NUMERICAL STUDY ON DOMINATING FACTOR AND KEY AREA OF EL NINO CYCLE FORMATION*

A coupled model,which is employed to study the dominating factor and key area of El Nino cycle formation,consists of a dynamical ocean model and a statistical atmospheric model.The coupled model with seasonal forcing successfully reproduces the El Nino event cycle which exhibits quasi-regular oscillations with a preferred period of about 4 years.The results show that the heat content(HC) is transported between the eastern and the western tropical Pacific areas.The spatial distribution of HC anomalies for four phases of the whole cycle clearly shows a possible formation mechanism of El Nino.Experiments further suggest that sea surface temperature in the tropical Pacific and HC in the central tropical Pacific are the most important factors and the central tropical Pacific is the most important area for determining formation of El Nino cycle.     

两类ENSO事件前期的热带太平洋海温距平场

分析了１９５６年以来两类ＥＮＳＯ事件热带太平洋海温距平场的特征。结果指出,东部型ＥｌＮｉｎｏ事件前期为ＬａＮｉｎａ事件年,热带中东太平洋为强的海温负距平,东部型ＬａＮｉｎａ事件前期为ＥｌＮｉｎｏ事件年,热带中不太平洋为强的海温正距平,中部型ＥｌＮｉｎｏ事件前期热带中西太平洋多为明显的海温正距平,中部型ＬａＮｉｎａ事件前期热带东太平洋多为明显的海渐负距平。两类ＥＮＳＯ事件前期海温距平场特殊基本相反。     

从有效位能变化来分析ElNino的年代际变化

利用近50a的美国Maryland大学全球海洋同化分析资料和NCEP全球大气再分析资料，分析了与ElNino相联系的热带太平洋有效位能（APE）的时空演变过程，从有效位能变化角度揭示了50年来ElNino的年代际变化特征。结果表明，热带太平洋ElNino年代际变化特征主要表现为在20世纪70年代末期以前与其后的显著不同。在有效位能异常变化的振幅和频率方面，20世纪70年代末期以后，振荡的振幅显著增,大直频率变慢。在有效位能异常变化的振幅和频率方面，70年代末期以前，ElNino事件的先兆是有效位能先在热带西太平洋异常积累，然后这些积累的有效位能沿赤道快速向东传播并释放能量，使得东太平洋出现能量的异常，东太平洋的有效位能减少，产生ElNino事件；70年代末期以后，ElNino事件的建立没有表现异常的有效位能在热带西太平洋的积累，其建立表现为赤道中太平洋直接出现的局地有效位能的异常，然后表现为不稳定海气耦合模态增幅性地缓慢向东传播，使得东太平洋的有效位能减少，发生ElNino事件。     

CMIP5模式对春季北极涛动影响后期冬季ENSO不对称性的模拟能力分析

根据观测资料的研究指出春季北极涛动(Arctic Oscillation, AO)对随后冬季厄尔尼诺-南方涛动(El Nino–Southern Oscillation, ENSO)的影响具有明显不对称性。春季AO处于正位相时,它对随后冬季厄尔尼诺(El Nino)事件的影响显著,然而春季AO负位相对随后冬季拉尼娜(La Nina)的影响不明显。本研究分析了30个来自CMIP5的耦合模式对春季AO与随后冬季ENSO不对称性关系的模拟能力。30个CMIP5耦合模式中,只有CNRM-CM5和GISS-E2-H-CC模式能较好地抓住春季AO与冬季ENSO的联系。进一步分析这两个模式中春季AO与冬季ENSO的不对称性关系,发现CNRM-CM5模式能较好地再现春季AO与冬季ENSO的非对称关系,即春季AO正(负)位相会导致赤道中东太平洋出现El Nino(La Nina)型海表温度增暖(冷却)。然而,GISS-E2-H-CC模式的模拟结果显示,春季AO对随后冬季ENSO的影响是对称的。本文随后解释了CNRM-CM5(GISS-E2-H-CC)模式能(不能)模拟出春季AO与冬季ENSO不对称关系的原因。对于CNRMCM5模式,在春季AO正位相年,副热带西北太平洋上空存在明显的异常气旋和正降水异常,正降水异常通过Gill型大气响应对赤道西太平洋异常西风的形成和维持起着重要作用,异常西风通过激发向东传播的暖赤道Kelvin波对随后冬季El Nino事件的发生产生显著的影响;然而,在春季AO负位相年,副热带北太平洋的异常反气旋和负降水异常较弱,导致赤道西太平洋的异常东风不明显,因此,春季AO负异常对随后冬季La Nina的影响不显著。所以,CNRM-CM5模式能够较好地抓住春季AO对随后冬季ENSO事件的非对称性影响。相比之下,对于GISS-E2-H-CC模式,春季AO正(负)位相年副热带西北太平洋上存在显著的正(负)降水异常,通过Gill型大气响应在赤道西太平洋激发出明显的异常西(东)风从而影响随后冬季的El Nino(La Nina)事件。因此,在GISS-E2-H-CC模式中,春季AO对随后冬季ENSO具有对称性影响。另外,模式捕捉春季AO对随后冬季ENSO非对称性影响的能力与模式对春季AO空间结构的模拟能力有一定的联系。     

厄尼诺现象对热带大气环流影响的研究

应用NMC 17年热带格点风及OLR资料，对比分析了厄尼诺年与反厄尼诺年热带环流的差异，发现厄尼诺现象使多年平均位于西太平洋上的最强盛的对流加热区东移到中太平洋，太平洋热带地区东西向对流加热梯度减弱，造成中东太平洋上的Walker环流及西太平洋与印度洋上的反Walker环流场比常年减弱；同时，厄尼诺现象也使中东太平洋地区经向反Hadley环流及美洲大陆上空的Hadley环流减弱。     

INFLUENCE OF TIBETAN PLATEAU ON AIR-SEA INTERACTION OVER TROPICAL PACIFIC IN OCEAN-ATMOSPHERIC COUPLED SYSTEM*

A hybrid coupled ocean-atmosphere model is designed,which consists of a global atmospheric general circulation model(L9R15 AGCM)and a simple ocean model(ZC ocean model over tropical Pacific).Using the model,experimental predictions are performed for the 1986/87 El Nino event and the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TP FORC and NTP FORC hereinafter).It is found as follows:(1)The coupled system can successfully predict the El Nino or La Nina event even if the Tibetan Plateau orography is not included in the model.The patterns of SSTA and wind anomalies in the model without the Tibetan Plateau are similar to those with the Tibetan Plateau,which further verifies the fact that ENSO process is mainly caused by the air-sea interaction in tropical Pacific.(2)However.the existence of the Tibetan Plateau exerts its influences on the intensity and duration of El Nino(or La Nina).It is unfavorable to the development and maintenance of westerly anomalies,so to some extent,restrains the development of El Nino,but favors the development of La Nina.(3)Effects of the Tibetan Plateau orography on the wind anomalies in the coupled system are different from those inuncoupled AGCM simulation.     

INFLUENCE OF TIBETAN PLATEAU ON AIR-SEA INTERACTION OVER TROPICAL PACIFIC IN OCEAN-ATMOSPHERIC COUPLED SYSTEM

A hybrid coupled ocean-atmosphere model is designed,which consists of a global atmosphericgeneral circulation model(L9R15 AGCM)and a simple ocean model(ZC ocean model over tropicalPacific).Using the model,experimental predictions are performed for the 1986/87 El Nino eventand the 1988/89 La Nina event with and without the Tibetan Plateau respectively(called TPFORC and NTP FORC hereinafter).It is found as follows:(1)The coupled system cansuccessfully predict the El Nino or La Nina event even if the Tibetan Plateau orography is notincluded in the model.The patterns of SSTA and wind anomalies in the model without the TibetanPlateau are similar to those with the Tibetan Plateau,which further verifies the fact that ENSOprocess is mainly caused by the air-sea interaction in tropical Pacific.(2)However.the existenceof the Tibetan Plateau exerts its influences on the intensity and duration of El Nino(or La Nina).It is unfavorable to the development and maintenance of westerly anomalies,so to some extent,restrains the development of El Nino,but favors the development of La Nina.(3)Effects of theTibetan Plateau orography on the wind anomalies in the coupled system are different from those inuncoupled AGCM simulation.     

PREDICTION AND VERIFICATION OF THE 1997-1999 EL NINOAND LA NINA BY USING AN INTERMEDIATE OCEAN-ATMOSPHERE COUPLED MODEL*

The numerical simulations,hindcasts and verifications of the tropical Pacific sea surface temperature anomaly(SSTA) have been conducted by using a dynamical tropical Pacific ocean-atmosphere coupled model named NCCo.The results showed that the model had performed reasonable simulations of the major El Nino episodes in the history,and the model forecast skill in 1990s had been significantly improved.NCCo model has been used to predict the tropical Pacific SSTA since January 1997.The comparisons between predictions and observations indicated that the occurrence,evolution and ending of the 1997-1998 El Nino episode have been predicted fairly well by using this model.Also,the La Nina episode that began in the autumn of 1998 and the developing tendency of the tropical Pacific SSTA during the year 1999 have been predictedsuccessfully.The forecast skills of NCCo model during the 1997-1999 El Nino and La Nina events are above 0.5 at 0-14 lead months.     

A mechanism for decadal changes of ENSO behavior: roles of background wind changes

This study explains why a number of El Nino properties (period, amplitude, structure, and propagation) have changed in a coherent manner since the late 1970s and why these changes had almost concurred with the Pacific decadal climate shift. Evidence is presented to show that from the pre-shift (1961-1975) to the post-shift (1981-1995) epoch, significant changes in the tropical Pacific are found in the surface winds and temperature, whereas changes in the thermocline are uncertain. Numerical experiments with the Cane and Zebiak model demonstrate that the decadal changes in the surface winds qualitatively reproduce the observed coherent changes in El Nino properties. The fundamental factor that altered the model's El Nino is the decadal changes of the background equatorial winds and associated upwelling. The annual cycle is also necessary for the mean state to modulate El Nino. From the pre- to post-shift epoch, the changes in the background winds and upwelling modify the structure of the coupled mode (eastward displacement of the equatorial westerly anomalies) by reallocating anomalous atmospheric heating and SST gradient along the equator. This structural change amplifies the ENSO cycle and prolongs the oscillation period by enhancing the coupled instability and delaying transitions from a warm to a cold state or vice versa. The changes in the mean currents and upwelling reduce the effect of the zonal temperature advection while enhance that of the vertical advection; thus, the prevailing westward propagation is replaced by eastward propagation or standing oscillation. Our results suggest a critical role of the atmospheric bridge that rapidly conveys the influences of extratropical decadal variations to the tropics, and the possibility that the Pacific climate shift might have affected El Nino properties in the late 1970s by changing the background tropical winds and the associated equatorial upwelling.     

A study on flow field and heat exchange anomalies on tropical pacific surface in the El Nino and La Nina events

Summary In this paper, based on the 2°×2° grid data COADS from 1950–1987 the flow field and heat exchange anomalies on the (11°S–11°N, 120° E-80°W) tropical Pacific surface (TPS) have been studied in El Nino and La Nina events. During El Nino, the zonal pressure gradient and trade winds decreased on the TPS, the tropical convergence strengthened on TPS, especially on the central TPS, the sensible and latent heat exchange increased, the net longwave radiation and incident solar radiation decreased and the net gain (loss)of heat reduced (increased) on the central and eastern TPS. During La Nina the results were opposite. Finally, two feedback mechanisms which include the dynamic, thermal and hydrological processes during El Nino and La Nina have been summarized and a conceptive model for El Nino-La Nina cycle is given. An estimated period for the El Nino-La Nina cycle is obtained.     

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES*

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short) of the tropical Pacific from January 1970 to December 1989,main spatial patterns of tropical Pacific SSTA and SSWA coupling features in the transform course from the warm phase to the cold phase of El Nino-southern Oscillation(ENSO) cycles are discussed.The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of El Nino(La Nina) are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at the mature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcing of distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with the California cold current and SSTA is mainly responsible for weakening of El Nino;(3)the second sea temperature increase along the South American coast in the decaying course of El Nino results from the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacific forced by anomalous west wind stress:(4)La Nina results from the joint effect of Walker circulation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.     

大气季节内振荡的活动与El Nino

用广东省４７个测站１９５４－１９９０年各月的平均气温、降水、日照时数的标准化资料分别作按时间点分解的主分量分析,取其前６个主分量作为各测站的气候特征量,用相关系数－重心法作聚类分析,作出各月、全年综合要素的气候分区。结果表明：（１）将广东各月分为有较显著差异的５个区,则冬半年（１０－翌年４月）各月的分区形式较类似且规律性明显,夏半年（５－９月）各月的分区逐月变化明显,且分区形式差别较大。（２）若用６个自然季节的平均气温、降水、日照时数标准化资料分别作主分量分析,各取前３个主分量作为全年气候分区的特征量,则广东全年的气候区可分为东南沿海、西南沿海、西北内陆３个区。     

AIR-SEA COUPLING FEATURES FROM THE WARM PHASE TO THE COLD PHASE OF ENSO CYCLES IN THE SEVENTIES TO THE EIGHTIES

Using the sea surface temperature and wind anomalies(SSTA and SSWA for short)of thetropical Pacific from January 1970 to December 1989,main spatial patterns of tropical PacificSSTA and SSWA coupling features in the transform course from the warm phase to the cold phaseof El Nino-southern Oscillation(ENSO)cycles are discussed.The main conclusions are as follows:(1)air-sea coupling patterns at the mature stage of ElNino(La Nina)are main spatial ones of tropical Pacific SSWA and SSTA coupling:(2)at themature stage of El Nino,the interaction of the anticyclonic anomaly wind,generated by the forcingof distinct meridional SSTA gradient in the Northern Hemisphere tropical central Pacific.with theCalifornia cold current and SSTA is mainly responsible for weakening of El Nino;(3)the secondsea temperature increase along the South American coast in the decaying course of El Nino resultsfrom the eastward movement of the weakened positive SSTA in the tropical central-eastern Pacificforced by anomalous west wind stress:(4)La Nina results from the joint effect of Walkercirculation,Ekman drift and negative SSTA in the tropical central-eastern Pacific.     

Factors that affect the amplitude of El Nino in global coupled climate models

Historically, El Nino-like events simulated in global coupled climate models have had reduced amplitude compared to observations. Here, El Nino-like phenomena are compared in ten sensitivity experiments using two recent global coupled models. These models have various combinations of horizontal and vertical ocean resolution, ocean physics, and atmospheric model resolution. It is demonstrated that the lower the value of the ocean background vertical diffusivity, the greater the amplitude of El Nino variability which is related primarily to a sharper equatorial thermocline. Among models with low background vertical diffusivity, stronger equatorial zonal wind stress is associated with relatively higher amplitude El Nino variability along with more realistic east–west sea surface temperature (SST) gradient along the equator. The SST seasonal cycle in the eastern tropical Pacific has too much of a semiannual component with a double intertropical convergence zone (ITCZ) in all experiments, and thus does not affect, nor is it affected by, the amplitude of El Nino variability. Systematic errors affecting the spatial variability of El Nino in the experiments are characterized by the eastern equatorial Pacific cold tongue regime extending too far westward into the warm pool. The time scales of interannual variability (as represented by time series of Nino3 SSTs) show significant power in the 3–4 year ENSO band and 2–2.5 year tropospheric biennial oscillation (TBO) band in the model experiments. The TBO periods in the models agree well with the observations, while the ENSO periods are near the short end of the range of 3–6 years observed during the period 1950–94. The close association between interannual variability of equatorial eastern Pacific SSTs and large-scale SST patterns is represented by significant correlations between Nino3 time series and the PC time series of the first EOFs of near-global SSTs in the models and observations. Received: 17 April 2000 / Accepted: 17 August 2000     

Indices of El Nino and El Nino Modoki： An Improved El Nino Modoki Index

In recent years, El Nino Modoki (pseudo-El Nino) has been distinguished as a unique large-scale ocean warming phenomenon happening in the central tropical Pacific that is quite different from the traditional El Nino. Empirical Orthogonal Function (EOF) analysis is used to successfully separate El Nino and El Nino Modoki. The abilities of the NINO3 index, NINO3.4 index, NINO1+2 index and NINO4 index in characterizing the El Nino are explored in detail. It is suggested that the NINO3 index is comparatively optimal to monitor the El Nino among the four NINO indices, since other NINO indices either cannot well distinguish El Nino and El Nino Modoki signals or are easily disturbed by El Nino Modoki signals. Further, an improved El Nino Modoki index (IEMI) is introduced to better represent the El Nino Modoki that is captured by the second leading EOF mode of monthly tropical Pacific sea surface temperature anomalies (SSTA). The IEMI is an improvement of the El Nino Modoki index (EMI) through adjusting the inappropriate weight coefficients of the three boxes of EMI, and it effectively overcomes the lack of EMI in monitoring the two historical El Nino Modoki events and also avoids the possible risk of EMI in excluding the interference of El Nino signal, indicating the realistic and potential advantages.     

A STUDY OF CLIMATOLOGICAL ASPECTS OF FLOW FIELD AND HEAT EXCHANGE ON TROPICAL PACIFIC SURFACE IN THE EL NINO AND LA NINA EVENTS

In this paper, based on the 2°× 2° grid data COADS from 1950—1987 the flow field and heatexchange anomalies on the tropical (11°S—11°N, 120°E—80°W) Pacific surface (TPS) are studiedin El Nino and La Nina events. During E1 Nino, the zonal pressure gradient and the trade windsdecrease on the TPS, the tropical convergence strengthens on the TPS, especially on the centralTPS, the sensible and latent heat exchanges increase, the net longwave radiation and incident solarradiation decrease and the net gain (loss) of heat reduces (increases) on the central and easternTPS. During La Nina the results turn out the contrary. Finally, two feedback mechanisms whichinclude the dynamic, thermal and hydrological processes during E1 Nino and La Nina are summa-rized and a conceptive model for El Nino-La Nina cycle is given.     

厄尔尼诺与拉尼娜的诊断

赤道太平洋海域海温异常事件包括厄尔尼诺 (EL- Nino)和拉尼娜 (L a- Nina)现象。通过对赤道东中太平洋特定区域海温指数的统计分析 ,确定厄尔尼诺和拉尼娜出现的诊断指标 ,分析了厄尔尼诺和拉尼娜的出现周期 ,在此基础上分析了厄尔尼诺与拉尼娜对广西气候变化的一些主要影响     

On the Mechanism of the Locking of the El Nino Event Onset Phase to Boreal Spring

The mechanism of the locking of the E1 Nino event onset phase to boreal spring （from April to June） in an intermediate coupled ocean-atmosphere model is investigated. The results show that the seasonal variation of the zonal wind anomaly over the equatorial Pacific associated with the seasonal variation of the ITCZ is the mechanism of the locking in the model. From January to March of the E1 Nino year, the western wind anomaly over the western equatorial Pacific can excite the downwelling Kelvin wave that propagates eastward to the eastern and middle Pacific by April to June. From April to December of the year before the E1 Nifio year, the eastern wind anomaly over the equatorial Pacific forces the downwelling Rossby waves that modulate the ENSO cycle. The modulation and the reflection at the western boundary modulate the time of the transition from the cool to the warm phase to September of the year before the E1 Nifio year and cause the strongest downwelling Kelvin wave from the reflected Rossby waves at the western boundary to arrive in the middle and eastern equatorial Pacific by April to June of the E1 Nino year. The superposition of these two kinds of downwelling Kelvin waves causes the El Nino event to tend to occur from April to June.     

On the Mechanism of the Locking of the El Nio Event Onset Phase to Boreal Spring

1. Introduction The observed facts show that the ENSO cycle has obvious phase-locking and oscillates irregularly (An and Wang, 2001; Kaplan et al., 1998). Based on Zibiak and Cane's (1987) model (hereafter, the Z-C model) and simple, coupled ocean-atmosph…     

中纬度北太平洋依赖于ENSO事件及独立于ENSO事件的变率特征:观测事实与海气耦合模式模拟

观测事实显示,在E1 Ni(?)o发生期间,伴随着赤道中东太平洋的增暖,中纬度北大平洋中部表层海温（SST）常出现冷距平,而北美大陆西海岸SST则出现暖距平。借助观测资料分析和海气耦合模式模拟两种手段,检验了北太平洋对ENSO事件的上述响应。观测证据和数值模拟都支持有关学者提出的“大气桥”概念,即大气对赤道中东太平洋SST异常增暖的响应,随后强迫中纬度北太平洋,并导致那里SST的变冷,从而起到了连接热带和热带外特别是中纬度北太平洋的“桥梁”的作用。关于其机制,本文认为主要是海洋对大气强迫的动力响应导致那里的SST变冷,尽管潜热通量的贡献也很显著。至少模式结果证明短波辐射、长波辐射和感热通量的贡献都是次要的。进一步的分析揭示,北太平洋存在着线性独立于ENSO事件的所谓“北太平洋模态”,在空间型上,它和线性地依赖于ENSO事件的模态非常相近,即它们的纬向结构都呈现出扁平的“双极”型,只是彼此间SST距平极大值的中心位置不同。模拟结果表明,北太平洋模态与大气的耦合作用,主要是通过海气热通量交换实现的,其中短波辐射和长波辐射的作用居主导地位,而潜热通量的贡献则基本可以忽略。