2012gre写作宝典:厄尔尼诺现象

2012-04-21 03:31:29 现象
GRE写作成为横在中国学生的理想和现实之间的一大障碍,因此,如何攻克写作这道屏障,成为摆在我们面前的首要任务。
Therecent,apparentlysuccessfulpredictionbymathematicalmodelsofanappearanceofEINino-thewarmoceancurrentthatperiodicallydevelopsalong(5)thePacificcoastofSouthAmerica-hasexcitedresearchers.JacobBjerknespointedoutover20yearsagohowwindsmightcreateeitherabnormallywarmorabnormallycoldwaterintheeastern(10)equatorialPacific.Nonetheless,untilthedevelopmentofthemodelsnoonecouldexplainwhyconditionsshouldregularlyshiftfromonetotheother,ashappensintheperiodicoscillations(15)betweenappearancesofthewarmEINinoandthecoldso-calledanti-ElNino.Theanswer,atleastifthecurrentmodelthatlinksthebehavioroftheoceantothatoftheatmosphereis(20)correct,istobefoundintheocean.IthaslongbeenknownthatduringanElNino,twoconditionsexist:(1)unusuallywarmwaterextendsalongtheeasternPacific,principallyalongthe(25)coastsofEcuadorandPeru,and(2)windsblowfromthewestintothewarmerairrisingoverthewarmwaterintheeast.Thesewindstendtocreateafeedbackmechanismbydrivingthewarmer(30)surfacewaterinta"pile"thatblocksthenormalupwellingofdeeper,coldwaterintheeastandfurtherwarmstheeasternwater,thusstrengtheningthewindstillmore.Thecontributionof(35)themodelistoshowthatthewindsofanElNino,whichraisesealevelinthecast,simultaneouslysendasignaltothewestloweringsealevel.Accordingtothemodel,thatsignalisgenerated(40)asanegativeRossbywave,awaveofdepressed,ornegative,sealevel,thatmoveswestwardparalleltotheequatorat25to85kilometersperday.TakingmonthstotraversethePacific,Rossby(45)wavesmarchtothewesternboundaryofthePacificbasin,whichismodeledasasmoothwallbutinrealityconsistsofquiteirregularislandchains,suchas,thePhilippinesandIndonesia.
(50)Whenthewavesmeetthewesternboundary,theyarereflected,andthemodelpredictsthatRossbywaveswillbebrokenintonumerouscoastalKelvinwavescarryingthesamenegative(55)sea-levelsignal.Theseeventuallyshoottowardtheequator,andthenheadeastwardalongtheequatorpropelledbytherotationoftheEarthataspeedofabout250kilometersperday.When(60)enoughKelvinwavesofsufficientamplitudearrivefromthewesternPacific,theirnegativesea-levelsignalovercomesthefeedbackmechanismtendingtoraisethesealevel,andtheybegin(65)todrivethesystemintotheoppositecoldmode.Thisproducesagradualshiftinwinds,onethatwilleventuallysendpositivesea-levelRossbywaveswestward,wavesthatwilleventually(70)returnascoldcycle-endingpositiveKelvinwaves,beginninganotherwarmingcycle.
数学模型最近对“厄尔尼诺”(ElNino)的出现所作出的显然是成功的预测使研究者们兴奋不已。所谓厄尔尼诺,即一股沿南美洲太平洋海岸周期性发展的海洋暖流。早在20年之前,雅各·皮叶克尼斯(JacobBjerknes)指出了风何以可能在东部赤道太平洋地区形成反常温暖或反常寒冷的水域。虽然如此,在上述数学模型得以发展之前,还没有任何人能够解释情况为什么应该有规律地从一种转变为另一种,正如在暖厄尔尼诺和冷厄尔尼诺(或所谓的反厄尔尼诺)出现之间周期性的交替变化中所发生的那样。问题的答案——至少如果目前将海洋的行为与大气层的行为联系起来的模型是正确的话——应从海洋中寻找。
长期以来众所周知,在厄尔尼诺出现期间,两个条件必须存在:(1)格外温暖的水流沿着东太平洋延伸,主要是沿厄瓜多尔和秘鲁海岸;(2)风从西部吹向在东部温暖的海水上空升起的较暖的空气。这些风倾向于造成一种反馈机制,迫使较为温暖的表层水形成一“堆状物”,阻挡住东部较深层的、寒冷的水的正常上涌,并进一步使东部的水温度上升,从而更进一步增强风的力度。该模型的贡献是旨在证明,致使海平面在东部海域升高的厄尔尼诺水流的风会同步向西部海域发送一信号,导致海平面降低。按照该模型,那一信号作为一种负罗斯比波(Rossby)而被产生形成,即一种致使海平面沉降、或负增长的波浪,此波浪会以每天85公里的速度平行于赤道西行,此海盆被模拟为一堵平滑的墙,但实际上却是由极不规则的群岛系列构成,如菲律宾群岛和印度尼西亚群岛。
当这些波浪抵达西部边缘时,它们被反弹回来,该模型预测,罗斯比波将会被分解成为诸多沿海岸流动的Kelvin波,携带着同样的负海平面信号。这些波最终涌向赤道,然后在地球旋转力量的推动下沿赤道以每天250公里的速度往东前进,当足够数量带有充分幅度的Kelvin波自西太平洋水域到达时,它们的负海面信号会压服那个倾向于提高海面的反馈机制,它们开始迫使整个系统转入相反的寒冷模式。这导致产生风向的逐渐转折变化,这一转折变化最终将正海面罗斯比波送往西部,这些罗斯比波将最终作为寒冷的、终结整个周期的正Kelvin波回来,从而引发另一个增温周期。 感谢您阅读《厄尔尼诺现象 》一文,出国留学网(liuxue86.com)编辑部希望本文能帮助到您。
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