赤道太平洋降水与厄·尼诺

根据1899年—1982年赤道太平洋降水资料,研究了10月—12月降水量的时空变化。发现中太平洋最主要的特点是赤道地区与20°N以北及10°S以南变化相反。与赤道东太平洋海温变化比较证实,海温高时中太平洋多雨。分析赤道中太平洋多雨及少雨时海平面气压场表明,多雨时赤道低压偏东,南太平洋高压弱,而澳大利亚气压上升。最后指出,1982—1983年的厄·尼诺事件,就其伴随的环流与气候异常来讲,与以前的事件并无本质上的不同,但其强烈程度则是几十年来少见的。     

The westerly anomalies over the tropical pacific and their dynamical effect on the enso cycles during 1980–1994

In this paper, the zonal wind anomalies in the lower troposphere over the tropical Pacific during 1980–1994 are analyzed by using the observed data. The results show that during the formation of the 1982/83, 1986/87 and 1991 / 92 ENSO events, there were the larger westerly anomalies in the lower troposphere over the equatorial Pacific. Moreover, it is explained by using the correlation analyses that the westerly anomalies over the equatorial Pacific could cause the warm episodes of the equatorial central and eastern Pacific. A simple air-sea coupled model is used to discuss theoretically the dynamical effect of the observed westerly anomalies of wind stress near the sea surface of the equatorial Pacific on the ENSO cycle occurred in the period of 1981–1983. It is shown by using the theoretical calculations of the equatorial oceanic Kelvin wave and Rossby waves responding to the forcing of the observed anomalies of zonal wind stress near the sea surface of the equatorial Pacific that the westerly anomalies of wind stress near the sea surface of the equatorial Pacific make significant dynamical effect on the ENSO cycles occurred in the period of 1982–1983.     

The Westerly Anomalies over the Tropical Pacific and Their Dynamical Effect on the ENSO Cycles during 1980～1994

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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。     

THE EFFECTS OF THE ARCTIC SEA ICE ON THE VARIATIONS OF ATMOSPHERIC GENERAL CIRCULATION AND CLIMATE

The SST anomaly of the central-eastern equatorial Pacific and the arctic sea ice anomalies of the four districts lo-cated respectively in 160°E—110°W,110°W—20°W,70°E—160°E and 20°W—70°E are taken as five separate factors.And the relationship between each factor and the atmospheric general circulation and the climate is investigated byobservational analysis and numerical experiments.It is shown that the effects of the arctic sea ice anomalies on the varia-tions of atmospheric circulation and climate are comparable to or even in some cases greater than that of EI Ninoevents.So one should pay much attention to the study of polar sea ice anomalies in climate research.     

夏季厄尔尼诺-Modoki和东部型ENSO海表温度异常分布型特征及其与海洋性大陆区域气候异常的联系

2007年,Ashok等揭示了赤道太平洋区域存在一种三极型分布海表温度异常并称之为厄尔尼诺-Modoki,同时定义了相应的海表温度异常指数EMI（记为I_EM）。在此基础上,利用英国哈得来中心逐月海表温度资料、美国NCEP/NCAR月平均再分析数据集、美国国家海洋和大气管理局（NOAA）逐月降水资料（CMAP）,通过在太平洋海表温度异常中扣除厄尔尼诺-Modoki信号后,在Nino1+2区域上定义了东太平洋型海表温度异常指数EPNI（I_EPN）。据此,由I_EPN和I_EM可构成描述热带太平洋海表温度异常变化的一对指数。分析了两个指数相应的海气状态及对海洋性大陆区域气候异常的影响。结果表明,厄尔尼诺-Modoki和东太平洋型海表温度异常及其影响存在显著差异。在北半球夏季,当I_EM处于正位相时,热带太平洋海表温度异常呈现“负-正-负”的结构,海洋性大陆大部分区域海表温度异常为负,此时对流层低层太平洋地区辐合,海洋性大陆地区辐散,对流层高层太平洋地区辐散,海洋性大陆地区辐合。对应于辐合辐散中心,存在着自赤道中太平洋分别向赤道东太平洋和海洋性大陆中东部地区的异常垂直环流圈,同时也存在自海洋性大陆西部向印度洋西部的垂直环流。大气在海洋性大陆区域北部加热,南部冷却;在太平洋地区西部加热而东部冷却;在海洋性大陆区域10°N以南降水偏少,而10°N以北降水偏多。当I_EPN处于正位相时,热带太平洋海表温度异常呈现“西负东正”分布型,海洋性大陆区域海表温度异常呈现“西正东负”分布,对流层低层海洋性大陆地区辐散中心范围偏大、位置偏东、强度偏强,太平洋地区辐合中心范围偏小、位置偏东,热带环流异常在垂直方向上呈斜压结构,海洋性大陆区域北部大气加热而南部冷却,太平洋地区大气均呈加热正异常,海洋性大陆大部分区域降水均偏少,赤道太平洋降水偏多。以上这些结果有利于深刻理解热带太平洋海表温度异常的特征及其对海洋性大陆区域气候的影响。      

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。      

Boreal summer quasi-monthly oscillation in the global tropics

The boreal summer intraseasonal oscillation (ISO) in the global tropics is documented here using a 7-year suite (1998–2004) of satellite measurements. A composite scenario was made of 28 selected events with reference to the oscillation in the eastern equatorial Indian Ocean (EIO), where the oscillation is most regular and its intensity is indicative of the strength of the subsequent northward propagation. The average oscillation period is about 32 days, and this quasi-monthly oscillation (QMO) is primarily confined to the tropical Indian and Pacific Oceans. Topics that were investigated are the partition of convective versus stratiform clouds, the vertical structure of precipitation rates, and the evolution of cloud types during the initial organization and the development of intraseasonal convective anomalies in the central Indian Ocean. During the initiation of the convective anomalies, the stratiform and convective rains have comparable rates; the prevailing cloud type experiences a trimodal evolution from shallow to deep convection, and finally to anvil and extended stratiform clouds. A major northwest/southeast-slanted rainband forms as the equatorial rainfall anomalies reach Sumatra, and the rainband subsequently propagates northeastward into the west Pacific Ocean. The enhanced precipitation in the west Pacific then rapidly traverses the Pacific along the Intertropical Convergence Zone, meanwhile migrating northward to the Philippine Sea. A seesaw teleconnection in rainfall anomalies is found between the southern Bay of Bengal (5–15°N, 80–100°E) and the eastern Pacific (5–15°N, 85–105°W). Local sea-surface temperature (SST)-rainfall anomalies display a negative simultaneous correlation in the off-equatorial regions but a zero correlation (quadrature phase relationship) near the equator. We propose that atmosphere–ocean interaction and the vertical monsoon easterly shear are important contributors to the northeastward propagation component of the intraseasonal rainband. The observed evidence presented here provides critical information for validating the numerical models, and it supports the self-induction mechanism theory for maintenance of the boreal summer ISO.     

太平洋海气相互作用的时空变化

本文根据1957—1976年赤道太平洋海温和北太平洋海平面气压的月平均资料,计算了它们之间全年(1—12月)逐月的时滞相关,分析了北太平洋副热带反气旋影响赤道海温和赤道海温对副高反馈的季节变化。发现它们之间的联系不同季节、不同地区有明显差异:副热带反气旋对赤道海温的影响(负相关)以春季最大,秋季最小;赤道海温的反馈,对副高的不同部分作用不同,对副高主体的作用(正相关)以冬半年最大、夏半年较小(尤其是盛夏),对西部副高脊的作用(负相关)相反,以夏半年最大,冬半年较小。其过渡期为5月和11月。同时对其季节变化的可能原因也提出了一些初步看法。其中特别强调了大型环流背景的基本状态(包括平均垂直环流)对海气相互作用过程的重要性。     

夏季海洋性大陆区域气候与赤道太平洋中部型海温异常的直接和间接联系

利用1979-2015年NCEP/NCAR月平均再分析资料、美国国家海洋和大气管理局（NOAA）的月平均降水资料（CMAP）以及英国哈得来中心海表温度月平均资料,采用2009年Kao等定义的中部型ENSO指数,给出了夏季中部型海表温度（SST）异常指数,并分析了中部型ENSO和海洋性大陆（MC）区域气候的联系。结果表明,当夏季中部型海表温度正异常事件发生时,海洋性大陆核心区域（中太平洋）出现显著降水和气温负（正）异常,此时海洋性大陆核心区域有明显的负（正）热源异常,大气受冷却（加热）而下沉（上升）,同时潜热释放之外的非绝热加热表现为负（正）异常,易于导致降水负（正）异常。海洋性大陆区域与中太平洋间主要通过水平环流和垂直环流建立联系。（1）中部型ENSO指数显著正异常时,在对流层低（高）层,海洋性大陆区域和中太平洋间存在由关于赤道的对称气旋性（反气旋性）环流对而形成的直接联系,并使得海洋性大陆区域东部辐散（辐合）偏弱,而海洋性大陆区域西部辐散（辐合）偏强。（2）在垂直剖面上,赤道中太平洋海表温度的正异常和海洋性大陆核心区域的大气异常冷却有利于促使该地区低层赤道西风异常增强并进而利于中部型海表温度正异常的维持,并由此通过反沃克环流圈促进海洋性大陆区域下沉运动增强。此为海洋性大陆与中太平洋间的直接联系,可由皮叶克尼斯机制进行解释。而位于中太平洋与秘鲁地区的异常垂直环流亦可用这一机制进行解释。海洋性大陆与中太平洋的间接联系主要表现在由赤道外低纬和中纬度地区均存在的沿弧形路径上的垂直环流而建立的海洋性大陆与中太平洋地区的联系上。这些弧形垂直剖面上的垂直环流不仅与局地哈得来环流有关,还与热带和中纬度的罗斯贝波动有关。这些结果有利于深刻认识中部型ENSO对海洋性大陆区域气候的影响机理以及与热带外环流异常的联系。     

热带西北太平洋与东南印度洋对流活动年际变化联系的年代际改变

采用美国NOAA卫星观测OLR (outing longwave radiation)资料以及NCEP/NCAR、CM AP月平均资料,利用合成分析等方法,研究了热带西北太平洋(125°～140°E,10°～20°N)与热带东南印度洋(90°～105°E,5°～15°S)对流活动异常的联系。结果表明:热带西北太平洋与东南印度洋对流活动异常的联系有显著的年代际变化; 20世纪80—90年代存在显著的正相关,20世纪90年代至21世纪初有显著的负相关,其后转变为正相关。合成分析表明,热带西北太平洋与东南印度洋对流活动正相关时,两地区均存在反气旋性环流,低层辐散、高层辐合,对流活动弱,不利于降水产生,有降水负异常;当热带西北太平洋与东南印度洋对流活动负相关时,两地区环流异常存在明显差别,热带东南印度洋有正的海温异常,高层辐散、低层辐合,有上升运动,对流活动强,有降水正异常,而热带西北太平洋则相反。热带西北太平洋和热带东南印度洋之间的斜向垂直环流圈将这两个地区联系起来,并决定了这两个地区对流活动负相关关系的形成。     

1991～1994年El Niño的异常特征的诊断研究

1991～1994年热带中东太平洋海温持续4a多出现正距平。Niño3指数一直为正值。在此正距平背景下,产生了两次振荡和3次ElNiño暖期。在此期间赤道东太平洋海温在±5°纬度范围内发生了两次负距平的变化,形成了一个狭窄的温度梯度很大的“冷核”,而赤道外的中纬度海洋则持续维持两个正海温距平。这一时期海表温度资料EOF分析的结果进一步表明,第一特征向量的空间分布实际上反映了上述冷核特征。1991～1994年的ENSO事件主要是低频分量发生了较大异常,赤道低层纬向风和高层西风在前期减弱(1991-01-1992-05),后期加强(1992-06-1994-011).无论是海温还是风场的低频分量都表现出一次ENSO循环的特征。因此作者认为,虽然Niño3指数等在这4a多期间均为正值,但是大气和海洋耦合系统的低频变化部分只发生了一次完整的ENSO循环过程。1991年至1992年上半年对应于ElNiño暖位相,1992年下半年至1994年底对应于LaNina冷位相。但是这个冷位相没有能够得到充分发展,只出现两次极狭窄的冷核。冷位相的明显“夭折”,而代之以出现两次较弱的增暖,可能与季节内尺度的大气强迫与低频变化部分的相互作用有关。     

On the Utility of ENSO and PNA Indices for Long-Lead Forecasting of Summer Weather over the Crop-Growing Region of the Canadian Prairies

Summary The well known ENSO (El Ni?o-Southern Oscillation) phenomenon is quantitatively identified in terms of SST (sea surface temperature) anomalies over the equatorial central and eastern Pacific and sea level pressure difference over eastern and western regions of the equatorial Pacific. The PNA (Pacific North American) atmospheric flow pattern, originally identified by Wallace and Gutzler (1981) is representative of a certain preferred configuration of the mid-tropospheric geopotential height field in the longitudinal sector extending from the mid-Pacific to the southeastern United States. The PNA index is defined as a linear combination of normalized geopotential height anomalies at the 700 mb level at four selected locations. Using multiple regression analysis, suitable linear combinations of predictors based on monthly values of ENSO and PNA indices are obtained which can foreshadow the summer season’s weather over the crop-growing region of the Canadian prairie provinces with a lead time of 2 to 7 months. The utility of the ENSO and PNA indices for advance indication of summer weather with implications for grain yields over the Canadian prairies is further discussed. Received September 10, 1996 Revised October 13, 1997     

2015/2016年超强El Nio局地海气特征及其特殊性

利用NOAA海表温度和NCEP/NCAR大气环流等全球再分析资料,讨论了2015/2016年超强El Nio事件局地海气过程的演变特征,并与1982/1983和1997/1998年两次强El Nio事件做了对比分析。结果表明,2015/2016年El Nio在峰值强度、持续时间、累计海温距平等指标上都略强于前两次El Nio,可视为有完整气象观测纪录以来的最强事件;与前两次事件相比,2015/2016年El Nio海温异常中心位置明显偏西,热带东太平洋海温相对较冷而中太平洋更暖,由于热带对流对海温的非线性响应,赤道东太平洋降水相对较弱,中太平洋则显著偏多,这在El Nio当年12月至次年4月尤为明显;此外,在前两次El Nio的成熟期至衰减期,中太平洋大气响应都存在明显的南移特征,西风异常和对流中心都从赤道南移到了5°S以南。而2015/2016年中太平洋大气响应一直位于赤道附近,南移特征相对较弱,ENSO和年循环相互作用的组合模态相比前两次较弱,西北太平洋反气旋的强度也弱于前两次。这主要是由于2015年冬季至2016年春季,热带太平洋暖海温异常位置偏西,中太平洋海温异常明显强于前两次,叠加气候平均态海温之后,赤道南北两侧海温都高于对流阈值,对流旺盛,这大大削弱了大气响应的经向移动和ENSO组合模态的强度。     

热带OLR异常准二年周期的初步研究

本文用带通滤波、单点相关和EOF研究了热带(40°N—40°S)OLR异常准二年周期的水平结构。指出其空间模态具有纬向偶极结构,它同热带行星波数2相联系。印尼—赤道中太平洋偶极型的准二年周期变化同赤道QBO有一定关系。后延相关和交叉谱分析揭示出赤道低纬OLR异常准二年周期在东非—赤道中太平洋(30°E—150°W)为驻波振荡,而在赤道东太平洋—赤道大西洋巴西海岸(140—40°W)则为向西传播。      

热带印度洋-西太平洋水汽输送异常对中国东部夏季降水的影响

利用1979~2015年NCEP/NCAR发布的月平均全球再分析资料,分析了热带印度洋-西太平洋水汽输送异常对中国东部夏季降水的影响及其形成机理。研究结果表明:热带印度洋-西太平洋地区（10°S~30°N,60°~140°E）夏季异常水汽输送主要包括两个模态,他们可以解释总的水汽输送异常34%的方差。其中,第一模态（EOF1）表现为异常水汽沿反气旋从热带西太平洋经过南海及孟加拉湾输送到中国东部上空,对应南海、孟加拉湾水汽路径输送均偏多,此时西太平洋副热带高压显著偏强,异常水汽在长江中下游地区辐合并伴随显著上升运动,有利于长江中下游降水偏多;第二模态（EOF2）表现为异常水汽从热带印度洋沿阿拉伯海、印度半岛、中南半岛等呈反气旋式输送,华南上空相应出现气旋式水汽输送异常,并对应异常水汽辐合和上升运动,有利于华南降水偏多。就可能的外部成因而言,EOF1与ENSO关系密切,表现为前冬热带中东太平洋显著偏暖,夏季同期热带北印度洋、南海上空显著偏暖,造成西太平洋副热带高压显著偏强,异常水汽主要来源于热带西太平洋和南海;EOF2与同期热带印度洋偶极子（TIOD）异常有关,TIOD为正位相时热带印度洋上空出现异常东风,华南上空出现异常气旋并伴随水汽异常辐合,异常水汽主要来源于热带南印度洋。     

El Niño－La Niña循环的海－气耦合机制研究

利用１９５０—１９８７年２°×２°格点综合海－气资料（ＣＯＡＤＳ），计算了赤道太平洋洋面（５°Ｓ—３°Ｎ）上逐月平均海温、气压、纬向风、比湿、云以及热量收支各分量的平均值和距平值，分析这些量的演变特征，得出：ＥｌＮｉｎｏ过程中，赤道太平洋洋面上气压梯度减小，东风减弱；赤道中、东太平洋洋面上空气中水汽和云量增加，洋面射入太阳辐射和有效长波辐射减少，感热和潜热交换加强，洋面净获得（损失）热量减少（增加）；ＬａＮｉｎａ过程中，情形相反。最后概括了ＥｌＮｉｎｏ－ＬａＮｉｎａ循环的两种海－气耦合反馈机制。     

THE EFFECT OF EQUATORIAL CENTRAL PACIFIC SSTAS ON THE INTERANNUAL VARIATION OF PRECIPITATION OVER SOUTHWEST CHINA DURING SPRING

The interannual variations of rainfall over southwest China (SWC) during spring and its relationship with sea surface temperature anomalies (SSTAs) in the Pacific are analyzed, based on monthly mean precipitation data from 26 stations in SWC between 1961 and 2010, NCEP/NCAR re-analysis data, and Hadley global SST data. Sensitivity tests are conducted to assess the response of precipitation in SWC to SSTAs over two key oceanic domains, using the global atmospheric circulation model ECHAM5. The interannual variation of rainfall over SWC in spring is very significant. There are strong negative (positive) correlation coefficients between the anomalous precipitation over SWC and SSTAs over the equatorial central Pacific (the mid-latitude Pacific) during spring. Numerical simulations show that local rainfall in the northwest of the equatorial central Pacific is suppressed, and a subtropical anticyclone circulation anomaly is produced, while a cyclonic circulation anomaly in the mid-latitude western Pacific occurs, when the equatorial Pacific SSTAs are in a cold phase in spring. Anomalous northerly winds appear in the northeastern part of SWC in the lower troposphere. Precipitation increases over the Maritime Continent of the western equatorial Pacific, while a cyclonic circulation anomaly appears in the northwest of the western equatorial Pacific. A trough over the Bay of Bengal enhances the southerly flow in the south of SWC. The trough also enhances the transport of moisture to SWC. The warm moisture intersects with anomalous cold air over the northeast of SWC, and so precipitation increases during spring. On the interannual time scale, the impacts of the mid-latitude Pacific SSTAs on rainfall in SWC during spring are not significant, because the mid-latitude Pacific SSTAs are affected by the equatorial central Pacific SSTAs; that is, the mid-latitude Pacific SSTAs are a feedback to the circulation anomaly caused by the equatorial central Pacific SSTAs.     

Wet and Dry Monthly Anomalies Across the Mediterranean Basin and their Relationship with Circulation, 1860–1990

Summary  Anomalously wet and dry months in the Mediterranean basin were identified during the period 1860–1990 from observations at five stations located along the west-east axis of the Mediterranean basin (Barcelona, Florence, Malta, Athens and Jerusalem), supplemented by data from Madrid and Lisbon. Wet and dry months were characterized by hydric indices (HI) based on values of the standardized precipitation anomalies. Different patterns of anomalously wet and dry months were qualitatively identified on the basis of the spatial distributions of the hydric indices. The standardized sea level pressure values at 56 grid points in the domain 35° N–65° N, 30° W–40° E, for each of the anomalously wet and dry months, were subjected to T-mode Principal Component Analysis. The mean hydric indices associated with each principal component in each season are arranged in four distinct different spatial distributions for wet months and in three for dry months as following: (a) Mediterranean wide distribution of positive/negative anomalies; (b1) Strong positive anomalies to the west, but weaker to eastern Mediterranean; (b2) Strong negative anomalies to the west, but weaker or normal to the east; (c1) Strong positive anomalies to the west and to the east and weaker ones to the central Mediterranean; (c2) Negative anomalies to the west and east, but weaker, or normal, or positive to the central Mediterranean; (d) Relatively strong positive anomalies to the east and weaker ones to the western Mediterranean. Finally, monthly mean charts of standardized anomaly and mean sea level pressure are presented for each principalcomponent in each season. These charts are used to interpret the spatial distribution of the positive and negative precipitation anomalies in terms of mean circulation over the domain. Received December 10, 1998 Revised June 14, 1999     

INTERDECADAL TRANSITIONS AND TWO EXCEPTIONAL YEARS OF JUNE PRECIPITATION OVER SOUTH CHINA

An analysis of high-resolution precipitation data for 1978-2006 indicates that the precipitation over southern China in June experienced a low-value period in 1980-1989 and a high-value period in 1992-2001.It also reveals that exceptional heavy(light) precipitation occurred in June 2005(2004) since 1951.For these variations on both interdecadal and interannual timescales,fairly uniform anomalies of precipitation appeared over Vietnam,southern China,and southeastern China.Corresponding to positive(negative) precipitation anomalies,anomalous southeasterly(northwesterly) flow at 850 hPa reached Vietnam and anomalous southwesterly(northeasterly) flow expanded to the coastal regions of southern and southeastern China.Precedent to the positive(negative) precipitation anomalies during 1992-2001(1980-1989),positive(negative) anomalies of sea surface temperature appeared over the extratropical northwestern Pacific in the winter and spring seasons,associated with a strong(weak) extension of the warm Kuroshio Current that affects the coastal region of eastern China.The above-normal precipitation in June 2005 was associated with the pseudo-ENSO event in the previous winter,and the below-normal precipitation in June 2004 was associated with negative anomalies of sea surface temperature over the equatorial central Pacific and positive anomalies over the equatorial western and eastern Pacific.