Seasonal and Sub-Seasonal Circulation Anomalies Associated with Persistent Rainy Days in 2018/2019 Winter in Shanghai,China

Shanghai experienced the longest rainy days in 2018/2019 winter since 1988. The physical cause of such an unusual climate condition was investigated through the diagnosis of observational data. From a seasonal perspective, a long persistent rainy winter was often associated with an El Ni?o condition in the equatorial Pacific. This abnormal oceanic condition induces a remote teleconnection pattern with pronounced low-level southerly anomalies over East China.The wind anomalies transported moisture from tropical oceans and caused persistent rainfall in East Asia. Meanwhile, the local rainfall time series exhibited a strong quasi-biweekly oscillation (QBWO). Three persistent rainy events were identified in the 2018/2019 winter and they all occurred during the active phase of the QBWO. The first two events were associated with a low pressure anomaly west of Shanghai. Southerly anomalies associated with the low pressure system advected high mean moisture into central eastern China, leading to the persistent rainfall there.The third event was associated with a high pressure anomaly in lower troposphere to the east of Shanghai, which induced anomalous southerlies to its west, favoring the occurrence of rainfall in Shanghai. The result suggests the importance of high-frequency variability in affecting seasonal rainfall anomalies.     

Distinct Evolution of the SST Anomalies in the Far Eastern Pacific between the 1997/98 and 2015/16 Extreme El Niños

The 2015/16 El Ni?o displayed a distinct feature in the SST anomalies over the far eastern Pacific(FEP)compared to the 1997/98 extreme case.In contrast to the strong warm SST anomalies in the FEP in the 1997/98 event,the FEP warm SST anomalies in the 2015/16 El Ni?o were modest and accompanied by strong southeasterly wind anomalies in the southeastern Pacific.Exploring possible underlying causes of this distinct difference in the FEP may improve understanding of the diversity of extreme El Ni?os.Here,we employ observational analyses and numerical model experiments to tackle this issue.Mixed-layer heat budget analysis suggests that compared to the 1997/98 event,the modest FEP SST warming in the 2015/16 event was closely related to strong vertical upwelling,strong westward current,and enhanced surface evaporation,which were caused by the strong southeasterly wind anomalies in the southeastern Pacific.The strong southeasterly wind anomalies were initially triggered by the combined effects of warm SST anomalies in the equatorial central and eastern Pacific(CEP)and cold SST anomalies in the southeastern subtropical Pacific in the antecedent winter,and then sustained by the warm SST anomalies over the northeastern subtropical Pacific and CEP.In contrast,southeasterly wind anomalies in the 1997/98 El Ni?o were partly restrained by strong anomalously negative sea level pressure and northwesterlies in the northeast flank of the related anomalous cyclone in the subtropical South Pacific.In addition,the strong southeasterly wind and modest SST anomalies in the 2015/16 El Ni?o may also have been partly related to decadal climate variability.     

Upper-Ocean Dynamical Features and Prediction of the Super El Ni?o in 2015/16:A Comparison with the Cases in 1982/83 and 1997/98

The 2015/16 super El Ni?o event has been widely recognized as comparable to the 1982/83 and 1997/98 El Ni?o events.This study examines the main features of upper-ocean dynamics in this new super event,contrasts them to those in the two historical super events,and quantitatively compares the major oceanic dynamical feedbacks based on a mixed-layer heat budget analysis of the tropical Pacific.During the early stage,this new event is characterized by an eastward propagation of SST anomalies and a weak warm-pool El Ni?o;whereas during its mature phase,it is characterized by a weak westward propagation and a westward-shifted SST anomaly center,mainly due to the strong easterly wind and cold upwelling anomalies in the far eastern Pacific,as well as the westward anomalies of equatorial zonal current and subsurface ocean temperature.The heat budget analysis shows that the thermocline feedback is the most crucial process inducing the SST anomaly growth and phase transition of all the super events,and particularly for this new event,the zonal advective feedback also exerts an important impact on the formation of the strong warming and westward-shifted pattern of SST anomalies.During this event,several westerly wind burst events occur,and oceanic Kelvin waves propagate eastwards before being maintained over eastern Pacific in the mature stage.Meanwhile,there is no evidence for westward propagation of the off-equatorial oceanic Rossby waves though the discharging process of equatorial heat during the development and mature stages.The second generation El Ni?o prediction system of the Beijing Climate Center produced reasonable event real-time operational prediction during 2014–16,wherein the statistical prediction model that considers the preceding oceanic precursors plays an important role in the multi-method ensemble prediction of this super.     

Parallel Comparison of the 1982/83,1997/98 and 2015/16 Super El Nios and Their Effects on the Extratropical Stratosphere

This study uses multiple sea surface temperature(SST) datasets to perform a parallel comparison of three super El Ni os and their effects on the stratosphere. The results show that, different from ordinary El Ni os, warm SST anomalies appear earliest in the western tropical Pacific and precede the super El Ni o peak by more than 18 months. In the previous winter,relative to the mature phase of El Ni o, as a precursor, North Pacific Oscillation-like circulation anomalies are observed. A Pacific–North America(PNA) teleconnection appears in the extratropical troposphere during the mature phase, in spite of the subtle differences between the intensities, as well as the zonal position, of the PNA lobes. Related to the negative rainfall response over the tropical Indian Ocean, the PNA teleconnection in the winter of 1997/98 is the strongest among the three super El Ni os. The northern winter stratosphere shows large anomalies in the polar cap temperature and the circumpolar westerly, if the interferences from other factors are linearly filtered from the circulation data. Associated with the positive PNA response in a super El Ni o winter, positive polar cap temperature anomalies and circumpolar easterly anomalies,though different in timing, are also observed in the mature winters of the three super El Ni os. The stratospheric polar vortex in the next winter relative to the 1982/83 and 1997/98 events is also anomalously weaker and warmer, and the stratospheric circulation conditions remain to be seen in the coming winter following the mature phase of the 2015/16 event.     

Upper-ocean dynamical features and prediction of the super El Niño in 2015/16: A comparison with the cases in 1982/83 and 1997/98

The 2015/16 super El Niño event has been widely recognized as comparable to the 1982/83 and 1997/98 El Niño events. This study examines the main features of upper-ocean dynamics in this new super event, contrasts them to those in the two historical super events, and quantitatively compares the major oceanic dynamical feedbacks based on a mixed-layer heat budget analysis of the tropical Pacific. During the early stage, this new event is characterized by an eastward propagation of SST anomalies and a weak warm-pool El Niño; whereas during its mature phase, it is characterized by a weak westward propagation and a westward-shifted SST anomaly center, mainly due to the strong easterly wind and cold upwelling anomalies in the far eastern Pacific, as well as the westward anomalies of equatorial zonal current and subsurface ocean temperature. The heat budget analysis shows that the thermocline feedback is the most crucial process inducing the SST anomaly growth and phase transition of all the super events, and particularly for this new event, the zonal advective feedback also exerts an important impact on the formation of the strong warming and westward-shifted pattern of SST anomalies. During this event, several westerly wind burst events occur, and oceanic Kelvin waves propagate eastwards before being maintained over eastern Pacific in the mature stage. Mean-while, there is no evidence for westward propagation of the off-equatorial oceanic Rossby waves though the discharging process of equatorial heat during the development and mature stages. The second generation El Niño prediction system of the Beijing Climate Center produced reasonable event real-time operational prediction during 2014–16, wherein the statistical prediction model that considers the preceding oceanic precursors plays an important role in the multi-method ensemble prediction of this super.     

Different Summer Rainfall Anomaly Patterns in Northeast China Associated with Two Kinds of El Niño Events

The relationship between summer rainfall anomalies in northeast China and two types of El Ni?o events is investigated by using observation data and an AGCM. It is shown that, for different types of El Ni?o events, there is different rainfall anomaly pattern in the following summer. In the following year of a typical El Ni?o event, there are remarkable positive rainfall anomalies in the central-western region of northeast China, whereas the pattern of more rainfall in the south end and less rainfall in the north end of northeast China easily appears in an El Ni?o Modoki event. The reason for the distinct differences is that, associated with the different sea surface temperature anomalies (SSTA) along the equatorial Pacific, the large-scale circulation anomalies along east coast of East Asia shift northward in the following summer of El Ni?o Modoki events. Influenced by the anomalous anticyclone in Philippine Sea, southwesterly anomalies over eastern China strengthens summer monsoon and bring more water vapor to Northeast China. Meanwhile, convergence and updraft is strengthened by the anomalous cyclone right in Northeast China in typical El Ni?o events. These moisture and atmospheric circulation conditions are favorable for enhanced precipitation. However, because of the northward shift, the anomalous anticyclone which is in Philippine Sea in typical El Ni?o cases shifts to the south of Japan in Modoki years, and the anomalous cyclone which is in the Northeast China in typical El Ni?o cases shifts to the north of Northeast China, leading to the “dipole pattern” of rainfall anomalies. According to the results of numerical experiments, we further conform that the tropical SSTA in different types of El Ni?o event can give rise to observed rainfall anomaly patterns in Northeast China.     

热带太平洋地区风场异常和与El Ni(~n)o有关的年际变化

利用1964～1993年NCEP/NCAR再分析风应力资料和中国科学院大气物理研究所发展的14层热带太平洋环流模式(OGCM),对热带太平洋与El Ni(~n)o有关的年际变化进行了研究.首先,分析了西太平洋暖池次表层海温异常(SOTA)与Ni(~n)o 3区海表温度异常(SSTA)的年际变化关系,发现在El Ni(~n)o事件之前,暖池的次表层海温都有明显正异常出现,它的东传导致了El Ni(~n)o的发生,并且SOTA的传播随纬度变化,沿赤道东传,在赤道外西传.然后,选取了20世纪70年代和80年代两次最强的El Ni(~n)o事件讨论了引起这种机制的可能原因--西风异常的作用.最后,对1964～1993所有的El Ni(~n)o年的风场、次表层海温和海表温度的异常进行了综合分析.     

CAM3模式模拟厄尔尼诺事件对大气环流异常的影响

采用CAM3（Community Atmosphere Model Version3）模式中海气湍流通量参数化原方案和改进方案,利用观测海温驱动CAM3模式进行气候模拟,以分析模式对厄尔尼诺事件影响气候变化的模拟能力。结果表明,采用CAM3模式海气湍流通量参数化改进方案,模式能够更好地模拟出由厄尔尼诺事件引起的北太平洋和北美地区大气环流的变化,尤其是对厄尔尼诺年冬季阿留申低压强度和与PNA遥相关型有关的500hPa位势高度异常的模拟。     

两类El Ni?o事件对我国西南地区冬季降水的影响

采用1960~2014年西南地区93个站的逐月降水资料以及NCEP/NCAR再分析月平均资料,分析了东部型El Ni?o（EP El Ni?o）和中部型El Ni?o（CP El Ni?o）对我国西南地区冬季降水的不同影响。结果表明,EP ElNi?o年冬季和CP El Ni?o事件中1994/1995年、2002/2003年冬季,我国西南地区受异常偏东风影响,获得来自孟加拉湾和南海的水汽供应。同时,西南地区存在明显的幅合上升运动,上升运动异常增强,对流加强,导致西南地区冬季降水偏多;而CP El Ni?o事件中1968/1969年、1977/1978年,2004/2005年、2009/2010年冬季,我国西南地区受异常西北风影响,来自孟加拉湾和南海的水汽减少。同时,西南地区存在明显的幅散下沉运动,上升运动异常减弱,对流减弱,导致西南地区冬季降水偏少。     

厄尔尼诺现象对北半球大气环流和中国降水的影响

本文主要讨论了厄尔尼诺现象对北半球大气环流和中国季风降水的影响。研究表明，在厄尔尼诺开始年的春夏季，对流层上部位势高度和副热带高压都处在由弱到强的转变状态，我国东部地区大范围少雨；秋季发生明显的趋势转折，秋季到次年夏季，对流层上部位势高度升高，副高增强西伸，除次年2、3月份全国大范围少雨外，其余月份大都为南多北少的分布型，这种分布形态在开始年秋季最典型。厄尔尼诺开始年、次年，长江中下游入梅期偏晚。     

Warming in the Northwestern Indian Ocean Associated with the El Ni(n)o Event

This paper investigates possible warming effects of an El Ni(n)o event on the sea surface temperature anomaly(SSTA)in the northwestern Indian Ocean.Most pure positive Indian Ocean dipole(IOD)events (without an El Ni(n)o event co-occurring) have a maximum positive SSTA mainly in the central Indian Ocean south of the equator.while most co-occurrences with an El Ni(n)o event exhibit a northwest-southeast typical dipole mode.It is therefore inferred that warming in the northwestern Indian Ocean is closely related to the El Ni(n)o event.Based on the atmospheric bridge theory,warming in the northwestern Indian Ocean during co-occurring cases may be primarily caused by relatively less latent heat loss from the ocean due to reduced wind speed.The deepened thermocline also contributes to the warming along the east coast of Africa through the suppressed upwelling of the cold water.Therefore,the El Ni(n)o event is suggested to have a modulating effect on the structure of the dipole mode in the tropical Indian Ocean.     

El Ni?o影响热带大气季节内振荡的动力学研究

根据El Ni?o期间热带大气状态的异常特征，确定了在简单大气动力模式中描写海温异常的参数化方法。然后利用斜压半地转两层模式从动力学上分析研究了El Ni?o对热带大气低频波的影响。理论分析同有关热带大气季节内振荡的资料分析结果完全一致，El Ni?o事件使热带大气季节内振荡减弱。