INDIAN OCEAN DIPOLE AND ITS RELATIONSHIP WITH ENSO MODE

Near 100-year observed data sets are analyzed, and the results show that the variation of sea surface temperature(SST)in the equatorial Indian Ocean has a feature as a dipole oscillation.The situation of the dipole oscillation mainly shows the positive phase pattern(higher SST in the west and lower SST in the east than normal)and the negative phase pattern(higher SST in the east and lower SST in the west).The amplitude of the positive phase is larger than that of the negative phase.The dipole is stronger in September—November and weaker in January—April than in other months.It principally shows obviously inter-annual(4—5 year period)and inter-decadal variation(25—30 year period).Although the Indian Ocean dipole in the individual year seems to be independent of ENSO in the equatorial Pacific Ocean,in general,the Indian Ocean dipole has obviously negative correlation with the Pacific Ocean “dipole”(similar to the inverse phase of ENSO).The atmospheric zonal(Walker) circulation is fundamental for relating the two dipoles to each other.

ENSO组合模态可预测性的季节-年代际变化

厄尔尼诺-南方涛动组合模态(C-mode)是厄尔尼诺衰减期西北太平洋上空异常反气旋发展的主要驱动力,通常可以形成水汽输送通道,导致我国南部地区降水增多并发生严重的洪涝灾害,而在拉尼娜期间则情况相反。利用1981—2020年美国国家环境预报中心和美国国家大气研究中心再分析数据集的表面风场资料分析了C-mode可预测性的季节-年代际变化。结果表明:C-mode在2000年以后的可预测性明显下降,主要原因是其变率减小、强度减弱、信噪比降低等。在季节尺度上,C-mode存在“秋季预报障碍”,这与信号的季节循环密切关联,当C-mode在秋季进入衰退期时,信号强度最弱、变率最小,因此其秋季的可预测性降低。

Influence of the Preceding Austral Summer Southern Hemisphere Annular Mode on the Amplitude of ENSO Decay

There is increasing evidence of the possible role of extratropical forcing in the evolution of ENSO. The Southern Hemi- sphere Annular Mode （SAM） is the dominant mode of atmospheric circulation in the Southern Hemisphere extratropics. This study shows that the austral summer （December-January-February; DJF） SAM may also influence the amplitude of ENSO decay during austral autumn （March-April-May; MAM）. The mechanisms associated with this SAM-ENSO relationship can be briefly summarized as follows： The SAM is positively （negatively） correlated with SST in the Southern Hemisphere middle （high） latitudes. This dipole-like SST anomaly pattern is referred to as the Southern Ocean Dipole （SOD）. The DJF SOD, caused by the DJF SAM, could persist until MAM and then influence atmospheric circulation, including trade winds, over the Nifio3.4 area. Anomalous trade winds and SST anomalies over the Nifio3.4 area related to the DJF SAM are further developed through the Bjerkness feedback, which eventually results in a cooling （warming） over the Nifio3.4 area followed by the positive （negative） DJF SAM.

Role of the Oceanic Channel in the Relationships between the Basin/Dipole Mode of SST Anomalies in the Tropical Indian Ocean and ENSO Transition

The relationships between the tropical Indian Ocean basin （IOB）/dipole （IOD） mode of SST anomalies （SSTAs） and ENSO phase transition during the following year are examined and compared in observations for the period 1958-2008. Both partial correlation analysis and composite analysis show that both the positive （negative） phase of the lOB and IOD （independent of each other） in the tropical Indian Ocean are possible contributors to the E1 Nino （La Nifia） decay and phase transition to La Nifia （El Nifio） about one year later. However, the influence on ENSO transition induced by the IOB is stronger than that by the IOD. The SSTAs in the equatorial central-eastern Pacific in the coming year originate from subsurface temperature anomalies in the equatorial eastern Indian and western Pacific Ocean, induced by the IOB and IOD through eastward and upward propagation to meet the surface. During this process, however the contribution of the oceanic channel process between the tropical Indian and Pacific oceans is totally different for the IOB and IOD. For the IOD, the influence of the Indonesian Throughflow transport anomalies could propagate to the eastern Pacific to induce the ENSO transition. For the IOB, the impact of the oceanic channel stays and disappears in the western Pacific without propagation to the eastern Pacific.

PRINCIPAL MODES OF THE SOUTH PACIFIC SSTA IN JUNE, JULY AND AUGUST AND THEIR RELATIONS TO ENSO AND SAM

The relationships of variations of sea surface temperature anomalies (SSTA) in the South Pacificwith ENSO and Southern Hemisphere Annular Mode (SAM) are examined in the present article byemploying the NCEP-NCAR reanalysis from 1951 to 2006. Two principal modes of South Pacific SSTA areobtained using the EOF (Empirical Orthogonal Function) analysis for austral winter (June, July and August).Our results suggest that EOF1 is closely related with ENSO and EOF2 links to SAM. The EOF1 varieslargely on an interannual and EOF2 on a decadal scale. The time series of coefficients of EOF1 is highlycorrelated simultaneously with Nino3 index. However, the time series of coefficients of EOF2 issignificantly correlated with the March-April-May mean SAM index. Both the EOF1 and EOF2 are found insignificant correlation to summer precipitation over China. With higher-than-normal SSTs in the easternSouth Pacific and simultaneously lower SSTs in the western South Pacific in June-July-August, thesummertime rainfall is found to be less than normal in northern China. As displayed in EOF2 of SSTA, inyears with lower-than-normal SSTs in mid-latitude southern and equatorial eastern Pacific andhigher-than-normal SSTs in the equatorial middle Pacific in March-April-May, the summer precipitation inAugust tends to be more than normal in regions south of Yangtze River.

Representation of the ENSO Combination Mode and its Asymmetric SST Response in Different Resolutions of HadGEM3

Previous studies have revealed a combination mode （C-mode） occurring in the Indo-Pacific region, arising from nonlinear interactions between ENSO and the western Pacific warm pool annual cycle. This paper evaluates the simulation of this C-mode and its asymmetric SST response in HadGEM3 and its resolution sensitivity using three sets of simulations at horizontal resolutions of N96, N216 and N512. The results show that HadGEM3 can capture well the spatial pattern of the C-mode associated surface wind anomalies, as well as the asymmetric response of SST in the tropical Pacific, but it strongly overestimates the explained variability of the C-mode compared to the ENSO mode. The model with the three resolutions is able to reproduce the distinct spectral peaks of the C-mode at the near annual combination frequencies, but the performance in simulating the longer periods is not satisfactory, presumably due to the unrealistic simulation of the ENSO mode. Increasing the horizontal resolution can improve the consistency between atmospheric and oceanic representations of the C-mode, but not necessarily enhance the accuracy of C-mode simulation compared with observation.

ENSO-Dependent and ENSO-Independent Variability over the Mid-Latitude North Pacific: Observation and Air-Sea Coupled Model Simulation

During El Niño events, the warm anomalies in the eastern tropical Pacific are seen to occur in conjunction with prominent warm anomalies in the North Pacific SSTs off the west coast of North America as well as with cold anomalies in the central North Pacific. This kind of North Pacific response to ENSO is examined in observational data and IPSL air-sea coupled model simulations. Analyses based on observational data and the model output data both support the hypothesis of an “atmospheric bridge concept”, i.e., the atmospheric response to ENSO, in turn, forces the extra-tropical SST anomalies associated with the El Ninno event, thereby serving as a bridge between the tropical and extra-tropical Pacific. Regarding the mechanism responsible for this, the ocean dynamical response to the atmospheric forcing is suggested to be active, while the contribution of latent heat flux is also significant. The role of solar radiation, longwave radiation, and sensible heat flux are of minor importance however, as indicated in the model. Further analysis shows that the North Pacific mode, which is linearly independent of ENSO, resembles the El Niño-type SST mode in the northern Pacific, i.e. both take the pattern of a zonally-oriented dipole in the subtropical Pacific, though differ slightly in the location of the anomaly center. The coupling between the North Pacific mode and the atmosphere is found to be mainly via air-sea heat flux exchange in the model. Both solar radiation and longwave radiation play important roles, while the contribution of latent heat flux is nearly negligible.

A PRINCIPAL MODE OF CIRCULATION COVARIATION BETWEEN THE NORTHERN AND SOUTHERN HEMISPHERES AND ITS ASSOCIATION WITH ENSO DURING BOREAL WINTER

By employing the singular value decomposition(SVD) analysis, we have investigated in the present paper the covariations between circulation changes in the Northern(NH) and Southern Hemispheres(SH) and their associations with ENSO by using the NCEP/NCAR reanalysis, the reconstructed monthly NOAA SST, and CMAP precipitation along with NOAA Climate Prediction Center(CPC) ENSO indices. A bi-hemispheric covariation mode(hereafter BHCM) is explored, which is well represented by the first mode of the SVD analysis of sea surface pressure anomaly(SLPA-SVD1). This SVD mode can explain 57.36% of the total covariance of SLPA. BHCM varies in time with a long-term trend and periodicities of 3—5 years. The long term trend revealed by SVD1 shows that the SLP increases in the equatorial central and eastern Pacific but decreases in the western Pacific and tropical Indian Ocean, which facilitates easterlies in the lower troposphere to be intensified and El Ni觡o events to occur with lower frequency. The spatial pattern of the BHCM looks roughly symmetric about the equator in the tropics, whereas it is characterized by zonal disturbances in the mid-latitude of NH and is highly associated with AAO in the mid-latitude of SH. On inter-annual time scales, the BHCM is highly correlated with ENSO. The atmosphere in both the NH and SH responds to sea surface temperature anomalies in the equatorial region, while the contemporaneous circulation changes in the NH and SH in turn affect the occurrence of El Ni觡o/La Ni觡a. In boreal winter, significant temperature and precipitation anomalies associated with the BHCM are found worldwide. Specifically, in the positive phase of the BHCM,temperature and precipitation are anomalously low in eastern China and some other regions of East Asia. These results are helpful for us to better understand interactions between circulations in the NH and SH and the dynamical mechanisms behind these interactions.

ENSO及其组合模态对中国东部各季节降水的影响

近期的研究发现,热带太平洋低层大气存在两种主要模态,即经向对称ENSO模态和ENSO与海表温度(SST)年循环相互作用产生的经向反对称组合模态。主要探讨了这两种不同ENSO模态对中国东部各季节降水的影响。结果表明,厄尔尼诺年秋季,中国西南、长江及华南大部分区域呈现显著正降水异常;冬季,正降水异常范围扩大,覆盖华南、华东及华北东南部地区。这两个季节的异常降水都主要受ENSO模态的影响。与ENSO模态相关的正异常海温局地强迫导致120°E以西出现反气旋性环流,其西北侧增强的西南暖湿气流使得中国东部地区降水增多。次年春季,从中国华南延伸到东北出现正的异常降水,主要是ENSO组合模态的贡献。因为次年春季热带太平洋地区ENSO模态信号只局限于赤道地区,并没有对中国东部降水有显著的影响,而ENSO与海温年循环相互作用的组合模态使得与ENSO相关的赤道大气异常可以扩展到赤道以外地区。ENSO组合模态对中国降水异常有重要影响,在今后的研究和短期预测中需引起重视。

海温距平的ENSO模和类ENSO模的三维结构

用美国马里兰大学提供的海洋同化(SODA)月平均资料,深入揭示了ENSO模的海洋三维结构及其年际和年代际变率。研究结果指出,ENSO海洋模随深度呈明显规律变化。在热带太平洋,它由热带中东太平洋表层显著海温异常分布型随深度增加逐渐过渡为热带西太平洋次表层显著反号海温异常分布型;在赤道太平洋以赤道西太平洋暖池次表层海温显著异常与赤道中东太平洋表层海温反号显著异常为主要特征。El Nino期间,热带中东太平洋表层为强海温正距平中心,西太平洋暖池次表层为强海温负距平中心,在年际尺度上,160°E以西的西北太平洋副热带海域还存在一个与西太平洋海温异常变化反号、与热带东太平洋同号的区域;La Nina期间正好相反。ENSO循环主要由ENSO年际变率所决定,年际ENSO模具有东部型ENSO事件的海温异常分布特征,其循环是东部型冷暖事件之间的转换,在200m以浅,它具ENSO模相同的三维结构和3-5年的显著年际变化周期;年代际类ENSO模具有中部型ENSO事件的海温异常分布特征,年代际ENSO循环是中部型冷暖事件之间的转换,其影响主要限制在200m以浅的海洋上层,具有ENSO模相似的三维结构和9-23年的显著周期。

热带太平洋气候变率的三类模态及与ENSO强度变化的关系

基于 Kaplan等重建的 1856—2001年全球海表水温距平(SSTA)资料,用小波变换分析了热带太平洋SSTA的气候变率,对热带太平洋SSTA分别进行2—8、8—30和30—100a带通滤波,然后进行EOF分解。结果发现,ENSO模态具有5a左右的年际变化和15a左右的年代际变化2种显著周期,当二者位相相同时,ENSO事件加强,当二者位相相反时,ENSO事件减弱,当年际变化不明显时,显著的年代际变化也可单独导致ENSO事件;热带太平洋SSTA气候态变率以西太平洋暖池和赤道两侧的热带中东太平洋明显海温同号异常为主要特征,具60a左右的周期,其相位变化与气候跃变及Dl Nino 事件的类型有密切联系;长期增温倾向加大了El Nino事件的振幅。文章最后指出,ENSO事件强度变化是由年际、年代际和气候态等3类模态变率共同作用的结果,在ENSO预报模式中考虑并引入年代际和气候态变化对ENSO循环的影响,是提高ENSO预测水平的有效途径之一。

北半球冬季南太平洋海表温度异常的主要模态及其与ENSO的关系

利用新版本的NOAA扩展重构的海温资料,研究了1950/1951年—2008/2009年北半球冬季(12月—翌年2月平均)南太平洋海温异常的主要模态,并探讨了海温主要模态在年际和年代际时间尺度上与ENSO的关系。研究结果表明:北半球冬季南太平洋海温变化主要存在两个模态,第1模态基本反映了南太平洋海温异常南-北的反相变化特征,第2模态基本反映了南太平洋海温异常东-西的反相变化特征。两个模态的年代际周期均比其年际周期显著,其中第1模态的主要年代际周期为11～16a,第2模态的主要年代际周期为18～24a。主要模态与ENSO关系的分析表明,第1模态与同期ENSO事件的关系最显著,主要显示其对ENSO的响应,而第2模态对滞后其1a的ENSO事件存在一定的影响。为进一步研究它们的年际和年代际关系特征,分别对它们进行滤波后的分析表明,在年际时间尺度上,南太平洋海温两个主模态同ENSO事件的关系与滤波前的基本一致;在年代际时间尺度上,第1模态的年代际变化可能对ENSO的年代际变化存在一定的响应,而第2模态可能不仅对前期的ENSO事件存在一定的响应,还可能对后期的EN-SO事件存在一定程度的影响作用。

两个典型ENSO季节演变模态及其与我国东部降水的联系

本文根据1950~2014年月平均海温和大气环流资料以及中国160站降水等资料,利用扩展经验正交函数(EEOF)分析、相关分析以及合成分析等方法,分析了太平洋海温季节演变的主导模态,并探讨了各模态与中国东部降水和东亚环流季节变异的关系及其联系的物理过程。结果表明,ENSO(El Ni?o/Southern Oscillation)季节演变存在2个主导模态,包含4种类型:El Ni?o持续型、La Ni?a持续型、La Ni?a转El Ni?o型和El Ni?o转La Ni?a型。发现不同模态和类型的ENSO季节变化过程我国东部降水距平的分布和强度都有明显差异。El Ni?o持续型和El Ni?o转La Ni?a型,冬春季和初夏均处在El Ni?o背景下,降水异常分布存在一定共性,但盛夏和秋季分别受El Ni?o和La Ni?a影响,降水异常分布差异十分明显,前者雨带北跳慢、位置偏南而后者雨带北跳快、位置偏北。La Ni?a持续型和La Ni?a转El Ni?o型也是如此,冬春季和初夏降水异常分布大致相似,但盛夏和秋季分别受La Ni?a和El Ni?o影响,前者雨带北跳快、位置偏北而后者雨带北跳慢、位置偏南。因此,利用ENSO做我国降水的气候预测时,不能只着眼于前期冬季El Ni?o或La Ni?a事件,还应考虑其未来演变所属的可能模态和类型。对他们之间联系的物理过程分析表明,不同ENSO季节演变模态和类型主要通过影响西太平洋副热带高压以及西风带经向型/纬向型环流调整及伴随的低纬暖湿水汽输送以及中高纬冷空气活动变化来影响我国东部降水。其中,西太平洋菲律宾群岛附近异常反气旋(或气旋)、赤道Walker环流和北半球Hadley环流分别是联系ENSO与西太平洋副热带高压活动和东亚西风带经向型/纬向型环流的重要环节。

HHT新技术及其对ENSO的诊断

年际及年代际时间变率是当代气候研究的重要问题之一。通过对近百年Ni灳o3区月平均海面温度(SST)资料做HHT多时间尺度分析,结果表明:SST的变化包含了7个不同时间尺度的准周期振荡和一个世纪尺度的气候态,SST的主周期发生了一次显著的变化:1891—1943年主周期是准2a,1944—2002年的主周期是3～4a;分析不同模态对ENSO事件的作用表明:3～4a和6～8a分量决定了ENSO事件发生,具有相位锁定的作用,而其余尺度上的分量起着振幅和持续时间的调制作用;近百年来,冷、暖气候态变化对ENSO的冷、暖事件的影响并不显著;一些研究认为的1977年以来的“暖态有利于ENSO暖事件发生”的主要原因是:准18～20a、40a和平均气候态三个时间尺度振荡的共同作用在1977—2002年期间呈强暖相位的缘故。自20世纪90年代末期,这三个尺度的共同作用在暖相位的振幅显著的减弱,对ENSO暖事件的影响可能会减小,这在ENSO的预报中应该引起注意。

北太平洋海温Victoria模态与ENSO年际关系的非对称特征

利用美国NOAA海表温度资料,重点分析了北太平洋海温异常EOF第二模态Victoria模态(VM)与ENSO年际关系的非对称特征。研究发现,VM和ENSO在年代际尺度上相关性较弱,而在年际尺度上有很好的相关关系,两者同期为负相关,VM超前1 a为正相关。然而,正负VM事件与ENSO冷暖位相在年际尺度上的联系存在着一定的非对称性。正VM事件与同年冬季热带中东太平洋海温异常的联系较弱,但次年常有厄尔尼诺事件发生;相比较而言,负VM事件在同年一般都有厄尔尼诺事件伴随发生,而与次年冬季热带中东太平洋海温没有显著联系,且很少有ENSO事件发生。由此可见,正VM事件对次年厄尔尼诺的发生发展似乎有促进作用,可作为ENSO前期预报因子之一,而负VM事件不能作为ENSO的前期预报因子。

一个海气耦合环流模式中的ENSO循环特征及控制机理

本文利用中科院大气所两层全球大气环流模式和十四层热带太平洋模式的耦合环流模式 10 0年积分中的后 30年的月平均输出资料 ,通过分析海表面温度、上层海洋热容量和海表面高度异常的年际变化 ,揭示了模式 ENSO循环 (包括其产生、发展、成熟和消亡过程 )的特征及其控制机理。结果表明 ,控制本文耦合环流模式中 ENSO循环的机理是“时滞振子”模态 ,这和由中间复杂程度耦合模式得到的 ENSO控制机理是一致的。反映了“时滞振子”机制对于解释 ENSO控制机理具有一定的普遍性。另外 ,分析结果也表明 ,海表面高度异常和上层海洋热容量异常以及温跃层变化具有动力上的一致性。

赤道太平洋次表层海温模态在ENSO循环中的作用

利用美国马里兰大学海洋同化月平均再分析资料(SODA),分离出赤道太平洋次表层海温异常(SOTA)的年际变率和年代际变率,利用经验正交分解(EOF)方法分别得到SOTA年际变率和年代际变率的第一模态和第二模态,重点分析了第二模态在ENSO循环中的作用。结果表明,赤道太平洋年际变率和年代际变率的第一模态为偶极子分布,此分布型是ENSO循环冷暖位相在次表层的同时表现。第二模态以次表层范围较广的海温异常趋势一致分布为显著特征,该模态是ENSO循环演变过程的重要环节。第二模态时间系数与Ni o-3.4指数具有较好的超前相关性,可作为ENSO事件的预测前兆信号,合成和个例分析验证了这一次表层信号的预测指示作用。

ENSO循环与低纬度大气的正压和斜压运动

:利用一个 2层海洋模式和 GISST资料、NCEP再分析资料 ,对大气正压 /斜压运动与 ESNO循环之间的联系进行了研究。结果表明 ,在赤道中、东太平洋地区 ,大气斜压模在 ENSO时间尺度上的变化位相略超前于正压模的变化 ,正压模的变化位相超前于 SSTA变化 ,SSTA变化位相超前斜压模的变化位相。斜压纬向风异常与SSTA具有正相关关系 (西太平洋西部相反 ) ;正压模则与 Nino3区海温异常有负相关关系 (西太平洋西部相反 )。斜压模与海表异常加热相互激励 ,促进异常海温增长 ,是增长模 ;而正压模则抑制斜压模的和海温异常的增长 ,是转换模。正压模的位相超前对预测 El Nino/ L a Nina的发生发展具有参考意义。

ENSO事件次表层海温的两个模态及其对大气环流的影响

利用SODA海洋同化资料和NCEP再分析大气资料,分析了热带太平洋次表层海温异常(subsurfaceoceantemperatureanomaly,SOTA)与厄尔尼诺与南方涛动(ElNi?o-SouthernOscillation,ENSO)循环的联系,及SOTA对大气环流的影响。回顾传统ENSO研究,指出存在的问题,提出了ENSO影响大气研究的新思路,得到以下结果:(1)以SOTA为基本资料的研究发现, ENSO事件有两个模态,主要出现在冬季的第一模态对冬季及夏季亚洲-北太平洋-北美地区上空中高纬大气环流有重要影响,主要出现在夏季的第二模态对该地区上空夏季热带和副热带大气系统有重要作用。(2)ENSO事件通过与ENSO相联系的热带太平洋海面温度异常(ENSO-relatedseasurface temperatureanomaly,RSSTA)对大气的异常热通量输送,强迫Walker环流和Hadley环流变化,导致热带和北太平洋及周边地区上空大气环流异常,进而影响相关地区冬季和夏季的气候。(3)海表面温度异常(seasurfacetemperatureanomaly,SSTA)包含RSSTA和大气异常导致的海温变化(sea temperature anomaly caused by atmospheric anomaly, STA)两部分, RSSTA是ENSO事件过程中海洋内部热动力结构调整导致的海面温度变化,在海洋对大气的热输送过程中,它随ENSO事件演变不断更新;STA是大气受RSSTA海洋异常加热后导致的大气环流异常对海面温度的影响,在海洋浅表层STA对RSSTA有重大影响。本文最后讨论了ENSO事件期间热带海洋对大气热输送过程,指出ENSO事件通过海洋内部热动力结构调整产生RSSTA,它直接对大气异常加热,导致大气环流和气候异常,局地海气之间负反馈过程产生STA,反过来抑制RSSTA。结果还指出,人们常用的SSTA变率实际上主要由秋冬季节RSSTA主导,丢失了春夏季ENSO信息,用SSTA研究ENSO事件存在局限性,这也可能是ENSO事件春季预报障碍的原因之一。

有关副热带太平洋对ENSO影响研究的综述

ENSO(El Nio-Southern Oscillation)的发生发展既受到来自热带西太平洋纬向海气过程的影响,也受到来自副热带太平洋经向海气过程的影响。本文概述了副热带太平洋海气异常影响ENSO研究方面的科学背景及研究进展,综述了前人提出的副热带太平洋大气海洋异常通过经向风应力以及北太平洋/南太平洋经向模态,影响ENSO发展演变的途径及相关物理机制,总结了近些年观测资料分析及数值模拟研究工作所提出的新观点,并讨论了相关研究中的学术分歧及有待进一步研究的问题。