Relationships between intraseasonal rainfall variability of coastal Tanzania and ENSO

Summary The influence of ENSO on intraseasonal variability over the Tanzanian coast during the short (OND) and long (MAM) rainy seasons is examined. In particular, variability in the rainfall onset, peak and end dates as well as dry spells are considered. In general, El Niño appears to be associated with above average rainfall while La Niña is associated with below average rainfall over the northern Tanzanian coast during OND, and to lesser extent MAM. Over the southern coast, the ENSO impacts are less coherent and this region appears to be a transition zone between the opposite signed impacts over equatorial East and southern Africa. The increased north coast rainfall during El Niño years is generally due to a longer than normal rainfall season associated with early onset while reduced rainfall during La Niña years tends to be associated with a late onset, and thus a shorter than average rainfall season. Wet conditions during El Niño years were associated with enhanced convection and low-level easterly anomalies over the equatorial western Indian Ocean implying enhanced advection of moisture from the Indian Ocean while the reverse is true for La Niña years. Hovmöller plots for OLR and zonal wind at 850 hPa and 200 hPa show eastward, westward propagating and stationary features over the Indian Ocean. It was observed that the propagating features were absent during strong El Niño years. Based on the Hovmöller results, it is observed that the convective oscillations over the Tanzanian coast have some of the characteristic features of intraseasonal oscillations occurring elsewhere in the tropics.     

ENSO对中国南方降水低频变率的可能影响

利用1979—2012年Hadley中心海表温度、中国2 474个台站逐日降水和NCEP/NCAR全球再分析资料,分析了不同类型ENSO事件秋冬季和次年春季中国南方地区10~30 d降水低频变率的变化特征。结果表明,中国南方地区10~30 d降水低频变率对不同类型ENSO事件的响应存在显著的季节差异。EP型El Ni1o的冬季和次年春季,低频降水变率显著增强; CP型El Ni1o秋冬季低频降水强度呈现相反的异常,秋季低频降水偏弱,而冬季则偏强; La Ni1a事件期间中国南方低频降水变率的变化较小且不稳定。进一步分析发现,ENSO对南方地区10~30 d低频降水变率的影响与西北太平洋地区季节平均大气环流背景场对ENSO的响应密切相关。相比正常年份,EP型El Ni1o冬春季菲律宾反气旋性异常环流的强度较强且范围较大,其西侧的异常西南风向中国南方地区输送了大量水汽,从而有利于低频降水的增强; CP型El Ni1o年秋季西北太平洋表现为气旋性环流异常,抑制了热带水汽向东亚大陆的输送,而冬季却产生了与EP型El Ni1o年类似的异常反气旋环流,只是强度有所减弱,因此中国南方地区低频降水强度在秋冬季呈相反异常。La Ni1a年菲律宾附近虽然存在气旋性环流异常,但强度较弱,因而我国南方地区低频降水变率的响应也较弱。     

Role of the southern Indian Ocean in the transitions of the monsoon-ENSO system during recent decades

The focus of this study is to document the possible role of the southern subtropical Indian Ocean in the transitions of the monsoon-ENSO system during recent decades. Composite analyses of sea surface temperature (SST) fields prior to El Niño-Southern Oscillation (ENSO), Indian summer monsoon (ISM), Australian summer monsoon (AUSM), tropical Indian Ocean dipole (TIOD) and Maritime Continent rainfall (MCR) indices reveal the southeast Indian Ocean (SEIO) SSTs during late boreal winter as the unique common SST precursor of these various phenomena after the 1976–1977 regime shift. Weak (strong) ISMs and AUSMs, El Niños (La Niñas) and positive (negative) TIOD events are preceded by significant negative (positive) SST anomalies in the SEIO, off Australia during boreal winter. These SST anomalies are mainly linked to subtropical Indian Ocean dipole events, recently studied by Behera and Yamagata (Geophys Res Lett 28:327–330, 2001). A wavelet analysis of a February–March SEIO SST time series shows significant spectral peaks at 2 and 4–8 years time scales as for ENSO, ISM or AUSM indices. A composite analysis with respect to February–March SEIO SSTs shows that cold (warm) SEIO SST anomalies are highly persistent and affect the westward translation of the Mascarene high from austral to boreal summer, inducing a weakening (strengthening) of the whole ISM circulation through a modulation of the local Hadley cell during late boreal summer. At the same time, these subtropical SST anomalies and the associated SEIO anomalous anticyclone may be a trigger for both the wind-evaporation-SST and wind-thermocline-SST positive feedbacks between Australia and Sumatra during boreal spring and early summer. These positive feedbacks explain the extraordinary persistence of the SEIO anomalous anticyclone from boreal spring to fall. Meanwhile, the SEIO anomalous anticyclone favors persistent southeasterly wind anomalies along the west coast of Sumatra and westerly wind anomalies over the western Pacific, which are well-known key factors for the evolution of positive TIOD and El Niño events, respectively. A correlation analysis supports these results and shows that SEIO SSTs in February–March has higher predictive skill than other well-established ENSO predictors for forecasting Niño3.4 SST at the end of the year. This suggests again that SEIO SST anomalies exert a fundamental influence on the transitions of the whole monsoon-ENSO system during recent decades.     

Reducing Vulnerability to Climate Variability in Southern Africa: The Growing Role of Climate Information

Since the devastating southern Africa drought of 1991/92 awareness has grown of the potential to better manage climate variability in the region through seasonal climate forecasting and monitoring of El Niño and the Southern Oscillation (ENSO). While other factors besides ENSO affect southern Africa's climate, and climate forecasting for the region is not based exclusively on ENSO, a major El Niño beginning in 1997 captured the attention of policy-makers and the public. Awareness of drought risks associated with the 1997/98 event was greater than during previous El Niños in 1991/92 and 1994/95. Mitigation and planning efforts also began earlier, with drought early warnings widely available and being taken seriously prior to the 1997/98 agricultural season. Actions taken include issuance of guidance to the public, on-going monitoring and preparedness efforts including the development of national preparedness plans in some countries, pre-positioning of food stocks, donor coordination, and greater reliance on the private-sector for meeting regional food needs. Although 1998 regional crop production was slightly below average, a major drought did not materialize. Nonetheless the experience is likely to ultimately strengthen capacity within the region to manage climate variability over the long term.     

Sea surface temperature associations with the late Indian summer monsoon

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.     

Modification of the southern African rainfall variability/ENSO relationship since the late 1960s

Analysis of 149 raingauge series (1946–1988) shows a weak positive correlation between late summer rainfalls (January–March) in tropical southern Africa and the Southern Oscillation Index (SOI). The correlation coefficients have been unstable since World War II. They were close to zero before 1970 and significant thereafter. Before 1970, southern African late summer rainfalls were more specifically correlated with regional patterns of sea surface temperature (SST), mainly over the southwestern Indian Ocean. After 1970, teleconnections with near global SST anomaly patterns, i.e. over the central Pacific and Indian oceans, dominate the regional connections. The increase in the sensitivity of the southern African rainfall to the global SO-related circulation anomalies is simultaneous with the correlation between SOI and more extensive SST anomalies, particularly over the southern Indian Ocean. This feature is part of longer term (decadal), global SST variability, as inferred from statistical analyses. Numerical experiments, using the Météo-France general circulation model ARPEGE-Climat, are performed to test the impact of the observed SST warming in the southern Indian and extratropical oceans during El Niño Southern Oscillation (ENSO) events on southern African rainfall. Simulated results show that ENSO events, which occurred in the relatively cold background of the pre-1970 period in the southern oceans, had a little effect on southern Africa climatic conditions and atmospheric circulation. By contrast, more recent ENSO events, with warmer SST over the southern oceans, lead to a climatic bipolar pattern between continental southern African and the western Indian Ocean, which is characterized by reduced (enhanced) deep convection and rainfall over the subcontinent (the western Indian Ocean). A weaker subtropical high-pressure belt in the southwestern Indian Ocean is also simulated, along with a reduced penetration of the moist southern Indian Ocean trade winds over the southern African plateau. These results are consistent with the strong droughts observed over all southern Africa during ENSO events since 1970.     

ENSO与北半球冬季大气环流异常年代际关系的数值模拟

利用NCAR/CCM3大气环流模式,模拟不同海温背景场强迫下,北半球冬季大气环流对ENSO事件的响应。结果表明,模式能够模拟出不同海温背景场下,北半球冬季大气环流对ENSO事件的不同响应,模拟结果与诊断结果基本一致。在20世纪70年代末之后的平均海温背景场强迫下,El Nio对阿留申低压的加强作用比70年代末之前更明显,而ENSO暖位相对蒙古高压的作用则由70年代末之前的轻微减弱变为明显加强。虽然70年代末以后ENSO事件的强度比70年代末之前有所增大,但ENSO对东亚大槽和东亚冬季风的减弱作用反而不再那么明显;同时在高空风场中,由ENSO激发出的PNA型异常环流在70年代末以后明显加强。     

El Niño- or La Niña-like climate change?

The potential for the mean climate of the tropical Pacific to shift to more El Niño-like conditions as a result of human induced climate change is subject to a considerable degree of uncertainty. The complexity of the feedback processes, the wide range of responses of different atmosphere–ocean global circulation models (AOGCMs) and difficulties with model simulation of present day El Niño southern oscillation (ENSO), all complicate the picture. By examining the components of the climate-change response that projects onto the model pattern of ENSO variability in 20 AOGCMs submitted to the coupled model inter-comparison project (CMIP), it is shown that large-scale coupled atmosphere–ocean feedbacks associated with the present day ENSO also operate on longer climate-change time scales. By linking the realism of the simulation of present day ENSO variability in the models to their patterns of future mean El Niño-like or La Niña-like climate change, it is found that those models that have the largest ENSO-like climate change also have the poorest simulation of ENSO variability. The most likely scenario (p=0.59) in a model-skill-weighted histogram of CMIP models is for no trend towards either mean El Niño-like or La Niña-like conditions. However, there remains a small probability (p=0.16) for a change to El Niño-like conditions of the order of one standard El Niño per century in the 1% per year CO₂ increase scenario.     

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。     

北半球夏季海洋性大陆区域气候与EP型ENSO:直接与间接联系

利用1979—2009年的NECP资料、Hadley海温月平均资料和CMAP降水资料,采用Kao and Yu(2009)的方法定义了夏季EP型ENSO指数EPI,用合成分析的方法分析了东部型ENSO与海洋性大陆降水的关系。结果表明:EPI与MC(Maritime Continent,海洋性大陆)区域降水变化间存在非常弱的负相关。造成这一弱相关的原因是EPI与MC区域降水在某些年份存在同号变化。在剔除Nio4信号后,海洋性大陆区域降水序列与EPI与存在着同号和反号两种关系。反号关系是通常所认为的,当经典的El Nio(La Nia)发生时MC区域降水出现显著地减少(增多)。此时,沿赤道的异常Walker环流建立了EP型ENSO与MC区域气候间的直接联系。而在同号关系时,菲律宾以东异常加热和SPCZ区域异常冷却引起的西北—东南走向的垂直环流圈削弱了MC区域与赤道东太平洋之间的异常Walker环流所建立的直接联系,或者说,赤道东太平洋区域SSTA与MC区域降水异常的形成是通过SPCZ区域SST的反号异常而产生间接联系的。这种机制的揭示为深刻认识ENSO影响海洋性大陆区域甚至东亚地区气候变动的联系提供了新的线索。     

The influence of climate regime shift on ENSO

Observations indicated that for the El Niño/Southern Oscillation (ENSO) there have been eastward displacements of the zonal wind stress (WS) anomalies and surface heat flux (short wave heat flux and latent heat flux) anomalies during El Niño episodes in the 1981–1995 regime relative to the earlier regime of 1961–1975 (without corresponding displacements during La Niña episodes). Our numerical experiments with the Zebiak–Cane coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed climatologies [of sea surface temperature (SST), surface WS and surface wind atmospheric divergence] and simulated climatologies (of oceanic surface layer currents and associated upwelling) for the 1981–1995 regime. Sensitivity tests indicated that the background atmospheric state played a much more important role than the background ocean state in producing the displacements, which enhanced the asymmetry between El Niño and La Niña in the later regime. The later regime climatology state resulted in the eastward shifts in the ENSO system (WS and SST) only during El Niño, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. That the change in the mean state of the tropical Pacific atmosphere and ocean after the mid 1970s could have produced the observed changes in ENSO properties is consistent with our findings.     

Historical ENSO teleconnections in the eastern hemisphere

Examination of instrumental data collected over the last one hundred years or so shows that rainfall fluctuations in various parts of the eastern hemisphere are associated with the El Niño-Southern Oscillation (ENSO) phenomenon. Using proxy rainfall data-sets from Indonesia, Africa, North China; and a chronology of droughts from India, we investigate the occurrence of ENSO-related floods and droughts over the last five hundred years. The aim of this work is to examine the stability of the pattern of ENSO teleconnections over this longer period, noting any changes in ENSO behaviour which may be relevant in estimating its future behaviour, such as its response to climate change due to the enhanced greenhouse effect.Comparisons of the various data sets with each other and with El Niño chronology from South America, showed statistically significant evidence of teleconnections characteristic of ENSO back to around 1750. Prior to that time, relationships characteristic of ENSO were weak or absent. The disappearance of the ENSO signal in the early period is considered to be most likely due to the poorer quality of the data at that time. From the 18th Century onwards, chronologies of ENSO and anti-ENSO events are given and compared with similar chronologies in the literature.     

Seasonality and time scales in the relationship between global SST and African rainfall

The main goal of this study is to determine the oceanic regions corresponding to variability in African rainfall and seasonal differences in the atmospheric teleconnections. Canonical correlation analysis (CCA) has been applied in order to extract the dominant patterns of linear covariability. An ensemble of six simulations with the global atmospheric general circulation model ECHAM4, forced with observed sea surface temperatures (SSTs) and sea ice boundary variability, is used in order to focus on the SST-related part of African rainfall variability. Our main finding is that the boreal summer rainfall (June–September mean) over Africa is more affected by SST changes than in boreal winter (December–March mean). In winter, there is a highly significant link between tropical African rainfall and Indian Ocean and eastern tropical Pacific SST anomalies, which is closely related to El Niño-Southern Oscillation (ENSO). However, long-term changes are found to be associated with SST changes in the Indian and tropical Atlantic Oceans, thus, showing that the tropical Atlantic plays a critical role in determining the position of the intertropical convergence zone (ITCZ). Since ENSO is less in summer, the tropical Pacific and the Indian Oceans are less important for African rainfall. The African summer monsoon is strongly influenced by SST variations in the Gulf of Guinea, with a response of opposite sign over the Sahelian zone and the Guinean coast region. SST changes in the subtropical and extratropical oceans mostly take place on decadal time scales and are responsible for low-frequency rainfall fluctuations over West Africa. The modelled teleconnections are highly consistent with the observations. The agreement for most of the teleconnection patterns is remarkable and suggests that the modelled rainfall anomalies serve as suitable predictors for the observed changes.     

Assessment of regional seasonal rainfall predictability using the CPTEC/COLA atmospheric GCM

This is a study of the annual and interannual variability of regional rainfall produced by the Center for Weather Forecasts and Climate Studies/Center for Ocean, Land and Atmospheric Studies (CPTEC/COLA) atmospheric global climate model. An evaluation is made of a 9-member ensemble run of the model forced by observed global sea surface temperature (SST) anomalies for the 10-year period 1982–1991. The Brier skill score and, Relative Operating Characteristics (ROC) are used to assess the predictability of rainfall and to validate rainfall simulations, in several regions world wide. In general, the annual cycle of precipitation is well simulated by the model for several continental and oceanic regions in the tropics and mid latitudes. Interannual variability of rainfall during the peak rainy season is realistically simulated in Northeast Brazil, Amazonia, central Chile, and southern Argentina–Uruguay, Eastern Africa, and tropical Pacific regions, where the model shows good skill. Some regions, such as northwest Peru–Ecuador, and southern Brazil exhibit a realistic simulation of rainfall anomalies associated with extreme El Niño warming conditions, while in years with neutral or La Niña conditions, the agreement between observed and simulated rainfall anomalies is not always present. In the monsoon regions of the world and in southern Africa, even though the model reproduces the annual cycle of rainfall, the skill of the model is low for the simulation of the interannual variability. This is indicative of mechanisms other than the external SST forcing, such as the effect of land–surface moisture and snow feedbacks or the representation of sub-grid scale processes, indicating the important role of factors other than external boundary forcing. The model captures the well-known signatures of rainfall anomalies of El Niño in 1982–83 and 1986–87, indicating its sensitivity to strong external forcing. In normal years, internal climate variability can affect the predictability of climate in some regions, especially in monsoon areas of the world.     

Tanzanian rainfall responses to El Niño and positive Indian Ocean Dipole events during 1951–2015

The relative impacts of Indian and Pacific Ocean processes on Tanzanian rainfall was evaluated using composite and correlation analyses. It was found that the seasonal responses of rainfall to positive Indian Ocean Dipole (pIOD) and El Niño events are substantial from September–October–November (SON) to December–January–February (DJF), whereas the Indian Ocean Dipole (IOD) exerts more control than El Niño–Southern Oscillation (ENSO) in both seasons. The associated relationship with the sea surface temperature (SST) and large-scale atmospheric circulations revealed distinct features. For the pure pIOD years, there is above-normal rainfall over the entire country. A strong rainfall condition is evident over the Lake Victoria basin and coastal and northeastern highland parts of the country during SON, while areas of the central and southern highlands exhibit substantial rains during DJF. For the pure El-Niño events, Tanzania has suffered from insignificant, weak, and non-coherent rainfall conditions during SON. However, a contrasting insignificant rainfall signature is found between the northern and southern parts of the country during the subsequent DJF season. For the co-occurrence of pIOD and El Niño, significant, excessive rainfall conditions are restricted to over the northern coast and northeastern areas of the country during SON, consistent with the rainfall pattern for pIOD. A weak, positive rainfall condition is observed over the entire country in the following season of DJF. Generally, in terms of Tanzanian rainfall, the IOD/ENSO variability and the associated impacts can be explained by the anomalous SST and circulation anomalies.     

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。     

季节预测模式对东亚夏季环流的预测能力及其对热带海洋的响应分析

东亚夏季环流变化对中国夏季降水的年际变化有重要影响,因此需要进一步理解季节预测模式对东亚夏季环流的预测能力。利用1991~2013年美国国家环境预测中心（NCEP）、中国气象局国家气候中心（NCC）和日本东京气候中心（TCC）的三个季节预测模式（CFS V2、BCC_CSM V2和MRI-CGCM）以及NCEP/NCAR再分析资料,定量评估了模式对东亚夏季风（EASM）和夏季西太平洋副热带高压（WPSH）强度的预测能力。在此基础上,分析了模式预测的EASM和WPSH对热带海温异常的响应能力,以及ENSO事件对EASM和WPSH预测的影响,阐述了预测误差产生的原因。结果表明:整体而言,三个模式对EASM和WPSH的预测技巧较高,但TCC模式对WPSH的预测技巧相对较低。三个模式预测的850 hPa风场在西北太平洋存在一个异常气旋,使得预测的EASM偏强和WPSH偏弱。同时,二者的年际变率整体比观测小。三个模式预测的EASM和WPSH对热带海洋海温异常的响应随季节演变特征与观测比较接近,但NCEP模式和TCC模式预测的EASM对前期热带太平洋和前期、同期热带印度洋的海温异常响应要强于观测,NCC模式预测的EASM对前期和同期的热带太平洋的海温异常响应明显比观测强。此外,三个模式预测的WPSH对前期和同期的热带太平洋、热带印度洋和热带大西洋的海温异常响应明显强于观测。三个模式预测的EASM和WPSH在ENSO年的平均绝对误差（MAE）整体而言要比正常年的小很多,NCEP模式和NCC模式预测的EASM和WPSH的MAE在La Ni?a年和El Ni?o年差别不大,而TCC模式预测的EASM和WPSH的MAE在El Ni?o年比在La Ni?a年大很多,表明ENSO事件是东亚夏季环流重要的可预报源。     

Nonlinear interdecadal changes of the El Niño-Southern Oscillation

Nonlinear interdecadal changes in the El Niño-Southern Oscillation (ENSO) phenomenon are investigated using several tools: a nonlinear canonical correlation analysis (NLCCA) method based on neural networks, a hybrid coupled model, and the delayed oscillator theory. The leading NLCCA mode between the tropical Pacific wind stress (WS) and sea surface temperature (SST) reveals notable interdecadal changes of ENSO behaviour before and after the mid 1970s climate regime shift, with greater nonlinearity found during 1981–99 than during 1961–75. Spatial asymmetry (for both SST and WS anomalies) between warm El Niño and cool La Niña events was significantly enhanced in the later period. During 1981–99, the location of the equatorial easterly anomalies was unchanged from the earlier period, but in the opposite ENSO phase, the westerly anomalies were shifted eastward by up to 25°. According to the delayed oscillator theory, such an eastward shift would lengthen the duration of the warm events by up to 45%, but leave the duration of the cool events unchanged. Supporting evidence was found from a hybrid coupled model built with the Lamont dynamical ocean model coupled to a statistical atmospheric model consisting of either the leading NLCCA or CCA mode.     

The signature of ENSO in the Northern Hemisphere midlatitude seasonal mean flow and high-frequency intraseasonal variability

Summary The impact of pronounced positive and negative sea surface temperature (STT) anomalies in the tropical Pacific associated with the El Niño/Southern Oscillation (ENSO) phenomenon on the atmospheric circulation in the Northern Hemisphere extratropics during the boreal winter season is investigated. This includes both the impact on the seasonal mean flow and on the intraseasonal variability on synoptic time scales. Moreover, the interaction between the transient fluctuations on these times scales and the mean circulation is examined. Both data from an ensemble of five simulations with the ECHAM3 atmospheric general circulation model at a horizontal resolution of T42 each covering the period from 1979 through 1992 and operational analyses from ECMWF for the corresponding period are examined. In each of the simulations observed SSTs for the period of investigation are given as lower boundary forcing, but different atmospheric initial conditions are prescribed.The simulations with ECHAM3 reveal a distinct impact of the pronounced SST-anomalies in the tropical Pacific on the atmospheric circulation in the Northern Hemisphere extratropics during El Niño as well as during La Niña events. These changes in the atmospheric circulation, which are found to be highly significant in the Pacific/North American as well as in the Atlantic/European region, are consistent with the essential results obtained from the analyses. The pronounced SST-anomalies in the tropical Pacific lead to changes in the mean circulation, which are characterized by typical circulation patterns. These changes in the mean circulation are accompanied by marked variations of the activity of the transient fluctuations on synoptic time scales, that are changes in both the kinetic energy on these time scales and the atmospheric transports of momentum and heat accomplished by the short baroclinic waves. The synoptic disturbances, on the other hand, play also an important role in controlling the changes in the mean circulation associated with the ENSO phenomenon. They maintain these typical circulation patterns via barotropic, but counteract them via baroclinic processes.The hypothesis of an impact of the ENSO phenomenon in the Atlantic/European region can be supported. As the determining factor the intensification (reduction) of the Aleutian low and the simultaneous reduction (intensification) of the Icelandic low during El Niño and during La Niña events respectively, is identified. The changes in the intensity of the Aleutian low during the ENSO-events are accompanied by an alteration of the transport of momentum caused by the short baroclinic waves over the North American continent in such a way that the changes in the intensity of the Icelandic low during El Niño as well as during La Niña events are maintained.With 16 Figures     

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。