海洋科学  2020, Vol. 44 Issue (8): 1-7   PDF    
http://dx.doi.org/10.11759/hykx20191213001

文章信息

冯俊乔, 鲁云龙, 官聪, 袁欣, 惠玉超, 武杰. 2020.
FENG Jun-qiao, LU Yun-long, GUAN Cong, YUAN Xin, HUI Yu-chao, WU Jie. 2020.
副热带太平洋海气变异对ENSO影响的研究进展和展望
Influence of subtropical Pacific variability on ENSO: Review and advance
海洋科学, 44(8): 1-7
Marina Sciences, 44(8): 1-7.
http://dx.doi.org/10.11759/hykx20191213001

文章历史

收稿日期:2019-12-13
修回日期:2020-01-20
副热带太平洋海气变异对ENSO影响的研究进展和展望
冯俊乔1,2,4,5, 鲁云龙1,2,3,4, 官聪1,2,4,5, 袁欣1,2,3,4, 惠玉超1,2,3,4, 武杰1,2,3,4     
1. 中国科学院 海洋环流与波动重点实验室, 山东 青岛 266071;
2. 中国科学院海洋研究所, 山东 青岛 266071;
3. 中国科学院大学, 北京 100049;
4. 中国科学院 海洋大科学研究中心, 山东 青岛 266071;
5. 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
摘要:El Niño-Southern Oscillation(ENSO)是热带太平洋海气作用最强的年际信号,其变化会引起全球气候异常,对东亚季风具有重要影响。2000年后中部型El Niño频繁发生,掀起了ENSO多样性研究热潮;El Niño的复杂性也对ENSO理论研究和预测提出了新的挑战。为进一步理解并深入研究ENSO物理机制,本文总结了近年来对两类ENSO的最新认识;特别对副热带太平洋通过海气界面“大气桥”和太平洋副热带-热带经向环流圈的内部经向翻转环流这一“海洋通道”与热带太平洋建立联系的相关成果进行了阐述,并对存在的关键问题进行了展望。
关键词太平洋副热带-热带经向翻转环流    副热带南太平洋经向模    副热带北太平洋经向模    ENSO    
Influence of subtropical Pacific variability on ENSO: Review and advance
FENG Jun-qiao1,2,4,5, LU Yun-long1,2,3,4, GUAN Cong1,2,4,5, YUAN Xin1,2,3,4, HUI Yu-chao1,2,3,4, WU Jie1,2,3,4     
1. Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao 266071, China;
2. Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China;
3. University of Chinese Academy of Sciences, Beijing 100049, China;
4. Center for Ocean Mega-Science, Chinese Academy of Sciences, Qingdao 266071, China;
5. Pilot National Laboratory for Marine Science and Technology(Qingdao), Marine Dynamics Process and Climate Functional Laboratory, Qingdao 266237, China
Abstract: The El Niño/Southern Oscillation (ENSO) is the strongest air-sea coupled interannual signal in the tropical Pacific. Its variation can induce severe global climate change and has a significant impact on the eastern Asian monsoon. Compared to the eastern Pacific El Niño, the occurrence frequency of the central Pacific El Niño has in-creased since 2000. The complexity of ENSO brings new challenges for its theoretical studies as well as its prediction. This paper reviews the related studies on two types of ENSO. We particularly pay attention to the influence of the subtropical variability on ENSO, through both the "atmospheric bridge" and "ocean tunnel." The North Pacific meridional mode (NPMM) and South Pacific meridional mode (SPMM) are two important components linking the subtropical and tropical Pacific via air-sea interaction; the interior branch of the subtropical-tropical cells (STC) is the important "ocean tunnel." The scientific issues are proposed, providing a reference for further study.
Key words: subtropical-tropical Pacific meridional overturning circulation    subtropical South Pacific meridional mode (SPMM)    subtropical North Pacific meridional mode (NPMM)    ENSO    

El Niño-Southern Oscillation(ENSO)是热带太平洋海气相互作用最强的年际信号。它的发生会在全球引起巨大的气候异常, 也是引起我国气候变化的重要因素。过去二十年, 相比东太平洋增暖的传统东部型El Niño, 海温异常中心位于中西太平洋的中部型El Niño频繁发生[1-3]。2015/16年发生了21世纪第一个极端El Niño事件, 造成世界许多地区出现强烈的灾害性天气。不同于1982/83和1997/98年均为东部型的极端El Niño事件, 此次事件成熟时赤道中西太平洋Niño4区的海表面温度异常(SSTA)创历史新高(达1.7℃), 对流中心较1982/83和1997/98时西移了20个经度, 赤道东太平洋SSTA扩展到日界线以西, 同时显示了中部型和东部型El Niño的共同特征[4-6]。鉴于其最大异常中心在赤道中太平洋, Palmeiro等[7]将其归为中部型El Niño事件。此次极端El Niño的复杂性为ENSO理论研究和预测提出了新的挑战, 甚至大部分模式未能成功预报2010—2014以及2017/18年的ENSO事件[8-10]。两类ENSO在时空分布、遥相关模态及演变上都有明显区别, 对全球气候的影响亦有很大差异[11-12]。ENSO对我国气候的影响也取决于其爆发时间、发展阶段、类型等[13-15]。比如, 在El Niño发展期的夏季, 华北偏旱; 而在其衰减期, 长江流域往往发生洪涝[16-18]。华南地区在东部型El Niño衰减期的春季降雨增多, 而在中部型El Niño衰减期的春季则偏少[11]。因此, 研究ENSO对深刻理解海气相互作用过程和机制, 提高我国乃至全球气候预测能力具有重要的科学意义和实用价值。

1 ENSO理论

关于ENSO传统的较成熟且广泛流行的理论都强调次表层海洋对大气驱动的滞后响应过程。比如, 源于赤道波动的延迟振子理论[19]、西太平洋振子理论[20]、基于赤道和赤道外水体交换的充放电理论(Recharge-Discharge, RD理论)[21-22]。“充放电”机制主要包括两种动力学过程[23]:一是常发生于赤道东太平洋的温跃层反馈机制—温跃层温度异常通过垂向平流反馈到海表面温度(SST)[22, 24]; 二是常发生于赤道中太平洋的水平温度平流反馈机制[25]。这两种动力过程通过温跃层深度和海洋环流的地转平衡建立动力学联系, 促进ENSO发展和演变[26-27]

2000年后中部型El Niño发生频繁, 对传统理论提出了挑战。有研究认为, 全球变暖影响下, 赤道东风减弱, 温跃层变平, 温跃层反馈从赤道东太平洋移至中太平洋, 引起中部型El Niño发生[1]。与此相反, 鉴于1998/99年以后, 赤道信风增强, 东/西太平洋明显变冷/暖, 有学者将其归因于东风和纬向SST梯度增强的结果[28-29]。许多学者认为温跃层反馈是推进东部型El Niño发展的重要机制; 中部型El Niño由水平温度平流反馈驱动[30-31]。Ramesh等[32]的研究结果表明两类El Niño都起源于西太平洋次表层, 早在其发生前一年的夏秋季节, 温跃层海温就开始出现暖异常并向东传播; 当暖信号到达中太平洋后, 也可以通过温跃层反馈机制影响SSTA, 促进中部型El Niño发展[33]。Ashok等[1]认为东传的赤道Kelvin波和西传的赤道Rossby波共同引起的中太平洋温跃层变化是中部型El Niño产生的物理机制。Xu等[33]指出虽然两类El Niño期间, 赤道次表层海温异常均显示向东传播的特征, 但中部型El Niño的初始建立和发展是海气相互作用的结果; 温跃层反馈在后期发展中起重要作用。Dewitte等[34]基于模式数据指出中部型El Niño充电期间中太平洋水平温度平流反馈的贡献大于东太平洋的温跃层反馈, 放电期间由垂向温度平流减弱SSTA。Guan等[35]的最新研究表明在ENSO期间温跃层反馈和水平温度平流反馈在赤道中、东太平洋均起重要作用, 二者在不同时期的不同特征是导致El Niño和La Niña在强度、持续时间上呈现不对称性的主要物理机制。Zhang等[36]提出了中太平洋北赤道逆流区次表层海温通过夹卷引起SST暖异常, 进而在海气耦合作用下导致El Niño发展的新机制; 他们认为赤道中太平洋次表层海温暖异常的持续存在对2015/2016年El Niño事件的产生起至关重要的作用[37]。最近, Abellán等[38]发现经向温度平流和垂向温度平流对2015/2016年El Niño事件的发展均具有很大贡献。

2 副热带南北太平洋经向模和ENSO

越来越多的学者强调赤道外过程对ENSO的贡献。Vimont等[39]最先提出季节性足迹机制, 认为北太平洋涛动作为北半球冬季大气变率的第二模态, 其南半支通过调节东北信风的强度, 从而改变潜热通量, 在副热带北太平洋留下海表面温度(SST)异常足迹, 同时与信风相互作用, 在风–蒸发–海表面温度反馈(WES)[40]作用下向西南传播, 在赤道太平洋引起类ENSO变率。Chiang等[41]提出SST经向梯度和热带辐合带(ITCZ)的变化存在一定联系, 并对副热带北太平洋经向模(NPMM, 图 1a)的产生有一定影响。Chang等[42]研究表明, 大多数El Niño事件之前都有明显的NPMM事件, 是北太平洋大气变率影响ENSO的重要通道。Larson等[43]肯定了NPMM作为ENSO预报因子的有效性, 但也发现其预报技巧很低。

图 1 (a) 副热带北太平洋SSTA和10 m风场在二者SVD分解系数上的回归场; (b)副热带南太平洋SSTA和10 m风场在二者SVD分解系数上的回归场 Fig. 1 Regression maps of the SVD leading mode SST-normalized expansion coefficients for the SST and 10-m wind vectors in the northern Pacific (a) and southern Pacific (b). The SVD analysis was conducted between the SSTA and both components of the 10-m wind in the extratropical northern and southern Pacific, respectively, in (a) and (b) 注:填色和矢量箭头分别为超过95%置信度检验的SSTA和10 m风场

Yu等[44]认为中部型El Niño的初始暖异常海温是由副热带北太平洋驱动。有研究认为2015/2016年极端中部型(或混合型)El Niño很大程度上由副热带驱动[45], 且与东北太平洋暖斑有关[46]; 其结束后2016年未出现La Niña也是源于副热带太平洋SSTA的作用[47]。在2014年初, 许多预测会发生强度堪比1997/1998年El Niño的暖事件, 然而, 到夏季暖异常增长停滞。有研究认为这可能与副热带南太平洋经向模(SPMM, 图 1b)有关[48]。Zheng等[49]发现SPMM东极子区的东南太平洋异常海温信号向赤道传播过程中, WES反馈不是必须的, 信风的变化也可引起赤道太平洋次表层变化, 从而引起ENSO事件。有研究表明, 南北半球的经向模态与不同类型的ENSO之间有重要的联系, SPMM有利于东部型El Niño, NPMM更有利于中部型El Niño的发生[50]。Min等[48]也指出副热带南、北太平洋经向模分别影响赤道东、中太平洋SST异常。有学者提出南太平洋经向模的强度和位相可有效预测El Niño发展及其类型[51]。南、北太平洋副热带经向模通过海气热力过程, 比如, WES机制、云-SST负反馈机制等, 影响赤道太平洋SSTA, 从而影响两类El Niño[47-48]

3 太平洋副热带–热带经向翻转环流与ENSO

副热带太平洋除了通过海气“大气桥”过程和热带建立联系外, 通过副热带-热带经向环流圈的内部输运(STC)影响赤道也是一个非常重要的动力过程[52]。STC主要由10°N/S—20°N/S之间的副热带风应力驱动[52], 通过经向热量输送和赤道上升流显著影响热带太平洋年际-年代际变化[53-54]。赤道SST和STC在年际和年代际尺度存在显著负相关关系, ENSO与STC的年际变化高度相关[55]。甚至有学者认为热带太平洋SST的年代际变化有可能通过ENSO对大气的响应而受控于STC[56]

太平洋副热带–热带经向环流圈(STCs)提供了联系副热带和热带的海洋通道。STCs包括:海表面50 m以浅极向的Ekman流、沿等密度面流向赤道的西边界流(北半球的棉兰老流和南半球的新几内亚沿岸流)和核心位于250 m以浅的内部经向翻转环流(STC), 以及赤道上升流[57-58]。由海表面风场驱动的STCs在调整热带-副热带之间的海洋热量和质量分布中起着至关重要的作用[59-60]。2000年后, 中部型El Niño频繁发生时期, 次表层海洋动力过程与ENSO的关系发生了显著变化, 是ENSO预报技巧降低的主要原因[61-62]。作为重要的次表层海洋动力过程之一的STC是驱动赤道水体充放的直接变量, 在ENSO演变中扮演重要角色。

源于赤道水体辐合/辐散的充放电(RD)理论亦将STC和ENSO紧密联系在一起。Chen等[63]定性描述了RD理论框架中, 赤道SST、内部STC和WWV之间显著的相关关系以及与之相关的ENSO演变过程。Ren等[64]认为RD理论在中部型ENSO中依然成立。Singh等[65]的研究结果显示该理论仅适用于东部型ENSO; 在中部型ENSO期间, 西太平洋的极向输运(放电)同东太平洋的向赤道输运(充电)相抵消, 赤道太平洋暖水体积变化(简称WWV)主要由水平输送决定。Feng等[66]基于海洋再分析资料发现赤道太平洋上层海洋热含量在东部型El Niño期间完成一次充放电过程约需42个月, 而中部型El Niño期间长达55个月。最近, Lu等[67]通过分析观测资料发现赤道西太平洋边界流区纬向输送对WWV起重要作用, 可超前STC辐合4~5个月。2000年前, 表征STC辐合/辐散的WWV与SST高度相关, 前者超前后者6~9个月, 是ENSO预测的有效因子; 2000年后, 与ENSO相关的SST异常振幅减弱、周期变短, WWV仅超前SST 2~3个月, 并且相关系数大大降低[68-69]。Lu等[70]认为赤道外风应力变化及其驱动的赤道热含量变化是引起ENSO周期变化的重要原因之一。同时, 20世纪60—90年代太平洋STCs减弱期间, 东部型El Niño比较活跃; 但2000年以后, STCs增强期恰好对应着中部型El Niño活跃[71-72]。因此, STC的变化可能与El Niño类型的变化有着千丝万缕的联系。

已有学者通过数值模拟数据对STC在两类El Niño中的变化规律进行了初步探索。Hasegawa等[73]发现1995—2003年期间的12次El Niño事件中, 仅有7次事件在其成熟后存在赤道上层热含量“放电”, 其中南太平洋STC输运起重要作用,并且这7次事件大部分是东部型El Niño事件:其余5次多数为中部型El Niño事件。也有研究认为与ENSO相关的STC输送在北半球大于南半球, 北半球WWV交换在充放电理论中更重要, 南半球STC输运与西边界流相互抵消[74-75]。Feng等[76]发现无论是沿9°N还是9°S的STC与ENSO的关系在不同类型期间均具有显著差异, 并提出STC通过西传Rossby波影响西太平洋温跃层深度是其影响ENSO的重要动力过程之一。

4 小结和展望

综上所述, 副热带-热带的物质能量交换对ENSO循环具有重要作用, 关于副热带变量对两类El Niño影响的研究已引起人们的广泛关注。以往学者对联系副热带-热带的“大气桥”进行了大量研究, 特别是SPMM和NPMM。也有学者关注联系二者的“海洋通道”, 即副热带-热带经向圈的内部分支经向翻转环流。目前仍然存在一些争议和悬而未决的问题, 比如, NPMM和SPMM和热带太平洋的关系尚不明确, 二者对两类El Niño发展演变的相对贡献和物理机制尚有争议; 由于海洋观测资料的匮乏及其时空分辨率的限制, STC变异规律在两类El Niño期间的差异及其对两类El Niño发展的作用和机理还不完全清楚, 是亟需解决的前沿科学问题。温跃层反馈、经向/纬向温度平流反馈是El Niño演变中至关重要的海洋动力过程, 各反馈机制的相对强度、时空分布特征决定了El Niño的发展和类型。副热带太平洋如何影响这些反馈机制最终影响El Niño将是问题的关键。当今世界, 海洋大气观测网日臻完善, 特别是在海洋, Argo资料、TAO/TRITON浮标资料以及融合观测数据的多源海洋再分析数据的丰富为我们从观测数据深入了解SPMM、NPMM以及STC在两类El Niño中的差异及其如何影响两类El Niño提供了机遇。

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