西太平洋次表层海温异常与北赤道流异常海温西传

通过对热带太平洋上层XBT温度资料分析,探讨了对厄尔尼诺(El Nino)事件发生起重要作用的西太平洋暖池区次表层海温变暖异常的变化规律,揭示了影响西太平洋暖池区次表层海温变暖异常的形成机制.分析表明:西太平洋暖池区的次表层海温异常变暖与赤道太平洋的北赤道流(10°N)的海温异常存在密切关系.在El Nino事件发生的前期,位于赤道中东太平洋的异常暖水沿北赤道流温跃层潜沉向西太平洋暖池区输送,在西太平洋暖池堆积并向赤道西太平洋扩展,当异常暖水达到一定强度,并在大气的强迫下,异常暖水沿温跃层东传至赤道中东太平洋并上浮于海面,最终导致El Nino事件的爆发.北赤道流的异常海温西传是导致西太平洋暖池区次表层海温异常的重要机制,是导致El Nino事件发生的关键.     

赤道太平洋潜流变化特征及其与异常海温东传

应用TOGA-TAO实测海流资料(ADCP)和SODA同化海流和海温资料,分析探讨了赤道太平洋潜流的季节和年际变化特征,以及与赤道异常暖水东传的内在联系.揭示了El Nino事件中异常暖水东传的物理机制.结果表明,赤道太平洋潜流具有明显的季节变化和年际变化特征.潜流的垂直最大深度可由海表层至300~400 m,其多年平均最大流速可达100 cm/s.潜流最大中心的纬向变化位置基本维持在160°W-130°W之间.在年际变化中,赤道潜流变化特征非常明显,尤其是在El Nino事件发生过程中,赤道潜流出现明显的加强现象.其最大流速可达140 cm/s以上.在西太平洋暖池区域次表层暖水东传之前,位于中东太平洋次表层的赤道潜流就已出现加强,其最大流速中心沿温跃层不断加强和东移,“引导”来自西太平洋暖池区域次表层的异常暖水向东传播.因此,可以认为西太平洋暖池区域异常暖水的东传与赤道潜流的强度和东传存在密切关系,也就是说在El Nino事件中,赤道潜流的变化是导致西太平洋暖池区域次表层异常暖水东传的重要机制.     

非线性模糊识别及其在海温异常检测中的应用

基于模糊推理和非线性模糊识别原理，讨论了从实际信号中检测识别主要影响因子，进而滤除干扰的方法，进行了相应的去噪试验。试验结果表明：由于模糊系统具有非线性、容错性和自适应学习等特性，因此，能够较为有效地辨识和检测出复杂非线性信号中的主要影响因子及其贡献大小。作为应用，研究了从观测资料中辩识El Niño/La Niña主要影响因子的诊断检测过程，并对 20世纪70年代以来出现的典型El Niño/La Niña事件中信风因子的影响作用进行了诊断检测和模糊识别，分析发现，70年代的几次ElNiño事件主要是由赤道西太平洋西风异常所触发，而80年代的几次El Niño事件（尤其是1982/1983年暖水事件）则主要是由赤道西太平洋与赤道东太平洋的信风活动异常共同所致，前者触发激励在先，后者巩固加强在后。     

两类ENSO事件赤道太平洋次表层海温异常的演变特征

基于美国马里兰大学提供的海洋同化(SODA)月平均资料,用个例和合成分析方法,剖析了两类ENSO事件赤道太平洋海温异常演变特征。结果指出,东部型ENSO事件的初始海温异常源来自赤道西太平洋次表层,海温异常中心沿气候温跃层向东向上传送,至赤道东太平洋表层形成ENSO事件。东部型ENSO冷暖事件互为初始场,在形成某一位相的ENSO事件的过程中也同时为相反位相的ENSO事件准备条件。中部型ENSO事件的初始海温异常源出现在赤道中太平洋次表层,海温异常中心沿气候温跃层向东向上传送至赤道中东太平洋表层形成ENSO事件。中部型ENSO事件多在前次事件减弱中断后出现。     

西太平洋暖池冰期旋回中的类ENSO式演化及其驱动机制

作为全球接受太阳辐射最多、表层海水温度最高的区域,西太平洋暖池区通过厄尔尼诺-南方涛动(El Niño-Southern Oscillation,ENSO)和季风等过程影响着全球气候的变化。越来越多的沉积记录证明,在地质历史时期西太平洋暖池也存在类似于现代ENSO过程的“类ENSO式”变化。而目前类ENSO式变化与冰期—间冰期旋回之间的响应关系和驱动机制及其与东亚季风的关联仍存在争议。本文利用位于暖池核心区的B10岩心浮游有孔虫氧同位素、Mg/Ca(质量分数比)和黏土矿物参数重建了暖池区氧同位素8期以来的古气候记录,并结合已有的热带海表温度记录、中国石笋氧同位素和南大洋地区海表温度记录,研究了西太平洋暖池冰期旋回中类ENSO状态的演化规律及其与东亚季风的关系,并探讨了暖池区类ENSO演化的驱动机制。结果发现:冰期时,西太平洋暖池区温跃层变浅,赤道东、西太平洋温差减小,同时,东亚夏季风减弱,暖池区降水量相对减少,与现代El Niño时期气候态类似;间冰期时,西太平洋暖池区温跃层加深,赤道东、西太平洋温差增大,东亚冬夏季风增强,暖池区降水量相对增加,与现代La Niña时期气候态类似。频谱分析结果表明,西太平洋暖池区海表温度的变化具有偏心率周期(96 ka)。冰消期时,低纬度太阳辐射量的增加,增大了纬向上的SST梯度,并使得次表层海水储存了更多的热量,积累的热量会通过调节次表层环流向暖池区的热传输,最终调控赤道太平洋地区Walker环流强度和ENSO活动的长期变化。而冰期时,南大洋地区降温所引起的东南信风和大洋环流异常可能对类ENSO式起到调控的作用。     

赤道太平洋次表层海水温度异常的信号通道

应用热带太平洋上层XBT温度资料,分析探讨了西太平洋暖池区次表层海温冷暖异常信号的变化规律,揭示了影响西太平洋暖池区次表层海温冷变异常的信号通道。分析表明,西太平洋暖池区的次表层海温异常变冷与太平洋北赤道流的海温冷异常信号西传有重要关系。北赤道流的海温异常冷(暖)信号是沿温跃层由赤道中东太平洋潜沉向西太平洋暖池区传播,与西太平洋次表层海温异常(冷)暖信号向赤道中东太平洋传播构成了热带海洋信号的气旋式"环流通道"。在这一"环流通道"中,北赤道流的海温异常信号西传是导致西太平洋暖池区及西太平洋次表层海温异常的重要机制,是影响厄尔尼诺(ElNino)和拉尼娜(LaNina)事件发生的关键。     

东太平洋暖池

利用1950-2002年间的海面温度(SST)资料,研究了东太平洋暖池的形态和热状态特征,并探讨了暖池变异与恩索(ENSO)的关系。结果表明,暖池形态和热状态均有明显的季节特征和年际变化,其年际变化与ENSO循环相联系;暖池热含量、面积和南界与ENSO有着十分密切的相关关系。合成分析结果显示,在厄尔尼诺(El Nino)事件爆发的前一年,暖池热含量偏少,面积偏小,南界偏北,而在事件爆发后,暖池热含量增多,面积增大,南界南移;在拉尼娜(La Ni?a)事件期间,暖池热含量、面积和南界的演变趋势基本与El Nino事件期间的情况相反。东太平洋暖池的经向变异可能对ENSO暖(El Nino)、冷(La Nina)事件的发展有重要作用。     

末次盛冰期太平洋赤道辐合带对暖池外热带海表温度变化的敏感性

利用美国NCAR CAM3大气环流模式,分析了末次盛冰期（LGM）两个不同的热带海表温度重建方案中,北半球冬季热带中、西太平洋对流活动及大气环流对暖池外（赤道东太平洋和热带大西洋）热带SST异常的敏感性。结果表明：  1）SST异常首先引起大气环流的改变。  赤道东太平洋对流层下沉增强,而作为经向补偿,副热带东太平洋上升运动增强,其中南半球尤为明显,同时南半球热带中、西太平洋上升运动增强,加剧了该区纬向逆时针环流,说明冰期热带海气耦合过程受气候背景场（如SST）影响很大;   2）大气环流格局改变引起热带中西太平洋的大气加热、对流活动、表层风场及降雨的巨大变化。  140°E以西的婆罗洲和菲律宾区域,总的大气加热减少是由于对流与辐射加热减少所致,对应于该区风场辐散和降雨减少;   而140°E以东的南半球热带中、西太平洋,大气吸收热量增加,对流与辐射加热均增强,总降雨量也随之增加,反映该区赤道辐合带南移并增强。该项研究为探索热带太平洋在冰期/间冰期旋回中的古海洋学变化提供了新的数据支撑。此外,不同重建SST对赤道辐合带的影响比较大,因此利用重建SST进行数值模拟或者利用耦合模式研究LGM热带海气相互作用时,应该十分重视全球热带SST分布特征。     

热带太平洋海洋环流与暖池的结构特征、变异机理和气候效应

热带西太平洋暖池是引发强烈的大气对流、驱动Walker环流和Hadley环流系统的主要热源之一,对全球、尤其是东亚气候有重要影响。针对我国在提升气候预测水平方面的重大和迫切需求,国家重点基础研究发展计划项目"热带太平洋海洋环流与暖池的结构特征、变异机理和气候效应"于2011年7月正式立项。项目拟解决的关键科学问题包括:①调控暖池形成和变异的海洋环流多尺度相互作用过程;②海洋动力过程在暖池热盐结构变异中的作用及其机理;③暖池变异对不同类型El Nio影响机理的异同和对东亚季风变异的调制机理。围绕上述关键科学问题,项目将以暖池变异为中心,关注影响和控制暖池结构与变异的关键海洋过程,以及暖池海气相互作用影响ENSO循环、东亚季风年际变异的过程和机理,重点组织开展以下3个方面有针对性的调查研究:①热带太平洋环流和暖池的结构和变异特征;②热带太平洋环流与暖池相互作用的关键过程和机理;③暖池变异的海洋—大气耦合过程及其气候效应。在此基础上,项目将力争阐明暖池影响东亚季风和我国气候变异的过程、机理与敏感区,改进模式的混合参数化方案,提出有效提高ENSO预报技巧的同化方案,为我国短期气候预测能力的提高提供科学支撑。     

热带西太平洋沉积物的环境磁学特征对东亚冬季风的响应

西太平洋暖池(Western Pacific Warm Pool,WPWP)指位于热带太平洋中、西部(包括南海南部和苏禄海)年平均水温超过 28 ℃的广大海域。由于巨厚的表层暖水覆盖,西太平洋暖池成为全球热量和水汽交互的重要源区,对驱动温盐环流、调节全球气候变化具有重要作用。东亚冬季风(EAWM)是全球气候系统中最活跃的组成部分之一,它可能会通过寒潮侵入热带地区,引起深层对流,以此加强暖池区的对流活动和降水异常,从而影响赤道地区的潜热释放。但地质历史时期西太平洋暖池与东亚冬季风的相互作用关系尚不明确。由于东亚冬季风携带的风尘中往往包含有大颗粒的高矫顽力磁性矿物,我们可以通过沉积物中磁性矿物的组合、含量、颗粒大小和形态变化,分析不同时间尺度上气候环境变化和风尘物质的输入情况,进而反演东亚冬季风的强度变化。本文对取自热带西太平洋B10钻孔的岩心样品进行了环境磁学测试,以揭示地质历史时期西太平洋暖池沉积物环境磁学特征对东亚冬季风的响应。实验结果表明,沉积物中的主要载磁矿物为低矫顽力的磁铁矿,属于亚铁磁性矿物,并含有少量高矫顽力磁性矿物。沉积物中的磁性颗粒以准单畴(PSD)颗粒为主。高矫顽力的磁性矿物含量和细颗粒磁铁矿相对含量在冰期和间冰期呈现出显著相对变化,对东亚冬季风的变化有敏感响应:冰期沉积物中高矫顽力矿物含量增多,磁性颗粒粒径变大;间冰期沉积物中高矫顽力矿物含量降低,磁性颗粒粒径变细。在干燥、寒冷的冰期,由风尘携带而来的高矫顽力磁性矿物相对含量增加,沉积物中的磁性颗粒粒径变大,反映冰期东亚冬季风强度增大;在气候温暖湿润的间冰期,风尘的输入量较小,由风尘携带的高矫顽力磁性矿物含量相对较低,沉积物中的磁性颗粒粒径变小,反映间冰期东亚冬季风的强度减弱。     

基于石笋记录的晚全新世太平洋东西两岸季风区气候事件对比研究

研究晚全新世季风气候演变有助于进一步认识与预测未来季风区气候变化。太平洋东西两岸是全球季风集中分布的地区,已经有大量的古气候记录发表,但是缺乏对各个季风区气候突变事件以及整体变化趋势的对比研究。针对这一问题,选取亚洲季风区、印澳季风区、北美季风区、南美季风区11个洞穴石笋δ18O和1个湖泊Ti含量,对比研究各个记录在3.5~0.5 ka B.P.期间指示的夏季风变化特征。通过对比发现四大季风区的石笋δ18O在晚全新世整体上呈现偏正趋势,指示夏季风减弱;2次重要的气候突变事件1.5 ka B.P.和2.7 ka B.P.弱夏季风事件在各个季风区内均有表现;同时也记录了一系列十年际-百年际尺度的弱夏季风事件,表明太平洋东西两岸和南北半球的夏季风都有减弱的趋势,这与先前研究认为的南北半球呈现"see-saw"模式表现出不一样的特征。晚全新世以来ENSO(El Nino-Southern Oscillation)活动的增强对太平洋东西两岸南北半球夏季风减弱具有重要影响。在El Nino事件发生时,Walker环流减弱,而且它的上升支向东移动远离西太平洋暖池,西太平洋副热带高压增强并向西移动,导致亚洲夏季风减弱。Walker环流的东移也会使得印度尼西亚-太平洋暖池(Indo-Pacific Warm Pool,简称IPWP)海温下降,热带季节内震荡减弱致使印澳夏季风减弱;此外,El Nino事件发生时,赤道东太平洋海水温度上升导致东西太平洋海水温度梯度减弱,在此状态下南美季风区低空急流(Low Level Jet,简称LLJ)减弱,导致南美夏季风减弱;同时,北美洲加勒比海低空急流增强,使得该季风区下沉气流增强,导致北美夏季风减弱。我们的研究表明,在晚全新世ENSO活动增强的状态下,太平洋东西两岸南北半球夏季风变化可能都呈现减弱趋势。     

Dynamics of upper tropospheric stationary wave anomalies induced by ENSO during the northern summer: A GCM study

Ensemble seasonal integrations are carried out with the COLA GCM, with a view to understand the dynamical connection between warm SST anomalies in the equatorial central-eastern Pacific Ocean and the upper level stationary wave anomalies seen during drought years over the Indian summer monsoon region. In addition, experiments with and without orography are performed in order to examine the role of the Himalayas in modulating the El Niño induced stationary wave anomalies over the summer monsoon region. The GCM simulations show a statistically significant weakening of the summer monsoon activity over India in response to the SST forcing in the equatorial Pacific Ocean. This weakening of the summer monsoon appears to be largely related to modifications of the local Hadley and Walker cells over the summer monsoon region. In addition, it is seen that the anomalous ENSO divergent forcing over the tropical Pacific Ocean can act as a potential source for Rossby wave dispersion. Here one finds the possibility of meridionally propagating Rossby waves, which emanate from the ENSO forcing region, to interact with the subtropical westerlies and generate anomalous highs and lows in the subtropics and extratropics. The quasi-stationary perturbations seen over west Asia, Pakistan and northwest India during drought years, seem to be generated by the above mechanism. An alternate mechanism that could be important for the persistence of the quasi-stationary perturbations seems to be based on the dynamic excitation of middle latitude normal modes which can extract energy from the zonally varying unstable basic flow. It is seen from the GCM simulations, that the Himalayan orography plays a crucial role in anchoring the El Niño induced extratropical westerly troughs far to the west in the high latitude belt. In the absence of orography it is seen that the ENSO induced extra-tropical cyclonic anomalies tend to intrude southward into the monsoon region thereby destroying the regional scale circulations completely. Another effect due to the Himalayas is to generate lee waves on the eastern side of the topographic barrier which encircle the globe in the subtropics and midlatitudes.     

Hindcast/forecast of ENSO events based upon the redistribution of observed and model dynamic height in the Western Tropical Pacific, 1964–1986

Anomalous sea level, anomalous observed dynamic height (0/400 db) and anomalous model dynamic height are examined at the locations of 13 island sea level stations in the tropical Pacific for each bimonth of the four year period 1979 to 1982. Starting in 1981, the anomalous dynamic height data show off-equatorial Rossby waves propagated toward the W boundary of the Pacific basin. At the W boundary, the model Rossby wave activity was found to have excited coastally trapped Kelvin-Munk waves which transmitted the anomalous dynamic height equatorward. At the equator, coastally trapped wave activity excited eastward propagating equatorial Kelvin waves, yielding a pair of anomalous peaks in dynamic height variability in the E equatorial Pacific associated with the 1982–1983 ENSO event. The evolution of the peaks in dynamic height associated with the Rossby and Kelvin wave activity reflects the redistribution of observed upper-ocean heat content in the W tropical Pacific, providing a qualitative hindcast for the 1982–1983 ENSO event. In consequence of these results, and the results of a related study (Inoue et al. 1985), the redistribution of both observed and model heat content, as evidenced in dynamic height in the W Pacific during the 23-year period 1964–1985, is examined for its ability to hindcast and forecast ENSO events in this period. Complex EOF analysis is applied to the Onset Phase of ENSO events occurring in 1968–69, 1972–73, 1976–77, and 1982–83; it is used to determine the characteristic redistribution of heat content (dynamic height) prior to the Mature Phase of ENSO events. This analysis found both model and observed dynamic height in the N hemisphere to be characterized by wind-driven, westward propagating, baroclinic Rossby wave activity, having a remarkably stable period of 3 years over the 23-year period. The complex time series associated with these first spatial eigen-functions are used to construct observed and model hindcast indices that yield high values one year prior to the Mature Phase of ENSO events of the period. These indices achieve these values due to the incidence upon the Philippine coast in fall/winter of a positive anomaly in dynamic height propagating from the east at nondispersive Rossby long wave speeds.     

Study of the global and regional climatic impacts of ENSO magnitude using SPEEDY AGCM

ENSO is considered as a strong atmospheric teleconnection that has pronounced global and regional circulation effects. It modifies global monsoon system, especially, Asian and African monsoons. Previous studies suggest that both the frequency and magnitude of ENSO events have increased over the last few decades resulting in a need to study climatic impacts of ENSO magnitude both at global and regional scales. Hence, to better understand the impact of ENSO amplitude over the tropical and extratropical regions focussing on the Asian and African domains, ENSO sensitivity experiments are conducted using ICTPAGCM (‘SPEEDY’). It is anticipated that the tropical Pacific SST forcing will be enough to produce ENSO-induced teleconnection patterns; therefore, the model is forced using NINO3.4 regressed SST anomalies over the tropical Pacific only. SPEEDY reproduces the impact of ENSO over the Pacific, North and South America and African regions very well. However, it underestimates ENSO teleconnection patterns and associated changes over South Asia, particularly in the Indian region, which suggests that the tropical Pacific SST forcing is not sufficient to represent ENSO-induced teleconnection patterns over South Asia. Therefore, SST forcing over the tropical Indian Ocean together with air–sea coupling is also required for better representation of ENSO-induced changes in these regions. Moreover, results obtained by this pacemaker experiment show that ENSO impacts are relatively stronger over the Inter-Tropical Convergence Zone (ITCZ) compared to extratropics and high latitude regions. The positive phase of ENSO causes weakening in rainfall activity over African tropical rain belt, parts of South and Southeast Asia, whereas, the La Niña phase produces more rain over these regions during the summer season. Model results further reveal that ENSO magnitude has a stronger impact over African Sahel and South Asia, especially over the Indian region because of its significant impact over the tropical Atlantic and the Indian Ocean through Walker circulation. ENSO-induced negative (positive) NAO-like response and associated changes over Southern Europe and North Africa get significantly strong following increased intensity of El Niño (La Niña) in the northern (southern) hemisphere in the boreal winter (summer) season. We further find that ENSO magnitude significantly impacts Hadley and Walker circulations. The positive phase of ENSO (El Niño) overall strengthens Hadley cell and a reverse is true for the La Niña phase. ENSO-induced strengthening and weakening of Hadley cell induces significant impact over South Asian and African ITCZ convective regions through modification of ITCZ/monsoon circulation system.     

Oceanic conditions in the eastern equatorial Pacific during the onset of ENSO in the Holocene

Records from South America show that modern ENSO (El Nino-Southern Oscillation) did not exist 7000 cal yr B.P. and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report quantitative observations on thermocline and mixed-layer conditions in the EEP during ENSO start up. We found important changes in both the thermocline and the mixed layer, indicating increased upwelling of cooler waters since 7000 cal yr B.P. This resulted from change in the source and/or properties of waters supplying the Equatorial Undercurrent, which feeds upwelling along the equator and the Peru margin. Modeling shows that ENSO is sensitive to subsurface conditions in the eastern equatorial Pacific and that the changes in the thermocline we observed were driven by extratropical processes, giving these a role in conditioning the development of ENSO. This is in contrast to models that call for control of equatorial Pacific oceanography by tropical processes only. These infer stronger upwelling and cooler surface waters for the EEP during the mid-Holocene, which is not supported by our results.