ENSO variability and the eastern tropical Pacific: A review

El Niño-Southern Oscillation (ENSO) encompasses variability in both the eastern and western tropical Pacific. During the warm phase of ENSO, the eastern tropical Pacific is characterized by equatorial positive sea surface temperature (SST) and negative sea level pressure (SLP) anomalies, while the western tropical Pacific is marked by off-equatorial negative SST and positive SLP anomalies. Corresponding to this distribution are equatorial westerly wind anomalies in the central Pacific and equatorial easterly wind anomalies in the far western Pacific. Occurrence of ENSO has been explained as either a self-sustained, naturally oscillatory mode of the coupled ocean–atmosphere system or a stable mode triggered by stochastic forcing. Whatever the case, ENSO involves the positive ocean–atmosphere feedback hypothesized by Bjerknes. After an El Niño reaches its mature phase, negative feedbacks are required to terminate growth of the mature El Niño anomalies in the central and eastern Pacific. Four requisite negative feedbacks have been proposed: reflected Kelvin waves at the ocean western boundary, a discharge process due to Sverdrup transport, western Pacific wind-forced Kelvin waves, and anomalous zonal advections. These negative feedbacks may work together for terminating El Niño, with their relative importance being time-dependent.ENSO variability is most pronounced along the equator and the coast of Ecuador and Peru. However, the eastern tropical Pacific also includes a warm pool north of the equator where important variability occurs. Seasonally, ocean advection seems to play an important role for SST variations of the eastern Pacific warm pool. Interannual variability in the eastern Pacific warm pool may be largely due to a direct oceanic connection with the ENSO variability at the equator. Variations in temperature, stratification, insolation, and productivity associated with ENSO have implications for phytoplankton productivity and for fish, birds, and other organisms in the region. Long-term changes in ENSO variability may be occurring and are briefly discussed. This paper is part of a comprehensive review of the oceanography of the eastern tropical Pacific.     

中国科学院气候系统模式模拟的ENSO循环

On the basis of more than 200-year control run, the performance of the climate system model of Chinese Academy of Sciences(CAS-ESM-C) in simulating the El Ni?o-Southern Oscillation(ENSO) cycle is evaluated, including the onset, development and decay of the ENSO. It is shown that, the model can reasonably simulate the annual cycle and interannual variability of sea surface temperature(SST) in the tropical Pacific, as well as the seasonal phase-locking of the ENSO. The model also captures two prerequisites for the El Ni?o onset, i.e., a westerly anomaly and a warm SST anomaly in the equatorial western Pacific. Owing to too strong forcing from an extratropical meridional wind, however, the westerly anomaly in this region is largely overestimated. Moreover, the simulated thermocline is much shallower with a weaker slope. As a result, the warm SST anomaly from the western Pacific propagates eastward more quickly, leading to a faster development of an El Ni?o. During the decay stage, owing to a stronger El Ni?o in the model, the secondary Gill-type response of the tropical atmosphere to the eastern Pacific warming is much stronger, thereby resulting in a persistent easterly anomaly in the western Pacific. Meanwhile, a cold anomaly in the warm pool appears as a result of a lifted thermocline via Ekman pumping. Finally, an El Ni?o decays into a La Ni?a through their interactions. In addition, the shorter period and larger amplitude of the ENSO in the model can be attributed to a shallower thermocline in the equatorial Pacific, which speeds up the zonal redistribution of a heat content in the upper ocean.     

SST年循环对El Niño事件局地海气过程的影响

利用Hadley中心逐月海表温度、欧洲中心ERA-40的10 m风场及CMAP降水资料探讨了年循环对热带太平洋El Niño海气相互作用过程的影响。尽管El Niño对应的海表温度异常主要出现在赤道东太平洋,经向上呈南北对称分布,然而其对应的大气响应在El Niño年衰减阶段却有着强的向南移动特征。在El Niño发展年的11月之前,强的西风和降水异常主要出现在赤道中太平洋;在12月份之后,赤道上的西风和降水异常迅速南移至5°S,随后西风一直维持在该位置直至衰亡。同时,西太平洋负降水和反气旋异常向北移动。这种SST异常与其大气响应的经向移动不一致,主要是由热带中太平洋气候态SST的季节性南移导致的。由于对流与海温之间存在非线性关系,即当总SST超过一定的阈值,对流降水才会迅速增强;因此相应的对流响应也随着总海温的南移而南移,风场响应也同时南移。此外,南半球增强的对流会通过经向环流进一步抑制北半球的降水,从而使西太平洋负降水和反气旋异常增强并北移。通过分析有/无年循环的两组数值试验结果验证了上述结论,即有年循环的试验较真实地模拟出了观测中异常西风南移和西北太平洋反气旋异常的出现;无年循环试验尽管能模拟出El Niño年赤道中太平洋的西风异常,但其却没有南北向的移动,西北太平洋的反气旋也没有出现。因此,热带中太平洋气候态暖海温的季节循环对El Niño事件大气响应有着至关重要的作用。     

副热带南北太平洋经向模与ENSO的关系

本文利用Hadley中心的海表面温度、海洋再分析资料ORAS4(Ocean Reanalysis System 4)的海表面高度、NCEP(National Centers for Environmental Prediction)的海气界面风场及热通量等数据,分析了1948-2018年期间副热带南、北太平洋经向模(S...     

Spatial and temporal structure of Tropical Pacific interannual variability in 20th century coupled simulations

Tropical Pacific interannual variability is examined in nine state-of-the-art coupled climate models, and compared with observations and ocean analyses data sets, the primary focus being on the spatial structure and spectral characteristics of El Niño-Southern Oscillation (ENSO). The spatial patterns of interannual sea surface temperature (SST) anomalies from the coupled models are characterized by maximum variations displaced from the coast of South America, and generally extending too far west with respect to observations. Thermocline variability is characterized by dominant modes that are qualitatively similar in all the models, and consistent with the “recharge oscillator” paradigm for ENSO. The meridional scale of the thermocline depth anomalies is generally narrower than observed, a result that can be related to the pattern of zonal wind stress perturbations in the central-western equatorial Pacific. The wind stress response to eastern equatorial Pacific SST anomalies in the models is narrower and displaced further west than observed. The meridional scale of the wind stress can affect the amount of warm water involved in the recharge/discharge of the equatorial thermocline, while the longitudinal location of the wind stress anomalies can influence the advection of the mean zonal temperature gradient by the anomalous zonal currents, a process that may favor the growth and longer duration of ENSO events when the wind stress perturbations are displaced eastwards. Thus, both discrepancies of the wind stress anomaly patterns in the coupled models with respect to observations (narrow meridional extent, and westward displacement along the equator) may be responsible for the ENSO timescale being shorter in the models than in observations. The examination of the leading advective processes in the SST tendency equation indicates that vertical advection of temperature anomalies tends to favor ENSO growth in all the CGCMs, but at a smaller rate than in observations. In some models it can also promote a phase transition. Longer periods tend to be associated with thermocline and advective feedbacks that are in phase with the SST anomalies, while advective tendencies that lead the SST anomalies by a quarter cycle favor ENSO transitions, thus leading to a shorter period.     

ENSO循环相关的海洋异常信号传播特征及其机制

通过分析最新的海洋模式同化资料(EstimatingtheCirculationandClimateoftheOcean,EC CO),研究了ENSO循环相关的海洋异常信号在太平洋中的传播过程。研究发现,导致ENSO位相变化的温跃层异常信号主要从北太平洋西传而来,该区与赤道东太平洋相反的温跃层异常信号到达西太暖池区,再从西太暖池沿赤道传到东太平洋,可使ENSO向反位相发展。该异常信号沿赤道东传过程中热带西南太平洋也会出现类似的温跃层异常变化,但是随着异常信号东移和从南太平洋东边界10°S左右传来的反异常信号入侵,热带西南太平洋的异常信号逐渐减弱并消失。稳定性分析表明,北太平洋较大面积区域存在斜压不稳定性或正压不稳定性,有利于ENSO相关的温跃层异常信号以Rossby波形式有效地西传;而在南太平洋,不稳定区的面积较小,且主要局限于海盆东侧,因而传播较弱,这样就造成了ENSO信号在太平洋南、北半球的非对称传播。一般来说,ENSO信号主要在以赤道波导区、东边界、北太平洋纬向区域和西边界组成的回路中循环,在南半球的传播不明显。     

Decadal modes of sea surface salinity and the water cycle in the tropical Pacific Ocean: The anomalous late 1990s

Limitations in sea surface salinity (SSS) observations and timescale separation methods have led to an incomplete picture of the mechanisms of SSS decadal variability in the tropical Pacific Ocean, where the El Niño Southern Oscillation (ENSO) dominates. Little is known regarding the roles of the North Pacific Gyre Oscillation (NPGO) and the Pacific Decadal Oscillation (PDO) in the large-scale SSS variability over the tropical basin. A self-organizing map (SOM) clustering analysis is performed on the intrinsic mode function (IMF) maps, which are decomposed from SSS and other hydrological fields by ensemble empirical mode decomposition (EEMD), to extract their asymmetric features on decadal timescales over the tropical Pacific. For SSS, an anomalous pattern appeared during 1997 to 2004, a period referred to as the anomalous late 1990s, when strong freshening prevailed in large areas over the southwestern basin and moderate salinization occurred in the western equatorial Pacific. During this period, the precipitation and surface currents were simultaneously subjected to anomalous fluctuations: the precipitation dipole and zonal current divergence along the equator coincided with the SSS increase in the far western equatorial Pacific, while the weak zonal current convergence in the southwestern basin and large-scale southward meridional currents tended to induce SSS decreases there. The dominant decadal modes of SSS and sea surface temperature (SST) in the tropical Pacific both resemble the NPGO but occur predominantly during the negative and positive NPGO phases, respectively. The similarities between the NPGO and Central Pacific ENSO (CP-ENSO) in their power spectra and associated spatial patterns in the tropics imply their dynamical links; the correspondence between the NPGO-like patterns during negative (positive) phases and the CP La Niña (CP El Niño) patterns for SSS is also discussed.     

热带太平洋海表二氧化碳分压的时空变化特征及其与物理场间的联系

为研究赤道太平洋海表二氧化碳分压(pCO₂^sw)年际变化的机制,基于中科院海洋所宋金明研究团队于2021年发布的中国首套全球海表二氧化碳分压数据产品,使用相关性分析、经验正交函数(empirical orthogonal function, EOF)分析和奇异值分解(singular value decomposition, SVD)等方法,研究了2005～2019年赤道太平洋pCO₂^sw气候态分布及其去趋势后的年际异常的时空演变特征;结合pCO₂^sw与多种参数的相关性和厄尔尼诺-南方涛动(El Ni?o-Southern Oscillation, ENSO)过程探讨了赤道中西太平洋pCO₂^sw年际异常中心形成的原因。研究结果显示,热带太平洋pCO₂^sw季节变化、年际异常及其EOF第一模态特征向量沿赤道均出现两个中心,其中一个在赤道中西太平洋日界线附近,另一个在赤道中东...     

El Niño in 1991–1992 and its manifestations in the tropical Atlantic

A variety of anomalous conditions in the tropical Pacific and Atlantic oceans during the 1991–1992 El Niño event are described. The current El Niño is shown to have specific features, evident in the behaviour of the southern oscillation index and sea surface temperature of the equatorial Pacific Ocean. Also, it was accompanied by warming of the eastern tropical Atlantic. Temperature anomalies of the tropical Atlantic surface water temperature reached their maximum (>3°C) in May–June 1991, i.e. 2–3 months earlier than in the Pacific Ocean.Translated by V. Puchkin.     

The circulation of the eastern tropical Pacific: A review

During the 1950s and 1960s, an extensive field study and interpretive effort was made by researchers, primarily at the Scripps Institution of Oceanography, to sample and understand the physical oceanography of the eastern tropical Pacific. That work was inspired by the valuable fisheries of the region, the recent discovery of the equatorial undercurrent, and the growing realization of the importance of the El Niño phenomenon. Here we review what was learned in that effort, and integrate those findings with work published since then as well as additional diagnoses based on modern data sets.Unlike the central Pacific, where the winds are nearly zonal and the ocean properties and circulation are nearly independent of longitude, the eastern tropical Pacific is distinguished by wind forcing that is strongly influenced by the topography of the American continent. Its circulation is characterized by short zonal scales, permanent eddies and significant off-equatorial upwelling. Notably, the Costa Rica Dome and a thermocline bowl to its northwest are due to winds blowing through gaps in the Central American cordillera, which imprint their signatures on the ocean through linear Sverdrup dynamics. Strong annual modulation of the gap winds and the meridional oscillation of the Intertropical Convergence Zone generates a Rossby wave, superimposed on the direct forcing, that results in a southwestward-propagating annual thermocline signal accounting for major features of observed thermocline depth variations, including that of the Costa Rica Dome, the Tehuantepec bowl, and the ridge–trough system of the North Equatorial Countercurrent (NECC). Interannual variability of sea surface temperature (SST) and altimetric sea surface height signals suggests that the strengthening of the NECC observed in the central Pacific during El Niño events continues all the way to the coast, warming SST (by zonal advection) in a wider meridional band than the equatorially trapped thermocline anomalies, and pumping equatorial water poleward along the coast.The South Equatorial Current originates as a combination of equatorial upwelling, mixing and advection from the NECC, and Peru coastal upwelling, but its sources and their variability remain unresolved. Similarly, while much of the Equatorial Undercurrent flows southeast into the Peru Undercurrent and supplies the coastal upwelling, a quantitative assessment is lacking. We are still unable to put together the eastern interconnections among the long zonal currents of the central Pacific.     

Plankton biomass and larval fish abundance prior to and during the El Niño period of 1997-1998 along the central Pacific coast of México

The temporal and spatial distributions of zooplankton biomass and larval fish recorded during 27 months (December 1995-December 1998) off the Pacific coast of central México are analyzed. A total of 316 samples were obtained by surface (from 40-68 to 0 m) oblique hauls at 12 sampling sites using a Bongo net. Two well-defined periods were observed: a pre-ENSO period (December 1995-march 1997) and an ENSO event (July 1997-September 1998) characterized by impoverishment of the pelagic habitat. The highest biomass concentrations occurred at coastal stations during the pre-ENSO period. During the El Niño period no spatial patterns were found in coastal waters. The months with highest biomass were those in which the lowest sea surface temperature (SST) occurred (January-May), and this pattern was also observed during the ENSO period. A typical, although attenuated, seasonal environmental pattern with enhanced phytoplankton (diatoms and dinoflagellates) was prevalent during the El Niño event in nearshore waters. During the El Niño period the phytoplankton was mainly small diatoms (microphytoplankton), while dinoflagellates were practically absent. The most parsimonious generalized linear models explaining spatial and temporal distribution of larval fish species included the ENSO index (MEI), upwelling index (UI) and distance to the coast. The environmental variability defined on an interannual time-scale by the ENSO event and the seasonal hydroclimatic pattern defined by the UI (intra-annual-scale) controlled the ecosystem productivity patterns. The small-scale distribution patterns (defined by a cross-shore gradient) of plankton were related to the hydroclimatic seasonality and modulated by interannual anomalies.     

The influence of ENSO on sea surface temperature variations in the China seas

Positive SST anomalies usually appear in remote ocean such as the China seas during an ENSO event.By analyzing the monthly data of HadISST from 1950 to 2007,it shows that the interannual component of SST anomalies peak approximately 10 months after SST anomalies peak in the eastern equatorial Pacific.As the ENSO event progresses,the positive SST anomalies spread throughout the China seas and eastward along the Kuroshio extension.Atmospheric reanalysis data demonstrate that changes in the net surface heat flux entering into the China seas are responsible for the SST variability.During El Ni o,the western north Pacific anticyclone is generated,with anomalous southwester lies prevailing along the East Asian coast.This anticyclone reduces the mean surface wind speed which decreases the surface heat flux and then increases the SST.The delays between the developing of this anticyclone and the south Indian Ocean anticyclone with approximately 3–6 months cause the 2–3 months lag of the surface heat flux between the China seas and the Indian Ocean.The northwestern Pacific anticyclone is the key process bridging the warming in the eastern equatorial Pacific and that in the China seas.     

中国近海海平面变化与ENSO的关系

利用中国沿海台站潮位和中国近海及赤道太平洋的卫星测高、海表温度、风及气压资料,分析了中国近海海平面变化与ENSO的关系。分析结果表明:中国沿海海平面季节变化受ENSO影响明显,在厄尔尼诺事件期间,中国沿海海平面Sa分潮的振幅明显减小,其中年振幅的历史极小值均出现厄尔尼诺年,不同区域历史极小值出现的年份不同;另外,中国沿海Sa分潮的振幅对厄尔尼诺事件的响应与其强弱有关,在强事件中,响应区域和幅度较大,弱事件中,响应区域和幅度偏小。在厄尔尼诺年,中国沿海海平面多低于相邻年份,并且其年际变化存在明显的2~3a、4~7a、准9a、11a和准19a的周期,其中4~7a的周期在冬春季节震荡最显著,其震荡幅度接近2cm。中国近海海平面与赤道东太平洋区域的海表温度年际变化之间存在反相关关系,其相关系数为-0.42;同时与Nio4和Nio3.4指数序列也呈现反相关关系。针对典型的1997/1998年尼诺事件发生前后的风场和气压场分析发现,尼诺发生前的冬半年,冬季风偏强,气压梯度加强,中国沿海海平面偏低;到了厄尔尼诺的盛期,出现较强的南风异常,气压梯度反向,季风转向,过渡到了厄尔尼诺事件的衰减期,为拉尼娜事件做准备,此时海平面偏高。     

Comparative characteristics of the sea surface temperature derived from data compiled by the Russian Hydrometeorological Centre and US National Meteorological Centre,in application to the El Niño event in 1991–1993

This paper provides calculations and an interpretation of the mean annual sea surface temperature (SST) anomalies and the characteristics of the SST annual harmonic in the equatorial Pacific Ocean from April 1991 to February 1993, using data provided by the Russian Hydrometeorological Centre (RHC) and the US National Meteorological Centre (NMC). The magnitude of the mean annual anomalies and the SST annual harmonic amplitude in typical El Niño Southern Oscillation (ENSO) regions, as derived from the RHC data set, are shown to be smaller than their NMC data-based counterparts, due to the considerable spatial smoothing of the original data provided by the RHC. However, in terms of quality, the two data sets agree well with one another, and quantitative discrepancies are rarely larger than the standard error of mean monthly SST determination.Translated by Vladimir A. Puchkin.     

The relationship between the canonical ENSO and the phase transition of the Antarctic oscillation at the quasi-quadrennial timescale

A correlation analysis is performed to investigate the relationship between El Nino-Southern Oscillation (ENSO) and the Antarctic oscillation (AAO) at the quasi-quadrennial (QQ) timescale.It is found that the cold tongue index (CTI) and the AAO index (AAOI) are negatively correlated with about a 7-month lead-time,while they are positively correlated with about a 15-month lag-time.To further explore this relationship,complex empirical orthogonal function analysis is employed in the QQ sea level pressure (SLP) anomalies from 1951 to 2002.The results indicate that,during the ENSO cycle,there exists one kind of global tropical wave of wavenumber 1 (GTW1) propagating eastward.With the traveling of GTW1,the tropical SLP anomaly tends to intrude into the southern mid-latitudes.Accordingly,three strong signals travel synchronously along the circumSouth-Pacific path,and a relatively weak signal extends eastward and poleward over the South Ocean in the Atlantic-Indian Ocean sector.Following the propagation of these signals,the AAO phase tends to be reversed progressively.As a result,there exists an evident lead-lag correlation between CTI and AAOI.It can be concluded that ENSO plays a key role in the phase transition of AAO at the QQ timescale.It is also noticed that this regular relationship is only evident in the canonical ENSO events,for which sea surface temperature (SST) anomalies extend westward from the tropical eastern Pacific.On the other hand,the similar relationships are not found among those atypical ENSO events for which SST anomalies spread eastward from the central Pacific,such as the 1982-1983 ENSO event.     

ENSO indices from sea surface salinity observed by Aquarius and Argo

Analysis of the first 26 months of data from the Aquarius satellite confirms the existence of a sharp sea surface salinity (SSS) front along the equator in the western equatorial Pacific. Following several earlier studies, we use the longitudinal location of the 34.8-psu isohaline as an index, termed Niño-S34.8, to measure the zonal displacement of the SSS front and consequently the eastern edge of the western Pacific warm pool. The on-going collection of the Array for Real-time Geostrophic Oceanography (ARGO) program data shows high correlations between Niño-S34.8 and the existing indices of El Niño, suggesting its potential important role in ENSO evolution. Further analysis of the ARGO data reveals that SSS variability in the southeastern tropical Pacific is crucial to identify the type of El Niño. A new SSS index, termed the southeastern Pacific SSS index (SEPSI), is defined based on the SSS variability in the region (0°–10°S, 150°–90°W). The SEPSI is highly correlated with the El Niño Modoki index, as well as the Trans-Niño index, introduced by previous studies. It has large positive anomalies during central Pacific El Niño or El Niño Modoki events, as a result of enhanced zonal sea surface temperature gradients between the central and eastern tropical Pacific, and can be used to characterize the type of El Niño. The processes that possibly control these SSS indices are also discussed.     

两类厄尔尼诺事件期间热带大气GILL响应分析

分析了1979-2018年两类厄尔尼诺事件期间月平均热带太平洋海面温度(sea surface temperature,SST)异常、对流降水异常、大气环流异常等特征,发现东部型、中部型厄尔尼诺期间海洋及大气加热场并不是赤道对称,赤道以南热源强度大于赤道以北.大气对热源的响应表现在:1)低层在大气热源西侧出现南、北半球...     

全球气候模式中赤道东太平洋海表温度的季节循环：从CMIP5到CMIP6

The sea surface temperature(SST) seasonal cycle in the eastern equatorial Pacific(EEP) plays an important role in the El Ni?o–Southern Oscillation(ENSO) phenomenon. However, the reasonable simulation of SST seasonal cycle in the EEP is still a challenge for climate models. In this paper, we evaluated the performance of 17 CMIP6 climate models in simulating the seasonal cycle in the EEP and compared them with 43 CMIP5 climate models. In general, only CESM2 and SAM0-UNICON are able to successfully capture the annual mean SST characteristics,and the results showed that CMIP6 models have no fundamental improvement in the model annual mean bias.For the seasonal cycle, 14 out of 17 climate models are able to represent the major characteristics of the observed SST annual evolution. In spring, 12 models capture the 1–2 months leading the eastern equatorial Pacific region 1(EP1; 5°S–5°N, 110°–85°W) against the eastern equatorial Pacific region 2(EP2; 5°S–5°N, 140°–110°W). In autumn,only two models, GISS-E2-G and SAM0-UNICON, correctly show that the EP1 and EP2 SSTs vary in phase. For the CMIP6 MME SST simulation in EP1, both the cold bias along the equator in the warm phase and the warm bias in the cold phase lead to a weaker annual SST cycle in the CGCMs, which is similar to the CMIP5 results. However,both the seasonal cold bias and warm bias are considerably decreased for CMIP6, which leads the annual SST cycle to more closely reflect the observation. For the CMIP6 MME SST simulation in EP2, the amplitude is similar to the observed value due to the quasi-constant cold bias throughout the year, although the cold bias is clearly improved after August compared with CMIP5 models. Overall, although SAM0-UNICON successfully captured the seasonal cycle characteristics in the EEP and the improvement from CMIP5 to CMIP6 in simulating EEP SST is clear, the fundamental climate models simulated biases still exist.     

Sea level response to ENSO along the central California coast: how the 1997–1998 event compares with the historic record

Long-term monthly sea level and sea surface temperature (SST) anomalies from central California show that during winter months, positive anomalies are associated with El Niño events and the negative ones with La Niña events. There is no significant impact on monthly mean anomalies associated with Pacific decadal oscillations, although there is a tendency for more extreme events and greater variance during positive decadal oscillations. The very strong 1997–1998 El Niño was analyzed with respect to the long-term historic record to assess the forcing mechanisms for sea level and SST. Beginning in the spring of 1997, we observed several long-period (>30 days) fluctuations in daily sea level with amplitudes of over 10 cm at San Francisco, California. Fluctuations of poleward long-period alongshore wind stress anomalies (AWSA) are coherent with the sea level anomalies. However, the wind stress cannot entirely account for the observed sea level signals. The sea level fluctuations are also correlated with sea level fluctuations observed further south at Los Angeles and Tumaco, Columbia, which showed a poleward phase propagation of the sea level signal. We suggest that the sea level fluctuations were, to a greater degree, forced by the passage of remotely generated and coastally trapped waves that were generated along the equator and propagated to the north along the west coast of North America. However, both local and remote AWSA can significantly modulate the sea level signals. The arrival of coastally trapped waves began in the spring of 1997, which is earlier than previous strong El Niño events such as the 1982–1983 event.