第八章大气环流的长期变化3

第八章全球大气环流的长期变化8.1全球大气环流的平均状况8.2大气活动中心与大气涛动8.3高空大气环流的变率

8.4季风系统

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,上下分别为不同方案重建结果)

Luterbacher(1999)第八章全球大气环流的长期变化8.1全球大气环流的平均状况8.2大气活动中心与大气涛动8.3高空大气环流的变率8.4季风系统

7月海平面气压分布(张家诚,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)

图2.3a1月海平面气压分布(张家诚,林之光,1985)

西伯利亚高压中心区平均气压变化一个标准差时温度的相应变化(

C).西伯利亚高压中心区平均气压变化一个标准差时降水量的相应变化(％)冬季西伯利亚高压强度指数亚洲大陆中高纬平均温度与降水序列及其与西伯利亚高压强度(虚线)的关系西伯利亚高压，北极涛动，欧亚遥相关型及南方涛动拟合的温度值(上图)及与实际观测值之间的差别(下图).虚线是线性趋势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