Locally and remotely forced atmospheric circulation anomalies of Ningaloo Niño/Niña

A recently identified climate mode called Ningaloo Niño (Niña) is associated with positive (negative) sea surface temperature (SST) anomalies off the west coast of Australia and negative (positive) sea level pressure (SLP) anomalies in the overlying atmosphere. By conducting a series of numerical experiments with an atmospheric general circulation model, generation mechanisms of the atmospheric circulation anomalies accompanied by Ningaloo Niño/Niña are examined. Even when SST is allowed to vary interannually only in the eastern South Indian Ocean, negative (positive) SLP anomalies are formed off the west coast of Australia in Ningaloo Niño (Niña) years, supporting the existence of local ocean–atmosphere interaction. When the model is forced by SST anomalies outside of the eastern South Indian Ocean, negative (positive) SLP anomalies are also generated in Ningaloo Niño (Niña) years owing to a Matsuno–Gill type response to atmospheric convection anomalies in the tropical Pacific. It is found that the latter impact is stronger in the current atmospheric general circulation model. Regarding climatic impacts, it is shown that Ningaloo Niño (Niña) induces wet (dry) anomalies over the northwestern part of Australia even when SST anomalies outside of the eastern South Indian Ocean are excluded from the SST forcing.     

Forecasting El Niño and La Niña events using decision tree classifier

Theoretical and Applied Climatology - The El Niño-Southern Oscillation (ENSO) phenomenon affects the global climate by changing temperature and precipitation patterns mainly in tropical...     

On the association between the recent episode of the quasi-biennial oscillation and the strong El Niño event

Theoretical and Applied Climatology - Andean tropical glaciers have shown a clear shrinkage throughout the last few decades. However, it is unclear how this general retreat is associated with...     

Statistical simulations of the future 50-year statistics of cold-tongue El Niño and warm-pool El Niño

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.     

Anomalous winter climate conditions in the Pacific rim during recent El Niño Modoki and El Niño events

Present work compares impacts of El Niño Modoki and El Niño on anomalous climate in the Pacific rim during boreal winters of 1979–2005. El Niño Modoki (El Niño) is associated with tripole (dipole) patterns in anomalies of sea-surface temperature, precipitation, and upper-level divergent wind in the tropical Pacific, which are related to multiple “boomerangs” of ocean-atmosphere conditions in the Pacific. Zonal and meridional extents of those “boomerangs” reflect their independent influences, which are seen from lower latitudes in the west to higher latitudes in the east. In the central Pacific, more moisture is transported from the tropics to higher latitudes during El Niño Modoki owing to displacement of the wet “boomerang” arms more poleward toward east. Discontinuities at outer “boomerang” arms manifest intense interactions between tropical and subtropical/extratropical systems. The Pacific/North American pattern and related climate anomalies in North America found in earlier studies are modified in very different ways by the two phenomena. The seesaw with the dry north and the wet south in the western USA is more likely to occur during El Niño Modoki, while much of the western USA is wet during El Niño. The moisture to the southwestern USA is transported from the northward shifted ITCZ during El Niño Modoki, while it is carried by the storms traveling along the southerly shifted polar front jet during El Niño. The East Asian winter monsoon related anticyclone is over the South China Sea during El Niño Modoki as compared to its position over the Philippine Sea during El Niño, causing opposite precipitation anomalies in the southern East Asia between the two phenomena.     

On the Atlantic–Pacific Niños connection: a multidecadal modulated mode

Atlantic and Pacific El Niño are the leading tropical oceanic variability phenomena at interannual timescales. Recent studies have demonstrated how the Atlantic Niño is able to influence on the dynamical processes triggering the development of the Pacific La Niña and vice versa. However, the stationarity of this interbasin connection is still controversial. Here we show for the first time that the Atlantic–Pacific Niños connection takes place at particular decades, coinciding with negative phases of the Atlantic Multidecadal Oscillation (AMO). During these decades, the Atlantic–Pacific connection appears as the leading coupled covariability mode between Tropical Atlantic and Pacific interannual variability. The mode is defined by a predictor field, the summer Atlantic Sea Surface Temperature (SST), and a set of predictand fields which represent a chain of atmospheric and oceanic mechanisms to generate the Pacific El Niño phenomenon: alteration of the Walker circulation, surface winds in western Pacific, oceanic Kelvin wave propagating eastward and impacting on the eastern thermocline and changes in the Pacific SST by internal Bjerknes feedback. We suggest that the multidecadal component of the Atlantic acts as a switch for El Niño prediction during certain decades, putting forward the AMO as the modulator, acting through changes in the equatorial Atlantic convection and the equatorial Pacific SST variability. These results could have a major relevance for the decadal prediction systems.     

Changes in El Niño and La Niña teleconnections over North Pacific–America in the global warming simulations

The change in the teleconnections of both El Niño and La Niña over the North Pacific and American regions due to a future greenhouse warming has been analyzed herein by means of diagnostics of the Intergovernmental Panel on Climate Change-Fourth Assessment Report (IPCC-AR4) coupled general circulation models (CGCMs). Among the IPCC-AR4 CGCM simulations, the composites of the eight-member multimodel ensemble are analyzed. In most CGCMs, the tropical Pacific warming due to the increase of CO₂ concentration in the atmosphere promotes the main convection centers in the equatorial Pacific associated with both El Niño and La Niña to the east. The eastward shift of the convection center causes a systematic eastward shift of not only El Niño but also La Niña teleconnection patterns over the North Pacific and America, which is demonstrated in the composite maps of 500 hPa circulation, surface temperature, and the precipitation against El Niño and La Niña, as observed in a comparison between the pre-industrial and CO₂ doubling experiments. Thus, a systematic eastward migration of convection centers in the tropical Pacific associated with both El Niño and La Niña due to a future global warming commonly causes the eastward shift of the atmospheric teleconnection patterns over the Northern Hemisphere.     

Impact of El Niño onset timing on the Indian Ocean: Pacific coupling and subsequent El Niño evolution

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.     

El Niño和La Niña事件对东亚冬、夏季风循环的影响

利用1961—2018年中国地面气象台站常规观测降水数据,探讨了不同分布型厄尔尼诺事件对中国东部地区降水持续性结构的影响,进一步分析了不同区域持续性降水变化的原因。东部型厄尔尼诺事件中北异常雨带（华北、内蒙古和东北南部）主要是由于短持续性降水频率和强度同步增大造成的,短持续性降水对该区域降水异常的贡献率超过80%;南异常雨带（长江流域）形成的原因则是长持续性降水频率和累计降水日数比例同步增多。中部型厄尔尼诺事件中的异常雨带（华北南部、东北南部、黄淮和江淮地区）形成最主要的原因是短持续性降水量异常增大,其贡献占比约为60%,其次是长持续性降水,占比约为24%;短持续性降水的增加主要表现为频率增加和强度增大,而长持续性降水增加主要发生在雨带南部,是频率和累计日数比例同时增大的结果。两类厄尔尼诺事件次年夏季,中国东部地区短持续性降水的变化对总降水量异常变化的贡献最大,在长江流域长持续性降水的增强对总降水量增加也有重要作用;异常雨带形成的最主要原因是短持续性和长持续性降水频率的明显增加。     

The El Niño stochastic oscillator

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.     

Influence of the pace of El Niño decay on tropical cyclone frequency over the western north pacific during decaying El Niño summers

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.     

Changes in the cloud properties in response to El Niño: a bivariate approach

We analyse the dependence of the cloud radiative effect (CRE) and cloud amount on mid-tropospheric pressure velocity (ω ₅₀₀) and sea surface temperature (SST) and point out the shortcomings of using these two proxies separately as means to separate cloud regimes. A bivariate approach is proposed to overcome these shortcomings and it is used to systematically investigate marine cloud properties at different spatial and time scales in the present-day (1985–2001) tropical climate. During the 1997–1998 El Niño, the greatest regional change in CRE and cloud cover coincides with the greatest local change in circulation and SST. In addition, we find that the cooling effect of the stratiform low clouds reduces at the rate of approximately 1 W/m² per percent of cloudiness reduction in the subsident cold pools of the Pacific ocean. During El Niño, the transition between different cloud regimes gives rise to opposing cloud feedbacks. The sign of the total feedback is controlled by the cloud optical thickness. More generally, we find that the largest part of the cloud response to El Niño, when averaged over the tropical Pacific, is not directly associated with ω ₅₀₀ and SST changes, so other factors must play a role as well.     

Warming in the northwestern Indian Ocean associated with the El Niño event

This paper investigates possible warming effects of an E1 Nifio event on the sea surface temperature anomaly （SSTA） in the northwestern Indian Ocean. Most pure positive Indian Ocean dipole （IOD） events （without an E1 Nifio event co-occurring） have a maximum positive SSTA mainly in the central Indian Ocean south of the equator, while most co-occurrences with an E1 Nifio event exhibit a northwest-southeast typical dipole mode. It is therefore inferred that warming in the northwestern Indian Ocean is closely related to the E1 Nifio event. Based on the atmospheric bridge theory, warming in the northwestern Indian Ocean during co-occurring cases may be primarily caused by relatively less latent heat loss from the ocean due to reduced wind speed. The deepened thermocline also contributes to the warming along the east coast of Africa through the suppressed upwelling of the cold water. Therefore, the E1 Nifio event is suggested to have a modulating effect on the structure of the dipole mode in the tropical Indian Ocean.     

Proposed scheme for modeling of ocean equatorial currents in the phase of El Niño and La Niña: Implementation of the mesoscale turbulence theory

El Niño is a phenomenon of the catastrophic increase of surface temperature in the eastern part of the Pacific Ocean. It has a significant impact to weather of the American continent and western regions of the tropical Pacific, as well as on the weather and climate of entirely the Earth. Most important factors influencing El Niño are the wind, ocean currents and slope of the water surface (and temperature resulting from these factors) at the equator in the Pacific Ocean. The paper considers results of mathematical modeling of the equatorial Pacific Ocean currents in the El Niño and La Niña phases using the theory of mesoscale turbulence. This theory has been successfully tested in modeling of global circulation of atmosphere and ocean (Arsen’yev et al., 2010) and it has been able to calculate the ocean current changes at equator under changing external conditions. It is shown that the water currents at the equator have a four-tier vertical structure. The surface trade-wind current is located above the subsurface undercurrent, below which we observe the intermediate current, turning into the equatorial deep counter flow. When El Niño begins, the currents are rearranged, change signs and sometimes merge with each other. In the phase of maximum development of the phenomenon there is a two-tier structure: (1) surface current heading the American coast is underlain (below the depth of 440 m) by (2) deep equatorial current directed to the Indonesian coast. The theoretical calculations are compared with the physical observations of ocean currents in the El Niño and La Niña phases. The obtained results indicate that the proposed mathematical apparatus makes it possible to explain the set of physical observations in the Pacific Ocean.