Understanding the Development of the 2018/19 Central Pacific El Niño

A central Pacific(CP)El Niño event occurred in 2018/19.Previous studies have shown that different mechanisms are responsible for different subtypes of CP El Niño events(CP-I El Niño and CP-II El Niño).By comparing the evolutions of surface winds,ocean temperatures,and heat budgets of the CP-I El Niño,CP-II El Niño,and 2018/19 El Niño,it is illustrated that the subtropical westerly anomalies in the North Pacific,which led to anomalous convergence of Ekman flow and surface warming in the central equatorial Pacific,played an important role in the 2018/19 El Niño event as well as in the CP-II El Niño.Although the off-equatorial forcing played a vital role,it is found that the equatorial forcing acted as a driving(damping)term in boreal spring(summer)of the 2018/19 El Niño.The 2018/19 El Niño provides a timely and vivid example that helps illustrate the proposed mechanism of the CP El Niño,which could be leveraged to improve El Niño predictability.

Different El Niño Flavors and Associated Atmospheric Teleconnections as Simulated in a Hybrid Coupled Model

A previously developed hybrid coupled model(HCM)is composed of an intermediate tropical Pacific Ocean model and a global atmospheric general circulation model(AGCM),denoted as HCMAGCM.In this study,different El Niño flavors,namely the Eastern-Pacific(EP)and Central-Pacific(CP)types,and the associated global atmospheric teleconnections are examined in a 1000-yr control simulation of the HCMAGCM.The HCMAGCM indicates profoundly different characteristics among EP and CP El Niño events in terms of related oceanic and atmospheric variables in the tropical Pacific,including the amplitude and spatial patterns of sea surface temperature(SST),zonal wind stress,and precipitation anomalies.An SST budget analysis indicates that the thermocline feedback and zonal advective feedback dominantly contribute to the growth of EP and CP El Niño events,respectively.Corresponding to the shifts in the tropical rainfall and deep convection during EP and CP El Niño events,the model also reproduces the differences in the extratropical atmospheric responses during the boreal winter.In particular,the EP El Niño tends to be dominant in exciting a poleward wave train pattern to the Northern Hemisphere,while the CP El Niño tends to preferably produce a wave train similar to the Pacific North American(PNA)pattern.As a result,different climatic impacts exist in North American regions,with a warm-north and cold-south pattern during an EP El Niño and a warm-northeast and cold-southwest pattern during a CP El Niño,respectively.This modeling result highlights the importance of internal natural processes within the tropical Pacific as they relate to the genesis of ENSO diversity because the active ocean–atmosphere coupling is allowed only in the tropical Pacific within the framework of the HCMAGCM.

CHARACTERISTICS OF TROPICAL CYCLONE GENESIS IN THE WESTERN NORTH PACIFIC DURING THE DEVELOPING AND DECAYING PHASES OF TWO TYPES OF EL NIO

During the developing phase of central Pacific El Nio(CPEN), more frequent TC genesis over the northwest quadrant of the western North Pacific(WNP) is attributed to the horizontal shift of environmental vorticity field.Such a northwestward shift resembles the La Nia composite, even though factors that cause the shift differ(in the La Nia case the relative humidity effect is crucial). Greater reduction of TC frequency over WNP happened during the decaying phase of eastern Pacific El Nio(EPEN) than CPEN, due to the difference of the anomalous Philippine Sea anticyclone strength. The TC genesis exhibits an upward(downward) trend over the northern(southern) part of the WNP,which is linked to SST and associated circulation changes through local and remote effects.

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 Niño 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 Niño 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 Niños.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 Niño 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 Niño may also have been partly related to decadal climate variability.

Why Does Extreme Rainfall Occur in Central China during the Summer of 2020 after a Weak El Niño?

In summer 2020,extreme rainfall occurred throughout the Yangtze River basin,Huaihe River basin,and southern Yellow River basin,which are defined here as the central China(CC)region.However,only a weak central Pacific(CP)El Niño happened during winter 2019/20,so the correlations between the El Niño–Southern Oscillation(ENSO)indices and ENSO-induced circulation anomalies were insufficient to explain this extreme precipitation event.In this study,reanalysis data and numerical experiments are employed to identify and verify the primary ENSO-related factors that cause this extreme rainfall event.During summer 2020,unusually strong anomalous southwesterlies on the northwest side of an extremely strong Northwest Pacific anticyclone anomaly(NWPAC)contributed excess moisture and convective instability to the CC region,and thus,triggered extreme precipitation in this area.The tropical Indian Ocean(TIO)has warmed in recent decades,and consequently,intensified TIO basinwide warming appears after a weak El Niño,which excites an extremely strong NWPAC via the pathway of the Indo-western Pacific Ocean capacitor(IPOC)effect.Additionally,the ENSO event of 2019/20 should be treated as a fast-decaying CP El Niño rather than a general CP El Niño,so that the circulation and precipitation anomalies in summer 2020 can be better understood.Last,the increasing trend of tropospheric temperature and moisture content in the CC region after 2000 is also conducive to producing heavy precipitation.

Wind-forced equatorial wave dynamics of the Pacific Ocean during 2014/2015 and 2015/2016 E1 Nino events

The equatorial wave dynamics of interannual sea level variations between 2014/2015 and2015/2016 El Nino events are compared using the Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics,Institute of Atmospheric Physics Climate Ocean Model(LICOM) forced by the National Centers for Environmental Prediction(NCEP) reanalysis I wind stre s s and heat flux during 2000-2015.In addition,the LICOM can reproduce the interannual variability of sea surface temperature anomalies(SSTA) and sea level anomalies(SLA) along the equator over the Pacific Ocean in comparison with the Hadley center and altimetric data well.We extracted the equatorial wave coefficients of LICOM simulation to get the contribution to SLA by multiplying the meridional wave structure.During 2014/2015 El Nino event,upwelling equatorial Kelvin waves from the western boundary in April2014 reach the eastern Pacific Ocean,which weakened SLA in the eastern Pacific Ocean.However,no upwelling equatorial Kelvin waves from the western boundary of the Pacific Ocean could reach the eastern boundary during the 2015/2016 El Nino event.Linear wave model results also demonstrate that upwelling equatorial Kelvin waves in both 2014/2015 and 2015/2016 from the western boundary can reach the eastern boundary.However,the contribution from stronger westerly anomalies forced downwelling equatorial Kelvin waves overwhelmed that from the upwelling equatorial Kelvin waves from the western boundary in 2015.Therefore,the western boundary reflection and weak westerly wind burst inhibited the growth of the 2014/2015 El Nino event.The disclosed equatorial wave dynamics are important to the simulation and prediction of ENSO events in future studies.

西北太平洋热带云团发展与两类El Nio事件的联系

利用1982—2009年全球热带云团数据集、NCEP/NCAR再分析资料和英国Hadley中心海温资料,并引入热带云团生成率(Genesis Productivity,GP)来分析EP(Eastern Pacific)El Nio和CP(Central Pacific)El Nio事件与西北太平洋热带云团发展的相关性。研究表明,1)夏秋两季GP与Nio3指数在东南区(SE)均为显著正相关,在西南区(SW)仅秋季呈显著正相关;GP与EMI(El Nio Modoki Index)指数在夏季SE区域为显著正相关,在秋季南中国海(South China Sea,SCS)区呈负相关。2)在EP El Nio年,夏季SE区域的GP增大与低层涡度、高层散度以及低层相对湿度的相对增大一致。夏季SW区域与秋季SE区域的GP增大与有利的高低空配置相关,La Nia年则与之相反。3)在CP El Nio年,夏季SE区域的GP增大伴随着低层涡度和高层散度的增加,同时与充足的水汽及弱风切变相吻合;而秋季SCS区域的GP下降源于正涡度带、正散度带以及水汽带的东移。

赤道太平洋初级生产力对El Nio事件响应的综合分析

本文依据热带海洋全球大气研究(TOGA),东太平洋海洋环流研究(EPOCS)、西赤道太平洋海洋环流研究(WEPOCS)、表层热带太平洋观测(SURTROPAC)以及太平洋区域观察)PROPPAC)等计划,对1980～1990年期间在赤道太平洋(141.5°E～85°W,10°N～10°S)所调查的生物化学资料进行了分析和比较。所得结果是:东赤道太平洋正常年份初级生产力为500～800mg/(m^2·d),El Ni(?)o期间约为150mg/(m^2·d);西赤道太平洋正常年份初级生产力为200～250rag/(m^2·d),El Ni(?)o期间约为300mg/(m^2·d)。研究结论是:El Ni(?)o期间东赤道太平洋初级生产力较正常年份显著降低,西赤道太平洋初级生产力较正常年份明显增加,生物响应均较显著。

北太平洋涛动与El Nino的关系及其年代际变化

基于HadISST海表温度和NCEP/NCAR的海平面气压等再分析资料,研究了北太平洋海平面气压主模态与El Nino的关系。结果发现：阿留申低压模态是对El Nino事件的同期响应,而北太平洋涛动模态可以诱导热带太平洋产生类似中部型El Nino的海温异常,且具有提前4～12个月的预报意义。冬春季的北太平洋涛动处于正位相时,阿留申低压与夏威夷高压同时减弱,北太平洋背景风场减弱。夏威夷高压东南侧西南风异常减弱北太平洋东北信风,使加利福尼亚海区SST暖异常,在＂风-蒸发-SST＂机制的作用下,异常暖海温向热带太平洋传播,使赤道地区海温升高并产生西风异常,热带太平洋产生类似中部型El Nino的异常海温。El Nino类型的年代际变化可能受到北太平洋涛动的影响,当北太平洋涛动信号活跃时,中部型El Nino事件的发生频率大。

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 INFLUENCES OF SSTA OVER KUROSHIO AND ITS EXTENSION ON RAINFALL IN NORTHEAST CHINA UNDER THE BACKGROUND OF TWO DIFFERENT EL NIO CASES

By using the gauged rainfall in 160 stations within China's Mainland and the NCEP/NCAR reanalysis data, the impacts of anomalous SST in Kuroshio and its extension on precipitation in Northeast China were investigated. The results show that a difference in the meridional circulation such as the East Asia/Pacific teleconnection pattern(EAP)may be responsible for the difference in rainfall between 1998 and 2010. In comparison with 1998, the anomalous meridional circulation pattern in 2010 shifted northeastward, and then the western subtropical high, the mid-latitudinal trough and the northeastern Asia blocking high also shifted northeastward, causing intensified convergence of the cold and warm air masses at the southern region and thus more rainfall in the southwestern region and less in the northwestern region. In 1998, the anomalous cyclone, one component of the meridional pattern, located at the Songhuajiang-Nengjiang River basin, resulted in more rainfall in the majority of the area. The results of observation and the model show that the difference in SSTA in Kuroshio and its extension under the background of different El Ni觡o events is the key point:(1) The anomalous warmth moved westward from the mid-Pacific to the east of the Philippine Sea during the central event, which led the heat resources shifting to the northeast in 2010; subsequently, a shift occurred to the north of the anomalous ascent and decent, followed by a warm SSTA in the region of Kuroshio's extension in 2010 and Kuroshio in 1998.(2) The warm SSTA in the Kuroshio extension causing the Rossby wave activity flux strengthened in 2010, and then the westerly jet shifted northward and extended eastward. A warm SSTA in Kuroshio and cold SSTA in its extension in 1998 caused the westerly jet to shift southward and weaken. As a result,the anomalous anticyclone and cyclone shifted northward in 2010, and the blocking high also shifted northward.

Sensitivity Difference in the Extratropical Atmosphere to Two Types of El Nio Events

A comparison of sensitivity in extratropical circulation in the Northern Hemisphere(NH)and Southern Hemisphere(SH)is conducted through observational analyses and diagnostic linear model experiments for two types of El Nio events,the traditional El Nio with the strongest warmth in the eastern tropical Pacific(EP El Nio)and the El Nio Modoki with the strongest warmth in the central tropical Pacific(CP El Nio).It is shown that CP El Nio favors the occurrence of a negative-phase Northern Annular Mode(NAM),while EP El Nio favors that of the Pacific-North American(PNA)pattern.In SH,both EP and CP El Nio induce a negative phase Southern Annular Mode(SAM).However,the former has a greater amplitude,which is consistent with the stronger sea surface temperature(SST)warmth.The difference in the two types of El Nio events in NH may originate from the dependence of heating-induced extratropical response on the location of initial heating,which may be associated with activity of the stationary wave.In SH,the lack of sensitivity to the location of heating can be associated with weaker activity of the stationary wave therein.

Energetic processes regulating the strength of MJO circulation over the Maritime Continent during two types of El Ni?o

不同类型的El Ni?o事件对海洋性大陆区域的MJO强度有显著的影响。其中,中太平洋型(CP)El Ni?o期间的秋冬季MJO环流在海洋性大陆区域变得更强。本文推导了新的MJO动能诊断方程,并以此来定量诊断两类El Ni?o期间调控海洋性大陆区域的MJO环流强度的物理过程。与EP El Ni?o相比,CP El Ni?o期间平均动能与MJO动能之间的正压能量转换增强。在CP El Ni?o期间,沃克环流在海洋性大陆区域出现上升异常、低层辐合和气旋异常,透过正压能量转换,MJO得以从平均气流场中获得较多的动能;并进一步透过斜压能量转换,来维持MJO自身的强度增长。CP El Ni?o期间,增强的MJO将其动能转给高频扰动,有利海洋性大陆区域的高频扰动发展。

中西太平洋鲣鱼空间聚类特征及其与ENSO的关系

为了解在不同厄尔尼诺-南方涛动(ENSO)事件下中西太平洋鲣鱼(Katsuwonus pelamis)资源的变动规律,本研究根据中西太平洋渔业委员会(WCPFC)2008—2018年中西太平洋鲣鱼的生产数据,结合海洋尼诺指数(ONI),利用聚类分析和灰色关联分析,研究季时间尺度下鲣鱼的自由鱼群和随附鱼群的渔场空间特征及其与ENSO事件的关系。季尺度下渔场重心聚类分析表明,各簇所包含季度发生的异常气候事件具有一致性。拉尼娜时期,两种鱼群的主要渔场都有向西移动的趋势,厄尔尼诺时期则相反。在异常气候事件下,随附鱼群的迁移幅度小于自由鱼群,且随附鱼群渔场的经向分布更稳定。不同ENSO模态下,资源丰度的空间分布存在差异。对自由鱼群而言,在拉尼娜事件发生于第1、2季度时165°E以西海域的资源丰度最高,灰色关联度为0.650,在拉尼娜事件发生于第3、4季度时165°E—180°海域的资源丰度最高,灰色关联度为0.411,在厄尔尼诺时期,180°以东海域的资源丰度最高,灰色关联度为0.727。对随附鱼群而言,165°E以西海域及165°E—180°海域资源丰度最高时期为拉尼娜时期,灰色关联度分别为0.852和1.000,180°以东海域资源丰度最高时期为厄尔尼诺时期,灰色关联度为1.000。研究结果可用于气候变化背景下鲣鱼渔情的预报。

Theories on Formation of an Anomalous Anticyclone in Western North Pacific during El Nino:A Review

The western Noah Pacific anomalous anticyclone （WNPAC） is an important atmospheric circulation system that conveys El Nifio impact on East Asian climate. In this review paper, various theories on the formation and maintenance of the WNPAC, including warm pool atmosphere-ocean interaction, Indian Ocean capacitor, a combination mode that emphasizes nonlinear interaction between ENSO and annual cycle, moist enthalpy advecfion/Rossby wave modulation, and central Pacific SST forcing, are discussed. It is concluded that local atmosphere-ocean interaction and moist enthalpy advection/Rossby wave modulation mechanisms are essential for the initial development and maintenance of the WNPAC during El Nifio mature winter and subsequent spring. The Indian Ocean capacitor mechanism does not contribute to the earlier development but helps maintain the WNPAC in El Nifio decaying summer. The cold SST anomaly in the western North Pacific, although damped in the summer, also plays a role. An inter- basin atmosphere-ocean interaction across the Indo-Pacific warm pool emerges as a new mechanism in summer. In addition, the central Pacific cold SST anomaly may induce the WNPAC during rapid El Nifio decaying/La Nina developing or La Nifia persisting summer. The near-annual periods predicted by the combination mode theory are hardly detected from observations and thus do not contribute to the formation of the WNPAC. The tropical Atlantic may have a capacitor effect similar to the tropical Indian Ocean.

Sea surface salinity-derived indexes for distinguishing two types of El Niño events in the tropical Pacific

In this study,sea surface salinity(SSS)indexes are derived from reanalysis and observational datasets to distinguish the two types of(Central Pacific(CP)and Eastern Pacific(EP))El Niño events in the tropical Pacific.Based on the SSS anomalous spatial and temporal pointwise correlations with sea surface temperature(SST)indexes of two types of El Niño events,the key areas with SSS variations for EP and CP El Niño events are identified.For EP El Niño events,the key areas are located over an arcuate area centered at(0°,130°E)and in the central equatorial Pacific covering(5°S–5°N,175°W–158°W).For CP El Niño events,the key areas are located in the northeastern western Pacific covering(2°N,142°E–170°E)and in the southeastern Pacific covering(20°S–10°S,135°W–95°W).The key areas for EP and CP El Niño events in this study are not located near the dateline in the equatorial Pacific and differ from those obtained from the regression or composite methods.Accordingly,these key areas are used to construct SSS indexes,termed as the CP/EP El Niño SSS index(CSI/ESI),to distinguish EP and CP El Niño events independently.The SSS indexes are verified by different datasets over varying time periods and they can be adequately used to identify the two types of El Niño events and serve as another useful tool for monitoring ENSO.These analyses offer novel insight into how to represent the diversity of El Niño events.

Cause of Extreme Heavy and Persistent Rainfall over Yangtze River in Summer 2020

Record-breaking heavy and persistent precipitation occurred over the Yangtze River Valley(YRV)in June-July(JJ)2020.An observational data analysis has indicated that the strong and persistent rainfall arose from the confluence of southerly wind anomalies to the south associated with an extremely strong anomalous anticyclone over the western North Pacific(WNPAC)and northeasterly anomalies to the north associated with a high-pressure anomaly over Northeast Asia.A further observational and modeling study has shown that the extremely strong WNPAC was caused by both La Niña-like SST anomaly(SSTA)forcing in the equatorial Pacific and warm SSTA forcing in the tropical Indian Ocean(IO).Different from conventional central Pacific(CP)El Niños that decay slowly,a CP El Niño in early 2020 decayed quickly and became a La Niña by early summer.This quick transition had a critical impact on the WNPAC.Meanwhile,an unusually large area of SST warming occurred in the tropical IO because a moderate interannual SSTA over the IO associated with the CP El Niño was superposed by an interdecadal/long-term trend component.Numerical sensitivity experiments have demonstrated that both the heating anomaly in the IO and the heating anomaly in the tropical Pacific contributed to the formation and maintenance of the WNPAC.The persistent high-pressure anomaly in Northeast Asia was part of a stationary Rossby wave train in the midlatitudes,driven by combined heating anomalies over India,the tropical eastern Pacific,and the tropical Atlantic.

El Nio and the Kppen–Geiger Classification: A Prototype Concept and Methodology for Mapping Impacts in Central America and the Circum-Caribbean

The aim of this pilot study conducted by the consortium for capacity building was to develop a prototype concept and methodology for the classification and visualization of the geographic impacts of El Nio on annual climates and seasonality. Our study is based on the Kppen–Geiger climate classification scheme for a set of selected countries affected by strong El Nios in Latin America. By identifying and visualizing the annual and seasonal changes in regional, national, or subnational climate regimes that generally accompany an El Nio event,this research proposes an efficient way to detect and describe climate shifts and variability across time and space. Such knowledge provides a support tool for risk analysis and can potentially enhance government efforts of climate risk management, including disaster risk reduction activities that prevent, mitigate, and improve coping responses to El Nio-related hydrometeorological threats.Details of the conceptual approach and methodology to classifying and mapping El Nio's impacts are described and explained using the Central American and circumCaribbean region as a case study. The potential applications for disaster risk reduction as well as its limitations and future work are also discussed.

基于AVHRR/SST的西太平洋暖池近期变化研究

考虑到热结构和纬度分布 ,提出了更加合理的暖池中心和面积计算方法。利用 AVHRR/SST月平均数据 ,分析了西太平洋暖池近期 (1 993～ 2 0 0 1年 )的变化特征 :暖池中心移动在经向上是单一的年周期 ,而纬向上存在 3.3(最显著 ) ,1和 0 .5年 3个周期分量 ;暖池面积和表面强度都有 3个主要的周期分量 ,分别是 3.3,1 ,0 .5年和 1 .5,1 ,0 .67年 ,且都以 1年周期为最显著 ;2者的主频在正常年份大致呈同相关系 ,而在厄尔尼诺 /拉尼娜期间存在位相差 ,甚至反相。厄尔尼诺 /拉尼娜现象在1 993～ 2 0 0 1年之间的平均周期为 3.3年。

西太平洋暖池变异及其对西太平洋次表层海温场的影响

应用热带太平洋上层XBT温度资料 ,分析研究了西太平洋暖池区 ( 0°～ 1 6°N ,1 2 5°～ 1 4 5°E)上层海洋的变化特征以及与西太平洋次表层海温场之间的关系 .研究表明 ,西太平洋暖池区的垂向温度存在显著的年际变化 ,尤其在次表层 ( 1 2 0～ 2 0 0m)的变化最为明显 .西太平洋暖池区的次表层冷暖信号明显早于西太平洋次表层的海温异常 .分析发现 ,西太平洋暖池区的海温异常是导致整个西太平洋次表层海温场变异的关键区 ,当西太平洋暖池区的次表层冷暖信号加强时 ,3～