesterlymonsoonflowandthehot,dry
northeasterlywindfromtheSahara.
AmericanMonsoons
Figure3.49
§  TheAmericanmonsoonsdonotmatchtheclassicalcriteriaformonsoonasthereis
noopposiDonalwindshiaduringthewinter(butreversalinthewindanomalies
comparedtotheannualmean).
§  TheNorthAmericanmonsoonischaracterizedbyheaDngovertheSonoranDesert
andtheestablishmentofasurfaceheatlowintowhichmoisttropicalairisdrawn
fromtheGulfofCalifornia,GulfofMexicoandthetropicalPacific.
§  TheSouthAmericanmonsoonreceivesmoisturefromthetropicalAtlanDcand
AmazonRiverbasin.
Australian-IndonesianMonsoon
§  TheAsianandAustralian
monsoonsformaninterconnected
system
§  OullowfromtheAsianwinter
monsoon(NEandNWmonsoons)
contributestoonshoreAustralian
summermonsoonflow
§  TheSHregionalmonsoonisknown
astheAustralian-Indonesianor
Australian-Mari.meCon.nent
monsoon
§  InthenextsecDon,wefocuson
theAustraliancomponent
cold
warm
warm
cool
3.TheAustralianmonsoon:
Amoredetailedview
Lowlevel(850hPa)windandOutgoingLongwaveRadia.on(OLR)
§  TheAustralianmonsoon
peakisinJanuaryand
February.
§  MonsoonOnsetoccurs
nearDarwinaround
December.
§  Theretreatofthe
monsoonatDarwinoccurs
inMarchorApril.
§  TheMonsoonShearLine
separatesmonsoon
westerliesfromtradewindeasterlies.
§  TheITCZismarkedbythe
lineofstrongest
convecDon(lowestOLR).
SEP
OCT
NOV
DEC
JAN
FEB
MAR
APR
Lowlevel(850hPa)streamlinesandprecipita.on
§  TheITCZcorrespondstothe
regionofconverging
streamlines,strongest
precipitaDon,andlowestOLR
§  Themonsoonshearlineisat
thezoneofstrongestcyclonic
shearandusuallyco-located
withthezoneoflowestsurface
pressure(themonsoontrough),
andsouthoftheITCZ.
December-January-February mean
Australianmonsoonevolu.on
Priortomonsoononset:Heatlowsandheattroughs
AstaDonarylowcanoaenoccur
overthehotinterioroverAustralia,
e.g.thePilbaraLowandthe
CloncurryLow(overQld).
SomeDmestheyjointoforma
“heattrough”.
Example:4January2006
monsoon trough
However,heatlowsandheattroughsareonly
shallowfeatures(1-2kmdeep)sotheyallowonly
isolatedthunderstormacDvity.
Heatlowsaremostcommonjustpriortomonsoon
onset.
Pilbara low
Australianmonsoonevolu.on
Priortomonsoononset:
§  Aheatlowdevelopsoverthe
Pilbararegion,oaenresulDng
inshallowwesterlyflowover
northernAustralia,which
moistensandpre-condiDons
theenvironmentfordeep
convecDon.
§  Thisperiodbeforemonsoon
onsetisknownasthe“buildup”.
MSLP(contours)and900hPavorDcity(shading)
priortomonsoononseton6December2005.
Figure9F1.3(a)
Australianmonsoonevolu.on
Ac.vemonsoon:Southwardexcursionofthemonsoontrough
Whenthemonsoonisfully
developed,themonsoontrough
(andshearline)canshiafartothe
south.
Theselowsarestrongerthanheat
lows,andareassociatedwith
widespreadheavyrains.
Example:29January2006
monso
on tro
ugh
Oaentheremaybeatropicalcycloneembedded
withinthezonally-orientedtrough.
monsoon trough
Australianmonsoonevolu.on
Ac.vemonsoon:
§  TheacDvemonsoonis
associatedwithwidespread,
deeptropicalconvecDon.
§  TheassociatedsynopDc
paiernisadeeplowoverthe
Kimberley,direcDngwestto
northwesterlywindsover
northwestAustraliaand
surroundingwaters.
§  Drosdowsky(1984)defined
theacDvemonsoonashaving
meanwesterlyzonalwindsto
500hPaandmeaneasterly
zonalwindsabove300hPa.
MSLP(contours)and900hPavorDcity(shading)
duringacDvemonsoonon23March2006.
Figure9F1.3(b)
4.Variabilityofthemonsoon
TroposphericBiennialOscilla.on(TBO)
§  TheTBOisamajormode
ofinterannualvariability
oftheAsian-Australian
monsoon
(a) Weak Aus monsoon
§  Thetendencyisfora
relaDvelystrongIndianor
Australianmonsoontobe
followedbyarelaDvely
weakone,andviceversa.
§  Asian-Australianmonsoon
intensityismodulatedby
boththeTBOandENSO.
(b) Strong Indian monsoon
Figure3.54
(c) Strong Aus monsoon
Meehletal.(2003)
ElNiño-SouthernOscilla.on(ENSO)
§  ENSOinfluencestheSouthAsiansummermonsoonthroughthestrength
oftheWalkerCirculaDon.Generally,ElNiñobringsaweakmonsoonand
LaNiñabringsastrongmonsoon,butthecorrelaDonvariesoverDme.
§  ENSOhasasignificantimpactontheAustralianmonsoon,withdrier
condiDonsexpectedovernorth-eastAustraliaduringElNiño.The
monsoononsetisgenerallylaterduringanElNiño,withoverallreduced
rainfall.
Figure3.55
IndianOceanDipole(ZonalMode)
§  TheIndianOceanDipole(IOD)
orIndianOceanZonalModeis
aneast-westoscillaDoninSST
anomaliesthatisaccompanied
byashiainprecipitaDon
anomalies.
§  TheposiDveIODphaseis
associatedwithheavierrainfall
overAfrica,lessoverIndonesia;
thereversepaiernoccurs
duringthenegaDvephase.
§  Generally,thedipoledevelops
inJune-July,reachesitspeakin
October-December,thenrapidly
decays.
Figure3.56
Bursts,breaksand
onset
Burstsareperiodsof
above-normalconvecDve
acDvity(lowOLR),and
enhancedrainfall.
Breaksareperiods
periodsofrelaDvelylow
rainfallembeddedwithin
thetradiDonalmonsoon
season
Break
Burst
Burst
3-day running mean OLR and rainfall
Break
Burst
Break
Break
Burst
Burst
Monsoononset
Burst
Bursts,breaksand
onset
Note:
1.  Theamplitudeofthe
burstsandbreaksis
comparabletothatof
themeanseasonal
cycle(dashedline).
2.  Monsoononsetis
ratherabrupt,and
almostalwaysoccurs
inconjuncDonwithan
intraseasonalburst.
Monsoononset
3-day running mean OLR and rainfall
MaddenJulianOscilla.on(MJO)
Approximate 1 month sequence:
Monsoon “break”
TC formation
Trade Wind surge
Monsoon westerlies
ACTIVE “BURST” OF INDONESIAN-AUSTRALIAN MONSOON
Westerly Wind Burst
§  TheMJOisa30-60day
oscillaDonintropicalrainfalland
circulaDon,firstdescribedby
MaddenandJulianintheearly
1970s.
§  TheMJOismostacDveoverthe
IndianandPacificOceans.
§  Itgeneratesmanyofthebursts
andbreaksoftheAustralian
monsoon.
§  Itisinvolvedinmonsoononset,
tropicalcyclonedevelopment,
andrainfallevents.
MJOandAustralianrainfall
§  Theaverageamplitudeofrainfall
variaDonsassociatedwiththeMJO
isabout5mm/daynearDarwin
§  DuringMJOwetphases,the
chanceofextremerainfallover
northernAustraliaisnearlytripled.
§  TheimpactofMJOonAustralian
monsoonrainfallvariesfromyear
toyear.
Andrew Marshall will
discuss the MJO in more
detail later in the week…
5.Monsoonsandclimatechange
Globalmonsoonprojec.ons
§  TheIPCCFiahAssessment
Reportstatesthatglobal
monsoonprecipitaDonis
likelytostrengthenover
the21stcentury
(Christensenetal.2013).
§  “IndianandEastAsian
monsoonprecipitaDonis
projectedtoincrease,
whileprojectedchangesin
Australiansummer
monsoonprecipitaDonare
small”
Figuresadaptedfrom
IPCCAR5WG1Chapter14
Globalmonsoonprojec.ons
§  Astheclimatewarms,watervapourtransportfromtheoceantolandincreases
becausewarmeraircontainsmorewatervapour.ThisalsoincreasesthepotenDalfor
heavyrainfall.
§  Warming-relatedchangesinlarge-scalecirculaDoninfluencethestrengthandextent
oftheoverallmonsooncirculaDon.
§  Landusechangeandatmosphericaerosolloadingcanalsoaffecttheamountofsolar
radiaDonthatisabsorbedbytheatmosphereandland,potenDallymoderaDngthe
land–seatemperaturedifference.
IPCCAR5WG1FAQ14.1
Regionalmonsoonprojec.ons
ProjectedchangesinEastAsian,SouthAsianandAustralian-Mari.meCon.nentrainfall:
§  Thereisameanincreasefor
EastandSouthAsian
monsoonsbutAustralian-MC
monsoonismoreuncertain.
§  Alsoshownareindicesof
monsoonvariability,
includingstandarddeviaDon
andonset,retreatand
duraDon.
§  Interannualvariabilityis
projectedtoincrease,and
alsomonsoonduraDon.
IPCCAR5WG1Figure14.4
SouthAsianmonsoonprojec.ons
§  CMIP5modelsprojectanincreaseinSouth
Asiansummermonsoonrainfalldueto:
-  enhancedland-seatemperaturecontrast
-  increasedmoisturetransportintothe
SouthAsianmonsoonregion
§  Modelsalsoprojectanincreaseinthe
interannualvariabilityofsummermonsoon
rainfall,relatedtothechangesintropical
PacificSST
§  Someothermodelstudies(e.g.Ashfaqet
al.2009)indicatethatSouthAsiansummer
monsoonrainfallandduraDonmay
decreaseinfuture.
Australianmonsoonprojec.ons
ModelsimulaDonsofchangeinAustraliansummermonsoonrainfallunder
highemissionsscenarioRCP8.5showstrongdisagreement.
+37%
(8.6%/K)
+1.1%
(0.4%/K)
-43%
(-11.1%/K)
ChangeinDJFAustralianmonsoonrainfall(%/K)forRCP8.5minusHistorical.
Monsoonrainfalliscalculatedforlandgridpointsinthedomain10-20S,120E-150E.
Brownetal.(2016),JournalofClimate
Australianmonsoonrainfallprojec.ons
WhatisthedifferencebetweenthegroupofmodelsprojecDngwet/dryfuture?
ALLmodelmean
DRYmodelmean
§  DRYmodelmeanhas
largestincreaseover
westernequatorial
Pacific
§  WETMMhasmoderate
increasesoverAustralia
andsouth-westPacific
WETmodelmean
§  DifferencebetweenDRY
andWETMMincludes
largeposiDveanomalyin
thewesternequatorial
Pacific
DRYminusWET
DJFrainfallchange(mm/day/K)forRCP8.5(2070-2099)–Historical(1970-1999)
Brownetal.(2016),JournalofClimate
Australianmonsoontemperatureprojec.ons
WhatisthedifferencebetweenthegroupofmodelsprojecDngwet/dryfuture?
ALLmodelmean
DRYmodelmean
WETmodelmean
DRYminusWET
§  DRYmodelmeanhas
enhancedequatorial
warming
§  WETMMhasless
warminginequatorial
Pacificandover
Australia
§  DRYminusWETMM
hasposiDveanomalyin
thewesternand
centralequatorial
Pacific
DJFsurfacetemperaturechange(K/K)forRCP8.5(2070-2099)–Historical(1970-1999)
Brownetal.(2016),JournalofClimate
Australianmonsoontemperaturevs.rainfall
WEPSSTbias(K)
WEPSSTchange(K)
Warmer in western Eq. Pac.
Drier
Wetter
§  Australian summer monsoon rainfall change is negaDvely correlated with the
amountofSSTwarmingintheWesternEquatorialPacific(WEP).
§  Monsoonrainfallchangeisalsoweaklycorrelatedwiththemodelbiasorerrorin
the SSTs in this same region, so models with a larger cold bias tend to project
larger decreases in monsoon rainfall (i.e. models that show large drying are less
reliable)
Brownetal.(2016),JournalofClimate
Furtherreading
Generalreferences:
§  “IntroducDontoTropicalMeteorology”,Chapter3,SecDon5andmany
referencesgivenattheendofChapter.
§  Webster,P.,1987:Theelementarymonsoon.Monsoons,J.S.Feinand
P.L.Stephens,Eds.,JohnWiley&Sons,3-32.
TheAsianMonsoon:
§  “TheAsianMonsoon”,BinWang,Springer,2006.
TheAustralianMonsoon:
§  “IntroducDontoTropicalMeteorology”,Chapter9,FocusSecDon1.
MonsoonsandClimateChange:
§  IPCC Fiah Assessment Report, “Climate Change 2013: The Physical
ScienceBasis”,Chapter14,SecDon2.
§  “The Monsoons and Climate Change: ObservaDons and Modeling”,
Editors:CarvalhoandJones,Springer,2016.
Indicesofmonsoonvariability
A(large)numberofindiceshavebeenproposedtomeasureregionalmonsoon
intensity:
§  WebsterandYangindex(WYI)–broad-scaleSouthAsiansummer
monsoon:verDcalshearofzonalwindanomaliesbetween850mband
200mb,(40-110°E,0-20°N),JJA;
§  MonsoonHadleyindex(MH)–SouthAsiansummermonsoon:verDcal
shearofmeridionalwindanomaliesbetween850mband200mb,
(70-110°E,10-30°N)JJA;
§  All-Indiamonsoonrainfall(AIMR)orIndianSummerMonsoonRain(ISMR)
–Indiansummermonsoonrainfall:averagedoverIndianconDnent,JJAS;
§  ExtendedIndianmonsoonrainfall(EIMR)–SouthAsiansummermonsoon
rainfallincludingadjacentoceans:(70-110°E,10-30°N),JJA;
§  Australianmonsoonindex(AUSMI)– 850hPazonalwindaveraged
(5°S-15°S,110°E-130°E),DJF.
§  +manymore…