On the “spring predictability barrier” for strong El Nio events as derived from an intermediate coupled model ensemble prediction system

Using predictions for the sea surface temperature anomaly(SSTA) generated by an intermediate coupled model(ICM)ensemble prediction system(EPS), we first explore the "spring predictability barrier"(SPB) problem for the 2015/16 strong El Nio event from the perspective of error growth. By analyzing the growth tendency of the prediction errors for ensemble forecast members, we conclude that the prediction errors for the 2015/16 El Nio event tended to show a distinct season-dependent evolution, with prominent growth in spring and/or the beginning of the summer. This finding indicates that the predictions for the 2015/16 El Nio occurred a significant SPB phenomenon. We show that the SPB occurred in the 2015/16 El Nio predictions did not arise because of the uncertainties in the initial conditions but because of model errors. As such, the mean of ensemble forecast members filtered the effect of model errors and weakened the effect of the SPB, ultimately reducing the prediction errors for the 2015/16 El Nio event. By investigating the model errors represented by the tendency errors for the SSTA component,we demonstrate the prominent features of the tendency errors that often cause an SPB for the 2015/16 El Nio event and explain why the 2015/16 El Nio was under-predicted by the ICM EPS. Moreover, we reveal the typical feature of the tendency errors that cause not only a significant SPB but also an aggressively large prediction error. The feature is that the tendency errors present a zonal dipolar pattern with the west poles of positive anomalies in the equatorial western Pacific and the east poles of negative anomalies in the equatorial eastern Pacific. This tendency error bears great similarities with that of the most sensitive nonlinear forcing singular vector(NFSV)-tendency errors reported by Duan et al. and demonstrates the existence of an NFSV tendency error in realistic predictions. For other strong El Nio events, such as those that occurred in 1982/83 and 1997/98, we obtain the tendency errors of the NFSV structure, which cause a significant SPB and yield a much larger prediction error. These results suggest that the forecast skill of the ICM EPS for strong El Nio events could be greatly enhanced by using the NFSV-like tendency error to correct the model.     

Genesis of the 2014–2016 El Nio events

The 2015/2016 El Nio was one of the strongest El Nio events in history, and this strong event was preceded by a weak El Nio in 2014. This study systematically analyzed the dynamical processes responsible for the genesis of these events. It was found that the weak 2014 El Nio had two warming phases, the spring-summer warming was produced by zonal advection and downwelling Kelvin waves driven by westerly wind bursts(WWBs), and the autumn-winter warming was produced by meridional advection, surface heating as well as downwelling Kelvin waves. The 2015/2016 extreme El Nio, on the other hand, was primarily a result of sustained zonal advection and downwelling Kelvin waves driven by a series of WWBs, with enhancement from the Bjerknes positive feedback. The vast difference between these two El Nio events mainly came from the different amount of WWBs in 2014 and 2015. As compared to the 1982/1983 and 1997/1998 extreme El Nio events, the 2015/2016 El Nio exhibited some distinctive characteristics in its genesis and spatial pattern. We need to include the effects of WWBs to the theoretical framework of El Nio to explain these characteristics, and to improve our understanding and prediction of El Nio.     

Evolution of the 2015/16 El Nio and historical perspective since 1979

The 2015/16 El Nio developed from weak warm conditions in late 2014 and NINO3.4 reached 3℃ in November 2015. We describe the characteristics of the evolution of the 2015/16 El Nio using various data sets including SST, surface winds,outgoing longwave radiation and subsurface temperature from an ensemble operational ocean reanalyses, and place this event in the context of historical ENSO events since 1979. One salient feature about the 2015/16 El Nio was a large number of westerly wind bursts and downwelling oceanic Kelvin waves(DWKVs). Four DWKVs were observed in April-November 2015 that initiated and enhanced the eastern-central Pacific warming. Eastward zonal current anomalies associated with DWKVs advected the warm pool water eastward in spring/summer. An upwelling Kelvin wave(UWKV) emerged in early November 2015 leading to a rapid decline of the event. Another outstanding feature was that NINO4 reached a historical high(1.7℃), which was 1℃(0.8℃) higher than that of the 1982/83(1997/98) El Nio . Although NINO3 was comparable to that of the 1982/83 and 1997/98 El Nio , NINO1+2 was much weaker. Consistently, enhanced convection was displaced 20 degree westward, and the maximum D20 anomaly was about 1/3.1/2 of that in 1997 and 1982 near the west coast of South America.     

Using CMIP5 model outputs to investigate the initial errors that cause the “spring predictability barrier” for El Nio events

Most ocean-atmosphere coupled models have difficulty in predicting the El Nio-Southern Oscillation(ENSO) when starting from the boreal spring season. However, the cause of this spring predictability barrier(SPB) phenomenon remains elusive. We investigated the spatial characteristics of optimal initial errors that cause a significant SPB for El Nio events by using the monthly mean data of the pre-industrial(PI) control runs from several models in CMIP5 experiments. The results indicated that the SPB-related optimal initial errors often present an SST pattern with positive errors in the central-eastern equatorial Pacific, and a subsurface temperature pattern with positive errors in the upper layers of the eastern equatorial Pacific, and negative errors in the lower layers of the western equatorial Pacific. The SPB-related optimal initial errors exhibit a typical La Ni-a-like evolving mode, ultimately causing a large but negative prediction error of the Nio-3.4 SST anomalies for El Nio events. The negative prediction errors were found to originate from the lower layers of the western equatorial Pacific and then grow to be large in the eastern equatorial Pacific. It is therefore reasonable to suggest that the El Nio predictions may be most sensitive to the initial errors of temperature in the subsurface layers of the western equatorial Pacific and the Nio-3.4 region, thus possibly representing sensitive areas for adaptive observation. That is, if additional observations were to be preferentially deployed in these two regions, it might be possible to avoid large prediction errors for El Nio and generate a better forecast than one based on additional observations targeted elsewhere. Moreover, we also confirmed that the SPB-related optimal initial errors bear a strong resemblance to the optimal precursory disturbance for El Nio and La Nia events. This indicated that improvement of the observation network by additional observations in the identified sensitive areas would also be helpful in detecting the signals provided by the precursory disturbance, which may greatly improve the ENSO prediction skill.     

Impact of El Nio on atmospheric circulations over East Asia and rainfall in China: Role of the anomalous western North Pacific anticyclone

This paper presents a review on the impact of El Nio on the interannual variability of atmospheric circulations over East Asia and rainfall in China through the anomalous anticyclone over western North Pacific(WNPAC). It explains the formation mechanisms of the WNPAC and physical processes by which the WNPAC affects the rainfall in China. During the mature phase of El Nio, the convective cooling anomalies over western tropical Pacific caused by the weakened convections trigger up an atmospheric Rossby wave response, resulting in the generation of the WNPAC. The WNPAC can persist from the winter when the El Nio is in its peak to subsequent summer, which is maintained by multiple factors including the sustained presence of convective cooling anomalies and the local air-sea interaction over western tropical Pacific, and the persistence of sea surface temperature anomalies(SSTA) in tropical Indian and tropical North Atlantic. The WNPAC can influence the atmospheric circulations over East Asia and rainfall in China not only simultaneously, but also in the subsequent summer after an El Nio year, leading to more rainfall over southern China. The current paper also points out that significant anomalies of atmospheric circulations over East Asia and rainfall over southern China occur in El Nio winter but not in La Nio winter, suggesting that El Nio and La Nio have an asymmetric effect. Other issues, including the impact of El Nio diversity and its impact as well as the relations of the factors affecting the persistence of the WNPAC with summer rainfall anomalies in China, are also discussed. At the end of this paper some issues calling for further investigation are discussed.     

日食、太阳黑子与地球自转对Ei Nio(La Nia)影响与预测

Ei Nio(La Nia)对全球气候变化产生显著影响并造成严重自然灾害,其形成与发生受多因素复杂影响.对其影响因素与事件发生预测的研究,有助于对其成因机制加深理解,减弱其造成的灾害影响.本文运用经验模态分解法(EMD)与动力力学预测模型等数学方法,对近60年地球自转、日食、太阳黑子等因素与Ei Nio(La Nia)事件相关性进行多尺度分析,探讨日食对Ei Nio(La Nia)热动力诱因,构建日食、太阳黑子、地球自转与Ei Nio(La Nia)事件间相关系数与预测定理(E1-E2),并对未来10年Ei Nio(La Nia)事件进行预测.研究结果表明:日食、太阳黑子、地球自转等现象与Ei Nio、La Nia事件具有密切相关性,是其发生重要影响因素.日食发生对Ei Nio(La Nia)事件具有热动力诱因,太阳黑子峰值年与Ei Nio事件发生有较好关联性,地球自转速率增加,诱发或增强La Nia事件,地球自转速率降低时,诱发或增强Ei Nio事件.通过构建日食、太阳黑子、地球自转与Ei Nio(La Nia)事件间相关系数与预测定理(E1-E2)进行Ei Nio(La Nia)事件的资料验证与预测,准确率达86.05%,得到预测结论:2016、2018、2019、2020、2023年发生Ei Nio事件,2017、2021、2024年发生La Nia事件.     

Processes involved in the second-year warming of the 2015 El Nio event as derived from an intermediate ocean model

The tropical Pacific experienced a sustained warm sea surface condition that started in 2014 and a very strong El Nio event in 2015. One striking feature of this event was the horseshoe-like pattern of positive subsurface thermal anomalies that was sustained in the western-central equatorial Pacific throughout 2014–2015. Observational data and an intermediate ocean model are used to describe the sea surface temperature(SST) evolution during 2014–2015. Emphasis is placed on the processes involved in the 2015 El Nio event and their relationships with SST anomalies, including remote effects associated with the propagation and reflection of oceanic equatorial waves(as indicated in sea level(SL) signals) at the boundaries and local effects of the positive subsurface thermal anomalies. It is demonstrated that the positive subsurface thermal anomaly pattern that was sustained throughout 2014–2015 played an important role in maintaining warm SST anomalies in the equatorial Pacific. Further analyses of the SST budget revealed the dominant processes contributing to SST anomalies during 2014–2015. These analyses provide an improved understanding of the extent to which processes associated with the 2015 El Nio event are consistent with current El Nio and Southern Oscillation theories.     

耦合条件非线性最优扰动及其在ENSO集合预报研究中的应用

考虑到目前集合预报初始扰动方法未能充分刻画地球系统不同圈层相互作用影响的局限性,本研究提出了能够考虑多圈层耦合初始不确定性的耦合条件非线性最优扰动(C-CNOP)方法.将C-CNOP方法应用于热带太平洋典型的海气耦合现象厄尔尼诺-南方涛动(ENSO)事件的集合预报研究,表明了C-CNOP方法能够产生ENSO集合预报的更恰当考虑海气耦合不确定性的快速增长初始扰动(CPs);从Ni?o3.4海表温度距平(SSTA)时间变率的预报和ENSO成熟位相SSTA空间变率的预报两方面,揭示了CPs能够有效促进ENSO集合平均预报的水平,尤其从春、夏季开始的预报,即使该季节热带太平洋海气耦合强度最弱, CPs也能够捕捉到该弱海气耦合信息的不确定性,从而大大抑制了ENSO预报误差在该季节由于最强海气耦合不稳定性导致的快速增长,有效延长了ENSO的预报时效.因此, C-CNOP方法是一个能够产生集合预报的更充分考虑初始耦合不确定性的初始扰动的方法,期望未来在继续研究的基础上,使C-CNOP在地球气候系统预测中发挥重要作用.     

Contrast of tropical cyclone frequency in the western North Pacific between two types of La Nia events

According to the different pattern of sea surface temperature anomaly(SSTA) in the previous year of La Nia events,we categorized La Nia events into two types to investigate the different characteristics of tropical cyclone(TC) activity over the western North Pacific(WNP) in TC peak season of two types La Nia events.One type is following the previous El Nio event(La Nia I);the other is following the previous neutral phase or developing La Nia event(La Nia II).Results show that TC genesis frequency in the WNP during TC peak season of La Nia I is less than normal year,whereas it has no differences from normal year during La Nia II.The main reason is attributed to the different amplitude SSTA in the East Indian Ocean(EIO) and the western Pacific Ocean(WPO).Similar to the capacitor effect,strongly positive SSTA in the EIO-WPO during La Nia I triggers an equatorial baroclinic Kelvin wave,which intensifies the easterly in the lower troposphere and weakens the East Asian summer monsoon,and thus the TC frequency decreased during La Nia I.However,the easterly anomaly shows a weak response to the SSTA in the EIO-WPO during La Nia II,and there is no significant change in the environmental pattern over the WNP;so is TC frequency.The modulation of strong EIO-WPO SSTA on large-scale circulation over the WNP reduces the environmental barotropic energy conversion into synoptic-scale disturbances during La Nia I,and also suppresses TC disturbances.The understanding of two different types of La Nia events could help improve the seasonal prediction of TC activity in the WNP during La Nia.     

区分热带太平洋两类厄尔尼诺事件的海表面盐度指数

本研究利用再分析资料和观测数据得到用来区分热带太平洋两种类型El Ni?o事件(中太平洋El Ni?o (CP El Ni?o)和东太平洋El Ni?o (EP El Ni?o))的海表面盐度(SSS)指数.通过计算SSS异常与海表面温度(SST)的空间和时间相关性,识别出了与CP和EP El Ni?o事件相关的SSS关键区.确定出的EP El Ni?o事件SSS关键区位于以(0°,130°E)为中心的弧形区域和由赤道太平洋覆盖(5°S～5°N, 175°W～158°W)的区域; CP El Ni?o事件SSS关键区位于西太平洋东北部(2°S～9°N, 142°E～170°E)和东南太平洋(20°S～10°S, 135°W～95°W).本研究中获得的CP和EP El Ni?o事件的关键区与传统的回归和合成方法不一致,并不位于赤道太平洋的日界期线附近.根据这些关键区,分别构建SSS指数,称为CSI指数(CP El Ni?o SSS index)和ESI指数(EP El Ni?o SSS index),用于独立区分CP和EP El Ni?o事件.利用不同时段的多种来源的数据进行检验,表明CSI和ESI可以有效地用于区分两类El Ni?o事件,并作为监测ENSO的另一个有效工具.本研究为表征和理解El Ni?o事件的多样性提供了可能的新方法.     

厄尔尼诺持续时间与大气环流异常形势

针对不同持续时间的El Nio事件,进行了大尺度大气环流及其演变的合成分析研究.其结果清楚地表明,不同持续时间的El Nio事件的发生、发展和消亡过程,对流层低层风场和对流层高层速度势场的距平都有极为显著差异.分析得到了对El Nio事件的发生和消亡起着重要作用的大气环流异常形势.还发现对于持续时间较长的El Nio事件,东北太平洋上850hPa异常气旋性环流减弱和西北太平洋上异常反气旋性环流增强较慢,因此赤道太平洋异常西风维持的时间也较长,而与澳大利亚冬季风加强相关联的南半球西太平洋的速度势正距平的维持,对El Nio的持续也起一定作用;对应持续时间较短的El Nio事件,西太平洋上200hPa速度势正距平的迅速东移,对El Nio的迅速消亡起重要作用.     

中国华北雾霾天气与超强El Ni?o事件的相关性研究

2015年11—12月,全国接连发生七次大范围、持续性雾霾天气过程,其中,11月27日—12月1日的雾霾天气过程持续时间长达五天,成为2015年最强的一次重污染天气过程;12月19-25日重度雾霾再次发展,影响面积一度达到35.2万km~2.本文利用多种数据资料通过个例对比和历史统计详细分析了超强El Ni?o背景下雾霾天气频发的天气气候条件.其结果清楚表明:2015年11—12月欧亚中高纬度以纬向环流为主,东亚冬季风偏弱,使得影响我国的冷空气活动偏少,我国中东部大部地区对流层低层盛行异常偏南风,大气相对湿度明显偏大,并且大气层结稳定,对流层底层存在明显逆温.上述大气环流条件使得污染物的水平和垂直扩散条件差,因此在有一定污染排放的情况下,造成了重度雾霾天气过程的频发.由此,超强El Ni?o事件所导致的大尺度大气环流异常是我国中东部,尤其华北地区冬季雾霾天气频发的重要原因之一.     

The unusual 2014–2016 El Ni?o events: Dynamics,prediction and enlightenments

The 2014–2016 El Ni?o events consist of a stalled El Ni?o event in the winter of 2014/2015 and a following extreme El Ni?o event in the end of 2015. Neither event was successfully predicted in operational prediction models. Because of the unusual evolutions of these events that rarely happened in the historical observations, few experience was ready for understanding and predicting the two El Ni?o events when they occurred. Also due to their specialties, considerable attention were attracted with aims to reveal the hidden mechanisms. This article reviews the recent progresses and knowledge that were obtained in these studies. Emerging from these studies, it was argued that the key factor that was responsible for the stalled El Ni?o in 2014 was the unexpected summertime Easterly Wind Surges(EWSs) or the lack of summertime Westerly Wind Bursts(WWBs). Most operational prediction models failed to reproduce such stochastic winds and thus made unrealistic forecasts. The two El Ni?o events awakened the research community again to incorporate the state-of-the-art climate models to simulate the stochastic winds and investigate their roles in the development of El Ni?o.     

不同强度El Ni?o的衰减过程.Ⅱ,中等和较弱El Ni?o的衰减过程

在第二部分,我们研究了中等和较弱El Ni?o的衰减过程. 结果表明,对中等El Ni?o而言,在其发展阶段和盛期,负异常信号在西太平洋产生,但由于强度不足,在El Ni?o盛期之后迅速衰减,这是一种夭折的类西太平洋振子过程. 因此,与强El Ni?o不同,中等El Ni?o衰减进入平常态. 而较弱El Ni?o以截然不同的另一种方式进行位相转换,伴随东南太平洋副高的加强和西移,东风异常和海表温度负异常自赤道东太平洋向西扩展,这是一种平流模态过程,导致较弱El Ni?o衰减进入La Nia.     

不同分布型El Ni?o期间MJO活动的差异

利用Hadley中心的月平均海温资料、NCEP/NCAR和ERA-Interim逐日再分析大气环流数据等,详细对比了赤道大气季节内振荡(MJO)活动在东部型El Ni?o与中部型El Ni?o发展期间的异同点.结果表明,与传统的东部型El Ni?o发展前MJO明显偏强的特征相似,在中部型El Ni?o迅速发展前的春夏季,MJO动能亦较强且持续东传特征显著.这说明无论是东部型El Ni?o还是中部型El Ni?o,与MJO能量的突然增长相联系的低频纬向西风和低频对流活动的增强及其持续东传是激发El Ni?o的重要因素.但是,在中部型El Ni?o发展成熟的冬季至次年春季,热带中西太平洋MJO动能的强度突然再次增强,所占大气总扰动动能的比重也再次增大,热带MJO动能的逐日演变达到第二次峰值,且较第一次峰值更强;MJO从热带印度洋向赤道中东太平洋持续东传的特征也更为显著.这与在传统东部型El Ni?o盛期MJO能量和东传都明显减弱的特征表现出显著的差异.进一步分析指出,中部型El Ni?o成熟期海温正距平中心位置的西移以及由中部型El Ni?o激发的范围偏小、位置偏西的菲律宾附近异常反气旋环流可能是导致中部型El Ni?o盛期MJO活动显著增强的主要原因.     

不同强度El Ni(n)o的衰减过程.Ⅱ,中等和较弱El Ni(n)o的衰减过程

在第二部分,我们研究了中等和较弱El Ni(n)o的衰减过程. 结果表明,对中等El Ni(n)o而言,在其发展阶段和盛期,负异常信号在西太平洋产生,但由于强度不足,在El Ni(n)o盛期之后迅速衰减,这是一种夭折的类西太平洋振子过程. 因此,与强El Ni(n)o不同,中等El Ni(n)o衰减进入平常态. 而较弱El Ni(n)o以截然不同的另一种方式进行位相转换,伴随东南太平洋副高的加强和西移,东风异常和海表温度负异常自赤道东太平洋向西扩展,这是一种平流模态过程,导致较弱El Ni(n)o衰减进入La Ni(n)a.     

The anomalous variation of the lightning activity in southern China during the 1997/98 El Nio event

The El Ni?o Southern Oscillation (ENSO) is a cli- mate anomaly responsible for worldwide weather im- pacts ranging from droughts to floods. It is of scien- tific importance to clarify the influences of the 1997/98 El Ni?o event (for simplicity, named the ENSO) on the regional and global lightning activity. Goodman et al.[1] noticed that during the El Ni?o ma- ture phase from December 1997 to February 1998, the total lightning frequencies recorded by LIS, lightning days and lightning ho…     

The critical role of Indian summer monsoon on the remote forcing between Indian and Northwest Pacific during El Nio decaying year

Recent studies have found a connection between Indian Ocean Basin Warming and the anomalous Northwest Pacific Anticyclone(ANPWA) during El Ni?o decaying year.This study focuses on the necessary condition for this connection by using observation and numerical simulation.The seasonal transition of the Indian Ocean sea surface wind is critical to the climatic effect of Indian Ocean Basin Warming.When the South Asian Summer Monsoon reaches its peak,the background wind becomes desirable for basin warming,which then affects the climate in the Northwest Pacific.Via the Kelvin waves and Ekman divergence,the wind anomalies exist in the lower atmosphere east of the Indian Ocean warm Sea Surface Temperature(SST) anomalies,and intensify and sustain the ANWPA throughout the El Nio decaying summer.This impact plays an important role in the inter-annual variability of the East Asian Summer Monsoon.     

地球系统模式CESM1-BGC模拟的西北太平洋海气CO_2通量随季节演变的年际变率主导模态

海气CO_2通量的年际变率是气候学界关注的一个前沿问题.本文基于地球系统模式CESM1-BGC,采用季节依赖的经验正交函数分解(S-EOF)分析方法,研究了热带西北太平洋(0°~35°N,110°E~150°E)海气CO_2通量随季节演变的年际变率主导模态.结果表明,该年际主导模态对应厄尔尼诺-南方涛动(ENSO)从发展到衰减的演变过程.在El Nio发展年的夏季和秋季,海水CO_2分压的贡献略大于交换系数的贡献(即风速的贡献),两者共同作用使得西北太平洋吸收的CO_2增多.在El Nio成熟位相的冬季,西北太平洋异常反气旋建立,其西北侧的异常西南风使得中国东南沿海及附近海域的风速减弱,从而降低交换系数,导致中国东南沿海及附近海域吸收的CO_2减少.随后的次年春季,西太平洋异常反气旋继续维持,但位置东移,西南风的异常减小了风蒸发进而增加了中国近海海水温度,从而引起海水CO_2分压增大,使得中国近海海域吸收的CO_2减少,这一影响超过了交换系数减少所带来的影响.     

中国东部土壤湿度异常和厄尔尼诺对中国东部夏季降水的作用比较

利用中国气象科学研究院气候系统模式CAMS-CSM中大气和陆面的耦合版本进行了土壤湿度和热带太平洋海温异常影响东亚夏季风的数值模拟,探讨了中国东部从长江中下游到华北(YRNC)春季土壤湿度和厄尔尼诺(El Ni?o)在影响夏季东亚环流和中国东部降水中的作用及其机理.结果表明,中国东部春季土壤湿度和El Ni?o海温异常均对东亚夏季风有显著的影响,其中土壤湿度对中国东部夏季降水的影响略大于海温的作用,然而两者对东亚夏季风环流和中国夏季降水的作用显著不同. YRNC土壤偏湿(干)引起的降水异常模态为中国北部和东南降水偏少(多),而长江流域和东北降水偏多(少),环流上YRNC土壤偏湿(干)能引起西太平洋副热带高压显著偏强(弱)偏西(东)和东亚大槽偏深(浅),表现为弱(强)夏季风形态. El Ni?o对降水的影响显著不同于土壤湿度的作用,在El Ni?o发展期的夏季,中国东北和华北地区为异常反气旋,长江中下游和华南地区为异常气旋,西太平洋副热带高压偏弱,引起长江下游、华南降水偏多,华北降水偏少.在El Ni?o衰减期的夏季,中国东北地区存在一个异常气旋,华南有一个异常反气旋,异常反气旋西部的偏南气流和异常气旋西部的偏北气流在中国中部和北部地区汇合,使得夏季华北和长江中游地区降水增多,其余地区降水偏少.