Simulation of the response of the North Pacific Ocean to the anomalous atmospheric circulation associated with El Niño

During El Niño events when positive sea surface temperature (SST) anomalies form in the equatorial Pacific, SST anomalies also tend to develop in the North Pacific. This study attempts to model and explain the large-scale features of the observed SST anomaly field in the North Pacific during the fall and winter of the El Niño year. The experiment design consists of a mixed layer ocean model of the North Pacific which is forced by atmospheric surface fields from two sets of Community Climate Model (CCM) integrations: the El Niño set with prescribed positive SST anomalies in the tropical Pacific; and the control set which is obtained from an extended CCM integration with prescribed climatological SSTs. The response of the midlatitude ocean to atmospheric surface fields associated with El Niño is obtained by compositing each set of model integrations (El Niño and Control) and then taking the difference between the composites. The ocean model is able to reproduce the general features of the observed midlatitude SST anomaly pattern: warm water in the northeast Pacific and an elliptically shaped cold pool in the central Pacific. In these regions, a large fraction of the temperature anomalies are significant at the 95% level as indicated by a two tailed t-test. The ocean temperature anomalies simulated by the model are primarily caused by changes in the sensible and latent heat flux and to a lesser extent the longwave radiation flux. Entrainment of cold water from below the mixed layer also influences ocean temperatures. However, the entrainment anomaly pattern has a complex spatial structure which does not always coincide with the simulated mixed layer temperature anomalies.This paper was presented at the International Conference on Modelling of Global Climate Change and Variability, held in Hamburg 11–15 September 1989 under the auspices of the Meteorological Institute of the University of Hamburg and the Max Planck Institute for Meteorology. Guest Editor for these papers is Dr. L. Dümenil     

The influence of climate regime shift on ENSO

Observations indicated that for the El Niño/Southern Oscillation (ENSO) there have been eastward displacements of the zonal wind stress (WS) anomalies and surface heat flux (short wave heat flux and latent heat flux) anomalies during El Niño episodes in the 1981–1995 regime relative to the earlier regime of 1961–1975 (without corresponding displacements during La Niña episodes). Our numerical experiments with the Zebiak–Cane coupled model generally reproduced such displacements when the model climatological fields were replaced by the observed climatologies [of sea surface temperature (SST), surface WS and surface wind atmospheric divergence] and simulated climatologies (of oceanic surface layer currents and associated upwelling) for the 1981–1995 regime. Sensitivity tests indicated that the background atmospheric state played a much more important role than the background ocean state in producing the displacements, which enhanced the asymmetry between El Niño and La Niña in the later regime. The later regime climatology state resulted in the eastward shifts in the ENSO system (WS and SST) only during El Niño, through the eastward shift of the atmosphere convergence heating rate in the coupled model. The ENSO period and ENSO predictability were also enhanced in the coupled model under the later regime climatology. That the change in the mean state of the tropical Pacific atmosphere and ocean after the mid 1970s could have produced the observed changes in ENSO properties is consistent with our findings.     

Coupled variability and air-sea interaction in the South Atlantic Ocean

A total of 52 years of data (1949–2000) from the NCEP/NCAR reanalysis are used to investigate mechanisms involved in forcing and damping of sea surface temperature (SST) variability in the South Atlantic Ocean. Organized patterns of coupled ocean–atmosphere variability are identified using EOF and SVD analyses. The leading mode of coupled variability consists of an SST pattern with a strong northeast–southwest gradient and an SLP monopole centered at 15°W, 45°S. The anomalous winds associated with this monopole generate the SST pattern through anomalous latent heat flux and mixed layer deepening. Other heat flux components and anomalous Ekman transport play only a secondary role. Once established, the SST pattern is attenuated through latent heat flux. The higher SST modes are also induced by anomalous winds and destroyed by latent heat flux. It thus appears that the coupled variability in the South Atlantic Ocean consists of atmospheric circulation anomalies that induce SST anomalies through anomalous latent heat fluxes and wind-induced mixed layer deepening. These SST anomalies are destroyed by latent heat flux with no detectable systematic feedback onto the atmospheric circulation. Atmospheric variability in the South Atlantic is found to be largely independent of that elsewhere, although there is a weak relation with ENSO (El Niño-Southern Oscillation).     

Impacts of different types of El Niño and La Niña on northern tropical Atlantic sea surface temperature

The present study investigates the dependence of the northern tropical Atlantic (NTA) sea surface temperature (SST) response to El Niño and La Niña events on the decay time and amplitude of tropical Pacific SST anomalies. It is found that the NTA SST response to La Niña events displays a notable difference between late and early decaying events, similar to that in response to El Niño events, but with a weaker signal. Latent heat flux is a dominant term in the NTA SST change in preceding winter-early spring in both El Niño and La Niña events and in the difference of the NTA SST anomaly between late and early decaying El Niño and La Niña events. The zonal and meridional advections have an opposite effect on the NTA SST warming in late decaying El Niño events. Although the warming in the NTA region is similar in late decaying moderate and strong El Niño events, the distribution of the SST anomalies in the mid-latitude North Atlantic Ocean shows a notable difference between the two types of late decaying El Niño events. The SST anomalies also display difference in the early decaying weak and moderate El Niño events. Surface heat flux differences are largely attributed to wind differences.     

Influence of sea surface temperatures on air temperatures in the tropics

Interannual variations of tropical tropospheric temperatures are closely related to sea surface temperature (SST) changes in the tropical eastern Pacific (TEP). This study investigated the physical mechanisms for such an air-sea interrelationship. SSTs and latent heat flux were analyzed to find the unique properties of their variations during El Niño. A Gill-type model was used to investigate how a local heat source communicates with the entire tropics. Radiative fluxes in the tropics were evaluated to search for the factors limiting air temperature increases when warm SSTs remain in the TEP. We found that interannual variabilities of SST and latent heat flux are dominated by the variations in the TEP region. The SST variations there have three unique properties that allow the ocean to influence the atmosphere effectively: large magnitude, long persistence, and spatial coherence. The Gill-type model shows that a local heat source can warm the entire tropical troposphere when the heat source is near the equator. Released latent heat in the heat source region and forced adiabatic subsidence elsewhere in the tropics warm the atmosphere. As a result, a local heat source warms the entire tropical strip. The forced subsidence depresses clouds, allowing more infrared radiation to leave the atmosphere and preventing further atmospheric warming when warm SSTs remain in the TEP. This finding is verified by reanalysis data from the National Centers for Environmental Prediction.     

春季北大西洋三极型海温异常变化及其与NAO和ENSO的联系

利用1951—2016年HadISST逐月海表温度(Sea Surface Temperature,SST)资料,NCEP/NCAR再分析资料以及1958—2016年美国伍兹霍尔海洋研究所(Woods Hole Oceanographic Institution,WHOI)提供的OAFlux数据集,运用经验正交函数分解(Empirical Orthogonal Function,EOF)和偏相关分析等统计方法,研究了春季北大西洋海温异常的主要特征及其与春季NAO和前期冬季ENSO联系。结果表明:春季北大西洋海温异常EOF的第一模态是自北而南出现的三极结构的海温距平型,其方差贡献率为35.7%。春季北大西洋三极型海温异常的形成主要受到春季NAO主导作用,还受到前期冬季热带中东太平洋海温异常的影响。消除前期冬季Niňo3.4的影响后,春季北大西洋三极型海温异常指数与同期北大西洋涛动(North Atlantic Oscillation,NAO)指数的偏相关系数分别为0.50,通过了99%置信度水平的显著性检验。消除春季NAO的影响后,春季北大西洋三极型海温异常指数与前期冬季Niňo3.4指数的偏相关系数为-0.26,通过了95%信度水平的显著性检验。春季NAO正(负)位相引起的海表风场和海表湍流热通量的异常,进而激发出正(负)位相的北大西洋三极型海温异常。前期冬季ENSO事件可以引起春季大气环流异常和热带外海温异常,进而调制春季NAO对北大西洋三极型海温异常的影响。     

Air–Sea Flux Estimates And The 1997–1998 Enso Event

Bulk formulae for wind stress, sensible and latent heat flux are presented that are suitable for strong mesoscale events such as westerly wind bursts that contribute to the El Niño-Southern Oscillation (ENSO). Their exchange coefficients for heat and momentum have a simple polynomial dependence on wind speed and a linear dependence on air–sea temperature difference. The accuracy of these formulae are validated with respect to air–sea fluxes estimated using the standard algorithm adopted by the Tropical Ocean-Global AtmosphereCoupled-Ocean Atmosphere Response Experiment (TOGA COARE). The comparison ismade for observations from 96 Tropical Atmosphere Ocean (TAO) array and National Oceanographic Data Center (NODC) moorings in the equatorial and North Pacific Ocean spanning years 1990–1999. The bulk formulae are shown to have very small median root–mean-square differences with respect to the TOGA COARE estimates: 0.003 N m^-2, 1.0 W m^-2, and 10.0 W m^-2 for the wind stress, sensible heat flux, and latent heat flux, respectively.The variability of air–sea fluxes during the 1997–1998 ENSO is also examined, along with a possible relationship between air–sea fluxes and surface ocean mixed layer depth (MLD). The wind stress and latent heat flux during the 1997 El Niño are found to be greater in the warm pool of the western Pacific than in the central Pacific where the ENSO is most clearly seen. These differences disappear upon the start of La Niña. The MLD in the equatorial Pacific is found to be moderately correlated to air–sea fluxes just before the start of the 1998 La Niña and poorly correlated otherwise.     

热带太平洋海表能量收支平衡特征及其与两类ENSO事件的联系

利用月平均的HadISST海表温度、NCEP再分析资料、OAFlux海表面热通量及相关物理量资料、NCAR/NOAA云量场资料,分析了热带太平洋海表热通量的年际特征,并且进一步分析了传统El Ni?o和El Ni?o Modoki事件中湍流热通量的异常演变特征以及影响因子。在热带太平洋上,净热通量的年际变化最大振幅出现在赤道太平洋上,且主要取决于潜热通量和短波辐射通量的变化。本文还利用两类ENSO事件旺盛期海温指数对不同时期海面热通量场的偏回归分析,考察了热带太平洋海表面热通量与两类ENSO事件中海温的联系。两类海温指数对各时期热带太平洋净热通量的回归均表现为赤道太平洋上存在显著的负异常,在Ni?o3指数偏回归下的负异常范围和强度都较El Ni?o Modoki指数回归的要大,且更偏于赤道东太平洋,而旺盛期海温对同期赤道东太平洋上湍流热通量的影响最大。     

冬季北太平洋海表热通量异常和海气相互作用——基于一个全球海气耦合模式长期积分的诊断分析

利用一个全球海气耦合模式长期积分所给出的资料,分析了冬季北太平洋海表湍流热通量（潜热和感热）异常及其对海表温度（SST）异常的影响,并比较了海表热通量诸分量和海洋内部的动力学过程对SST变化的相对重要性。结果表明,冬季热带外海洋上的湍流热通量是影响SST的主要因子,但在北太平洋中部海水的平流作用也不可忽视。冬季热带外海洋向大气释放的潜热和感热通量与SST倾向（而不是SST本身）之间存在着显著的相关,这同Cayan和Reynolds等利用COADS资料和NCEP资料同化模式分析的结果是一致的。模式诊断的结果支持这样一种看法：和热带海洋不同,冬季热带外海洋上的海气相互作用主要地表现为大气对海洋的强迫作用,而不是相反。模式给出的SST倾向的第一个EOF分量及其与海平面气压场的相关特征同Wallace等从观测资料分析所得到的结果是一致的;进一步的分析表明：在冬季北太平洋的大部分区域（特别是西太平洋）,大尺度大气环流异常在很大程度上决定着SST的异常,而这种决定作用正是通过它对湍流热通量的强烈影响来实现的。     

热带印度洋海温的年际变化与ENSO

文中讨论了热带印度洋海表温度距平空间分布的年际变化与赤道中东太平洋海温的关系。EOF分析的结果表明 ,印度洋海温的变化主要存在全区符号一致的单极型和西部与东南部符号相反的偶极型 ,它们具有显著的年际变化。小波凝聚谱揭示了单极、偶极的变化与Nino3区海表温度距平存在密切关系 ,印度洋海温距平从偶极到单极的变化对应着ElNi no事件从发展到衰减的过程。平均而言 ,印度洋偶极超前Nino3区海温距平约 4个月 ,单极滞后约 6个月。整个热带印度洋 -太平洋地区海气耦合特征的演变表明 ,与ElNino从发展到衰减相联系的热带西太平洋海气耦合相互作用在印度洋海温距平从偶极到单极的演变过程中起着非常重要的作用。     

印度洋对ENSO事件的响应:观测与模拟

观测事实显示,在El Ni(n～)o期间,伴随着赤道中东太平洋表层海温(SST)的升高,热带印度洋SST出现正距平.作者利用海气耦合模式模拟了印度洋对ENSO事件的上述响应,并进而讨论了其物理机制.所用模式为法国国家科研中心Pierre-Simon-Laplace 全球环境科学联合实验室(IPSL)发展的全球海气耦合模式.该模式成功地控制了气候漂移,能够合理再现印度洋的基本气候态.观测中与ENSO相关的热带印度洋SST变化,表现为全海盆一致的正距平,并且这种变化要滞后赤道中东太平洋SST变化大约一个季度,意味着它主要是对东太平洋SST强迫的一种遥响应,模式结果也支持这一机制,尽管模式中的南方涛动现象被夸大了,使得模拟的与ENSO相关联的SST正距平的位置南移,阿拉伯海和孟加拉湾被负距平(而不是正距平)所控制.研究表明,东太平洋主要通过大气桥影响潜热释放来影响印度洋SST变化.赤道东太平洋El Ni(n～)o事件的发展,导致印度洋上空风场异常自东而西传播;伴随着风场的变化,潜热发生相应变化,并最终导致SST异常的发生.非洲东海岸受索马里急流控制的海域,其SST的变化不能简单地利用热通量的变化来解释.证据显示,印度洋的增暖是ENSO事件发生的结果而不是其前期信号.     

Epochal changes in the seasonal evolution of tropical Indian Ocean warming associated with El Niño

The epochal changes in the seasonal evolution of El Niño induced tropical Indian Ocean (TIO) warming in the context of mid-1970s regime shift is investigated in this study. El Niño induced warming is delayed by one season in the northern TIO during epoch-2 (post mid-1970) and southern TIO during epoch-1 (pre mid-1970). Significant spatiotemporal changes in TIO (especially in the north) warming are apparent during the developing phase of El Niño. The ocean dynamics is the major driver in the basin wide warming during epoch-2 whereas heat fluxes are the dominant processes during epoch-1. Strong coupling between thermocline and sea surface temperature (SST) in epoch-2 indicates that El Niño induced oceanic changes are very significant in the seasonal evolution of basin-wide warming. The thermocline-SST coupling is strengthened by the upward propagating subsurface warming in epoch-2. The westward propagating barrier layer over southern TIO supports persistence of warm SST (over southwest TIO in epoch-2), which in turn induce spring asymmetric mode in winds and precipitation. The asymmetric wind pattern and persistent subsidence over maritime continent are primarily responsible for stronger spring warming in epoch-2. The strong east equatorial Indian Ocean cooling in epoch-2 is mainly driven by coastal upwelling over Java–Sumatra coast, whereas in epoch-1 the weak cooling is controlled by the latent heat flux. The spatiotemporal changes in TIO SST warming and their evolution have strong impact on atmospheric circulation and rainfall distribution over the Indian Oceanic rim through local air–sea interaction.     

中东太平洋ITCZ对两类厄尔尼诺SST异常的敏感性试验

利用MPAS-A(The Model for Prediction Across Scales-Atmosphere)模式设计了中东太平洋热带辐合带CEP-ITCZ(Intertropical Convergence Zone over Central and Eastern Pacific)对两类厄尔尼诺SST(Sea Surface Temperature)异常的敏感性试验,通过试验结果与两类厄尔尼诺年实际大气异常的对比,初步解释了CEP-ITCZ在两类厄尔尼诺年产生不同异常的可能原因。通过CP-EL试验发现,热带太平洋SST异常的第一模态会使中东太平洋低层风场辐合增强,但对辐合带的位置影响不大,与中部型厄尔尼诺对CEP-ITCZ的影响基本一致。通过EP-EL试验发现,热带太平洋SST异常的第二模态会使中东太平洋低层风场产生较大异常,辐合带中心向南移动,辐合带明显减弱增宽,与东部型厄尔尼诺对CEP-ITCZ的影响基本一致。     

Sea surface temperature associations with the late Indian summer monsoon

Recent gridded and historical data are used in order to assess the relationships between interannual variability of the Indian summer monsoon (ISM) and sea surface temperature (SST) anomaly patterns over the Indian and Pacific oceans. Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoon season (June–September) are strongly influenced by rainfall and circulation anomalies observed during August and September, or the late Indian summer monsoon (LISM). Anomalous monsoons are linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-west Walker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISM circulation is weakened and shifted eastward during weak ISM years. Therefore, we focus on the predictability of the LISM. Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in the southeastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomalies are mainly linked to south Indian Ocean dipole events, studied by Besera and Yamagata (2001) and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies are highly persistent and affect the northwestward translation of the Mascarene High from austral to boreal summer. The southeastward (northwestward) shift of this subtropical high associated with cold (warm) SST anomalies off Australia causes a weakening (strengthening) of the whole monsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, it is suggested that the Mascarene High interacts with the underlying SST anomalies through a positive dynamical feedback mechanism, maintaining its anomalous position during the LISM. Our results also explain why a strong ISM is preceded by a transition in boreal spring from an El Niño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warm SST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in the development of a strong ISM by strengthening the local Hadley circulation during the LISM. On the other hand, a developing La Niña event in boreal spring and summer may also enhance the east–west Walker circulation and the monsoon as demonstrated in many previous studies.     

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。     

中国东北地区冬季气温趋势及反相模态分析

利用1951—2010年我国东北地区共97个台站的逐月气温资料,应用trend-EOF和EOF分析方法研究了我国东北地区冬季气温的趋势模态和反相模态及其影响因子。trend-EOF的结果表明,东北地区均为一致型的趋势变化,trend-PC1有明显的年代际周期变化和更长期的上升趋势变化特征。去除全球变暖信号后的EOF分析结果表明:第一模态仍为全区一致型的空间分布,而第二模态的空间分布呈现南北反相型的分布特征,是东北冬季气温变化模态中极为重要的一部分,对应的时间序列有明显的年际周期变化,前两个模态可以解释总方差80%以上的变化。东北地区全区一致的上升趋势是在全球变暖大背景下发生的,既是对全球变暖的局地响应,同时全球变暖也使大气环流发生了变化,西伯利亚高压减弱,纬向环流增强,导致了东北地区冬季气温全区的上升趋势。赤道东太平洋的异常海温对第二模态的出现具有一定的预示意义,当太平洋出现典型的厄尔尼诺年海温距平场分布特征时,东北南部较常年偏暖,北部地区却较常年偏冷。     

A note on the simulation of winter monsoon anomalies during an El Niño year

Summary This paper presents the results of the Florida State University atmospheric general circulation model that addresses the impact of sea surface temperature anomalies on an El Niño year. Northern Hemisphere winter season simulation. Specifically, our interest is in the simulation of seasonal winter monsoonal rainfall, the planetary scale divergent motions and the westerly wind anomalies of an El Niño year.The El Niño episode of 1982–1983 was interesting due to its higher than average amplitude and its overall evolution. By late 1982 the anomalous circulations associated with the sea surface temperature forcing had begun to take shape even though the anomalies did not attain their peak amplitude until February 1983. The atmosphere-ocean teleconnections set up a strong pattern of geopotential height anomalies during the Northern Hemisphere winter that coincides with El Niño conditions in the tropical Pacific Ocean.Wallace and Gutzler (1981) defined a Pacific North American (PNA) teleconnection pattern index based on data from within this region. The El Niño episode of 1982–1983 has been shown to be strong via the PNA Index and illustrates an importance for climate models to correctly simulate these teleconnections. The importance of the forced anomalies can be seen in the long-range forecasting of conditions over North America as well as the winter monsoon intensity and location.In this study, we utilize a general circulation model with a resolution of triangular truncation at 42 waves to investigate the effects of prescribed sea surface temperature anomalies. We are able to simulate the majority of the large-scale atmospheric response although on regional climatic scales some phase shifts seem apparent.With 7 Figures     

北太平洋海温Victoria模态与ENSO年际关系的非对称特征

利用美国NOAA海表温度资料,重点分析了北太平洋海温异常EOF第二模态Victoria模态(VM)与ENSO年际关系的非对称特征.研究发现,VM和ENSO在年代际尺度上相关性较弱,而在年际尺度上有很好的相关关系,两者同期为负相关,VM超前1 a为正相关.然而,正负VM事件与ENSO冷暖位相在年际尺度上的联系存在着一定的...     

Surface flux response to interannual tropical Pacific sea surface temperature variability in AMIP models

 A systematic comparison of observed and modeled atmospheric surface heat and momentum fluxes related to sea surface temperature (SST) variability on interannual time scales in the tropical Pacific is conducted. This is done to examine the ability of atmospheric general circulation models (AGCMs) in the Atmospheric Model Intercomparison Project (AMIP) to simulate the surface fluxes important for driving the ocean on interannual time scales. In order to estimate the model and observed response to such SST variability, various regression calculations are made between a time series representing observed ENSO SST variability in the tropical Pacific and the resulting surface flux anomalies. The models exhibit a range of differences from the observations. Overall the zonal wind stress anomalies are most accurately simulated while the solar radiation anomalies are the least accurately depicted. The deficiencies in the solar radiation are closely related to errors in cloudiness. The total heat flux shows some cancellation of the errors in its components particularly in the central Pacific. The performance of the GCMs in simulating the surface flux anomalies seems to be resolution dependent and low-resolution models tend to exhibit weaker flux responses. The simulated responses in the western Pacific are more variable than those of the central and eastern Pacific but in the west the observed estimates are less robust as well. Further improvements in atmospheric GCM flux simulation through better physical parametrization is clearly required if such models are to be used to their full potential in coupled modeling and climate forecasting. Received: 24 August 1999 / Accepted: 11 September 2000     

Remotely forced variability in the tropical Atlantic Ocean

An ensemble of eight hindcasts has been conducted using an ocean-atmosphere general circulation model fully coupled only within the Atlantic basin, with prescribed observational sea surface temperature (SST) for 1950–1998 in the global ocean outside the Atlantic basin. The purpose of these experiments is to understand the influence of the external SST anomalies on the interannual variability in the tropical Atlantic Ocean. Statistical methods, including empirical orthogonal function analysis with maximized signal-to-noise ratio, have been used to extract the remotely forced Atlantic signals from the ensemble of simulations. It is found that the leading external source on the interannual time scales is the El Niño/Southern Oscillation (ENSO) in the Pacific Ocean. The ENSO signal in the tropical Atlantic shows a distinct progression from season to season. During the boreal winter of a maturing El Niño event, the model shows a major warm center in the southern subtropical Atlantic together with warm anomalies in the northern subtropical Atlantic. The southern subtropical SST anomalies is caused by a weakening of the southeast trade winds, which are partly associated with the influence of an atmospheric wave train generated in the western Pacific Ocean and propagating into the Atlantic basin in the Southern Hemisphere during boreal fall. In the boreal spring, the northern tropical Atlantic Ocean is warmed up by a weakening of the northeast trade winds, which is also associated with a wave train generated in the central tropical Pacific during the winter season of an El Niño event. Apart from the atmospheric planetary waves, these SST anomalies are also related to the sea level pressure (SLP) increase in the eastern tropical Atlantic due to the global adjustment to the maturing El Niño in the tropical Pacific. The tropical SLP anomalies are further enhanced in boreal spring, which induce anomalous easterlies on and to the south of the equator and lead to a dynamical oceanic response that causes cold SST anomalies in the eastern and equatorial Atlantic from boreal spring to summer. Most of these SST anomalies persist into the boreal fall season.

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