Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Ni ?no Events?—Role of Midlatitude Circulation in August

It is widely recognized that rainfall over the Yangtze River valley(YRV) strengthens considerably during the decaying summer of El Ni ?no,as demonstrated by the catastrophic flooding suffered in the summer of 1998.Nevertheless,the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998,despite the intensity of the 2016 El Nio having been as strong as that in 1998.A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years,principally in August.The precipitation anomaly was negative in August 2016—different to the positive anomaly of 1998.Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes.The intensified "Silk Road Pattern" and upper-tropospheric geopotential height over the Urals region,both at their strongest since 1980,resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere.This easterly flow reduced the climatological wind,weakened the water vapor transport,and induced the weaker YRV rainfall in August 2016,as compared to that in 1998.Given the unique sub-seasonal variation of the YRV rainfall in summer 2016,more attention should be paid to midlatitude circulation—besides the signal in the tropics—to further our understanding of the predictability and variation of YRV summer rainfall.     

IMPACT OF ENSO ON THE PRECIPITATION OVER CHINA IN WINTER HALF-YEARS

In this paper, the impact of ENSO on the precipitation over China in the winter half-year is investigated diagnostically. The results show that positive precipitation anomalies with statistical significance appear over southern China in El Nio episodes, which are caused by the enhanced warm and humid southwesterlies along the East Asian coast in the lower troposphere. The enhanced southwesterlies transport more water vapor to southern China, and the convergence of water vapor over southern China increases the precipitable water and specific humidity. In La Nia episodes,although atmospheric elements change reversely, they are not statistically significant as those in El Nio periods. The possible physical mechanism of the different impact of ENSO cycle on the precipitation over southern China is investigated by analyzing the intraseasonal oscillations(ISOs) in El Nio and La Nia winter half-years, respectively. By comparing the characteristics of ISOs in El Nio and La Nia, a physical mechanism is proposed to explain the different responses of the precipitation over China to ENSO in the winter half-year. In El Nio episodes, over western North Pacific(WNP) and South China Sea(SCS) the ISOs are inactive and exert little effect on water vapor transport and convergence, inducing positive precipitation anomalies with statistical significance over southern China in El Nio episodes. In La Nia episodes, however, the ISOs are active, which weaken the interannual variation signals of ENSO over WNP and southern China and lead to the insignificance of the interannual signals related to ENSO. Therefore, the different responses of precipitation over China to ENSO in the winter half-year are possibly caused by the difference of intraseasonal oscillations over WNP and SCS between El Nio and La Nia.     

Variable and Robust East Asian Monsoon Rainfall Response to El Nio over the Past 60 Years(1957–2016)

Severe flooding occurred in southern and northern China during the summer of 2016 when the 2015 super El Nio decayed to a normal condition. However, the mean precipitation during summer(June–July-August) 2016 does not show significant anomalies, suggesting that — over East Asia(EA) — seasonal mean anomalies have limited value in representing hydrological hazards. Scrutinizing season-evolving precipitation anomalies associated with 16 El Nio episodes during 1957–2016 reveals that, over EA, the spatiotemporal patterns among the four categories of El Nio events are quite variable, due to a large range of variability in the intensity and evolution of El Nio events and remarkable subseasonal migration of the rainfall anomalies. The only robust seasonal signal is the dry anomalies over central North China during the El Nio developing summer. Distinguishing strong and weak El Nio impacts is important. Only strong El Nio events can persistently enhance EA subtropical frontal precipitation from the peak season of El Nio to the ensuing summer, by stimulating intense interaction between the anomalous western Pacific anticyclone(WPAC) and underlying dipolar sea surface temperature anomalies in the Indo-Pacific warm pool, thereby maintaining the WPAC and leading to a prolonged El Nio impact on EA. A weak El Nio may also enhance the post-El Nio summer rainfall over EA, but through a different physical process: the WPAC re-emerges as a forced response to the rapid cooling in the eastern Pacific. The results suggest that the skillful prediction of rainfall over continental EA requires the accurate prediction of not only the strength and evolution of El Nio, but also the subseasonal migration of EA rainfall anomalies.     

Relationships between the East Asian-western north pacific monsoon and ENSO simulated by FGOALS-s2

The relationships between ENSO and the East Asian-western North Pacific monsoon simulated by the Flexible Global Ocean-Atmosphere-Land System model, Spectral Version 2 (FGOALS-s2), a state-of-the-art coupled general circulation model (CGCM), are evaluated. For El Nio developing summers, FGOALS-s2 reproduces the anomalous cyclone over the western North Pacific (WNP) and associated negative precipitation anomalies in situ. In the observation, the anomalous cyclone is transformed to an anomalous anticyclone over the WNP (WNPAC) during El Nio mature winters. The model reproduces the WNPAC and associated positive precipitation anomalies over southeastern China during winter. However, the model fails to simulate the asymmetry of the wintertime circulation anomalies over the WNP between El Nio and La Nia. The simulated anomalous cyclone over the WNP (WNPC) associated with La Nia is generally symmetric about the WNPAC associated with El Nio, rather than shifted westward as that in the observation. The discrepancy can partially explain why simulated La Nin a events decay much faster than observed. In the observation, the WNPAC maintains throughout the El Nio decaying summer under the combined effects of local forcing of the WNP cold sea surface temperature anomaly (SSTA) and remote forcing from basinwide warming in the tropical Indian Ocean. FGOALS-s2 captures the two mechanisms and reproduces the WNPAC throughout the summer. However, owing to biases in the mean state, the precipitation anomalies over East Asia, especially those of the Meiyu rain belt, are much weaker than that in the observation.     

ECHAM5-Simulated Impacts of Two Types of El Nio on the Winter Precipitation Anomalies in South China

The authors used an atmospheric general circulation model(AGCM) of European Centre Hamburg Model(ECHAM5.4) and investigated the possible impacts of eastern Pacific(EP) and central Pacific(CP) El Nio on the winter precipitation anomalies in South China.A composite analysis suggested much more rainfall during the mature phase of EP El Nio than in the case of CP El Nio,and their corresponding observed wet centers to be located in the southeast coast and the region to the south of the Yangtze River,respectively.Results obtained on the basis of model-sensitive run imply that the modelsimulated rainfall anomalies agree well with the observation,and the magnitude of simulated rainfall anomalies were found to be reduced when the amplitude of sea surface temperature anomaly(SSTA) forcing of EP and CP El Nio was cut down.These results imply that the rainfall anomaly in South China is very sensitive not only to the type of El Nio but also to its intensity.     

The Amplitude-Duration Relation of Observed El Nino Events

The authors demonstrate that the El Ni o events in the pre-and post-1976 periods show two ampli-tude-duration relations. One is that the stronger El Ni o events have longer durations, which is robust for the moderate El Ni o events; the other is that the stronger El Ni o events have shorter durations but for strong El Nio events. By estimating the sign and amplitude of the nonlinear dynamical heating (NDH) anomalies, the authors illustrate that the NDH anomalies are negligible for moderate El Nio events but large for strong El Nio events. In particular, the large NDH anomalies for strong El Nio events are positive during the growth and mature phases, which favor warmer El Nio events. During the decay phase, however, the negative NDH anomalies start to arise and become increasingly significant with the evolution of the El Nio events, in which the negative NDH anomalies dampen the sea surface temperature anomalies (SSTA) and cause the El Nio events to reach the SST normal state earlier. This pattern suggests that the nonlinearity tends to increase the intensities of strong El Nio events and shorten their duration, which, together with the previous results showing a positive correlation between the strength of El Nio events and the significance of the effect of nonlinear advection on the events (especially the suppression of nonlinearity on the SSTA during the decay phase), shows that the strong El Nio events tend to have the amplitude-duration relation of the stronger El Nio events with shorter durations. This result also lends support to the assertion that moderate El Nio events possess the amplitude-duration relation of stronger El Nio events with longer durations.     

The Amplitude-Duration Relation of Observed El Nio Events

The authors demonstrate that the El Ni o events in the pre-and post-1976 periods show two ampli-tude-duration relations. One is that the stronger El Ni o events have longer durations, which is robust for the moderate El Ni o events; the other is that the stronger El Ni o events have shorter durations but for strong El Nio events. By estimating the sign and amplitude of the nonlinear dynamical heating (NDH) anomalies, the authors illustrate that the NDH anomalies are negligible for moderate El Nio events but large for strong El Nio events. In particular, the large NDH anomalies for strong El Nio events are positive during the growth and mature phases, which favor warmer El Nio events. During the decay phase, however, the negative NDH anomalies start to arise and become increasingly significant with the evolution of the El Nio events, in which the negative NDH anomalies dampen the sea surface temperature anomalies (SSTA) and cause the El Nio events to reach the SST normal state earlier. This pattern suggests that the nonlinearity tends to increase the intensities of strong El Nio events and shorten their duration, which, together with the previous results showing a positive correlation between the strength of El Nio events and the significance of the effect of nonlinear advection on the events (especially the suppression of nonlinearity on the SSTA during the decay phase), shows that the strong El Nio events tend to have the amplitude-duration relation of the stronger El Nio events with shorter durations. This result also lends support to the assertion that moderate El Nio events possess the amplitude-duration relation of stronger El Nio events with longer durations.     

Contribution of Water Vapor to the Record-Breaking Extreme Meiyu Rainfall along the Yangtze River Valley in 2020

A record-breaking extreme Meiyu rainfall occurred along the Yangtze River valley(YRV) in 2020 since 1961, persisting from 11 June to 31 July with the largest amount and the highest intensity. From the aspect of water vapor, the causes of its formation are revealed in this study. The 2020 Meiyu rainfall amount is directly attributed to the greatly enhanced vertically integrated water vapor transport(IVT) convergence, which is in turn primarily determined by the mean circulation dynamic(MCD) contribution associated with anomalous East Asian summer monsoon(EASM) and the thermodynamic component(TH) contribution due to water vapor anomaly. The MCD contribution is mainly responsible for the extreme Meiyu rainfall amount and abundant water vapor convergence in the YRV, whereas the TH contribution tends to shift Meiyu rain belt northward to the Yangtze–Huaihe River valley, extending the Meiyu rainfall coverage area. Furthermore, the excessive moist static energy(MSE) associated with the largest water vapor anomaly could substantially increase the atmospheric instability, favoring the extreme 2020 Meiyu rainfall intensity. In terms of the tremendous IVT to the YRV from both the South China Sea and Bay of Bengal during the 2020 Meiyu period, the low-level anticyclone anomalies over the western North Pacific(WNP) and Bay of Bengal provide appropriate atmospheric circulation conditions, and they are generated by the super suppressed WNP convective activities as a Matsuno–Gill type response, which are further attributed to the combined warm SST anomalies in both the tropical western Indian Ocean(TWIO) and tropical Atlantic Ocean(TAO) eventually.     

华南春季降水和水汽输送的年代际变化特征

The characteristics of spring precipitation and water vapor transport in South China were analyzed by using observational data and the National Centers for Environmental Prediction （NCEP） reanalysis data. The results show that, during the spring, each component of the water cycle （precipitation, wind field, specific humidity, water vapor transport, etc.） in South China exhibits a notable interdecadal variability. An abrupt increase in spring precipitation occurred in the early 1970s. During the dry period from 1958 to 1971, a water vapor flux divergence （positive divQ） existed in South China, which may have led to the deficiency in rainfall. However, during the wet period from 1973 to 1989, there was a remarkable water vapor flux convergence （negative divQ） in South China, which may have resulted in the higher rainfall. The interdecadal variability of water vapor transport is closely related to the interdecadal variability of wind fields, although the interdecadal variability of specific humidity also plays a role to some extent, and the interdecadal variability of the zonal water vapor transport contributes much more to the interdecadal variability of spring precipitation than the meridional water vapor transport.     

The 2016 Summer Floods in China and Associated Physical Mechanisms:A Comparison with 1998

The characteristics of droughts and floods in China during the summers(May–August)of 2016 and 1998 were compared in great detail,together with the associated atmospheric circulations and external-forcing factors.Following results are obtained.(1)The precipitation was mostly above normal in China in summer 2016,with two main rainfall belts located in the Yangtze River valley(YRV)and North China.Compared with 1998,a similar rainfall belt was located over the YRV,with precipitation 100%and more above normal.However,the seasonal processes of Meiyu were different.A typical"Secondary Meiyu"occurred in 1998,whereas dry conditions dominated the YRV in2016.(2)During May–July 2016,the Ural high was weaker than normal,but it was stronger than normal in 1998.This difference resulted from fairly different distributions of sea surface temperature anomalies(SSTAs)over the North Atlantic Ocean during the preceding winter and spring of the two years.(3)Nonetheless,tropical and subtropical circulation systems were much more similar in May–July of 2016 and 1998.The circulation systems in both years were characterized by a stronger than normal and more westward-extending western Pacific subtropical high(WPSH),a weaker than normal East Asian summer monsoon(EASM),and anomalous convergence of moisture flux in the mid and lower reaches of the YRV.These similar circulation anomalies were attributed to the similar tropical SSTA pattern in the preceding seasons,i.e.,the super El Ni?o and strong warming in the tropical Indian Ocean.(4)Significant differences in the circulation pattern were observed in August between the two years.The WPSH broke up in August 2016,with its western part being combined with the continental high and persistently dominating eastern China.The EASM suddenly became stronger,and dry conditions prevailed in the YRV.On the contrary,the EASM was weaker in August 1998 and the"Secondary Meiyu"took place in the YRV.The Madden–Julian Oscillation(MJO)was extremely active in August 2016 and stayed in western Pacific for 25 days.It triggered frequent tropical cyclone activities and further influenced the significant turning of tropical and subtropical circulations in August2016.In contrast,the MJO was active over the tropical Indian Ocean in August 1998,conducive to the maintenance of a strong WPSH.Alongside the above oceanic factors and atmospheric circulation anomalies,the thermal effect of snow cover over the Qinghai–Tibetan Plateau from the preceding winter to spring in 2016 was much weaker than that in 1998.This may explain the relatively stronger EASM and more abundant precipitation in North China in 2016than those in 1998.     

厄尔尼诺事件对中国夏季水汽输送和降水分布影响的新研究

基于1961-2016年中国地面台站降水观测资料和多种再分析资料,分析了东部型和中部型两类厄尔尼诺事件对中国夏季水汽输送和降水的不同影响。结果表明：（1）厄尔尼诺事件对中国夏季降水的影响在发生当年和次年有明显的不同,主要影响是在其发生的次年,中国大部分地区的夏季降水明显偏多。（2）东部型厄尔尼诺事件当年夏季,西北太平洋副热带高压（副高）偏东偏弱,水汽输送条件较弱,不利于中国大范围降水的发生;中部型事件当年夏季,低纬度印度洋和西太平洋蒸发异常偏强,来自阿拉伯海、孟加拉湾和西北太平洋向华南地区的水汽输送和净水汽收支增加,有利于华南地区降水的异常增多。（3）东部型厄尔尼诺事件次年夏季,副热带太平洋蒸发异常偏强,副高西伸,由于东亚-太平洋（EAP）遥相关型的建立,副高西侧的强西南气流将来自太平洋蒸发的大量水汽持续输送至中国中东部地区。此外,在东亚-太平洋遥相关型影响下中高纬度地区建立了亚洲双阻型环流,其间的低槽冷涡与上游阻高之间的强偏北气流有利于北冰洋的水汽持续输送到西北和华北北部地区,中国大部分地区净水汽收支均增加,中国北方和南方地区的降水均产生了明显的同步性增多响应,形成了南北两条异常雨带。中部型厄尔尼诺事件次年夏季,副高较常年偏西且偏北,来自太平洋蒸发的大量水汽输送到江淮地区,使其净水汽收支增加和降水偏多。因此,厄尔尼诺事件的发生不仅对长江流域和淮河流域等南方地区的降水有重要影响,对华北、东北和西北地区的降水异常也有相当的作用。     

ENSO位相转换对华北雨季降水的影响

利用国家气候中心华北地区站点雨季降水数据及美国国家环境预报中心和大气研究中心（NCEP/NCAR）的再分析数据等资料,采用统计诊断方法,分析了1961~2014年华北雨季降水异常偏多（少）年对应的Ni?o3.4指数变化特征,发现华北雨季降水异常偏多年通常发生在El Ni?o结束且当年转为La Ni?a的年份,而华北雨季降水异常偏少年通常发生在赤道中东太平洋冷水位相结束且当年发展成El Ni?o事件的年份。并且在华北雨季降水异常偏多年,7~8月华北平均日降水量超过10 mm的天数约占25%,日降水量在4~10 mm的天数也约占25%,两者所占比率远高于华北雨季降水异常偏少年。对华北雨季降水异常偏多年的同期环流特征分析显示:西北太平洋副热带高压（副高）北跳偏早且有明显阶段性偏北情况,同时东亚高空副热带西风急流北移,从而造成华北雨季天气过程频繁、降水偏多。进一步诊断环流特征与ENSO位相转换的关系发现:相比El Ni?o衰减年,在El Ni?o转为La Ni?a的年份,7~8月500 hPa日本海到渤海正位势高度距平加强,更符合华北雨季降水偏多年的典型环流特征;同时热带沃克环流显著加强,造成东亚高空副热带西风急流北移更为显著,从而更有利于华北盛夏和雨季降水偏多。文中揭示的现象启示我们在做华北盛夏降水和雨季降水的预测时,需要关注ENSO事件前期的变化速度以及未来的演变特征。     

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。     

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。     

两类ENSO背景下中国东部夏季降水的环流特征及关键系统

利用全国160站逐月降水资料、74项环流指数、HadISST月平均海温资料以及NCEP/NCAR月平均再分析资料,对比分析了两类ENSO事件衰减阶段中国东部夏季降水及相应大气环流的差异,并探讨其可能成因。结果表明:1)EP型El Ni?o(La Ni?a)事件次年夏季,中国东部降水由北至南呈正负正(负正负)的三极型反相分布;CP型El Ni?o(La Ni?a)事件次年夏季,中国东部降水由北至南呈正负(负正)的偶极型反相分布;2)El Ni?o事件次年夏季,西北太平洋副热带高压(以下简称西太副高)及南亚高压均偏强,EP型西太副高偏西、南亚高压偏东,CP型西太副高范围更大、强度更强;La Ni?a事件次年夏季,西太副高及南亚高压强度偏弱,CP型强于EP型但弱于气候平均;3)El Ni?o事件期间西北太平洋上存在异常反气旋,EP型位置偏南,强度更强,持续时间更长,CP型位置偏北,范围更大;La Ni?a事件期间,西北太平洋区域至中国东南部存在异常气旋,EP型异常气旋的强度及范围均不及CP型;4)两类El Ni?o事件期间异常反气旋的差异可能与印度洋海盆增暖及太平洋海温持续性偏冷有关;两类La Ni?a事件期间异常气旋的差异可能由赤道西太平洋海温持续偏暖造成。     

ENSO对冬季北太平洋水汽输送及大气河的影响

利用1979-2016年ERA-Interim再分析资料,分析了ENSO对冬季北太平洋地区水汽输送特征的影响,包括整层水汽含量、整层水汽输送及其散度和大气河频率。结果表明,在El Ni1o年冬季,东北太平洋地区的气旋式环流异常增强了自副热带太平洋向北美西海岸的水汽输送,导致区域性的水汽辐合与辐散异常;La Ni1a年冬季的水汽输送特征与厄尔尼诺年大致相反。根据尺度分解的方法,对水汽输送及其散度的异常成因进行分析,得到结论如下:除El Ni1o年黑潮及其续流区外,ENSO年冬季北太平洋的水汽输送异常主要由环流异常导致;水汽输送散度异常则主要由环流异常的散度、气候态比湿的经向梯度和异常比湿的纬向梯度三部分决定。此外,El Ni1o年冬季北太平洋大气河频率分布的向极弯曲增强,分布更加集中;而La Ni1a年冬季大气河频率分布更加分散,纬向跨度减小,经向跨度增加。     

春季/夏季型El Ni?o对中国夏季降水变化的影响

利用中国气象局160站的逐月降水资料,NCAR/NCEP再分析资料以及美国CPC提供的Ni?o3. 4区指数,对比分析了SP、SU两类El Ni?o事件发展年、衰减年中国夏季降水和东亚环流特征的差异。结果表明:1) SP型El Ni?o事件发展年全国以少雨为主要特征,衰减年降水呈"北多南少"分布; SU型发展年,降水异常呈"三明治"型分布特征,衰减年降水偏多区域增大。2) 850 h Pa风场上,SP型El Ni?o事件发展年夏季风偏强,但反气旋性风场异常位置偏东偏北,衰减年夏季风还保持强劲,且反气旋性风场异常偏西偏南;而SU型发展年东亚夏季风偏弱,衰减年夏季风偏强。3) 500 hPa高度场上,SP型事件发展年夏季,我国受东亚夏季风影响总体偏弱,来自低纬地区的暖湿水汽向我国大陆输送不足;衰减年,受偏强东亚夏季风影响,来自低纬地区的暖湿水汽向我国腹地输送。SU型发展年,受相对活跃的冷空气影响,内蒙到华北地区多雨;衰减年,受偏强夏季风影响,冷暖气流在我国黄河及以北地区交汇。4)比较两类El Ni?o事件发生时对流层低层和高层特征表明:在SP型事件发展年,我国大陆上空,除内蒙和东北外,基本为下沉异常,衰减年基本为上升异常;在SU型发展年,我国大陆上空皆为显著的下沉异常,衰减年为弱下沉异常,但在偏强夏季风的作用下,降水偏多区域增大了。     

Why Was the Strengthening of Rainfall in Summer over the Yangtze River Valley in 2016 Less Pronounced than that in 1998 under Similar Preceding El Niño Events?——Role of Midlatitude Circulation in August

It is widely recognized that rainfall over the Yangtze River valley (YRV) strengthens considerably during the decaying summer of El Niño, as demonstrated by the catastrophic flooding suffered in the summer of 1998. Nevertheless, the rainfall over the YRV in the summer of 2016 was much weaker than that in 1998, despite the intensity of the 2016 El Niño having been as strong as that in 1998. A thorough comparison of the YRV summer rainfall anomaly between 2016 and 1998 suggests that the difference was caused by the sub-seasonal variation in the YRV rainfall anomaly between these two years, principally in August. The precipitation anomaly was negative in August 2016——different to the positive anomaly of 1998. Further analysis suggests that the weaker YRV rainfall in August 2016 could be attributable to the distinct circulation anomalies over the midlatitudes. The intensified "Silk Road Pattern" and upper-tropospheric geopotential height over the Urals region, both at their strongest since 1980, resulted in an anticyclonic circulation anomaly over midlatitude East Asia with anomalous easterly flow over the middle-to-lower reaches of the YRV in the lower troposphere. This easterly flow reduced the climatological wind, weakened the water vapor transport, and induced the weaker YRV rainfall in August 2016, as compared to that in 1998. Given the unique sub-seasonal variation of the YRV rainfall in summer 2016, more attention should be paid to midlatitude circulation——besides the signal in the tropics——to further our understanding of the predictability and variation of YRV summer rainfall.     

ENSO年东亚夏季风异常对中国江、淮流域夏季降水的影响

利用Nio3指数,把ENSO循环不同位相的夏季划分为4类并进行聚类分析,发现ElNio发展期和LaNia衰减期可以聚为一类,其夏季淮河流域降水往往偏多,长江中下游降水偏少;ElNio衰减期和LaNia发展期可以聚为一类,其夏季长江中下游地区降水往往偏多,淮河流域降水往往偏少。而后对这两大类中的年份分别聚类和合成分析。结果发现,这次聚类的结果反映了强弱夏季风对江、淮地区降水的影响。这一方面表明ENSO循环的同一位相既可能对应强东亚夏季风也可能对应弱夏季风,另一方面表明ENSO循环通过影响东亚夏季风环流异常的范围而使雨带位置发生变化,东亚夏季风强弱主要使雨量多少发生变化。     

Impacts of different types of El Niño and La Niña on northern tropical Atlantic sea surface temperature

The present study investigates the dependence of the northern tropical Atlantic (NTA) sea surface temperature (SST) response to El Niño and La Niña events on the decay time and amplitude of tropical Pacific SST anomalies. It is found that the NTA SST response to La Niña events displays a notable difference between late and early decaying events, similar to that in response to El Niño events, but with a weaker signal. Latent heat flux is a dominant term in the NTA SST change in preceding winter-early spring in both El Niño and La Niña events and in the difference of the NTA SST anomaly between late and early decaying El Niño and La Niña events. The zonal and meridional advections have an opposite effect on the NTA SST warming in late decaying El Niño events. Although the warming in the NTA region is similar in late decaying moderate and strong El Niño events, the distribution of the SST anomalies in the mid-latitude North Atlantic Ocean shows a notable difference between the two types of late decaying El Niño events. The SST anomalies also display difference in the early decaying weak and moderate El Niño events. Surface heat flux differences are largely attributed to wind differences.