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1、Lecture8ElNinoandClimateVariabilityInternalClimateVariability:Somekeyconceptstoreview:•Recallthathighpressureisassociatedwithcoldtemperaturesandsinkingmotion.Thesinkingmotionoccursbecausesurfaceairmovesawayfromthehighpressurecenterundertheinfluenceofg
2、ravity,anddrawsdownairfromabove.•Conversely,lowpressureisassociatedwithwarmtemperaturesandrisingmotion.Therisingmotionoccursbecausesurfaceairconvergesonanareaoflowpressureundertheinfluenceofgravity,forcingrisingmotionoverthelow.•Whatkindofhydrologicco
3、nditionsareassociatedwithahighandlow?•Recallalsothatthepredominantwinddirectionattheequatoris…In1899,theIndianmonsoonfailed,leadingtodroughtandfamineinIndia.ThisledGilbertWalker,theheadoftheIndianMeteorologicalService,tosearchforawaytopredicttheIndian
4、monsoon.Bytheearly20thcentury,hehadidentifiedapeculiarsee-sawrelationshipbetweenpressureoverthemaritimecontinentandIndiaandthePacificnearSouthAmerica.WhenpressureishighovertheeasternPacific,itislowoverthemaritimecontinent,andviceversa.Hecalledthisrela
5、tionshiptheSouthernOscillation.Sincethe1800s,Peruvianfishermannoticedthattheirharvestcompletelyfailedeveryfewyears.ThisperiodiceventwasassociatedwithunusuallywarmwatersoffthecoastofPeru.Thesewarmwatersresultedfromashutdownoftheupwellingcirculationnorm
6、allyfoundalongtheequator.Sinceupwellingsuppliesnutrientstothesurfacewaters,thisresultedinmassstarvationofplantandanimallifeintheeasternequatorialPacific.SincetheperiodicwarmingalmostalwaysoccurredaroundDecember,thefishermannameditElNiño,inreferencetot
7、heChristchild.In1969,UCLAprofessorJacobBjerkneswasthefirsttorecognizethatElNiñoandtheSouthernOscillationareactuallymanifestationsofthesamephysicalphenomenonandthatitresultsfromanunstableinteractionbetweentheatmosphereandtheocean.ThisresultedinthetermE
8、NSOtorefertothisphenomenon.HowareatmosphereandoceanicconditionsrelatedduringanElNiño?Under“normal”conditions,oftenreferredtobythetermLaNiña,theeasterlytradewindsblowacrossthePacific,generatingupwellingalongtheequatoracrossmostthePacific,andpil
