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赤道太平洋海洋过程在ENSO 及其变异中的作用
其他题名Roles of equatorial Pacific Ocean processes in ENSO and its variability
官聪1,2
学位类型博士
导师王凡 ; Michael J McPhaden
2017-05-27
学位授予单位中国科学院大学
学位授予地点北京
学位专业物理海洋学
关键词Enso 年代际跃迁 Enso 非对称性 El Nino Enso 反馈机制
其他摘要    厄尔尼诺-南方涛动(El Niño-Southern Oscillation,ENSO)是全球气候系统中最显著的年际变化信号,对太平洋乃至全球气候系统都有着重要的影响。虽然关于ENSO的研究已有长达半个世纪,近年来ENSO的低频变化及其在气候变化下的变异给已有的研究带来挑战。对ENSO动力学过程的理解是准确预报ENSO的基础,但迄今为止,在ENSO动力学方面仍有诸多重要的问题尚待解决。
    本论文构建了温度方差收支方程,并首次将其应用于ENSO机制研究。通过在赤道太平洋关键海域展开基于温度收支方程和温度方差收支方程的热收支分析,本文详细地定量诊断了各海洋反馈过程在ENSO的21世纪初期纬向跃迁、非对称性及2014–2016年暖事件中的不同贡献。使用多资料平均的方法提高了研究的可信度。论文得到的主要结论如下:
   
(1)21世纪初期ENSO海表温度(SST)变率发生纬向跃迁,相对于1980–1999年(P1时段),在2000年代(P2时段)ENSO SST变率在赤道东太平洋显著减弱,而在赤道中太平洋有增强趋势。同时,在P2时段中部(CP)型El Niño出现频率增加,而P1时段则主要是东部(EP)型El Niño主导。
    本论文使用七种再分析及模式资料,分别在Niño3区(5°S–5°N,150°W–90°W)和Niño4区(5°S–5°N,160°E–150°W)进行了温度收支方程和温度方差收支方程的计算。结果显示,温跃层反馈(TCF)和纬向平流反馈(ZAF)是产生ENSO SST变率的重要正反馈过程,而来自海表热通量的热阻尼作用(TD)是减弱SST变率的主要负反馈过程。伴随着2000年以后CP型El Niño事件增多,TCF显著减弱,而ZAF则变化不大。负反馈项同样也在2000年以后发生减弱,这主要体现在Niño3区TD的减小以及非线性作用(NL)在两个尼诺区域的减弱。对比P1时段,在P2时段Niño3区ENSO SST变率的减弱是由TCF的显著减弱导致的,而Niño4区ENSO SST变率的增强则与起负反馈作用的NL减弱有关。
    本文通过温度方差收支方程研究了各反馈过程在充放电振荡机制中的作用。本文将TCF拆分成与东西向“跷跷板”效应的分项和赤道-赤道外充放电过程的分项。结果显示,前者为对ENSO SST变率的Bjerknes正反馈作用,而后者体现了充放电过程在ENSO位相转换期间的贡献。后者在2000年代的减小导致了ENSO充放电过程的减弱。
   
(2)基于温度方差收支方程,本文诊断了各海洋反馈过程在ENSO非对称性中的作用,主要从以下三个方面展开:在Niño3区El Niño事件成熟期振幅高于La Niña事件,在Niño4区La Niña事件振幅高于El Niño事件,La Niña的持续时间要比El Niño长。
    结果发现,在Niño3区的ENSO事件发展期,El Niño比La Niña有更强的增长率,其原因是El Niño发展过程中更强的正反馈过程,其中ZAF的差别最大,Ekman反馈(EKF)次之,然后是TCF项。负反馈项的差别主要来源于TD的阻尼作用。而在Niño4区,La Niña较El Niño在发展期有更强的增长率,使其事件振幅强于El Niño。这主要取决于La Niña事件中较强的TCF,而ZAF在两者中的贡献相同。在Niño3区,ENSO事件在持续时间上的不对称性,是由于在事件衰退期La Niña较El Niño的衰减率小约0.2 °C2 mon-1。La Niña衰退期中仍存在较强的TCF正反馈过程阻碍了海温异常的衰亡。而TCF在其衰退期的非对称性来自于赤道与赤道外的充放电过程的不对称性。
   
(3)针对2014年El Niño事件为什么会夭折而又在2015年发生超级El Niño的原因,目前虽有少量研究并提出一些猜想,但并没有从定量分析海洋反馈过程角度来进行系统分析。本文基于温度收支方程,使用ECCO2并结合观测资料,在Niño3、Niño3.4和Niño4区详细诊断了各海洋反馈过程在2014–2016 ENSO暖事件中的具体贡献。2014年春季在强的西风事件驱动下,Niño3区混合层平均温度(MLT)异常最大,在5月份接近1°C,而后在6月份逐渐衰亡。从温度收支方程结果来看, 在3–5月份出现正异常,峰值最大振幅约0.8 °C mon-1,继而在5月中旬开始转为负值,由异常下沉流导致的EKF是支配MLT异常增暖的主要动力机制,最大贡献达到0.4 °C mon-1。由Niño3区西边界异常纬向流主导的ZAF是第二大正反馈过程(约为0.25 °C mon-1);TCF较小且滞后于 异常的变化。同时,EKF在2014年事件夭折过程中也扮演了重要角色,异常下沉流自4月份开始转变为异常上升流,将次表层冷水带到混合层使得MLT暖异常消亡。在6月份纬向流转为向西的异常流动,ZAF变成冷平流,加速了Niño3区暖异常的消亡。
    在2015年,西风爆发事件几乎贯穿整年,1–6月份出现在西太平洋,7月份以后爆发区域向东移动到日界线附近。在一系列的海气耦合过程中,赤道西太平洋出现SST冷异常,在中东太平洋SST暖异常加强,并在当年11月底成熟时峰值高达4°C。从Niño3区的温度收支方程结果来看, 首先在2015年春季出现峰值(约为0.45°C mon-1),ZAF和TCF对MLT的增暖发挥了重要的正反馈作用。在偏西的中东太平洋的Niño3.4区和Niño4区,TCF的正反馈过程要弱很多,ZAF成为主要的正反馈过程。
; El Niño-Southern Oscillation (ENSO) is the most prominent interannual signal in the global climate system, which has severely impacts on Pacific and even global climate system. Although lots of efforts have been made to ENSO study for half a century, its low-frequency variability and changes under climate changes keep challenging our current understanding. Understanding of ENSO dynamics is a basis of correct prediction of ENSO, but many important issues remain in ENSO dynamics to date.

In this study, we constructed a novel temperature variance equation, andapplied it into ENSO dynamics study for the first time. Through heat budget analysis based on temperature equation and temperature variance equations in key Niño regions, we investigated quantificationally the role of oceanic feedbacks in ENSO sea surface temperature (SST) variability shift at the beginning of 21st century, ENSO asymmetry and 2014–2016 ENSO warm events. Besides, our multi-model product based approach provides a robust assessment of dominant mechanisms that account for these ENSO variabilities in key Niño regions. Main results are shown below.

(1) SST variability associated with ENSO shows zonal shift at the beginning of 21st century. ENSO-related SST variability during 2000s (period P2) weakened significantly in the equatorial eastern Pacific but slightly increased in the central Pacific Ocean relative to 1980–1999 (period P1). Meanwhile, greater prominence of central Pacific (CP) El Niño events is clear during P2 relative to P1 when eastern Pacific (EP) events are dominant.
Temperature and temperature variance budgets are examined in the mixed layer of the Niño3 (5°S–5°N, 150°W–90°W) and Niño4 (5°S–5°N, 160°E–150°W) regions from seven reanalysis datasets and model products. Results show that, the thermocline feedback (TCF) and zonal advective feedback (ZAF) are the most important positive feedbacks for generating ENSO SST variance, while the thermodynamic damping (TD) from sea surface heat flux is the largest negative feedback for damping ENSO variance. As more CP El Niños after 2000, TCF experienced a substantial reduction while ZAF were less affected. Negative feedbacks likewise weakened after 2000, particularly TD in the Niño3 region and the nonlinear sink of variance in both Niño3 and Niño4 regions. Compared to P1, the decreased ENSO SST variability in Niño3 in the 2000s, is attributed to the significant decease in TCF, while the decease of negative NL contributes to the slight increase of ENSO SST variability in the Niño4 region.

The role of thermocline feedbacks in the recharge oscillator theory is also examined using the temperature variance equation.TCF is divided into two parts: one related to the zonal-gradient thermocline variability which contributes to the ENSO SST growth as a part of Bjerknes positive feedback; The other related to zonal-mean thermocline variability which reveals the recharge/ discharge oscillator and contributes to ENSO phase transitions. The decrease in the latter in the 2000s reveals the weakening of recharge/ discharge processes.

(2) Based on temperature variance budget, we diagnosed oceanic roles in these ENSO asymmetric features from three aspects: the amplitude of anomalous temperature during the mature phase of El Niño events is larger than La Niña in the Niño3 region;In the Niño4 region, the amplitude in mature La Niña is larger than El Niño;La Niña events basically last longer than El Niños.

In the Niño3 region, the larger temperature amplitude of El Niño than La Niña is attributed to a greater growth rate during El Niño development phase than La Niña events. Stronger positive oceanic feedbacks are found during El Niño development phase with the most contribution from ZAF, following by Ekman feedback (EKF) and TCF. Difference of the negative feedbacks between the El Niño and La Niñacevents is mostly from the damping effects of TD. However, in Niño4, La Niña has greater growth rate than El Niño, which is induced by larger effects of positive TCF in its developing phase while ZAF contributes equally to developing these two events. ENSO asymmetry in duriation in the Niño3 region, is because the decay rate in La Niña events is smaller than that of El Niño events by 0.2 °C2 mon-1. Strong positive TCF still persists during the decaying of La Niña, which slows down the fade of the cold events. It is shown that the difference of TCF between El Niño and La Niña decaying phases is induced by the asymmetry of discharge/ recharge processes between the equator and off-equator.

(3) Why did the predicted “super El Niño” fade out in the summer of 2014 and the following 2015 developed into one of the three strongest El Niño on record? Though there are already some hypothesis and studies, a systematic analysis from the perspective of quantitative analysis of oceanic feedbacks’ contribution is still absent yet. Using ECCO2 simulation outputs and satellite-based observations, we diagnosed oceanic processes in the evolution of 2014–2016 events, by executing temperature budgets in the Niño3, Niño3.4 and Niño4 regions. Driven by the westley wind busts (WWBs) in the spring of 2014, largest temperature anomalies appeared in the Niño3 region, close to 1°C in May, but suddently declined in June–July. From the perspective of temperature budget, the mixed layer temperature (MLT) anomaly shows positive tendency during March–May with a large peak of about 0.8 °C mon-1, and tends negative in mid-May. EKF induced by anomalous downwelling is the dominant process in leading this warming, with a peak of 0.4°C mon-1. ZAF by the anomalous eastward current at the west boundary contributes secondly (about 0.25°C mon-1). TCF is relatively smaller and lags behind the MLT tendency. EKF also plays a significant role in its decay, which is caused by anomalous upwelling beginning in April. This upwelling brings relatively colder water into the mixed layer and thus contributes to the decay of warm MLT. ZAF also helps kill the warm MLT as a cold advection induced by westward current anomalies in June.

Strong WWBs appeared almost throughout the whole year 2015, in the west Pacific during the first half year and moved eastward to the dateline afterward. Under intense air-sea interactions, warm SSTs show up in the mid-east Pacific with a peak about 4°C in November while cold SSTs appear in the west. Based on temperature budget in the Niño3 region, there is a positive MLT tendency in boreal spring peaking at 0.45°C mon-1 ZAF and TCF play significant roles in this warming tendency as positive feedbacks. In the west Niño3.4 and Niño4 regions, the role of TCF is weakened and thus ZAF becomes the dominant positive feedback. 
学科领域物理海洋学
语种中文
文献类型学位论文
条目标识符http://ir.qdio.ac.cn/handle/337002/136651
专题海洋环流与波动重点实验室
作者单位1.中国科学院海洋研究所
2.中国科学院大学
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官聪. 赤道太平洋海洋过程在ENSO 及其变异中的作用[D]. 北京. 中国科学院大学,2017.
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