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1、英语阅读专项特训⑴Before1965manyscientistspicturedthecirculationoftheocean'swatermassasconsistingoflarge,slow-movingcurrents,suchastheGulfStream.Thatview,basedon100yearsofobservationsmadearoundtheglobe,producedonlyaroughapproximationofthetruecirculation.Butinthe1950'sandthe1960's,researchersbegantoemployn
2、ewlydevelopedtechniquesandequipment,includingsubsurfacefloatsthatmovewithoceancurrentsandemitidentificationsignals,andoceancurrentmetersthatrecorddataformonthsatfixedlocationsintheocean.Theseinstrumentsdisclosedanunexpectedlevelofvariabilityinthedeepocean.Ratherthanbeingcharacterizedbysmooth,larg
3、e-scalecurrentsthatchangeseasonally(ifatall),theseasaredominatedbywhatoceanographerscallmesoscalefields:fluctuating,energeticflowswhosevelocitycanreachtentimesthemeanvelocityofthemajorcurrents・Mesoscalephenomena一theoceanicanalogueofweathersystems―ftenextendtodistancesof100kilometersandpersistfor1
4、00days(weathersystemsgenerallyextendabout1,000kilometersandlast3to5daysinanygivenarea).Morethan90percentofthekineticenergyoftheentireoceanmaybeaccountedforbymesoscalevariabilityratherthanbylargescalecurrents・Mesoscalephenomenamay,infact,playasignificantroleinoceanicmixing,air-seainteractions,ando
5、ccasional一butfar-reaching一climaticeventssuchasElNino,theatmospheric-oceanicdisturbanceintheequatorialPacificthataffectsglobalweatherpatterns.Unfortunately,itisnotfeasibletouseconventionaltechniquestomeasuremesoscalefields.Tomeasurethemproperly,monitoringequipmentwouldhavetobelaidoutonagridatinter
6、valsofatmost50kilometers,withsensorsateachgridpointlowereddeepintheoceanandkeptthereformanymonths.Becauseusingthesetechniqueswouldbeprohibitivelyexpensiveandtimeconsuming,itwasproposedin1979thattomographybeadaptedtomeasuringthephysicalpropertiesoftheocean.InmedicaltomographyX-raysmapthehumanbody9
7、sdensityvariations(andhenceinternalorgans);theinformationfromtheX-rays,transmittedthroughthebodyalongmanydifferentpaths,isrecombinedtoformthree-dimensionalimagesofthebody'sinterior.Itisprimarilythismultiplicativeincrea
