第八章大氣環(huán)流的長期變化3

第八章全球大氣環(huán)流的長期變化8.1全球大氣環(huán)流的平均狀況8.2大氣活動中心與大氣濤動8.3高空大氣環(huán)流的變率

8.4季風系統(tǒng)

Stratosphericandmesosphericwindsystems

Polar-nightwesterlies

Summereasterlies

Inmiddleandhighlatitudesthereisindeedareversingvortexinbothhemispheres.Inthewinterhemispherethemesosphericwesterlies(socalledbecausetheirjetcoreslieinthemesosphere)arebelievedstrongestatabout65kilometresinmid-latitudes,wherewindsaveragemorethan80metrespersecond.Thepolar-nightareaisbitterlycold(downtobelow-80Cinthe25–35-kmlayer),andstrongwesterliesareusuallypresentintheintenselybarocliniczonearoundthiscoldcore.Thesewesterlies,whichareoftenvisibleonchartsaslowas20kilometres,havebecomefamoussince1952,whenthefirstexplosivewarminginthemwasobservedoverBerlin.Theirrelationtothemainmesosphericjetisnotclear,andtheymaybedistinct.WinterwesterliesThepolar-nightwesterliesoftheNorthernHemispherearedeformedbystandingwaves(mostlyofthetwo-wavesort),witharidgeoverandwestofAlaskaandtroughsovereasternSiberiaandeasternNorthAmerica,whichareoftencontinuouswithsimilarwavesintheFerrelwesterliesbelow.Thesouthernpolar-nightsystemliesovertheoceanformuchofthewinterandislesswellknown,butitisprobablymorenearlyapurezonalwesterly.Bothsystemsaresubjecttotransientdisturbancesinvolvingstrongverticalmotion,asisthewholedepthofthemesosphericwesterlies.Thesedisturbancesshowthemselvesbysuddenwarmings,sometimesmorethan30Cinaday,throughsubstantialdepthsofthestratosphere.Thewarmings,oncethoughttoresultfromsomekindofextraterrestrialimpulse,arealmostcertainlyduetosubsidenceontheorderofoneortwokilometresperday.Theytemporarilydestroyorevenreversethewesterlycirculation.Thelastofthesewarmingsbringswintertoanabruptandspectacularend,usuallyatorbeforethespringequinox.Theseinvisibleeventsalsotransfermuchozonedownwardintothelowerstratospherefromthelayersofrapidformationhigherup,givingthehigh-latitudebeltsthecuriousphenomenonofaspringozoneSummereasterliesSummereasterliesathighlevels,establishedaftersomeweeksoffurthervacillation,blowcontinuouslyuntilSeptember(MarchintheSouthernHemisphere).Theyareprobablystrongestatabout70kilometresinabout50°latitude,wheretheyattain60metrespersecond.Theyprobablytiltequatorwardasonedescends,andbelow40kilometresthestrongestwindsareinabout15°latitude.Theseeasterlieslackthespectaculardisturbancesofwinterbutcontainratherfeeblewestward-driftingtroughsandridges.Betweenthesevastzonalcurrents,inbroadlyequatoriallatitudes,meanwindspeedsarelow.Inthelayeraboveabout15kilometresanduptoatleast40kilometreswithinthe30°parallels,thereisaremarkableregimeinwhichthezonalwindreversesonaunique26-monthcyclewitheasterliesgivingwaytowesterliesallaroundtheEarthandthenreappearing26monthslater.Ithasbeenshownthata26-monthcomponentofvariationoccursinmanyatmosphericphenomena,butintheequatorialstratosphereitdominatesthemotionofthewinds.QBOVericallinesindicatetheJanuaries.TheindexistheconcatenationofvaluesatCantonIsland(3S,172W)forJan1953-Aug1967;Gan/Maledives(1S,73E)forSep1967-Dec1975;andSingapore(1N,104E)forJan1976-Feb1999.

B.NaujokatandC.Marquardt200030hPazonalwind(m/s)

WETime-heightsectionofzonalwind,1993-98WEWWEETeleconnectionsEAPNAWAWPEUPPJEU序列(冬季,上下分別為不同方案重建結果)

Luterbacher(1999)第八章全球大氣環(huán)流的長期變化8.1全球大氣環(huán)流的平均狀況8.2大氣活動中心與大氣濤動8.3高空大氣環(huán)流的變率8.4季風系統(tǒng)

7月海平面氣壓分布(張家誠,林之光,1985)

1)low-levelcrossequatorialflow,2)southwesterlies,3)EASMtrough,4)subtropicalhigh,5)Mei-Yufrontzone,6)midlatitudewesterlies,7)Tibetianhigh,and8)upperlevelreturnflow.(J.Zhou,K.-M.Lau,1998)

圖2.3a1月海平面氣壓分布(張家誠,林之光,1985)

西伯利亞高壓中心區(qū)平均氣壓變化一個標準差時溫度的相應變化(

C).西伯利亞高壓中心區(qū)平均氣壓變化一個標準差時降水量的相應變化(％)冬季西伯利亞高壓強度指數(shù)亞洲大陸中高緯平均溫度與降水序列及其與西伯利亞高壓強度(虛線)的關系西伯利亞高壓，北極濤動，歐亞遙相關型及南方濤動擬合的溫度值(上圖)及與實際觀測值之間的差別(下圖).虛線是線性趨勢AtmosphericCirculation1)Thegreatestannualinputofsolarenergyoccursattheequator.2)Warmairmassesrise;astheyrisetheycoolandprecipitate;tropicalrainforestsaround0°4)AsairmassesmovelatitudinallytheyaredeflectedduetothedifferentspeedsoftheEarth’srotationatdifferentlatitudes(Corioliseffect)Majordrivingforces:3)Thesecool,dryairmassesmoveawayfromtheequatoranddescend;desertsfoundat30°NandS2WVgsinf=-(1/r)Dp/Dy

Vg=-(1/2rWsinf)Dp/DyGeostrophicBalance:Responsibleforeast-westcomponentofwindatsurfacePressure