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1.
A stochastic model is fitted to the observed NINO3.4 time series between 1951–1995. The model is nothing more than the complex version of a first-order autoregressive process. The autocorrelation of this stochastic oscillator model is an exponentially decaying cosine, specified by three parameters: a period, a decay time, and a phase shift. It fits the observed NINO3.4 autocorrelation quite well. Anomalies during an El Niño can be characterized to a large extent by a single, irregularly oscillating, index. Equatorial wave dynamics and delayed-oscillator models have been used to explain this behaviour, and it has been suggested that El Niño might be a stable phenomenon excited by weather noise. Assuming this is the case, the stochastic oscillator has a direct physical interpretation: the parameters of the oscillation can be linked to dynamical models of the delayed-oscillator type, and the noise terms represent random influences, such as intraseasonal oscillations. Long Monte Carlo simulations with the stochastic oscillator show substantial decadal variability and variation in predictability. The observed decadal variability is comparable, except for the rather large rise in the long-term mean around 1980. The observed seasonal dependence of El Niño behaviour is not compatible with the natural variability of a stationary stochastic oscillator. Formulating the model in terms of standardized anomalies takes into account some of the seasonal dependence. A stochastic oscillator forecast model has a skill approaching that of more comprehensive statistical models and may thus serve as an appropriate baseline for the skill of El Niño forecasting systems. 相似文献
2.
Recent extensive studies have suggested that the occurrence of warm-pool El Niño has increased since the late 1970s and will increase in future climate. Occurrence frequencies of cold-tongue and warm-pool El Niño have been investigated in the observational record (1980–2006) and in the future 50 years (2007–2056) based on 100 synthetic SST datasets with estimates of statistical confidence. In the observational record, 80% of the warm-pool El Niño occurred since 1980 over a period of 27 years; only 20% of the warm-pool El Niño occurred prior to 1980 over a period of 110 years. The 100 synthetic datasets, on average, produce 142 months of cold-tongue El Niño in 2007–2056 as opposed to an average 107 months in the same length of the observational data; this is a 20.7% increase in the occurrence of cold-tongue El Niño compared with the observational period. Warm-pool El Niño occurred for 112 months in 2007–2056 as opposed to an average occurrence of 42 months in the observational record; this is 2.5 times the occurrence frequency in the 1980–2006 period in the synthetic datasets. As a result, occurrence frequencies of cold-tongue and warm-pool El Niño in the period of 2007–2056 become quite comparable to each other in the synthetic datasets. It is expected in the next 50 years that warm-pool El Niño will be nearly as frequent as cold-tongue El Niño. 相似文献
5.
Teleconnections associated with warm El Niño/southern oscillation (ENSO) events in 20 climate model intercomparison project 5 (CMIP5) models have been compared with reanalysis observations. Focus has been placed on compact time and space indices, which can be assigned a specific statistical confidence. Nearly all of the models have surface temperature, precipitation and 250 hPa geopotential height departures in the Tropics that are in good agreement with the observations. Most of the models also have realistic anomalies of Northern Hemisphere near-surface temperature, precipitation and 500 hPa geopotential height. Model skill for these variables is significantly related to the ability of a model to accurately simulate Tropical 250 hPa height departures. Additionally, most models have realistic temperature and precipitation anomalies over North America, which are linked to a model’s ability to simulate Tropical 250 hPa and Northern Hemisphere 500 hPa height departures. The skills of temperature and precipitation departures over the Northern Hemisphere and North America are associated with the ability to realistically simulate realistic ENSO frequency and length. Neither horizontal nor vertical resolution differences for either the model atmosphere or ocean are significantly related at the 95 % level to variations in El Niño simulation quality. Overall, recent versions of earlier models have improved in their ability to simulate El Niño teleconnections. For instance, the average model skills of temperature and precipitation for the Tropics, Northern Hemisphere and North America for 11 CMIP5 models are all larger than those for prior versions. 相似文献
6.
A relation between the timing of the El Niño onset and its subsequent evolution is examined by emphasizing its association with the Indian Ocean (IO) SST variation. Two types of El Niño events based on the timing of their onset are classified and their characteristics are examined and compared. In general, spring onset (SP) events grow greater in magnitude and their evolutions have a faster transition. On the contrary, summer onset (SU) events are relatively weaker in magnitude and have a slower transition. Moreover, in contrast to the SU events, the SP events have a strong tendency for accompanying an IO dipole and basin-wide type of warming pattern in the El Niño developing and mature phases, respectively. It is demonstrated here that the distinctive evolutions in transition phase of the two events are resulted from the difference in IO SST. The warm IO SST in the SP El Niño event, lead an anomalous easterlies over the western Pacific, which forces a fast termination of El Niño events. 相似文献
7.
Theoretical and Applied Climatology - The El Niño-Southern Oscillation (ENSO) phenomenon affects the global climate by changing temperature and precipitation patterns mainly in tropical... 相似文献
9.
Previous studies suggested that the wintertime SST in the North Pacific that are generated by the concurrent North Pacific Oscillation (NPO) are able to force El Niño during subsequent winter via the so-called ‘seasonal footprinting mechanism’ (SFM). We examine how the NPO effectively generates the El Niño via the SFM in the observations and models. The occurrence ratio for El Niño under conditions of NPO forcing during the previous winters is about 41 % for the period of 61 years (1949–2009), indicating that the atmospheric forcing from the mid-latitudes through the SFM does not always trigger an El Niño. We observed certain favorable conditions under which the SFM may effectively induce El Niño. We directly compared these observations with two cases: when the wintertime NPO leads to El Niño during the following winter through the SFM, and when the wintertime NPO is not followed by El Niño. Our analysis demonstrates that the spatial structures of the NPO, associated wind speed and net heat flux in the northeast Pacific, differ between the two cases. Such differences determine the existence of a footprint SST in the northeastern Pacific during the late spring and summer, which plays a key role in initiating the El Niño via the projection of westerly wind stress anomalies onto the equatorial Pacific during the same seasons. By conducting linear baroclinic model experiments, it is found that the positions of La Niña SST forcing during the previous winter are able to modify the spatial structures of the NPO, which produces favorable conditions for the El Niño during subsequent winter via the SFM. 相似文献
10.
Through an oceanic mixed-layer heat budget analysis, the dominant processes contributing to the largest decay rate (− 0.37 °C/mon) in EP El Nino, the moderate delay rate (− 0.22 °C/mon) in CP El Nino and the smallest decay rate (0.13 °C/mon) in La Nina, are identified. The result shows that both dynamic (wind induced equatorial ocean waves and thermocline changes) and thermodynamic (net surface solar radiation and latent heat flux changes) processes contribute to a fast decay and thus phase transition in EP El Niño composite, whereas the thermodynamic process has less effect on the decay rate for both CP El Niño and La Niña due to the westward shift of sea surface temperature anomaly (SSTA) centers. Thus, the difference in surface wind stress forcing is critical in contributing to evolution asymmetry between CP El Niño and La Niña, while the difference in both the wind stress and heat flux anomalies contribute to evolution asymmetry between EP El Niño and La Niña. It is interesting to note that El Nino induced anomalous anticyclone over the western North Pacific is stronger and shifts more toward the east during EP El Niño than during CP El Niño, while compared to CP El Niño, the center of an anomalous cyclone during La Niña shifts further to the west. As a consequence, both EP and CP El Niño decay fast and transform into a La Niña episode in the subsequent year, whereas La Niña has a much slower decay rate and re-develops in the second year. 相似文献
11.
This study investigated the distinct responses of western North Pacific (WNP) tropical cyclone (TC) activity during different decaying El Niño summers. The El Niño events were classified into two types according to the periodicity of the ENSO cycle, with positive SST anomalies in the equatorial central-eastern Pacific maintaining positive values into the following summer as the slow decaying (SD) cases, but transforming to negative values in the following summer as the rapid decaying (RD) cases. Compared with that in SD El Niño summers, the TC occurrence frequency over the WNP is significantly lower in RD El Niño summers, led by a much weaker WNP monsoon trough with more unfavorable environmental factors for TC genesis and development. Further examination showed that the apparent warming over the tropical Indian Ocean basin and cooling over the equatorial central-eastern Pacific contribute together to an enhanced lower-tropospheric anticyclone through modulation of the descending branch of the large-scale Walker circulation over the WNP, which may play a crucial role in suppressing the TC activity during the decaying summer of RD El Niño cases. In contrast, the warming equatorial central-eastern Pacific and remote western Indian Ocean induce a weakening WNP anticyclone and less suppressed deep convection during the decaying summer of SD El Niño cases. Thus, the different evolution of SST anomalies associated with different paces of El Niño decay results in the linkage between the preceding winter El Niño and the decreased WNP TC frequency in summer being more (less) robust for RD (SD) El Niño cases.摘要本文分析了El Niño事件衰减速度的差异对衰退年夏季西北太平洋热带气旋 (tropical cyclone, TC) 频数的不同影响. 按照El Niño事件衰减速度不同, 将其划分为迅速衰减 (rapid decaying, RD) 和缓慢衰减 (slow decaying, SD) 的El Niño事件. SD (RD) El Niño事件的衰退年夏季, 赤道中东太平洋海温仍维持正异常 (衰减为负异常) . 与SD El Niño事件相比, RD El Niño事件衰退年夏季西北太平洋TC频数显著减少. 进一步的分析揭示了导致TC频数差异的大尺度环境要素, 指出热带印度洋-太平洋海温异常密切相关的西北太平洋低层反气旋异常在其中起到了关键作用. 相似文献
12.
In recent years, El Nio Modoki (a type of pseudo-El Nio) 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 Nio. In this study, EOF analysis was used to successfully separate El Nio and El Nio Modoki. The abilities of the NINO3 index, NINO3.4 index, NINO1+2 index and NINO4 index in characterizing El Nio were explored in detail. The resulting suggestion was that, comparatively, NINO3 is the op... 相似文献
13.
This paper investigates the optimal observational array for improving the initialization of El Niño-Southern Oscillation predictions by exploring the sensitive areas for target observations of two types of El Niño predictions. The sensitive areas are identified by calculating the optimally growing errors (OGEs) of the Zebiak–Cane model, as corrected by the optimal forcing vector that is determined by assimilating the observed sea surface temperature anomalies (SSTAs). It is found that although the OGEs have similar structures for different start months of predictions, the regions covered by much large errors for the SSTA component tend to locate at different zonal positions and depends on the start months. Furthermore, these regions are also in difference between two types of El Niño events. The regions covered by large errors of OGEs represent the sensitive areas for target observations. Considering the dependence of the sensitive areas on related El Niño types and the start months of predictions, the present study propose a quantitative frequency method to determine the sensitive areas for target observations associated with two types of El Niño predictions, which is expected to be applicable for both types of El Niño predictions with different start months. As a result, the sensitive areas that describe the array of target observations are presented with a reversal triangle-like shape locating in the eastern Pacific, specifically the area of 120°W–85°W, 0°S–11°S, and an extension to the west along the equator and then gathering at the 180° longitude and the western boundary. “Hindcast” experiments demonstrated that such observational array is very useful in distinguishing two types of El Niño and superior to the TAO/TRITON array. It is therefore suggested that the observational array provided in the present study is towards the optimal one and the original TAO/TRITON array should be further optimized when applied to predictions of the diversities of El Niño events. 相似文献
14.
Using both observational and reanalysis data, evolution processes of a regional climate phenomenon off Western Australia named recently “Ningaloo Niño (Niña)” are studied in detail. It is also shown that the Ningaloo Niño (Niña) has significant impacts on the precipitation over Australia. The Ningaloo Niño (Niña), which is associated with positive (negative) sea surface temperature (SST) anomalies and atmospheric anomalies off the western coast of Australia, peaks during austral summer and is classified into two types based on the difference in the evolution process. The first type called a locally amplified mode develops through an intrinsic unstable air–sea interaction off the western coast of Australia; an anomalous cyclone (anticyclone) generated by positive (negative) SST anomalies forces northerly (southerly) alongshore wind anomalies, which induce coastal downwelling (upwelling) anomalies, and enhance the positive (negative) SST anomalies further. The second type called a non-locally amplified mode is associated with coastally trapped waves originating in either the western tropical Pacific, mostly related to El Niño/Southern Oscillation, or the northern coast of Australia. Positive (negative) SST anomalies in both modes are associated with an anomalous low (high) off the western coast of Australia. The sea level pressure (SLP) anomalies in the locally amplified mode are regionally confined with a cell-like pattern and produce a sharp offshore pressure gradient along the western coast of Australia, whereas those in the non-locally amplified mode tend to show a zonally elongated pattern. The difference is found to be related to conditions of the continental SLP modulated by the Australian summer monsoon and/or the Southern Annular Mode. 相似文献
15.
The optimally growing initial errors (OGEs) of El Niño events are found in the Community Earth System Model (CESM) by the conditional nonlinear optimal perturbation (CNOP) method. Based on the characteristics of low-dimensional attractors for ENSO (El Niño Southern Oscillation) systems, we apply singular vector decomposition (SVD) to reduce the dimensions of optimization problems and calculate the CNOP in a truncated phase space by the differential evolution (DE) algorithm. In the CESM, we obtain three types of OGEs of El Niño events with different intensities and diversities and call them type-1, type-2 and type-3 initial errors. Among them, the type-1 initial error is characterized by negative SSTA errors in the equatorial Pacific accompanied by a negative west–east slope of subsurface temperature from the subsurface to the surface in the equatorial central-eastern Pacific. The type-2 initial error is similar to the type-1 initial error but with the opposite sign. The type-3 initial error behaves as a basin-wide dipolar pattern of tropical sea temperature errors from the sea surface to the subsurface, with positive errors in the upper layers of the equatorial eastern Pacific and negative errors in the lower layers of the equatorial western Pacific. For the type-1 (type-2) initial error, the negative (positive) temperature errors in the eastern equatorial Pacific develop locally into a mature La Niña (El Niño)-like mode. For the type-3 initial error, the negative errors in the lower layers of the western equatorial Pacific propagate eastward with Kelvin waves and are intensified in the eastern equatorial Pacific. Although the type-1 and type-3 initial errors have different spatial patterns and dynamic growing mechanisms, both cause El Niño events to be underpredicted as neutral states or La Niña events. However, the type-2 initial error makes a moderate El Niño event to be predicted as an extremely strong event. 相似文献
16.
Many recent studies have reported the presence of two types of El Niño events in observation: Cold Tongue (CT) El Niño and Warm Pool (WP) El Niño. We investigate the sensitivity of a model simulating two types of El Niño by changing a convective triggering parameter (Tokioka parameter). When deep convections are highly suppressed with a large Tokioka parameter, the model is capable of simulating distinct two-types of El Niño. However, the model has a problem in simulating two-types of El Niño distinctively when the Tokioka parameter is small, because the location of the maximum precipitation anomaly related to the CT El Niño is significantly shifted westward, leading to an atmospheric response pattern similar to that of the WP El Niño. Our results suggest that the mean precipitation over the eastern equatorial Pacific and the resultant zonal distribution in atmospheric feedback associated with ENSO can be one of the crucial factors for simulating two-types of El Niño. 相似文献
18.
分析了 2019年一次El Ni?o事件发展的基本特征,并探讨了该次事件背景下副高偏强偏南的原因.研究发现:赤道中东太平洋纬向流反馈偏弱和西太平洋海域区域性的东风异常,都对该次事件表现出向中部型转变的趋势有一定程度的贡献.西太平洋海域上层大气的强烈下沉运动以及对流层低层存在的异常反气旋环流在一定程度上促进了副高的发展,在该次El Ni?o事件发展期间,导致西太平洋副高向南增强,位置偏南;Hadley环流下沉支的异常进一步加强了西太平洋副高的强度.2018年期间,从东太平洋海域西传的冷性Rossby波到达西太平洋海域且维持,产生异常Rossby波响应,同时2019年5月,MJO来到印度洋海域第8位相,西太平洋海域为反气旋性环流异常,二者共同影响,有利于热带西太平洋及我国南海地区出现异常的反气旋环流,进一步促进西太平洋副高加强西伸、强度偏强、位置偏南. 相似文献
19.
The influences of strong El Niño events (1997/98 and 2015/16) on summertime near-surface ozone (O 3) concentrations over China are investigated using the GEOS-Chem model. The results show that near-surface O 3 concentrations increased by a maximum of 6 ppb (parts per billion) during the summer of the developing phase of the 1997/98 El Niño in northeastern China, mainly due to the increased chemical production related to the hot and dry conditions. Besides, the O 3 concentration increased by 3 ppb during the developing summer of both the 1997/98 and 2015/16 El Niño in southern China. It was linked to the weakened prevailing monsoon winds, which led to the accumulation of O 3 in southern China. In contrast, in the summer of the decaying phase of the two El Niño events, O 3 concentrations decreased over many regions of China when the El Niño reversed to the cooling phase. This highlights that El Niño plays an important role in modulating near-surface O 3 concentrations over China.摘要利用全球大气化学三维模式 (GEOS-Chem) 模拟研究两次强厄尔尼诺事件 (1997/98和2015/16) 对中国夏季近地面臭氧 (O 3) 浓度的影响. 结果表明1997/98年厄尔尼诺事件发展期夏季中国东北区域O 3浓度升高, 最大值超过6ppb, 这主要归因于高温晴朗低湿等气象因素导致O 3化学生成升高. 此外, 两次厄尔尼诺事件发展期夏季O 3浓度在中国南部均增加了3ppb, 这与盛行季风减弱导致中国南方O 3局地积累有关. 相反, 在两次强厄尔尼诺衰减期夏季, 中国大部分地区O 3浓度下降伴随着海温模态转变为拉尼娜事件. 这表明厄尔尼诺在调节中国近地面O 3浓度中发挥着重要作用. 相似文献
20.
Our early work (Wang and Wang in J Clim 26:1322–1338, 2013) separates El Niño Modoki events into El Niño Modoki I and II because they show different impacts on rainfall in southern China and typhoon landfall activity. The warm SST anomalies originate in the equatorial central Pacific and subtropical northeastern Pacific for El Niño Modoki I and II, respectively. El Niño Modoki I features a symmetric SST anomaly distribution about the equator with the maximum warming in the equatorial central Pacific, whereas El Niño Modoki II shows an asymmetric distribution with the warm SST anomalies extending from the northeastern Pacific to the equatorial central Pacific. The present paper investigates the influence of the various groups of El Niño events on the Indian Ocean Dipole (IOD). Similar to canonical El Niño, El Niño Modoki I is associated with a weakening of the Walker circulation in the Indo-Pacific region which decreases precipitation in the eastern tropical Indian Ocean and maritime continent and thus results in the surface easterly wind anomalies off Java-Sumatra. Under the Bjerknes feedback, the easterly wind anomalies induce cold SST anomalies off Java- Sumatra, and thus a positive IOD tends to occur in the Indian Ocean during canonical El Niño and El Niño Modoki I. However, El Niño Modoki II has an opposite impact on the Walker circulation, resulting in more precipitation and surface westerly wind anomalies off Java-Sumatra. Thus, El Niño Modoki II is favorable for the onset and development of a negative IOD on the frame of the Bjerknes feedback. 相似文献
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