地球科学进展  2018 , 33 (8): 775-782 https://doi.org/10.11867/j.issn.1001-8166.2018.08.0775

科技重大计划进展

太平洋暖池冷舌交汇区盐度变异机制及气候效应研究

王凡, 刘传玉, 胡石建, 高山, 贾凡, 张林林, 汪嘉宁, 冯俊乔

1.中国科学院海洋研究所海洋环流与波动重点实验室,山东 青岛 266071
2.青岛海洋科学与技术试点国家实验室海洋动力过程与气候功能实验室,山东 青岛 266237
3.中国科学院海洋大科学研究中心,山东 青岛 266071

Variability and Climate Effect of the Salinity in the Pacific Warm Pool-cold Tongue Confluence Region

Wang Fan, Liu Chuanyu, Hu Shijian, Gao Shan, Jia Fan, Zhang Linlin, Wang Jianing, Feng Junqiao

1.Key Laboratory of Ocean Circulation and Waves, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China
2.Function Laboratory for Ocean Dynamics and Climate, Pilot National Laboratoryfor Marine Science and Technology Qingdao, Qingdao 266237, China
3.Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China

中图分类号:  P731.12

文献标识码:  A

文章编号:  1001-8166(2018)08-0775-08

收稿日期: 2018-04-2

修回日期:  2018-06-10

网络出版日期:  2018-08-10

版权声明:  2018 地球科学进展 编辑部 

基金资助:  国家自然科学基金重点项目“暖池冷舌交汇区盐度变异机制及气候效应研究”(编号:41730534)中国科学院前沿科学重点研究项目“赤道太平洋温跃层混合研究”(编号:QYZDB-SSW-DQC030)资助.

作者简介:

First author:Wang Fan (1967-),male,Qingdao City, Shandong Province,Professor. Research areas include ocean circulation dynamics.E-mail:fwang@qdio.ac.cn

作者简介:王凡(1967-),男,山东青岛人,研究员,主要从事海洋环流动力学研究.E-mail:fwang@qdio.ac.cn

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摘要

热带中太平洋暖池冷舌交汇区既是冷暖水交汇的区域,也是高盐低盐水交汇之地,形成了以强烈的海表盐度锋、较浅的混合层和较厚的障碍层为显著特征的温盐结构。该区域还是不同类型厄尔尼诺(El Niño)发生发展的关键区域,也是气候模式模拟偏差比较集中的区域。为了研究该区域盐度过程及其时空变异,及其在多大程度上影响热带太平洋上层海洋热力动力结构和ENSO的发展变异这一重要科学问题,国家自然科学基金重点项目“暖池冷舌交汇区盐度变异机制及气候效应研究”于2017年7月正式立项。该项目拟解决的关键科学问题包括:①融合多源数据刻画交汇区盐度的三维结构及变异规律;②阐明影响盐度锋和障碍层不同时间尺度变异的主要过程及作用机理;③阐明交汇区盐度变异,特别是障碍层和盐度锋变异,是如何以及在多大程度上影响ENSO循环及其变异的。通过该项目的实施,有望在热带海洋动力学理论和ENSO动力学理论方面取得突破,为提高ENSO预报水平提供新的思路和依据。

关键词: 暖池冷舌交汇区 ; 盐度 ; 三维结构 ; 变异机制 ; ENSO循环

Abstract

This study focused on the warm pool-cold tongue confluence region (WCCR) in the central tropical Pacific Ocean where the warm and fresh water from the warm pool encounters with cold and saline water from the cold tongue. The WCCR is characterized by strong surface salinity front, shallow mixed layer and thick barrier layer. The WCCR is the key area for the development of different types of El Niño, and also the area with significant systematic bias in climate models. In order to reveal what role the structure and variability of salinity will play in the ocean dynamic and thermal conditions, and the cycle of the ENSO, a key project was approved by the National Natural Science Foundation of China (NSFC) in July, 2017. The key scientific issues that will be addressed in the project are as follows: ①to depict the three-dimensional structure and variability of salinity in the WCCR; ②to reveal the mechanism for the variability of salinity front and barrier layer; and ③to illustrate the main processes that control the impact of salinity on the upper-ocean variation in the tropical Pacific and the cycle of the ENSO. The present study will improve our understanding of the tropical ocean dynamics and ENSO dynamics, and will enhance the prediction skill of the ENSO.

Keywords: Warm Pool-Cold Tongue Confluence Region ; Salinity ; Three-dimensional Structure ; Variability mechanism ; ENSO cycle

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王凡, 刘传玉, 胡石建, 高山, 贾凡, 张林林, 汪嘉宁, 冯俊乔. 太平洋暖池冷舌交汇区盐度变异机制及气候效应研究[J]. 地球科学进展, 2018, 33(8): 775-782 https://doi.org/10.11867/j.issn.1001-8166.2018.08.0775

Wang Fan, Liu Chuanyu, Hu Shijian, Gao Shan, Jia Fan, Zhang Linlin, Wang Jianing, Feng Junqiao. Variability and Climate Effect of the Salinity in the Pacific Warm Pool-cold Tongue Confluence Region[J]. Advances in Earth Science, 2018, 33(8): 775-782 https://doi.org/10.11867/j.issn.1001-8166.2018.08.0775

1 引 言

西太平洋暖池(简称暖池)和东太平洋冷舌(简称冷舌),分别定义为水温高于28 ℃的暖水体和水温低于25 ℃的冷水体,是热带太平洋上层海洋热力结构的最显著特征。两者交汇的热带中太平洋“暖池冷舌交汇区”(160°E~140°W,20°S~20°N),既是东部型厄尔尼诺(El Niño)发展的前兆区和必经之地,也是中部型El Niño发生、发展、消退的主要区域[1],其海洋变异直接影响并参与了2类El Niño事件的发生、发展和变异,在热带太平洋海洋与气候系统中扮演着非常重要的角色。

暖池冷舌交汇区具有极其复杂和独特的热盐结构,除了强烈的海表温度(Sea Surface Temperature,SST)锋面和西深东浅的温跃层之外,还存在强烈的海表盐度(Sea Surface Salinity,SSS)锋面、较浅的盐度跃层以及介于盐度跃层与温跃层之间的障碍层。盐度可以影响海洋层结和障碍层及其“热障”效应,进而对热带太平洋上层海洋热力和动力结构产生重要影响。世界气候研究计划(World Climate Research Program, WCRP)耦合模式比较计划(Coupled Model Intercomparison Project,CMIP)几乎所有的气候模式都存在显著的热带模拟偏差,所模拟的暖池偏西、偏小,冷舌过强和过度西伸,而这些偏差均集中体现在暖池冷舌交汇区。ENSO预报模式在模拟和预报热带太平洋SST异常方面也存在明显缺陷,多数模式对2014—2015年“夭折”的ENSO事件预报失败。已有研究结果表明,在模式中是否考虑盐度会显著影响对热带太平洋SST的模拟能力和对ENSO的预报效果[2]。然而,绝大多数气候模式对海洋盐度的模拟存在显著偏差,大部分中等复杂程度的ENSO预报模式和ENSO统计预报模型甚至没有引入盐度过程。因此,当前气候模式和ENSO预报模式中存在的偏差,很可能是因对暖池冷舌交汇区盐度相关的海洋和海气过程认识不足、表征不准造成的。

因此,深刻揭示暖池冷舌交汇区盐度的结构特征、变异规律、控制机理及其对ENSO的影响过程,对理解热带海洋与气候变异机制、提高ENSO模拟预报水平具有重要的科学意义。

由中国科学院海洋研究所申请的国家自然科学基金重点项目“暖池冷舌交汇区盐度变异机制及气候效应研究”于2017年7月正式立项。该项目将利用多源数据和综合性研究方法,从刻画暖池冷舌交汇区盐度的三维结构特征和变异规律入手,以揭示盐度锋和障碍层的多尺度变异机制为核心,阐明盐度影响热带太平洋上层海洋变异,进而影响ENSO循环的主要控制过程。通过该项目的实施,有望完善热带海洋动力学理论和ENSO动力学理论,为提高ENSO预报水平提供新的思路和依据。

2 国内外研究现状及发展动态分析

2.1 暖池冷舌交汇区盐度的结构特征与变异规律

暖池冷舌交汇区的海洋热力和动力环境的分布特征、变异规律及其对ENSO的影响是国际海洋和大气研究的重点之一。但由于观测资料缺乏,以往研究多侧重于海洋温度结构和变异。近年来,随着Argo和卫星遥感盐度等盐度观测的大幅增多,海洋盐度的结构特征和变异规律也正日益受到关注,成为当前一个重要的前沿研究方向。

暖池冷舌交汇区存在显著且位置和走向与温度结构迥异的盐度锋和障碍层。与热带辐合带(Intertropical Convergence Zone,ITCZ)相对应,西太平洋低盐水与赤道东太平洋高盐水之间形成一个分布在赤道以北的纬向盐度锋和一个分布在日界线以西的经向盐度锋[3,4,5,6,7,8]。这2个锋面共同形成了暖池冷舌交汇区SSS西北侧低、东南侧高的分布格局,与大体上关于赤道南北对称的SST分布形成鲜明反差。同时,在盐度锋以西的盐度跃层与温跃层之间还存在着较厚的障碍层[9,10,11],其最大值中心位于160°E附近[12]

迄今关于该区域盐度锋和障碍层的研究多以平均态和个例研究为主,关于不同时间尺度变异规律及其机制和过程的研究不多。研究发现盐度锋、障碍层与温度锋之间具有密切联系且存在显著的年际变化。例如,2002—2004年暖池东边界(温度锋)与纬向盐度锋空间位置重叠,前者与障碍层和西风爆发之间存在强相关[13],2000—2007年障碍层的厚度与盐度锋的强度成正比[11]。盐度锋在ENSO循环中东西移动可达8 000 km[14,15]、障碍层和经向盐度锋中心位置东西移动超过6 000 km,东部型El Niño事件可导致暖池东边界(温度锋)东移3 000 km以上,并导致日界线附近SSS变淡1 psu左右[16]。然而,在中部型El Niño期间,异常厚的障碍层会局限在160°~180°E的SSS锋面区域[17]。该区域障碍层的形成机制存在潜沉[9,10]、强降雨[18,19]和淡水水平平流[20]等多种不同观点。也有研究认为,低云、强降雨、向东低盐水输运、纬向流的垂向剪切,与/或伴随发生的赤道下沉Kelvin和Rossby波导致的上层水柱垂向拉伸等均有助于障碍层形成,但没有一个过程起支配作用[11]。因此,暖池冷舌交汇区障碍层的形成机制仍是一个极具争议的科学问题,亟需进一步研究。

另一个未解之谜是盐度锋和障碍层对ENSO的响应过程与机制。有研究认为,El Niño期间东向表层流异常导致纬向低盐水输送加强,同时暖水和大气深对流东移,导致赤道中太平洋降水加强、SSS降低[14]。另一种观点认为,ITCZ或南太平洋辐合带(South Pacific Convergence Zone,SPCZ)位置的经向移动也会导致显著的降水异常[21]。基于2001年4月调查数据的个例研究发现,暖池冷舌交汇区的盐度锋消失,主要与南太平洋高盐水在上温跃层向北扩张有关,而与局地淡水输入无关[4]。可见,盐度锋和障碍层的年际变化主要与ENSO循环中的降水和淡水平流密切相关,但哪个因子占主导、具体过程是什么、对ENSO有何反馈等目前仍不清楚。

2.2 太平洋暖池冷舌交汇区盐度三维结构与变异的主要控制机理

影响暖池冷舌交汇区盐度三维结构及变异的过程包括海洋—大气、混合层—障碍层、障碍层—温跃层等界面是物质能量交换过程、大尺度平流效应、波致平流与混合效应、湍流混合效应等,分别在不同时间尺度上起重要作用。其中,能够参与或影响ENSO循环的关键过程是本文关注的重点。

在界面物质能量交换中,海表淡水通量的影响至关重要,但其不确定性很大。由ITCZ和SPCZ引起的局地降水是赤道太平洋SSS分布和变化的主要控制因素[22,23,24,25]。海气界面淡水通量的空间分布不均匀及其形成的盐度锋为高盐水潜沉形成障碍层提供了条件[10,19]。然而,由于直接观测的海面资料较为稀疏,大多数海气淡水通量产品都是依据经验和半经验公式估算得到的[26]。分析发现,不同数据库提供的产品相互之间存在极大差异;而且,与浮标数据对比发现,卫星资料估算的淡水通量准确率甚至只达到浮标观测的20%~30%[27]。因此,确定准确的海表淡水通量,是研究盐度分布和变异特征首要解决的问题。

大尺度环流的纬向和经向平流效应均应受到重视。在赤道太平洋区域,赤道流系(包括西向的南、北赤道流、东向的北赤道逆流及赤道潜流)的平流效应在暖池冷舌交汇区的热盐结构形成及变异中扮演着至关重要的角色。如前所述,SSS、盐度锋和障碍层都随着赤道流系在不同时间尺度上的变化而东西移动[14,28]。近期基于Argo数据的研究表明,西太平洋的盐度异常信号可以通过纬向平流向东传输到中太平洋[29]。同时有研究指出了经向平流的重要影响,认为只有充分考虑来自副热带的经向盐度平流效应才能使暖池盐度收支在季节和更长时间尺度上保持闭合[22,30]。可以看出,迄今关于赤道流系和经向平流影响盐度的具体过程和机制的系统研究较少。

迄今热带不稳定波(Tropical Instability Wave,TIW)对暖池冷舌交汇区盐度(包括温度)的影响未开展研究。一个广为接受的观点是,赤道东太平洋冷舌区的TIW对上层海洋热结构具有明显的影响。TIW可通过经向热对流实现冷舌与北侧暖水的交换,从而对冷舌起到加热作用,这个过程抵消了上升流给上层海洋带来的降温[31,32,33,34,35,36,37,38,39],对混合层的增温也可超过海表面热通量的加热作用[40]。其他研究则发现,TIW对应的纬向热平流所起的加热作用甚至比经向热平流作用更大[35,41]。然而,基于直接湍流观测的研究发现,TIW能够通过增强从混合层到温跃层上部的垂向混合,引起每月1~2 ℃的表层降温[42]。相比较而言,有关TIW对盐度影响的研究较少。事实上其他海域的少量研究已经展现了TIW与盐度的密切关系,如从SSS中发现了明显的TIW信号,且该信号更集中于赤道且西传速度更快;大西洋SSS能够把TIW势能提高50%等[43,44]。在暖池冷舌交汇区,经向和纬向盐度锋均很强,而TIW具有更大的波幅且以赤道为轴心纬向分布,与赤道东太平洋冷舌区的结构有很大不同[45],可能会形成与盐度(及温度)结构之间复杂的相互调制作用。其波动力学性质对盐度结构和变异的贡献,值得深入研究。

由湍流导致的垂向混合在盐度收支等分析中也很少受到关注。少量研究发现,混合在混合层盐度收支中具有与水平对流和表面淡水通量相当的贡献[46];暖池障碍层和盐度锋的变异不仅明显受到海洋平流的影响,而且与次表层的垂向混合和夹卷过程存在很高的相关性,特别是它们均超前ENSO变化2~3个月[47]。除此之外,鲜见有探讨海洋混合过程对盐度结构影响的文献。考虑到湍致混合对海洋上层温度结构和变异在季节内变化、季节变化、年际变化中均起着重要作用[48],且混合层底的湍流通量是控制ENSO循环的主要因子之一[40,49,50],可以推测,垂向混合必然也对混合层、障碍层的盐度结构和变异起着重要作用。然而,暖池冷舌交汇区混合过程的深度分布特征、与障碍层之间的关系以及受何种过程的控制,因缺乏观测尚不清楚。同时,暖池冷舌交汇区盐度结构和SSS在不同时间尺度上受混合影响的程度、过程和机制亦需要进一步研究。

另外,冷舌形态的变异及暖池增暖的空间不均匀性等大尺度热力学过程,会伴随温度和盐度空间结构的大尺度改变,而其如何在较小尺度上引发盐度三维结构的响应,也值得深入研究。

2.3 暖池冷舌交汇区盐度对ENSO的影响

绝大多数ENSO预报模型没有预报出2014—2015年El Niño“夭折”事件,此次预报失败是当前ENSO预测水平的一个缩影,也表明现有的ENSO动力学理论和预报模式中可能忽视了一些未知的重要过程。其中,暖池冷舌交汇区盐度过程的作用是一个重要的潜在突破方向。

统计模式和敏感性试验结果表明盐度在ENSO发展中起到重要作用。在热带太平洋,盐度的年际变化以往被视为对ENSO的被动响应[7,14]。事实上,SSS和障碍层的变化可超前ENSO数个月达到最大相关[38,51];障碍层有助于暖池强度的维持以及向东扩展,如果在模式中消除障碍层,El Niño事件会减弱甚至不会发生[52,53]

盐度可通过多种途径影响热带太平洋上层海洋热力动力结构,进而对ENSO的发生与变异产生反馈作用。一般认为,障碍层的存在改变混合层厚度,抑制了混合层底的混合与夹卷过程,使暖池区内的风应力、海表热通量等外部强迫作用被限制在较浅的混合层内而无法向下有效传递。这种“热障”效应造成了SST对海表热通量的变化更加敏感,使上层暖水能在赤道风场的作用下更加迅速地纬向移动[14,30,54],从而进一步影响大气环流及海气耦合过程。有研究结果表明,盐度是控制热带太平洋混合层平均厚度的关键因素[10,55];热带西、中太平洋盐度变化对混合层厚度、密度、海洋层结的影响比温度的作用还要大[56];ENSO与海洋上层盐度及相关的淡水通量之间可以形成正反馈,帮助ENSO期间的SST异常不断成长[56,57]

盐度的影响在ENSO预报中也得到了直观的体现。例如,将观测的SSS数据加入模式中,可较好地修正“春季预报障碍”[58];热带太平洋尤其是西太平洋的SSS初始场对于ENSO的准确预测非常关键[59];加入SSS场之后可准确地后报2007—2008年拉尼娜事件[2]。值得注意的是,绝大多数中等复杂程度的ENSO预测模式或者ENSO统计预报模型都没有引入盐度,这也意味着盐度可能是ENSO预报得以进步的突破点之一。受限于盐度观测数据的缺乏,前人的主要工作以个例分析或者模式研究为主,既缺乏针对性,又缺乏系统性,暖池冷舌交汇区盐度通过影响海洋上层热力动力结构进而影响ENSO循环和变异的具体过程尚仍需进一步研究。

总之,不管从完善热带海洋动力学理论和ENSO理论的角度,还是从提高预测能力的角度,暖池冷舌交汇区都是值得高度关注的区域。深入研究和认知该海域的海洋热力、动力环境,特别是盐度的分布特征、变异机制和影响过程,建立起该海域与ENSO之间的内在联系,是ENSO研究进一步深化的出发点和关键点。

3 关键科学问题及主要研究内容

3.1 关键科学问题

(1)暖池冷舌交汇区盐度三维结构特征和多时间尺度变异规律尚不清楚:该海域有着比西太暖池区和东太冷舌区更为复杂、以盐度锋和障碍层为显著特征的温盐结构,由于观测数据不足,之前的研究主要是基于航次数据和早期少量的Argo观测,对三维结构和变异规律的刻画不够准确和全面。当前,Argo观测和卫星遥感数据的大量积累,以及海洋数据同化产品的不断优化,为解决这一问题提供了条件。

(2)影响暖池冷舌交汇区盐度三维结构和变异的控制机理尚不明确:一是盐度锋和障碍层的生成变化机制不清楚,对于降水、平流和潜沉等过程的相对影响程度和作用过程存在争议;二是在海洋模拟中,是否真正给定了真实的海表淡水通量(还包括热通量和风应力)还存在疑问,“准确”的海表淡水通量对盐度锋面和障碍层的生成和变异的影响有待考察;三是对于中尺度海洋过程(如TIW平流效应,或侧向混合效应)及小尺度过程(如垂向湍流混合)的影响尚缺少研究。

(3)暖池冷舌交汇区盐度通过影响海洋上层热力动力结构进而影响ENSO循环和变异的作用过程尚无定论:受限于盐度观测数据的缺乏,前人的主要工作以个例分析或者模式研究为主,缺乏针对性和系统性。另外,海洋波动(开尔文波、罗斯贝波)作为ENSO动力学的重要部分,盐度的三维结构对其有何影响,在2类ENSO和极端ENSO事件中有什么不同的作用,尚没有涉及。

3.2 主要研究内容

围绕上述关键科学问题,本项目拟开展的主要研究内容包括:

3.2.1 暖池冷舌交汇区盐度的三维结构特征及变异规律

(1)系统分析研究该区域盐度的基本分布特征,重点分析纬向和经向盐度锋的位置和强度,障碍层的范围、深度和厚度,两者之间的关系,及其与季节内、季节、年际变化规律以及ENSO循环的关系。

(2)量化分析研究区域盐度收支和各界面通量,估算海表淡水通量、平流/对流过程、垂向夹卷以及侧向/垂向混合等分量对盐度收支和盐度锋、障碍层变化的贡献;对比分析研究区域的热收支和各界面通量;对比研究盐度收支和热收支各分量变化对混合层温度变化的贡献,及其与ENSO循环的相关性。

(3)研究构成盐度锋和障碍层的主要水团性质、分布和相互影响;重点研究盐度锋两侧低盐和高盐水团的来源和路径,评估平流和海表淡水通量对其形成和变异的贡献;分析障碍层及其上下层水团的来源,评估关于障碍层形成机制的几种不同观点的合理性,重点关注来自盐度锋的潜沉过程的贡献,探讨盐度锋与障碍层分布和形成之间的联系。

3.2.2 暖池冷舌交汇区盐度变异的控制机理

(1)系统评估代表性海洋模式和气候模式对暖池冷舌交汇区盐度、盐度锋和障碍层的模拟能力,从海表热盐通量、平流/对流、混合层底夹卷和混合、潜沉等方面分析不同类模式的差异,揭示影响其模拟能力的主要物理过程。

(2)研究构建最优海气淡水通量数据,进而研究及其对盐度结构的影响:利用四维变分(伴随)数据同化系统确定最优海面淡水通量;研究其与当前数据产品的差异,研究其对暖池冷舌交汇区盐度结构的影响过程。

(3)研究TIW的影响:研究TIW在暖池冷舌交汇区的空间结构和传播特征及其与ENSO的联系,并与其在冷舌区的特征进行比较;研究不同深度层次上TIW平流效应对赤道及赤道外区域盐度变化的贡献。

(4)研究垂向湍流混合的影响:重点研究垂向混合特征在研究区域复杂海洋环境下的结构特征和时间变异特征,研究湍致盐度通量在各层次之间的传递,研究其对盐度在不同时间尺度上的影响。

3.2.3 暖池冷舌交汇区盐度变异对ENSO循环的影响

(1)综合研究盐度收支和热收支各分量在ENSO循环位相转换、ENSO强度变异及ENSO类型变换中的贡献,梳理主要控制因子或反馈机制。

(2)揭示混合层和障碍层内纬向和经向热、盐平流辐聚效应,以及海表面、温跃层和盐度锋面等处的跨界面物质能量交换过程对El Niño的关键调控机理。

(3)揭示盐度变异通过改变密度跃层和混合层热状况而改变海气耦合过程及改变赤道波动,进而影响ENSO循环和变异的机理。

3.3 学术思路

总体来说,本项目结合ENSO动力学研究与热带海洋动力学研究两大海洋科学前沿,围绕揭示太平洋暖池冷舌交汇区以盐度锋和障碍层为典型的盐度变异机制这一核心,从研究影响该区域盐度的三维结构特征和变异规律入手,进而研究其对El Niño的发生、发展和变异的影响及作用过程。

4 预期目标

4.1 总体目标

系统刻画太平洋暖池冷舌交汇区上层海洋盐度的三维结构特征与变异规律,揭示海洋动力过程和海气界面通量对其变异的调控机制,阐释该海域热盐结构、收支及相关内部过程和界面过程对不同类型和强度El Niño发生、发展和可预报性的影响及其机制。

4.2 具体目标

(1)揭示太平洋暖池冷舌交汇区盐度的三维结构特征,估算该区域热盐收支,以及混合层、温跃层、障碍层和盐度锋等不同时间尺度变异(特别是季节内、季节和年际变异)规律。

(2)阐释太平洋暖池冷舌交汇区及周边海域垂向/侧向混合、中尺度过程/赤道波动、纬向/经向环流等不同尺度海洋动力过程及海气界面过程对盐度结构和收支变异的影响过程,并评估其相对重要性;建立盐度锋、障碍层和盐度收支变异与热收支、SST变异之间的内在联系。

(3)确定太平洋暖池冷舌交汇区盐度影响ENSO循环,特别是不同类型和强度El Niño发生、发展和变异的主要控制过程;揭示混合层和障碍层内纬向、经向平流效应,海表面、混合层、障碍层、温跃层和盐度锋附近界面物质能量交换过程对El Niño的关键调控机理,以及盐度变异通过改变密度层结、混合层热状态和赤道波动而改变海气耦合过程,进而影响ENSO循环和变异的机理。

5 结 语

通过本项目的实施,有望在赤道太平洋暖池冷舌交汇区海洋盐度结构变异、控制机理及其影响El Niño方面取得一系列研究成果。在全球变暖和中部型El Niño增多的背景下,本项目将深化对热带海洋动力学和ENSO动力学的认识,为改善赤道太平洋海洋模拟和ENSO模拟预报能力提供重要的科学依据。

The authors have declared that no competing interests exist.


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[J]. Journal of Geophysical Research,1998,103(C1):1 087-1 098.

DOI      URL      [本文引用: 1]     

[16] Singh A,Delcroix T,Cravatte S.

Contrasting the flavors of El Niño-Southern Oscillation using sea surface salinity observations

[J]. Journal of Geophysical Research,2011,116(C6).DOI:10.1029/2010JC006862.

[本文引用: 1]     

[17] Wang Xidong,Liu Hailong.

Seasonal-to-interannual variability of the barrier layer in the western Pacific warm pool associated with ENSO

[J]. Climate Dynamics,2015,47(1/2):1-18.

[本文引用: 1]     

[18] Sprintall J,Tomczak M.

Evidence of the barrier layer in the surface layer of the Tropics

[J]. Journal of Geophysical Research,1992,97(C5):7 305-7 316.

DOI      URL      [本文引用: 1]      摘要

Comparisons between isothermal depth to the top of the thermocline, and the mixed layer depth based on a 0303t criterion were undertaken for the tropical world oceans. In three equatorial regions, a shallower mixed layer than isothermal layer occurs, implying the presence of a strong halocline above the thermocline. This distance separating the top of the thermocline and the bottom of the mixed layer is referred to as the 090008barrier layer090009, in relation to its impediment to vertical heat flux out of the base of the mixed layer. Different mechanisms are responsible for maintaining the barrier layer in each of the three regions. In the western equatorial Pacific Ocean a salinity budget confirmed that heavy local precipitation most likely results in the isothermal but salt-stratified layer. In the northwest equatorial Atlantic, it is hypothesized that high salinity waters are subducted at the subtropics during winter and advected westward as a salinity maximum in the upper layers of the tropics, resulting in the barrier layer. In the eastern equatorial Indian Ocean, monsoonal related rainfall and river runoff contribute significantly to the freshwater flux, producing salt stratification in the surface. These results suggest the need to include the effects of salinity stratification when determining mixed layer depth.
[19] Chen D,Rothstein L M.

Modeling the surface mixed layer structure in the western equatorial Pacific

[J]. TOGA-Notes,1991,2:13-16.

[本文引用: 2]     

[20] McPhaden M J,Peters H.

Diurnal cycle of internal wave variability in the equatorial Pacific Ocean: Results from moored observations

[J]. Journal of Physical Oceanography,1992,22(11):1 317-1 329.

DOI      URL      [本文引用: 1]     

[21] Trenberth K E,Caron J M.

The southern oscillation revisited: Sea level pressures, surface temperatures, and precipitation

[J]. Journal of Climate,2000,13(24):4 358-4 365.

DOI      URL      [本文引用: 1]     

[22] Delcroix T,Henin C,Porte V,et al.

Precipitation and sea-surface salinity in the tropical Pacific Ocean

[J]. Deep Sea Research Part I: Oceanographyic Research Papers,1996,43(7):1 123-1 141.

DOI      URL      [本文引用: 2]      摘要

Monthly sea-surface salinity ( SSS ) and precipitation ( P ) in the tropical Pacific region are examined for the 1974–1989 period. The SSS data are derived mainly from water sample measurements obtained from a ship-of-opportunity program, and the rainfall data are derived from satellite observations of outgoing longwave radiation. The mean and standard deviation patterns of SSS and P exhibit good correspondence in the heavy-rainfall regions characterising the Intertropical Convergence Zone (ITCZ), the South Pacific Convergence Zone (SPCZ) and part of the western Pacific warm pool. An Empirical Orthogonal Function (EOF) analysis indicates two dominant modes of variation linking P and SSS changes, one mode at the seasonal timescale in both convergence zones, and the other at the ENSO timescale in the central-western equatorial Pacific (165°E–160°W) and in the SPCZ. The inferences derived from the EOF analysis are used in a simple linear regression model in order to try to specify P changes from known SSS changes. A comparison between hindcast and observed P changes suggests that, at seasonal and ENSO timescales, SSS changes could be used to infer the timing, but not the magnitude, of P in the central-western equatorial Pacific (165°E–160°W) and in the SPCZ mean area. The effects of evaporation, salt advection and mixed-layer depth on the results are discussed.
[23] Luo J,Yamagata T.

Long-term El Niño-Southern Oscillation (ENSO)-like variation with special emphasis on the South Pacific

[J]. Journal of Geophysical Research,2001,106(C10):22 211-22 227.

DOI      URL      [本文引用: 1]     

[24] Martins M,Stammer D.

Pacific Ocean surface freshwater variability underneath the double ITCZ as seen by satellite sea surface salinity retrievals

[J]. Journal of Geophysical Research,2015,120(8):5 870-5 885.

[本文引用: 1]     

[25] Yu Lisan.

Sea-surface salinity fronts and associated salinity-minimum zones in the tropical ocean

[J]. Journal of Geophysical Research,2015,120(6):4 205-4 225.

DOI      URL      [本文引用: 1]      摘要

Abstract The Intertropical Convergence Zone (ITCZ) is a major source of the surface freshwater input to the tropical open ocean. Under the ITCZ, sea-surface salinity (SSS) fronts that extend zonally across the basins are observed by the Aquarius/SAC-D mission and Argo floats. This study examined the evolution and forcing mechanisms of the SSS fronts. It is found that, although the SSS fronts are sourced from the ITCZ-freshened surface waters, the formation, structure, and propagation of these fronts are governed by the trade wind driven Ekman processes. Three features characterize the governing role of Ekman forcing. First, the SSS fronts are associated with near-surface salinity-minimum zones (SMZs) of 50–80 m deep. The SMZs are formed during December–March when the near-equatorial Ekman convergence zone concurs with an equatorward displaced ITCZ. Second, after the formation, the SMZs are carried poleward away at a speed of 653.5 km d611 by Ekman transport. The monotonic poleward propagation is a sharp contrast to the seasonal north/south oscillation of the ITCZ. Lastly, each SMZ lasts about 12–15 months until dissipated at latitudes beyond 10°N/S. The persistence of more than 1 calendar year allows two SMZs to coexist during the formation season (December–March), with the newly formed SMZ located near the equator while the SMZ that is formed in the previous year located near the latitudes of 10–15° poleward after 1 year's propagation. The contrast between the ITCZ and SMZ highlights the dominance of Ekman dynamics on the relationship between the SSS and the ocean water cycle.
[26] Fairall C W,Bradley E F,Rogers D P,et al.

Bulk parameterization of air-sea fluxes for Tropical Ocean-Global Atmosphere Coupled-Ocean Atmosphere Response Experiment

[J]. Journal of Geophysical Research,1996,101(C2):3 747-3 764.

DOI      URL      [本文引用: 1]     

[27] Bourras D.

Comparison of five satellite-derived latent heat flux products to moored buoy data

[J]. Journal of Climate,2006,19(24):6 291-6 313.

DOI      URL      [本文引用: 1]      摘要

Five satellite products of latent heat flux at the sea surface were compared to bulk fluxes calculated with data from 75 moored buoys, on almost 36 successive months from 1998 to 2000. The five products compared are the Hamburg Ocean Atmosphere Parameters and Fluxes from Satellite Dataset (HOAPS-2), the Japanese Ocean Flux Datasets with Use of Remote Sensing Observations (J-OFURO), the Jones dataset, the Goddard Satellite-Based Surface Turbulent Fluxes, version 2 (GSSTF-2), and the Bourras–Eymard–Liu dataset (BEL). The comparisons were performed under tropical and midlatitude environmental conditions, with three datasets based on 66 Tropical Atmosphere–Ocean array (TAO) buoys in the tropical Pacific, nine National Data Buoy Center (NDBC) buoys off the U.S. coasts, and four Met Office/Météo-France (UK–MF) moorings west of the United Kingdom and France, respectively. The satellite products did not all compare well to surface data. However, for each in situ dataset (TAO, NDBC, or UK–MF) at least one satellite product was found that had a good fit to surface data, that is, an rms deviation of 15–30 W m2. It was found that HOAPS-2, J-OFURO, GSSTF-2, and BEL satellite products had moderate systematic errors with respect to surface data, from 13 to 26 W m2, and small biases at midlatitudes (6–8 W m2). Most of the satellite products were able to render the seasonal cycle of the latent heat flux calculated with surface data. The estimation of near-surface specific humidity was found to be problematic in most products, but it was best estimated in the HOAPS-2 product. GSSTF-2 and J-OFURO strongly overestimated the surface flux variations in time and space compared to surface data and to a flux climatology. With respect to TAO data, Jones fluxes yielded good results in terms of rms deviation (27 W m2) but also presented a large systematic deviation. Overall, for application of the satellite fluxes to the world oceans, it was found that HOAPS-2 was the most appropriate product, whereas for application to the Tropics, BEL fluxes had the best performance in rms with respect to TAO data (24 W m2).
[28] McPhaden M J,Picaut J.

El Niño-Southern oscillation displacements of the western equatorial Pacific Warm pool

[J]. Science,1990,250(4 986):1 385-1 388.

DOI      URL      PMID      [本文引用: 1]      摘要

The western equatorial Pacific warm pool (sea-surface temperatures >29°C) was observed to migrate eastward across the date line during the 1986-1987 El Ni09o-Southern Oscillation event. Direct velocity measurements made in the upper ocean from 1986 to 1988 indicate that this migration was associated with a prolonged reversal in the South Equatorial Current forced by a large-scale relaxation of the trade winds. The data suggest that wind-forced zonal advection plays an important role in the thermodynamics of the western Pacific warm pool on interannual time scales.
[29] Hasegawa T,Ando K,Ueki I,et al.

Upper-ocean salinity variability in the tropical Pacific: Case study for quasi-decadal shift during the 2000s using TRITON Buoys and Argo Floats

[J]. Journal of Climate,2013,26(20):8 126-8 138.

DOI      URL      [本文引用: 1]     

[30] Chen Dake,Busalacchi A J,Rothstein L M.

The roles of vertical mixing, solar radiation, and wind stress in a model simulation of the sea surface temperature seasonal cycle in the tropical Pacific Ocean

[J]. Journal of Geophysical Research,1994,99(C10):20 345-20 359.

DOI      URL      [本文引用: 2]     

[31] Bryden H L,Brady E C.

Eddy momentum and heat fluxes and their effects on the circulation of the equatorial Pacific Ocean

[J]. Journal of Marine Research,1989,47(1):55-79.

DOI      URL      [本文引用: 1]     

[32] Baturin N G,Niiler P P.

Effects of instability waves in the mixed layer of the equatorial Pacific

[J]. Journal of Geophysical Research,1997,102(C13):27 771-27 793.

DOI      URL      [本文引用: 1]      摘要

Data from more than 1900 Lagrangian drifters in the equatorial Pacific Ocean from 1980 to 1994 together with velocity records from two Tropical Ocean-Global Atmosphere - Tropical Atmosphere Ocean (TOGA-TAO) equatorial moorings at 1100° and 14000°W, advanced very high resolution radiometer (AVHRR) sea surface temperature (SST) product, and European Centre for Medium-Range Weather Forecasts (ECMWF) winds were used to investigate the effects and energetics of currents associated with the tropical instability waves (TIWs). Adaptive multitaper spectral analysis was used to estimate how spectral energy varied in the 15-to-30-day period TIW band. The drifter data was analyzed separately for high and low values of the TIW energy in regions of 2000° longitude by 2000° latitude centered at 000°N, 11000°W and 000°N, 14000°W to construct meridional profiles of energetics of the TIW region. High TIW energy values typically occurred around October when the South Equatorial Current (SEC) and the North Equatorial Countercurrent (NECC) both became stronger and the eddy kinetic and potential energy production at 14000°W was noticeably larger. At 11000°W the eddy kinetic and potential energy production existed all the time without large differences between the periods of high and low TIW activity. The meridional kinetic energy was enhanced in the region between the equator and 1000°N from 15000° to 10000°W, with the largest values occurring between 11000° and 14000°W in longitude and around 500°N in latitude. The largest terms in the horizontal kinetic energy production equation were and with maxima in the region of anticyclonic shear between SEC and NECC, from 200° to 600°N. The temperature variance, or the potential energy production, peaked closer to the equator at 300°N. The linear growth timescale of the instability was about 10 days.The time-variable wind supplied energy to the current fluctuations during the TIW off period, but for the TIW on period the wind energy input was reduced (at 110W) or even reversed (at 14000°W, between 100°S and 700°N), suggesting that air-sea interaction was important in the total energy balance of the waves. The effect of instability was to reduce the shear of the mean current and to warm the equatorial cold tongue. These calculations suggest that there exists a balance between energy production and dissipation in the TIWs.
[33] Kessler W S,Rothstein L M,Chen D K.

The annual cycle of SST in the eastern tropical Pacific, diagnosed in an ocean GCM

[J]. Journal of Climate,1998,11(5):777-799.

DOI      URL      [本文引用: 1]      摘要

The annual onset of the east Pacific cold tongue is diagnosed in an ocean GCM simulation of the tropical Pacific. The model uses a mixed-layer scheme that explicitly simulates the processes of vertical exchange of heat and momentum with the deeper layers of the ocean; comparison with observations of temperature and currents shows that many important aspects of the model fields are realistic. As previous studies have found, the heat balance in the eastern tropical Pacific is notoriously complicated, and virtually every term in the balance plays a significant role at one time or another. However, despite many complications, the three-dimensional ocean advection terms in the cold tongue region tend to cancel each other in the annual cycle and, to first order, the variation of SST can be described as simply following the variation of net solar radiation at the sea surface (sun minus clouds). The cancellation is primarily between cooling due to equatorial upwelling and warming due to tropical instability waves, both of which are strongest in the second half of the year (when the winds are stronger). Even near the equator, where the ocean advection is relatively intense, the terms associated with cloudiness variations are among the largest contributions to the SST balance. The annual cycle of cloudiness transforms the semiannual solar cycle at the top of the atmosphere into a largely 1 cycle yrvariation of insolation at the sea surface. However, the annual cycle of cloudiness appears closely tied to SST in coupled feedbacks (positive for low stratus decks and negative for deep cumulus convection), so the annual cycle of SST cannot be fully diagnosed in an ocean-only modeling context as in the present study. Zonal advection was found to be a relatively small influence on annual equatorial cold tongue variations; in particular, there was little direct (oceanic) connection between the Peru coastal upwelling and equatorial annual cycles. Meridional advection driven by cross-equatorial winds has been conjectured as a key factor leading to the onset of the cold tongue. The results suggest that the SST changes due to this mechanism are modest, and if meridional advection is in fact a major influence, then it must be through interaction with another process (such as a coupled feedback with stratus cloudiness). At present, it is not possible to evaluate this feedback quantitatively.
[34] Menkes C,Vialard J,Kennan S C,et al.

A modeling study of the impact of tropical instability waves on the heat budget of the eastern equatorial Pacific

[J]. Journal of Physical Oceanography,2006,36(5):847-865.

DOI      URL      [本文引用: 1]     

[35] Jochum M,Murtugudde R.

Internal variability of the tropical Pacific ocean

[J]. Geophysical Research Letters,2004,31(14):110-111.

DOI      URL      [本文引用: 2]      摘要

A 40 year integration of an eddy resolving numerical model of the tropical Pacific ocean is analyzed to quantify the interannual variability caused by internal variability of ocean dynamics. It is found that along the Pacific cold tongue internal variability contributes a significant amount to the observed interannual variability. This suggests that in this location the predictability of SST is limited to the persistence time of SST anomalies which is approximately 100 days. Furthermore, a comparison with other sources of variability suggests that internal variability may play an important role in modifying or setting up El Ni o.
[36] Seo H,Jochum M,Murtugudde R,et al.

Effect of ocean mesoscale variability on the mean state of tropical Atlantic climate

[J]. Geophysical Research Letters,2006,33(9):179-212.

DOI      URL      [本文引用: 1]      摘要

A regional coupled ocean-atmospheric model is used to investigate the effect of oceanic mesoscale features on the mean climate of the tropical Atlantic. It is shown that, compared to a non-eddy resolving ocean model, resolving oceanic mesoscale variability leads to a cooler mean equatorial cold tongue and a cooler coastal upwelling zone. This changes the meridional SST gradient, and the resulting weaker low-level convergence reduces the mean of rainfall in the marine Inter-Tropical Convergence Zone (ITCZ). The reduced rainfall and the cooler coastal upwelling regions represent a clear improvement of the model solution.
[37] Graham T.

The importance of eddy permitting model resolution for simulation of the heat budget of tropical instability waves

[J]. Ocean Modelling,2014,79:21-32.

DOI      URL      [本文引用: 1]     

[38] Ballabrera-Poy J,Murtugudde R,Busalacchi A J.

On the potential impact of sea surface salinity observations on ENSO predictions

[J]. Journal of Geophysical ResearchOceans,2002,107(C12).DOI:10.1029/2001JC000834.

URL      [本文引用: 2]      摘要

[1] Multiple regression analysis is used here to construct statistical prediction models for the El Ni01±o/Southern Oscillation (ENSO) to explore the potential impact of monitoring Pacific Ocean sea surface salinity (SSS) on prediction of equatorial Pacific sea surface temperature (SST). This study, one of the firsts focusing on the direct role of SSS in ENSO predictions, is motivated by proposed missions for remote sensing of SSS. A forward stepwise method is used to extract significant predictors of the Ni01±o-3 SST index from observed monthly anomalies of tropical SST, SSS, sea level, freshwater flux, and components of the wind stress. The results indicate that SSS monitoring would have small impact on the statistical nowcast (reconstruction) of ENSO but a potential role in the 609000912 month forecasts. Correlation maps show two regions of high correlation: an equatorial region (between 17000°E and 16000°W) and an off-equatorial region (between 17000°E and 14000°W and 500°S and 2000°S). Short lag correlations display the negative relationship between the warm phase of ENSO and the negative equatorial SSS anomalies related with the increase of local rainfall. Such an equatorial negative correlation coexists with an area of positive correlations off the equator. The region with positive correlations moves eastward as the lag increases, reaching the geographical limit of the SSS observations at 6 months lag. The region of negative correlation moves northward and becomes weaker as the lag increases (it is nonsignificant for 9 months lag). For lags longer than 9 months, significant positive correlations are found south of the equator (500°S0900091000°S). At these lags, positive salinity anomalies have the potential to modify the subsurface stratification of the western Pacific as they are subducted westward. Thus, the availability of continuous remotely sensed SSS data might add considerably to ENSO predictions at longer lead times as a result of SSS-induced changes in the subsurface density field.
[39] Kennan S C,Flament P J.

Observations of a tropical instability vortex

[J]. Journal of Physical Oceanography,2000,30(9):2 277-2 301.

DOI      URL      [本文引用: 1]     

[40] Vialard J,Menkes C,Boulanger J P,et al.

A model study of oceanic mechanisms affecting equatorial pacific sea surface temperature during the 1997-98 El Niño

[J]. Journal of Physical Oceanography,2001,31(17): 1 649-1 675.

DOI      URL      [本文引用: 2]     

[41] Jochum M,Cronin M F,Kessler W S,et al.

Observed horizontal temperature advection by tropical instability waves

[J]. Geophysical Research Letters,2007,34(9):252-254.

DOI      URL      [本文引用: 1]      摘要

Velocity data from moored current meters is combined with satellite sea surface temperature (SST) to compute oceanic mixed layer temperature advection by tropical instability waves (TIWs). For the years 2002 to 2005 it is found that this process heats the equatorial mixed layer at an annual mean rate of +0.8°C/month at 0°N, 140°W and +2.8°C/month at 0°N, 110°W. At 0°N, 110°W, approximately 25% of the heating is contributed by zonal temperature advection, a process that has often been assumed to be negligible. From a nine month segment of data (May 2004-February 2005), the zonal temperature advection at 2°N, 140°W has been estimated to be approximately 0.7°C/month, much larger than the equatorial value for the same time period. Thus, the data supports a recent hypothesis that tropical instability waves contribute a significant mean zonal temperature advection with off-equatorial maxima to the equatorial mixed-layer heat budget. Comparisons with numerical model results suggest that current ocean general circulation models can realistically simulate important aspects of tropical eddy-mixed layer interactions.
[42] Moum J N,Lien R C,Perlin A,et al.

Sea surface cooling at the Equator by subsurface mixing in tropical instability waves

[J]. Nature Geoscience,2009,2(11):761-765.

DOI      URL      [本文引用: 1]      摘要

Each month, Nature Geoscience will bring you top-quality research papers, reviews and opinion pieces - in print and online.
[43] Lee T,Lageriorf G,Gierach M M,et al.

Aquarius reveals salinity structure of tropical instability waves

[J]. Geophysical Research Letter,2012,39(12):12 610.

DOI      URL      [本文引用: 1]      摘要

Sea surface salinity (SSS) measurements from the Aquarius/SAC-D satellite during September-December 2011 provide the first satellite observations of the salinity structure of tropical instability waves (TIWs) in the Pacific. The related SSS anomaly has a magnitude of approximately 卤0.5 PSU. Different from sea surface temperature (SST) and sea surface height anomaly (SSHA) where TIW-related propagating signals are stronger a few degrees away from the equator, the SSS signature of TIWs is largest near the equator in the eastern equatorial Pacific where salty South Pacific water meets the fresher Inter-tropical Convergence Zone water. The dominant westward propagation speed of SSS near the equator is approximately 1 m/s. This is twice as fast as the 0.5 m/s TIW speed widely reported in the literature, typically from SST and SSHA away from the equator. This difference is attributed to the more dominant 17-day TIWs near the equator that have a 1 m/s dominant phase speed and the stronger 33-day TIWs away from the equator that have a 0.5 m/s dominant phase speed. The results demonstrate the important value of Aquarius in studying TIWs.
[44] Lee T,Lageriorf G,Kao H,et al.

The influence of salinity on tropical Atlantic instability waves

[J]. Journal of Geophysical Research,2014,119(12):8 375-8 394.

DOI      URL      [本文引用: 1]      摘要

Abstract Sea surface salinity (SSS) data derived from the Aquarius/SAC-D satellite mission are analyzed along with other satellite and in situ data to assess Aquarius' capability to detect tropical instability waves (TIWs) and eddies in the tropical Atlantic Ocean and to investigate the influence that SSS has on the variability. Aquarius data show that the magnitude of SSS anomalies associated with the Atlantic TIWs is 卤0.25 practical salinity unit, which is weaker than those in the Pacific by 50%. In the central equatorial Atlantic, SSS contribution to the mean meridional density gradient is similar to sea surface temperature (SST) contribution. Consequently, SSS is important to TIW-related surface density anomalies and perturbation potential energy (PPE). In this region, SSS influences surface PPE significantly through the direct effect and the indirect effect associated with SSS-SST covariability. Ignoring SSS effects would underestimate TIW-related PPE by approximately three times in the surface layer. SSS also regulates the seasonality of the TIWs. The boreal-spring peak of the PPE due to SSS leads that due to SST by about one month. Therefore, SSS not only affects the spatial structure, but the seasonal variability of the TIWs in the equatorial Atlantic. In the northeast Atlantic near the Amazon outflow and the North Brazil Current retroflection region and in the southeast Atlantic near the Congo River outflow, SSS accounts for 80 90% of the contribution to mean meridional density gradient. Not accounting for SSS effect would underestimate surface PPE in these regions by a factor of 10 and 4, respectively.
[45] Chelton D B,Wentz F J,Gentemann C L,et al.

Satellite microwave SST observations of transequatorial tropical instability waves

[J]. Geophysical Research Letters,2000,27(9):1 239-1 242.

DOI      URL      [本文引用: 1]      摘要

Satellite measurements of sea-surface temperature (SST) by the TRMM Microwave Imager reveal previously unreported features of tropical instability waves (TIWs). In the Pacific, TIW-related variability is observed from the eastern boundary to at least 160掳E. Cusp-shaped distortions of SST fronts and associated trains of anticyclonic vortices both north and south of the equator propagate westward at ~0.5 m s-1 with approximately 50% larger meridional displacements in the north. In the Atlantic, TIWs and associated anticyclonic vortices are clearly observed only on the north side of the equator where they propagate from the eastern boundary to the western boundary at ~0.3 m s-1.
[46] Hasson A,Delcroix T,Dussin R.

An assessment of the mixed layer salinity budget in the tropical Pacific Ocean: Observations and modelling (1990-2009)

[J]. Ocean Dynamics,2013,63(2/3):179-194.

DOI      URL      [本文引用: 1]     

[47] Gao Shan,Qu Tangdong, Nie Xunwei.

Mixed layer salinity budget in the tropical Pacific Ocean estimated by a global GCM

[J]. Journal of Geophysical Research,2014,119(12):8 255-8 270.

DOI      URL      [本文引用: 1]      摘要

Abstract The mixed layer salinity (MLS) budget of the tropical Pacific is investigated using results from a model of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO). The results focusing on the western Pacific freshwater pool indicate that the long-term averaged surface freshwater flux is well balanced by ocean dynamics, in which the subsurface processes account for the major part. The MLS budget shows significant seasonal and interannual variability, as a consequence of interplay among surface freshwater flux, advection, mixing, and vertical entrainment. On seasonal time scale, both the MLS and mixed layer depth are largely controlled by surface freshwater flux. The opposite phase between the subsurface processes and the barrier layer thickness confirms the important influence of the barrier layer on vertical mixing and entrainment from below. On interannual time scale, all the MLS budget terms show significant ENSO signal, which in turn is highly correlated with the salinity front and barrier layer thickness in the equatorial Pacific.
[48] Moum J N,Perlin A,Nash J D,et al.

Seasonal sea surface cooling in the equatorial Pacific cold tongue controlled by ocean mixing

[J]. Nature,2013,500(7 460):64-67.

DOI      URL      PMID      [本文引用: 1]      摘要

Sea surface temperature (SST) is a critical control on the atmosphere, and numerical models of atmosphere-ocean circulation emphasize its accurate prediction. Yet many models demonstrate large, systematic biases in simulated SST in the equatorial `cold tongues' (expansive regions of net heat uptake from the atmosphere) of the Atlantic and Pacific oceans, particularly with regard to a central but little-understood feature of tropical oceans: a strong seasonal cycle. The biases may be related to the inability of models to constrain turbulent mixing realistically, given that turbulent mixing, combined with seasonal variations in atmospheric heating, determines SST. In temperate oceans, the seasonal SST cycle is clearly related to varying solar heating; in the tropics, however, SSTs vary seasonally in the absence of similar variations in solar inputs. Turbulent mixing has long been a likely explanation, but firm, long-term observational evidence has been absent. Here we show the existence of a distinctive seasonal cycle of subsurface cooling via mixing in the equatorial Pacific cold tongue, using multi-year measurements of turbulence in the ocean. In boreal spring, SST rises by 2 kelvin when heating of the upper ocean by the atmosphere exceeds cooling by mixing from below. In boreal summer, SST decreases because cooling from below exceeds heating from above. When the effects of lateral advection are considered, the magnitude of summer cooling via mixing (4 kelvin per month) is equivalent to that required to counter the heating terms. These results provide quantitative assessment of how mixing varies on timescales longer than a few weeks, clearly showing its controlling influence on seasonal cooling of SST in a critical oceanic regime.
[49] Meinen C S,McPhaden M J.

Warm water displacements in the equatorial Pacific during 1993-1999

[J/OL]. Journal of Climate,2000.[2018-03-20]. .

URL      [本文引用: 1]     

[50] Wang Weimin,McPhaden M J.

What is the mean seasonal cycle of surface heat flux in the equatorial Pacific?

[J]Journal of Geophysical ResearchOceans,2001,106(C1):837-857.

DOI      URL      [本文引用: 1]      摘要

The mean seasonal cycles of six state-of-the-art surface heat flux products (three based on widely available data and three based on numerical model reanalysis fields) are compared in the equatorial Pacific with heat fluxes computed from Tropical Atmosphere-Ocean (TAO) buoy data. Net surface heat flux and individual flux components derived from these products exhibit large deviations from TAO. We find that a significant contribution to these differences, which are often 50 W mor more for net flux, is the use of systematically biased bulk variables in the computation of turbulent surface heat fluxes. We also find that for some products, compensating errors in bulk variables lead to fortuitous agreement with turbulent heat fluxes estimated from TAO. Finally, comparisons of TAO-derived fluxes with tuned and untuned heat flux estimates from the Comprehensive Ocean Atmosphere Data Set indicate better agreement with untuned fluxes, suggesting that commonly used ad hoc strategies to close the global ocean heat budget are not strictly valid.
[51] Zheng Fei,Fang Xianghui,Yu Jinyi,et al.

Asymmetry of the Bjerknes positive feedback between the two types of El Niño

[J]. Geophysical Research Letters,2014,41(21):7 651-7 657.

DOI      URL      [本文引用: 1]      摘要

082014. American Geophysical Union. All Rights Reserved. Corresponding to the pronounced amplitude asymmetry for the central Pacific (CP) and eastern Pacific (EP) types of El Ni09o, an asymmetry in the strength of the Bjerknes positive feedback is found between these two types of El Ni09o, which is manifested as a weaker relationship between the zonal wind anomaly and the zonal gradient of sea surface temperature (SST) anomaly in the CP El Ni09o. The strength asymmetry mainly comes from a weaker sensitivity of the zonal gradient of sea level pressure (SLP) anomaly to that of diabatic heating anomaly during CP El Ni09o. This weaker sensitivity is caused by (1) a large cancelation induced by the negative SST-cloud thermodynamic feedback to the positive dynamical feedback for CP El Ni09o, (2) an off-equator shift of the maximum SLP anomalies during CP El Ni09o, and (3) a suppression of the mean low-level convergence when CP El Ni09o events occur more often. Key Points Asymmetry of the Bjerknes positive feedback exits between the CP and EP El Ni09osThe strength of the Bjerknes positive feedback acts to induce the ENSO diversityThe asymmetry is caused by a different response of zonal SLP to diabatic heating
[52] Maes C,Picaut J, Belamari S.

Salinity barrier layer and onset of El Niño in a Pacific coupled model

[J]. Geophysical Research Letters,2002,29(24):5 951-5 954.

DOI      URL      [本文引用: 1]      摘要

The importance of the barrier layer during the onset of El Ni09o is investigated using a coupled ocean-atmosphere general circulation model. Sensitivity experiments are done by removing or keeping the salinity stratification in the upper layer of the western equatorial Pacific warm pool. The barrier layer favors the maintenance and displacement of the warm pool into the central Pacific by isolating the mixed layer from the entrainment cooling at depth and by confining the response of westerly wind events (WWEs) to a shallow mixed layer. The increased zonal fetch of WWEs through the coupling with sea surface temperature (SST) enhances downwelling equatorial Kelvin waves and thus leads to El Ni09o. In the absence of salinity stratification, slightly cooler SST and a reduced eastward displacement of the warm pool result in a reduced El Ni09o or a return to the mean seasonal cycle of the model. The possibility that the barrier layer affects the onset of El Ni09o pleads for a careful consideration of the salinity stratification in climate forecasts.
[53] Maes C,Picaut J, Belamari S.

Importance of the salinity barrier layer for the buildup of El Niño

[J]. Journal of Climate,2005,18(1):104-118.

DOI      URL      [本文引用: 1]      摘要

Several studies using sea level observations and coupled models have shown that heat buildup in the western equatorial Pacific is a necessary condition for a major El Nino to develop. However. none of these studies has considered the potential influence of the vertical salinity stratification on the heat buildup and thus on El Nino. In the warm pool, this stratification results in the presence of a barrier layer that controls the base of the ocean mixed layer. Analyses of in situ and TOPEX/Poseidon data, associated with indirect estimates of the vertical salinity stratification, reveal the concomitant presence of heat buildup and a significant barrier layer in the western equatorial Pacific. This relationship occurs during periods of about one year prior to the mature phase of El Nino events over the period 1993-2002. Analyses from a coupled ocean-atmosphere general circulation model suggest that this relationship is statistically robust. The ability of the coupled model to reproduce a realistic El Nino together with heat buildup, westerly wind bursts, and a salinity barrier layer suggests further investigations of the nature of this relationship. In order to remove the barrier layer, modifications to the vertical ocean mixing scheme are applied in the equatorial warm pool and during the 1-yr period of the heat buildup. At the bottom of the ocean mixed layer, the heat buildup is locally attenuated, as expected from switching on the entrainment cooling. At the surface, the coupled response over the warm pool increases the fetch of westerly winds and favors the displacement of the atmospheric deep convection toward the central equatorial Pacific. These westerly winds generate a series of downwelling equatorial Kelvin waves whose associated eastward currents drain the heat buildup toward the eastern Pacific Ocean. The overall reduction of the heat buildup before the onset of El Nino results in the failure of El Nino. These coupled model analyses confirm that the buildup is a necessary condition for El Nino development and show that the barrier layer in the western equatorial Pacific is important for maintaining the heat buildup.
[54] Chen Dake.

Upper ocean response to surface momentum and freshwater fluxes in the wester

[J]. Journal of Tropical Oceaography,2004,23(6):1-15.

[本文引用: 1]     

[55] Miller A J,Angell J K,Korshover J.

Tropical waves and the Quasi-Biennial Oscillation in the lower stratosphere

[J]. Journal of the Atmospheric Sciences,1976,33(3):430-435.

DOI      URL      [本文引用: 1]      摘要

By means of spectrum analysis of 11 years of lower stratospheric daily winds and temperatures at Balboa, Ascension and Canton-Singapore, evidence is presented supporting the existence of two principal wave modes with periods of about 11-17 days (Kelvin waves) and about 4-5 days (mixed Rossby-gravity waves). The structure of the two wave modes, as well as the vertical eddy momentum flux by the waves, is shown to be related to the quasi-biennial cycle, although for the mixed Rossby-gravity waves this is obvious only at Ascension. In addition, the Coriolis term, suggested as a source of vertical easterly momentum flux for the mixed Rossby-gravity waves, is investigated avid found to be of the same magnitude as the vertical eddy flux term. Finally, we have examined the mean meridional motion and the meridional eddy momentum flux for its possible association with the quasi-biennial variation.
[56] Zheng Fei,Zhang Ronghua.

Interannually varying salinity effects on ENSO in the tropical Pacific: A diagnostic analysis from Argo

[J]. Ocean Dynamics,2015,65(5):691-705.

DOI      URL      [本文引用: 2]     

[57] Zhang Ronghua,Busalacchi A J,Wang X J,et al.

Role of ocean biology-induced climate feedback in the modulation of El Niño-Southern Oscillation

[J]. Geophysical Research Letters,2009,36(3).DOI:10.1029/2008G2036568.

[本文引用: 1]     

[58] Hackert E,Ballabrera-Poy J,Busalacchi A J,et al.

Impact of sea surface salinity assimilation on coupled forecasts in the tropical Pacific

[J]. Journal of Geophysical ResearchOceans,2011:116(C5).DOI:10.1029/2010JC006708.

[本文引用: 1]     

[59] Zhao Mei,Hendon H H, Alves O,et al.

Impact of improved assimilation of temperature and salinity for coupled model seasonal forecasts

[J]. Climate Dynamics,2014,42(9/10):2 565-2 583.

DOI      URL      [本文引用: 1]      摘要

We assess the impact of improved ocean initial conditions for predicting El Ni09o-Southern Oscillation (ENSO) and Indian Ocean dipole (IOD) using the Bureau of Meteorology’s Predictive Ocean Atmosphere Model for Australia (POAMA) coupled seasonal prediction model for the period 1982–2006. The new ocean initial conditions are provided by an ensemble-based analysis system that assimilates subsurface temperatures and salinity and which is a clear improvement over the previous optimal interpolation system which used static error covariances and was univariate (temperature only). Hindcasts using the new ocean initial conditions have better skill at predicting sea surface temperature (SST) variations associated with ENSO than do the hindcasts initialized with the old ocean analyses. The improvement derives from better prediction of subsurface temperatures and the largest improvements come during ENSO–IOD neutral years. We show that improved prediction of the Ni09o3.4 SST index derives from improved initial depiction of the thermocline and halocline in the equatorial Pacific but as lead time increases the improved depiction of the initial salinity field in the western Pacific become more important. Improved ocean initial conditions do not translate into improved skill for predicting the IOD but we do see an improvement in the prediction of subsurface temperatures in the Indian Ocean (IO). This result reflects that the coupling between subsurface and surface temperature variations is weaker in the IO than in the Pacific, but coupled model errors may also be limiting predictive skill in the IO.

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