Contrasting the Skills and Biases of Deterministic Predictions for the Two Types of El Nio

The tropical Pacific has begun to experience a new type of El Nio, which has occurred particularly frequently during the last decade, referred to as the central Pacific(CP) El Nio. Various coupled models with different degrees of complexity have been used to make real-time El Nio predictions, but high uncertainty still exists in their forecasts. It remains unknown as to how much of this uncertainty is specifically related to the new CP-type El Nio and how much is common to both this type and the conventional Eastern Pacific(EP)-type El Nio. In this study, the deterministic performance of an El Nio–Southern Oscillation(ENSO) ensemble prediction system is examined for the two types of El Nio. Ensemble hindcasts are run for the nine EP El Nio events and twelve CP El Nio events that have occurred since 1950. The results show that(1) the skill scores for the EP events are significantly better than those for the CP events, at all lead times;(2) the systematic forecast biases come mostly from the prediction of the CP events; and(3) the systematic error is characterized by an overly warm eastern Pacific during the spring season, indicating a stronger spring prediction barrier for the CP El Nio. Further improvements to coupled atmosphere–ocean models in terms of CP El Nio prediction should be recognized as a key and high-priority task for the climate prediction community.

A Diagnostic Study of the Impact of El Nion on the Precipitation in China

The impact of E1 Nino on the precipitation in China for different seasons are investigateddiagnostically. It is found that E1 Nino can influence the precipitation in China significantly duringits mature phase. In the Northern winter, spring and autumn, the positive precipitation anomaliesare found in the southern part of China during the E1 Nino mature phase. In the Northernsummer, the patterns of the precipitation anomalies in the E1 Nifio mature phase are different fromthose in the other seasons. The negative precipitation anomalies appear in both southern andnorthern parts of China, while in between around the lower reaches of the Yangtze River and theHuaihe River valleys the precipitation anomalies tend to be positive.In the Northern winter, spring and autumn, the physical process by which E1 Nino affects theprecipitation in the southern part of China can be explained by the features of the circulationanomalies over East Asia during the E1 Nino mature phase (Zhang et al. 1996). The appearance ofan anticyclonic anomaly to the north of the maritime continent in the lower troposphere during theE1 Nino mature phase intensifies the subtropical high in the western Pacific and makes it shiftwestward. The associated southwesterly flow is responsible for the positive precipitation anomaliesin the southern part of China. In the Northern summer, the intensified western Pacific subtropicalhigh covers the southeastern periphery of China so that the precipitation there becomes less. In addition, the weakening of the indian monsoon provides less moisture inflow to the northern part ofChina.

Variable and Robust East Asian Monsoon Rainfall Response to El Nio 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 Nio 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 Nio episodes during 1957–2016 reveals that, over EA, the spatiotemporal patterns among the four categories of El Nio events are quite variable, due to a large range of variability in the intensity and evolution of El Nio 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 Nio developing summer. Distinguishing strong and weak El Nio impacts is important. Only strong El Nio events can persistently enhance EA subtropical frontal precipitation from the peak season of El Nio 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 Nio impact on EA. A weak El Nio may also enhance the post-El Nio 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 Nio, but also the subseasonal migration of EA rainfall anomalies.

Responses of CO_2 in surface water in subtropical West Pacific to El Nio event

In order to investigate the relationships between the change of T CO 2 , Δ P CO 2 and SST, current, upwelling and biological activities during El Nio event in the subtropical Pacific, the responses of T CO 2 and Δ P CO 2 in surface water in the subtropical Pacific during El Nio and La Nina have been simulated using a three dimension carbon cycle model with biota pump. The results of numerical simulations show that T CO 2 in sea water increases with reducing of SST during mature phase of El Nio in the subtropical West Pacific . At the same period ,the Kuroshio in this region was weakened ,the zonal currents were divergence , the upwelling carried the water with high concentrations of CO 2 to the sea surface , so both of T CO 2 and Δ P CO 2 in surface water were increased . But T CO 2 and Δ P CO 2 were decreased during La Nina period. These simulated results confirmed the observations in 1982/1983 , 1986/1987 , 1991/1995 and 1997/1998 El Nio events .

THE INFLUENCES OF SSTA OVER KUROSHIO AND ITS EXTENSION ON RAINFALL IN NORTHEAST CHINA UNDER THE BACKGROUND OF TWO DIFFERENT EL NIO 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.

REGIONAL DISCREPANCIES OF THE IMPACT OF TROPICAL INDIAN OCEAN WARMING ON NORTHWEST PACIFIC TROPICAL CYCLONE FREQUENCY IN THE YEARS OF DECAYING EL NIO

This study investigates the influences of tropical Indian Ocean(TIO) warming on tropical cyclone(TC)genesis in different regions of the western North Pacific(WNP) from July to October(JASO) during the decaying El Nio. The results show significant negative TC frequency anomalies localized in the southeastern WNP. Correlation analysis indicates that a warm sea surface temperature anomaly(SSTA) in the TIO strongly suppresses TC genesis south of 21°N and east of 140°E in JASO. Reduced TC genesis over the southeastern WNP results from a weak monsoon trough and divergence and subsidence anomalies associated with an equatorial baroclinic Kelvin wave. Moreover,suppressed convection in response to a cold local SSTA, induced by the increased northeasterly connected by the wind-evaporation-SST positive feedback mechanism, is found unfavorable for TC genesis. Positive TC genesis anomalies are observed over higher latitudinal regions(at around 21°N, 140°E) and the western WNP because of enhanced convection along the northern flank of the WNP anomalous anticyclone and low-level convergence,respectively. Although local modulation(e.g., local SST) could have greater dominance over TC activity at higher latitudes in certain anomalous years(e.g., 1988), a warm TIO SSTA can still suppress TC genesis in lower latitudinal regions of the WNP. A better understanding of the contributions of TIO warming could help improve seasonal TC predictions over different regions of the WNP in years of decaying El Nio.

Connections Between Different Types of El Nio and Southern/Northern Oscillation

It has long been acknowledged that there are two types of El Nio events, i.e., the eastern Pacific El Nio （EE） and the central Pacific El Nio （CE）, according to the initial position of the anomalous warm water and its propagation direction. In this paper, the oceanic and atmospheric evolutions and the possible mechanisms of the two types of El Nio events were examined. It is found that all the El Nio events, CE or EE, could be attributed to the joint impacts of the eastward advection of warm water from the western Pacific warm pool （WPWP） and the local warming in the equatorial eastern Pacific. Before the occurrence of CE events, WPWP had long been in a state of being anomalous warm, so the strength of eastward advection of warm water was much stronger than that of EE, which played a major role in the formation of CE. While for the EE events, most contribution came from the local warming of the equatorial eastern Pacific. It is further identified that the immediate cause leading to the difference of the two types of El Nio events was the asynchronous variations of the Southern Oscillation （SO） and the Northern Oscillation （NO） as defined by Chen in 1984. When the transition from the positive phase of the NO （NO＋） to NO- was prior to that from SO＋ to SO-, there would be eastward propagation of westerly anomalies from the tropical western Pacific induced by NO and hence the growth of warm sea surface temperature anomalies in WPWP and its eastward propagation. This was followed by lagged SO-induced weakening of southeast trade winds and local warming in the equatorial eastern Pacific. These were conducive to the occurrence of the CE. On the contrary, the transition from SO＋ to SO- leading the transition of NO would favor the occurrence of EE type events.

Genesis of the 2014–2016 El Nio events

The 2015/2016 El Nio was one of the strongest El Nio events in history, and this strong event was preceded by a weak El Nio in 2014. This study systematically analyzed the dynamical processes responsible for the genesis of these events. It was found that the weak 2014 El Nio 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 Nio, 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 Nio events mainly came from the different amount of WWBs in 2014 and 2015. As compared to the 1982/1983 and 1997/1998 extreme El Nio events, the 2015/2016 El Nio exhibited some distinctive characteristics in its genesis and spatial pattern. We need to include the effects of WWBs to the theoretical framework of El Nio to explain these characteristics, and to improve our understanding and prediction of El Nio.

Processes involved in the second-year warming of the 2015 El Nio 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 Nio 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 Nio 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 Nio event are consistent with current El Nio and Southern Oscillation theories.

On the “spring predictability barrier” for strong El Nio 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 Nio 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 Nio 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 Nio occurred a significant SPB phenomenon. We show that the SPB occurred in the 2015/16 El Nio 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 Nio 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 Nio event and explain why the 2015/16 El Nio 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 Nio 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 Nio events could be greatly enhanced by using the NFSV-like tendency error to correct the model.

A new advance in_~El Nio prediction──The next El Nio will occur in 1997-1998

A statistic model of predicting El Nio based on the multiple regression analysis is developed by using sea surface temperature anomalies in the NINO3 region (5°S-5°N), 90-150°W) as a predictant and the Asian meridional circulation index as the predictors. This model cannot only simulate most of El Nio during 1950-1980 (except for El Nio in 1965), but also hindcast 1982/1983, 1986/1987, 1991/1992 and 1994 El Nio and predict 1997/1998 El Nio six months in advance. At the same time, three different sample series (periods of 1950-1969, 1970-1989, 1950-1989) are used to examine the stability and skill of the statistic model. The models have successfully hindcasted and predicted El Nio before and after the selected samples except for the 1965 El Nio. In addition, the forecast in the El Nio year is clearly superior to that in the following year. The model prediction reaches the lowest skill during March to April.

INTERANNUAL VARIATION OF EARTH ROTATION,El Nio EVENTS AND ATMOSPHERIC ANGULAR MOMENTUM

In this paper the multi-stage digit filter is used to analyse the data of Earth rotation represented by the length of day, ΔLOD. The results show that the interannual variations of Earth rotation, which are in the time scale of several years but not quasi-periodic terms, exist in the long periodic fluctuations. They induce the relative variation in the length of day of 0.3×10^(-8).Comparing the series of length of day with the data of temperature departure of the sea surface in the equatorial area of the eastern Pacific, we found that the deceleration and acceleration of the interannual rate of Earth rotation are consistent with the warming up and down of sea temperature in the equatorial area very well. This means that every El Nio event always occurs after the turning of acceleration of the interannual rate of Earth rotation to deceleration.According to the strong interannual variation in the length of day and strong warming of the sea surface temperature in the equatorial area between 1982 and 1983, we analysed the data from atmospheric angular momentum (AAM) calculated by using the global zonal wind data, and found that the interannual variation in AAM has an excess of two to three months. We suggest that the interannual variations in Earth rotation and the El Nio events are probably responses of solid earth and ocean, respectively, to the anomaly of atmospheric circulation.It is also shown in oar analysis that the minimum of ΔLOD series, which is deduced from UT1 data observed regularly with astrometry, can predict the occurrence of the El Nio events for a long range forecast about one year.

1997—1998年El-Nio至La-Nia期间东海黑潮的变异

基于日本“长风号”调查船在 1 997与 1 998年 1 0个航次的CTD资料 ,采用改进逆方法及改进动力计算方法对东海黑潮的流速、流量进行计算 .1 997年 5月出现了El Ni no现象 ,东海黑潮流量在 1 997年夏季减少 ,1 997年东海黑潮的平均流量也减少 .在 1 997年 1月与 6— 7月 ,即El Ni no现象出现前后 ,东海环流的流态有些不同 .在 1 998年 4至 1 1月黑潮在PN断面出现多流核心的结构 ,特别在 1 0— 1 1月出现 3个流核心 ,黑潮主流核的位置秋季时东移 .1 995年与 1 998年都是东海黑潮异常年 ,这些异常现象可能与冲绳岛以南出现的反气旋涡的强度变化以及从El Ni no现象过渡到La Ni

El Nio事件发生和消亡中热带太平洋纬向风应力的动力作用 I.资料诊断和理论分析

通过资料分析，研究了发生在热带西太平洋海表面西风或东风应力异常与ＥｌＮｉｎｏ事件的关系。分析结果表明，对应着ＥｌＮｉｎｏ事件从发生到消亡的过程，热带西太平洋纬向风应力存在着从西风应力异常到东风应力异常的变化，并且在这个过程中，西风应力异常向东传，东风应力异常紧接其后也向东传。本文还根据观测资料的分析结果建立了理想风应力，并利用简单热带海洋模式，对热带西太平洋纬向风应力异常及其东传在ＥＮＳＯ循环中的作用进行了动力学分析，指出了它们在ＥｌＮｉｎｏ事件发生和消亡中起着重要的作用。西风应力异常通过激发出海洋中东传的暖Ｋｅｌｖｉｎ波及其在大洋东边界反射产生的暖Ｒｏｓｓｂｙ波、以及西风应力异常本身东传到赤道东太平洋，引起正的海洋混合层扰动厚度异常，导致了ＥｌＮｉｎｏ事件的发生；而异常东风应力则通过激发出东传的冷Ｋｅｌｖｉｎ波及其在大洋东边界反射产生的冷Ｒｏｓｂｙ波、以及东风应力异常本身东传到赤道东太平洋，引起负的海洋混合层扰动厚度异常，导致了ＥｌＮｉｎｏ事件的消亡。对于热带西太平洋上风应力异常的形式是东部为异常西风应力而其西部为异常东风应力，并且它们同时向东传时，则大洋东部混合层厚度对异常风应力的响应随异常东风和西风?

东亚季风El Nio与中国松辽平原夏季低温关系初探

分析了１９５１～１９９５年东亚夏季风、ＥｌＮｉｎｏ与中国松辽平原的代表站长春站夏季农作物生长期（５～９月）气温关系，结果表明：２０世纪５０～７０年代，东亚夏季风偏弱，ＥｌＮｉｎｏ增温始于上半年，长春夏季多气温偏低或有低温冷害发生；１９８０～１９９５年，东亚夏季风偏强，即使ＥｌＮｉｎｏ增温始于上半年，当年长春夏季气温多为稍高到偏高，次年夏季气温稍低，但不致出现偏低和低温冷害。初步分析了典型ＥｌＮｉｎｏ年ＯＬＲ季内振荡特征，研究了ＥｌＮｉｎｏ，ＯＬＲ季内振荡、东亚夏季和冬季风对松辽平原夏季低温影响的某些成因。

1986～1987年El Nio期间热带西太平洋及南海海气热量交换研究

文中根据热带西太平洋海气相互作用研究（ＴＯＧＡ）第１～５及第８航次和南海科学考察结果，对热带西太平洋和南海海气热量交换作了分析。结果表明：ＥｌＮｉｎｏ事件发生前，热带西太平洋及南海海气热量交换非常强烈；ＥｌＮｉｎｏ事件发生后，热带西太平洋及南海海气热量交换反而减弱。

地球自转与El Nio——海气耦合理论

从低纬的海气耦合的浅水模式方程组出发 ,运用正交模和特殊函数的方法进一步讨论地球自转速率变化对海气耦合系统的影响 .研究表明 :地球自转速率的变化通过海气耦合一方面使大气和海洋的Kelvin波和Rossby波的移动及稳定性发生变化 ,另一方面使纬向风、洋流和海表温度发生变化 .特别是在地球自转减慢时 ,通过海气耦合 ,出现纬向风和洋流异常和大洋东部海表温度增加 ,从而导致引起全球气候异常的ElNi

1991～1995年El Nio事件的特征及其对中国天气气候异常的影响

利用多种资料分析了１９９１～１９９５年异常ＥｌＮｉｎｏ事件的特征及东亚环流变异和中国异常天气气候的特征。根据所采用的划分ＥｌＮｉｎｏ事件的标准和指数，９０年代初的５年里共发生了３次ＥｌＮｉｎｏ事件，即１９９１年６月～１９９２年９月、１９９３年４～１２月、１９９４年６月～１９９５年２月（简称１９９１～１９９５年３次ＥｌＮｉｎｏ事件）。这３次事件都是中部型，间隔时间短（以年尺度为主）是它们的一个显著特点。研究表明１９９１～１９９５年３次ＥｌＮｉｎｏ事件对中国天气气候的影响有许多不同的方面。其中１９９１年（ＥｌＮｉｎｏ当年）夏季和１９９２年（ＥｌＮｉｎｏ次年）夏季降水异常分布与１９５１～１９９０年间１０次ＥｌＮｉｎｏ事件的平均结果相似，但１９９３、１９９４年夏季降水与平均结果相反。这说明ＥＮＳＯ事件不是唯一决定中国夏季天气气候的强信号，还有别的气候因子影响中国夏季的天气气候异常。在东亚，夏季风的活动可能是另一个关键气候信号。通过对１９９１年和１９９４年夏季东亚大尺度环流变化和南海地区经纬向风时间演变的分析发现，两者虽都处于ＥｌＮｉｎｏ爆发的当年，但相反的环流形势造成了这两年夏季相反的降水形势。这一方面表明ＥｌＮｉ?

ELNio事件与我国月降水和温度的关系

用对比度分析方法,研究了ELNino事件对我国月降水和温度的影响,并进行了信度检验.结果表明:ELNino年,全国降水1～9月偏少,10～12月偏多.当5月降水异常少时,ELNino发生的概率达50%以上.当4月温度异常高时,ELNino发生的概率为零,而4月气温异常低时,ELNino发生的概率达50%～83%.这对ELNino的预测,提出了一个重要的信号依据.

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 Nino 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 E1 Nifio, 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 E1 Nifio 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.