热带太平洋西风异常对ENSO事件发生的作用

本文从观测资料对80年代两次ENSO事件产生过程中，热带太平洋西风异常及其对赤道中、东太平洋表层海温增暖的作用进行了分析和比较。分析结果表明:在这两次ENSO事件的产生过程中，赤道西太平洋上空均有较大的西风异常，并且它由赤道西太平洋向赤道中、东太平洋传播，随着西风异常从西向东传播，赤道中、东太平洋的表层相继增温。分析还表明，1982／1983年ENSO事件发生过程中，热带太平洋西风异常的强度要比1986／1987年热带太平洋西风异常强得多，这使得1982／1983 ENSO事件的强度比1986／1987_ENSO事件强得多。为了说明热带西太平洋西风异常对赤道中、东太平洋ENSO事件发生的作用，本文还利用IAP太平洋环流模式对西风异常在ENSO事件产生过程中的作用进行了数值模拟。模拟的结果说明了热带太平洋的西风异常对赤道太平洋暖水的向东传播和赤道中、东太平洋的增温起了很重要作用，这与观测事实分析一致。     

赤道印度洋—太平洋地区海气系统的齿轮式耦合和ENSO事件

利用历史观测数据,研究了印度洋海表温度（SST）的季节变化特征,证实赤道印度洋和东太平洋SST年际变化有显著的正相关,指出这种正相关是由于沿赤道印度洋上空纬向季风环流和太平洋上空Walker环流之间显著的耦合造成的。这两个异常的纬向环流圈之间的耦合形式看起来很象是存在于赤道印度洋和太平洋上空的一对齿轮（简写为GIP）,当一个作顺时向变化时,另一个则作反时向变化。文中还证明ENSO事件与GIP的年际异常存在很好的对应关系,暖事件时GIP为反向运转;冷事件时GIP为正向运转;异常的GIP的啮合点位于印尼群岛附近。对80年代以来的ENSO事件的分析表明,每次事件前期异常的GIP的啮合点首先出现在印度洋上空,然后逐渐传入太平洋,引起GIP东侧的大气纬向风#Au#a和SST同时发生异常变化。当这种风场和SST的异常变化发展东传到达赤道中东太平洋时,导致ENSO事件最终出现。本文由此指出印度洋上空纬向环流的异常可以通过印度洋和太平洋上空大气系统的齿轮式耦合去影响赤道中东太平洋的海-气相互作用并触发ENSO事件发生。     

太平洋年代际振荡冷、暖背景下ENSO循环的特征

利用英国气象局哈德莱中心的月平均海温距平资料、美国Scripps海洋研究所联合环境分析中心(JEDAC)的海表和次表层海温观测资料以及NCEP/NCAR再分析资料,研究了太平洋年代际振荡(PDO)不同背景下ENSO循环的特征.结果表明,PDO为ENSO循环提供了一个年代际气候背景,在PDO的暖位相时期,El Nino事件发生的频率较高,强度较强;反之,在PDO的冷位相时期,La Nina事件发生的频率较高,强度较强.而且在不同的太平洋年代际振荡背景下,ENSO循环表现出不同的特征.在PDO冷位相时期,发生El Nino(La Nina)事件时,正(负)的SOTA从西太平洋沿温跃层向东传播,正(负)的SSTA从赤道东太平洋向西扩展到中太平洋,ENSO事件先在赤道东太平洋爆发.在PDO的暖位相时期,发生El Nino(La Nina)事件时,正(负)的SOTA首先出现在赤道中太平洋,然后沿温跃层向东传播,正(负)的SSTA从赤道中太平洋向东扩展到东太平洋,ENSO事件首先在中太平洋爆发.这为ENSO预测提供了新的线索.     

印度洋对ENSO事件的响应:观测与模拟

观测事实显示,在El Ni(n～)o期间,伴随着赤道中东太平洋表层海温(SST)的升高,热带印度洋SST出现正距平.作者利用海气耦合模式模拟了印度洋对ENSO事件的上述响应,并进而讨论了其物理机制.所用模式为法国国家科研中心Pierre-Simon-Laplace 全球环境科学联合实验室(IPSL)发展的全球海气耦合模式.该模式成功地控制了气候漂移,能够合理再现印度洋的基本气候态.观测中与ENSO相关的热带印度洋SST变化,表现为全海盆一致的正距平,并且这种变化要滞后赤道中东太平洋SST变化大约一个季度,意味着它主要是对东太平洋SST强迫的一种遥响应,模式结果也支持这一机制,尽管模式中的南方涛动现象被夸大了,使得模拟的与ENSO相关联的SST正距平的位置南移,阿拉伯海和孟加拉湾被负距平(而不是正距平)所控制.研究表明,东太平洋主要通过大气桥影响潜热释放来影响印度洋SST变化.赤道东太平洋El Ni(n～)o事件的发展,导致印度洋上空风场异常自东而西传播;伴随着风场的变化,潜热发生相应变化,并最终导致SST异常的发生.非洲东海岸受索马里急流控制的海域,其SST的变化不能简单地利用热通量的变化来解释.证据显示,印度洋的增暖是ENSO事件发生的结果而不是其前期信号.     

南太平洋6—8月SST异常的主要模态及其与ENSO和SAM的联系

利用1951—2006年NCEP/NCAR再分析资料,采用EOF分析方法提取了南太平洋6—8月SST异常变化的两个主要模态,结果表明:EOF1主要反映了ENSO信息,EOF2与南半球环状模(SAM)存在关联。EOF1以年际时间尺度变化为主,EOF2的周期接近于年代变化。EOF1的时间序列与同期Ni?o3指数存在显著相关,EOF2的时间序列与3—5月SAM指数存在显著相关。分析EOF1、EOF2与我国夏季降水的相关场,得出6—8月南太平洋SST出现西低东高型分布的年份我国华北地区夏季降水偏少,出现西高东低型分布的年份则降水偏多。3—5月出现中纬度和赤道东太平洋SST异常偏低,而赤道中太平洋SST异常偏高分布型的年份,我国江南地区夏季降水偏多;出现相反的分布型则降水偏少。     

厄尔尼诺过程赤道太平洋海温增暖与传播特征

本文首先使用标准EOF时间系数主分量的谱分析探讨了赤道太平洋海面温度（EPSST）的长期变化情况，尔后使用CEOF及其恢复场分析了厄尔尼诺过程EPSST的增暖和传播特征。结果表明，按照厄尔尼诺过程的增暖范围，可概括为三类:①中东太平洋增暖型；②整个赤道太平洋增暖型；③东太平洋增暖型。而按厄尔尼诺发展过程的传播特征，也可将其分为三类:①东太平洋增暖西传型；②东西太平洋同时增暖尔后向中太平洋传播型；③西太平洋首先增暖尔后东传型。     

GOALS模式对热带太平洋ENSO年际变化特征的模拟评估

文中重点分析了中国科学院大气物理研究所LASG最新发展的全球大气环流谱模式(R42L9)与一全球海洋环流模式(T63L30)耦合形成的全球海洋-大气-陆面气候系统模式(GOALS/LASG)新版本已积分30 a的模拟结果,通过与多种观测资料的对比分析,讨论了赤道太平洋海表温度(SST)的年际变化及其纬向传播、赤道东太平洋SST异常与其他洋面SST变化之间的遥相关关系、赤道太平洋浅表层海温的年际变化特征等研究内容.结果表明,COALS模式模拟出了赤道太平洋SST异常出现不规则的年际变化特点;赤道东太平洋SST异常的向西传播过程;赤道太平洋混合层海温变化由西向东、由深层向浅层的传播过程;同时也模拟出了赤道东太平洋SST变化与赤道西太平洋以及与西南太平洋海温之间的反相关关系,与南印度洋和副热带大西洋SST之间的正遥相关关系等实际观测现象.但COALS模式也存在明显的不足,如对赤道东、中太平洋SST异常的年际变化幅度明显偏小,没能模拟出赤道东太平洋的SST变化比赤道中太平洋强的特点;赤道太平洋SST从东向西的传播速度明显比实际观测慢得多,但混合层海温极值变化由西向东的传播速度明显比实际情况快得多;没能模拟出赤道东太平洋SST变化同西北太平洋SST的负相关和北印度洋海温变化的正相关现象,因此也影响了对南亚、东南亚降水年际变化的模拟能力.     

冬季北太平洋海温主模态在1990年前后调整及其成因初探?

太平洋年代际振荡(PDO)和北太平洋涡旋振荡(NPGO)是北太平洋(20°~60°N,120°E~120°W)海温(SST)的EOF前两个模态,本文通过比较1990年前后北太平洋冬季SST EOF前两个模态,揭示了PDO和NPGO在1990年前后特征,并从关键区海温变化、北太平洋涛动(NPO)、赤道太平洋中部变暖(CPW)和北极涛动(AO)的影响,揭示了北太平洋主模态在1990年之后调整的成因。我们发现,1990年之前,北太平洋SST场的EOF前两个模态与PDO和NPGO的空间结构类似,但是在1990年之后,SST的EOF第一模态的最大荷载中心向日界线移动,40°N以北的太平洋被正的SST异常控制,表现出与NPGO模态的负位相相似的空间分布特征,而EOF第二模态由偶极子演变成了三极子结构。北太平洋中部(28°~36°N,152°~178°W)和北太平洋北部(44°~49°N,151°~177°W)海温距平在1990年之后呈显著的负相关变化,是导致在1990年之后冬季NPGO成为主模态的内部原因,而NPO在1990年之后的显著增强则是重要的外部原因。分析显示,NPO在1980年开始表现出增强趋势,通过风生流机制,NPO可以增强北太平洋45°N附近的气压梯度和西风异常幅度,从而导致了1990年之后NPGO海温模态的加强。虽然CPW和AO对NPO的南支(夏威夷)和北支(阿拉斯加)的海平面气压异常中心加强有贡献,但是上述两个因子与NPGO之间的关系在1990年之前并不明显。因此,CPW和AO与NPGO之间并不存在稳定的物理联系。     

1976—1977年及1982—1983年厄尔尼诺事件过程差异的年代际背景

本文根据NMC客观分析资料进行诊断分析,揭示了1976－77年及1982－83年这两次ElNino事件海气相互作用过程中的若干特征。结果表明：70年代末赤道西风异常背景场、SST背景场及其准4年周期振荡均存在年代际变化;SST背景场冷（暖）、80°W-0°E赤道区域850hPa纬向风异常西（东）风以及赤道太平洋SSTA准4年周期振荡东传型（西传型）共同决定了这两次事件的强度与类型的差异,即1976／77年事件为较弱的东部型EINino事件,而1982／83年事件表现为很强的西部型EINino事件。     

热带海表温度和低层风场的年际变率及与ENSO循环的相关特征研

采用NCEP/NCAR的1979～1998年逐月平均的海表温度及1000hPa风场资料,进行滤波和均方差计算,得到了热带太平洋、印度洋、大西洋海表温度(SST)和风场的年际变化特征.用旋转主分量(RPC)方法和投影法对热带三大洋海表温度距平(SSTA)进行分析,得到了各大洋SSTA演变的主要时空特征和相应的距平风场特征;并用相关分析研究热带三大洋与ENSO相关的特征,得到三大洋间的同期相关关系为印度洋SSTA与赤道东太平洋SSTA成正相关,而赤道东大西洋SSTA与赤道东太平洋SSTA成弱的负相关;赤道印度洋在落后于赤道东太平洋3个月左右时正相关达到最大,赤道大西洋在超前于赤道东太平洋6个月左右时负相关达到最大;热带印度洋和大西洋与ENSO相关的分量对各自大洋海表温度年际变化的方差贡献数值相近,最大在40%以上,平均解释方差分别为14%和12%.     

冬季赤道太平洋不同类型海温异常表征指数的再构建

利用1963—2013年Hadley中心月平均海表温度资料,以及NCEP/NCAR再分析资料,根据两类厄尔尼诺事件发生时北半球冬季赤道太平洋地区海温异常的不同空间分布特征,即赤道中太平洋CP型和东太平洋EP型海温异常空间分布,从寻找与之相似的空间型角度出发,设计了一组新的海温异常指数I_(CP)和I_(EP)。与以往ENSO指数相比,新指数组I_(CP)和I_(EP)不仅表示了空间上相互独立的海温异常分布,而且在相同的研究时段内,因时间域上相互独立而能更好地表征和区分两类El Ni?o/La Ni?a事件。据此,采用该新指数组探讨了与中部型和东部型海温异常事件相关的热带太平洋的主要海气耦合特征。结果表明,与传统的东部型El Ni?o事件发生时最大暖海温中心位于赤道东太平洋地区不同,中部型El Ni?o事件,异常增暖中心位于赤道中太平洋。中部型时异常Walker环流的上升支向西偏移,异常降水集中于热带中太平洋,不似东部型时异常限定于赤道东太平洋地区。不论哪类事件,海洋性大陆均可受到影响,即CP或EP型El Ni?o发生时,海洋性大陆区域降水偏少。但比较而言,中部型ENSO对海洋性大陆区域的影响更大。     

NCEP/CFSR再分析耦合资料中MJO对ENSO的影响研究

基于1979—2008年NCEP/CFSR再分析耦合数据集,研究了冬季MJO对ENSO事件的影响。结果表明,在年际时间尺度以及长期的年代际时间尺度上,热带印度洋MJO活动的强弱性都可以影响热带中东太平洋ENSO事件的发生和发展。在年际时间尺度上,ENSO发生前期征兆的赤道中东太平洋的西风爆发事件(Westerly Wind Burst,WWB),作为MJO影响ENSO的主要途径,存在着显著的次季节时间尺度的变化。相对于气候平均的赤道太平洋西部暖池区上升而东部下沉的Walker环流,MJO正位相东传后的西风异常,减弱了低层东风和赤道东太平洋海水上翻。这一上升海流的减弱导致了中东赤道太平洋的海温升高,从而有利于ENSO暖海温事件的发生。而在年代际时间尺度上,MJO范围和强度在1998年前后出现了明显的转变,1998年之前MJO的东移范围更东,强度更强,从而导致了西太平洋西风爆发区的次季节西风异常事件更加显著,在Bjeknes正反馈机制下对应了年代际时间尺度下的强尼诺事件出现,1998年之后则与之相反。冬季MJO对ENSO影响的这一年代际特征主要体现在晚冬季节,而在早冬伴随着印度洋的增暖,MJO强度一直在逐年增加。     

Role of TBO and ENSO scale ocean–atmosphere interaction in the Indo-Pacific region on Asian summer monsoon variability

Interannual variability of the Indian summer monsoon rainfall has two dominant periodicities, one on the quasi-biennial (2–3 year) time scale corresponding to tropospheric biennial oscillation (TBO) and the other on low frequency (3–7 year) corresponding to El Niño Southern Oscillation (ENSO). In the present study, the spatial and temporal patterns of various atmospheric and oceanic parameters associated with the Indian summer monsoon on the above two periodicities were investigated using NCEP/NCAR reanalysis data sets for the period 1950–2005. Influences of Indian and Pacific Ocean SSTs on the monsoon season rainfall are different for both of the time scales. Seasonal evolution and movement of SST and Walker circulation are also different. SST and velocity potential anomalies are southeast propagating on the TBO scale, while they are stationary on the ENSO scale. Latent heat flux and relative humidity anomalies over the Indian Ocean and local Hadley circulation between the Indian monsoon region and adjacent oceans have interannual variability only on the TBO time scale. Local processes over the Indian Ocean determine the Indian Ocean SST in biennial periodicity, while the effect of equatorial east Pacific SST is significant in the ENSO periodicity. TBO scale variability is dependent on the local factors of the Indian Ocean and the Indian summer monsoon, while the ENSO scale processes are remotely controlled by the Pacific Ocean.     

热带太平洋海表能量收支平衡特征及其与两类ENSO事件的联系

利用月平均的HadISST海表温度、NCEP再分析资料、OAFlux海表面热通量及相关物理量资料、NCAR/NOAA云量场资料,分析了热带太平洋海表热通量的年际特征,并且进一步分析了传统El Ni?o和El Ni?o Modoki事件中湍流热通量的异常演变特征以及影响因子。在热带太平洋上,净热通量的年际变化最大振幅出现在赤道太平洋上,且主要取决于潜热通量和短波辐射通量的变化。本文还利用两类ENSO事件旺盛期海温指数对不同时期海面热通量场的偏回归分析,考察了热带太平洋海表面热通量与两类ENSO事件中海温的联系。两类海温指数对各时期热带太平洋净热通量的回归均表现为赤道太平洋上存在显著的负异常,在Ni?o3指数偏回归下的负异常范围和强度都较El Ni?o Modoki指数回归的要大,且更偏于赤道东太平洋,而旺盛期海温对同期赤道东太平洋上湍流热通量的影响最大。     

Linear trends in sea surface temperature of the tropical Pacific Ocean and implications for the El Niño-Southern Oscillation

A principal component decomposition of monthly sea surface temperature (SST) variability in the tropical Pacific Ocean demonstrates that nearly all of the linear trends during 1950–2010 are found in two leading patterns. The first SST pattern is strongly related to the canonical El Niño-Southern Oscillation (ENSO) pattern. The second pattern shares characteristics with the first pattern and its existence solely depends on the presence of linear trends across the tropical Pacific Ocean. The decomposition also uncovers a third pattern, often referred to as ENSO Modoki, but the linear trend is small and dataset dependent over the full 61-year record and is insignificant within each season. ENSO Modoki is also reflected in the equatorial zonal SST gradient between the Niño-4 region, located in the west-central Pacific, and the Niño-3 region in the eastern Pacific. It is only in this zonal SST gradient that a marginally significant trend arises early in the Northern Hemisphere spring (March–May) during El Niño and La Niña and also in the late summer (July–September) during El Niño. Yet these SST trends in the zonal gradient do not unequivocally represent an ENSO Modoki-like dipole because they are exclusively associated with significant positive SST trends in either the eastern or western Pacific, with no corresponding significant negative trends. Insignificant trends in the zonal SST gradient are evident during the boreal wintertime months when ENSO events typically mature. Given the presence of positive SST trends across much of the equatorial Pacific Ocean, using fixed SST anomaly thresholds to define ENSO events likely needs to be reconsidered.     

Predictable patterns and predictive skills of monsoon precipitation in Northern Hemisphere summer in NCEP CFSv2 reforecasts

The predictable patterns and predictive skills of monsoon precipitation in the Northern Hemisphere summer (June–July–August) are examined using reforecasts (1983–2010) from the National Center for Environmental Prediction Climate Forecast System version 2 (CFSv2). The possible connections of these predictable patterns with global sea surface temperature (SST) are investigated. The empirical orthogonal function analysis with maximized signal-to-noise ratio is used to isolate the predictable patterns of the precipitation for three regional monsoons: the Asian and Indo-Pacific monsoon (AIPM), the Africa monsoon (AFM), and the North America monsoon (NAM). Overall, the CFSv2 well predicts the monsoon precipitation patterns associated with El Niño-South Oscillation (ENSO) due to its good prediction skill for ENSO. For AIPM, two identified predictable patterns are an equatorial dipole pattern characterized by opposite variations between the equatorial western Pacific and eastern Indian Ocean, and a tropical western Pacific pattern characterized by opposite variations over the tropical northwestern Pacific and the Philippines and over the regions to its west, north, and southeast. For NAM, the predictable patterns are a tropical eastern Pacific pattern with opposite variations in the tropical eastern Pacific and in Mexico, the Guyana Plateau and the equatorial Atlantic, and a Central American pattern with opposite variations in the eastern Pacific and the North Atlantic and in the Amazon Plains. The CFSv2 can predict these patterns at least 5 months in advance. However, compared with the good skill in predicting AIPM and NAM precipitation patterns, the CFSv2 exhibits little predictive skill for AFM precipitation, probably because the variability of the tropical Atlantic SST plays a more important than ENSO in the AFM precipitation variation and the prediction skill is lower for the tropical Atlantic SST than the tropical Pacific SST.     

PRELIMINARY RESEARCH OF THE PACIFIC-INDIAN OCEAN SSTA MODE AND DEFINITION OF ITS INDEX

Applying the empirical orthogonal function (EOF) analysis to the sea surface temperature (SST) field of the tropical Pacific and Indian Oceans for determination of the first eigenvector field, the current work reveals that there are significant zonal gradients of SST in all seasons of the year in the northwestern and eastern Indian Ocean and equatorial central and eastern Pacific and western Pacific. It is also found that the variance contribution rates of the first EOF mode of every season is more than 33%. This shows that this kind of spatial distribution of the SST is stable. This pattern is named Pacific-Indian Oceans SSTA mode. Through careful analysis and comparison, an index of the mode was defined.     

Mechanism of Kelvin and Rossby waves during ENSO events

Summary ?The fields of sea-level height anomaly (SLHA) and surface zonal wind anomaly (SZWA) have been analyzed to investigate the typical evolution of spatial patterns during El Ni?o-Southern Oscillation (ENSO) events. Sea surface temperature (SST) changes during ENSO events are represented as an irregular interplay of two dominant modes, low-frequency mode and biennial mode. Cyclostationary principal component (PC) time series of the former variables are regressed onto the PC time series of the two dominant SSTA modes to find the spatial patterns of SLHA and SZWA consistent with the two SSTA modes. The two regressed patterns of SLHA explain a large portion of SLHA total variability. The reconstruction of SLHA using only the two components reasonably depicts major ENSO events. Although the low-frequency component of SST variability is much larger than the biennial component, the former does not induce strong Kelvin and Rossby waves. The biennial mode induces much stronger dynamical ocean response than the low-frequency mode. Further decomposition of the SLHA modes into Kelvin and Rossby components shows how these two types of equatorial waves evolve during typical ENSO events. The propagation and reflection of these waves are clearly portrayed in the regressed patterns leading to a better understanding of the wave mechanism in the tropical Pacific associated with ENSO. A close examination suggests that the delayed action oscillator hypothesis is generally consistent with the analysis results reported here. Rossby wave development in the central Pacific in the initiation stage of ENSO and the subsequent reflection of Kelvin waves at the western boundary seems to be an important mechanism for further development of ENSO. The development of Kelvin waves forced by the surface wind in the far-western Pacific cannot be ruled out as a possible mechanism for the growth of ENSO. While Kelvin waves in the far-western Pacific serve as an intiation mechanism of ENSO, they also cause the termination of existing ENSO condition in the central and eastern Pacific, thereby leading to a biennial oscillation over the tropical Pacific. The Kelvin waves from the western Pacific erode the thermocline structure in the central Pacific preventing further devlopment of ENSO and ultimately terminating it. It should be emphasized that this wave mechanism is clear and active only in the biennial mode. Received August 15, 2001; revised March 6, 2002     

ENSO phase-locking to the boreal winter in CMIP3 and CMIP5 models

In this study, the El Nino-Southern Oscillation (ENSO) phase-locking to the boreal winter in CMIP3 and CMIP5 models is examined. It is found that the models that are poor at simulating the winter ENSO peak tend to simulate colder seasonal-mean sea-surface temperature (SST) during the boreal summer and associated shallower thermocline depth over the eastern Pacific. These models tend to amplify zonal advection and thermocline depth feedback during boreal summer. In addition, the colder eastern Pacific SST in the model can reduce the summertime mean local convective activity, which tends to weaken the atmospheric response to the ENSO SST forcing. It is also revealed that these models have more serious climatological biases over the tropical Pacific, implying that a realistic simulation of the climatological fields may help to simulate winter ENSO peak better. The models that are poor at simulating ENSO peak in winter also show excessive anomalous SST warming over the western Pacific during boreal winter of the El Nino events, which leads to strong local convective anomalies. This prevents the southward shift of El Nino-related westerly during boreal winter season. Therefore, equatorial westerly is prevailed over the western Pacific to further development of ENSO-related SST during boreal winter. This bias in the SST anomaly is partly due to the climatological dry biases over the central Pacific, which confines ENSO-related precipitation and westerly responses over the western Pacific.