On the Ningaloo Niño/Niña

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.     

El Niño teleconnections in CMIP5 models

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.     

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.     

South-coast cyclone in Japan during El Niño-caused warm winters

La Niña conditions during boreal winter sometimes brings excessive snowfall in Japan, especially on the East Sea/Sea of Japan coastal and mountain areas through intensified northwesterly cold winds caused by La-Niña related atmospheric teleconnection. Meanwhile, snowfall events also increase in the Pacific coast area of Japan during the El Niño state due to extratropical cyclones passing along the south coast of Japan (hereafter referred to as South-coast cyclone). In the present study, we investigated year-to-year snowfall/rainfall variations based on meteorological station data and cyclone tracks identified by using the Japanese 55-year Reanalysis. The result clearly indicates increase of the South-coast cyclone during El Niño-developing winters, which is consistent with excessive snow-fall in the northern part of the Pacific coast. Strong subtropical jet hampers cyclogenesis due to less vertical interaction through the trapping of upper-level eddies. During El Niño-developing winters, the subtropical jet is weakened over East Asia, indicating dynamic linkage to increased cyclone frequency. In addition to this, both the deepening of the upper-tropospheric trough over East Asia and anomalous low-tropospheric northwest anticyclones extending from the Philippines toward Japan are also consistent with the enhancement of cyclogenesis over the East China Sea as well as warm winter in Japan.     

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...     

Why were some La Niñas followed by another La Niña?

This paper investigates why some La Niña events are followed by another La Niña and some others are not. We propose two preconditions that result in continuation of a La Niña. The first one is that La Niña must be a strong event (a major La Niña). This ensures that the reflected Rossby wave signal at the eastern boundary of the Pacific has a strong westward propagating cold ocean temperature anomaly over the off-equatorial region. The off-equator cold anomaly may not be conducive to the equatorial recharge process, and as a result, may favor the persistence of cold ocean subsurface temperature anomaly and prevent the transition from La Niña to El Niño. The second precondition is whether there are eastward propagating downwelling Kelvin waves during the decay phase of a major La Niña. Eastward propagating downwelling Kelvin waves could lead to demise for a tendency for a follow-up La Niña. The equatorial Kelvin wave activities are associated with fluctuations of surface wind in the equatorial far-western Pacific. The analysis suggests that both the surface wind in the equatorial far-western Pacific and the recharge/discharge of the equatorial Pacific are indicators for occurrence or no occurrence of a follow-up La Niña event.     

Spatial variability and mechanisms underlying El Niño-induced droughts in Mexico

The El Niño Southern Oscillation plays a key role in modulating interannual rainfall variability in Mexico. While El Niño events are linked to drought in Mexico, uncertainty exists about the spatial pattern and causal mechanisms behind El Niño-induced drought. We use lead/lag correlation analysis of rainfall station data to identify the spatial pattern of drought associated with the summer before, and the spring following, the peak of warm SST anomalies in the eastern equatorial Pacific. We also use atmospheric fields from the North American Regional Reanalysis to calculate the anomalous moisture budget and diagnose the mechanisms associated with El Niño-induced drought in Mexico. We find that reduced rainfall occurs in Mexico in both the summer before and the spring after a peak El Niño event, especially in regions of climatologically strong convection. The teleconnection in the developing phase of El Niño is primarily driven by changes in subsidence resulting from anomalous convection in the equatorial Pacific. The causes of drought during the decaying phase of El Niño events are varied: in some years, descent anomalies dominate other moisture budget terms, while in other years, drying of the boundary layer on the Mexican plateau is important. We suggest that the latter may result from the interaction of weakened southeasterly winds in the Intra-Americas Sea with high topography along the Atlantic coast of Mexico. Weakened winds are likely driven by a reduced sea level pressure gradient between the Atlantic and the Pacific. Changes in easterly wave activity may contribute to drought in the developing phase of El Niño, but may be less important in the decaying phase of El Niño.     

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.     

A plausible reason for changes in El Niño parameters in the 2000s

An attempt is made to find a plausible reason for the weakening of the interrelation between the variability in wind and water volume in the tropical warm pool in the western equatorial Pacific and the onset of El Niño–Southern Oscillation event (ENSO). It is demonstrated that variability in the atmospheric dynamics near the Drake Passage can affect the ENSO development. The weakening of the interrelation between ENSO and the variability in wind together with water volume in the tropical warm pool is caused by the fact that the processes of atmosphere–ocean interaction in the tropical Pacific started exerting smaller influence on the ENSO development (as compared with the processes in the Southern Ocean). This is due to warmer ocean conditions registered since the late 1990s that favored the decrease in the zonal gradient of temperature in the ocean surface layer in the tropics and led to lower atmospheric variability in the tropical Pacific whereas this variability remained the same over the Southern Ocean.     

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.     

Optimally growing initial errors of El Niño events in the CESM

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.

     

Precipitation response to La Niña and global warming in the Indo-Pacific

Recent studies have highlighted the nonlinear rainfall response to El Niño sea surface temperature (SST) events in the Indo-Pacific region and how this response might change over coming decades. Here we investigate the response to La Niña SST anomalies with and without global warming by performing idealised SST-forced experiments with an atmospheric general circulation model. The La Niña SST anomaly is multiplied by a factor \(1 \le \alpha \le 4\) and added to climatological SSTs. Similar experiments using El Niño SST anomalies were previously performed, in which large nonlinearities in the precipitation response were evident. We find that: (i) Under current climatic conditions, as \(\alpha\) increases, the precipitation responds in three ways: the intertropical convergence zone (ITCZ) dries and moves poleward, the maximum precipitation along the equator moves west, and the South Pacific convergence zone (SPCZ) narrows, intensifies, and elongates. For weak ( \(\alpha = 1\) ) La Niña events, the precipitation anomalies approximately mirror those from the El Niño events along the ITCZ and SPCZ, though there are some marked differences in the central-eastern Pacific. For stronger La Niña events ( \(\alpha > 1\) ), precipitation responds nonlinearly to SST anomalies, though the nonlinearities are smaller and differ spatially from the nonlinearities in the El Niño runs. (ii) The addition of a global warming SST pattern increases rainfall in the western Pacific and SPCZ, enhances the narrowing of the SPCZ, and increases the nonlinear response in the western Pacific. However, large La Niña events reduce the impact of global warming along the central-eastern equatorial Pacific as the global warming and La Niña SST anomalies have opposite signs in that region. (iii) The response to La Niña SST anomalies is driven primarily by changes in the atmospheric circulation, whereas the response to the global warming SST pattern is mainly driven by increases in atmospheric moisture. (iv) Large changes in La Niña-driven rainfall anomalies can occur in response to global warming, even if the La Nina SST anomalies relative to the warmer background state are completely unchanged.     

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.     

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 Southern Oscillation and economic growth in the developing world

The El Niño Southern Oscillation (ENSO) affects weather around the globe, particularly in regions where developing countries typically lie. These countries are known to be most vulnerable to weather anomalies, and ENSO thereby has the potential to influence their economic growth. In this study, we investigate the effect of ENSO on economic growth in 69 developing countries, using annual data from 1961 to 2015. We find regime-dependent nonlinearity in the growth response to ENSO shocks. An El Niño event, equivalent to a 1 °C deviation in sea surface temperatures in the Niño3.4 region of the equatorial Pacific, results in one-to-two percent annual growth reduction during the El Niño regime, but the effect is absent during the La Niña regime. In addition, we find that the effect of El Niño is twice-as-large in the tropics relative to temperate areas, and particularly pronounced in Africa and Asia-Pacific. The findings of this study have two important implications. From the modeling standpoint, we find that the growth impacts of ENSO shocks are nonlinear, and vary across regions and climatic zones. From the policy-making standpoint, our findings suggest opportunities for short-term adjustments to climate shock management and international aid programs, depending on the existing state and the intermediate-term patterns of the ENSO cycle.     

Nonlinear precipitation response to El Niño and global warming in the Indo-Pacific

Precipitation changes over the Indo-Pacific during El Niño events are studied using an Atmospheric General Circulation Model forced with sea-surface temperature (SST) anomalies and changes in atmospheric CO₂ concentrations. Linear increases in the amplitude of the El Niño SST anomaly pattern trigger nonlinear changes in precipitation amounts, resulting in shifts in the location and orientation of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). In particular, the maximum precipitation anomaly along the ITCZ and SPCZ shifts eastwards, the ITCZ shifts south towards the equator, and the SPCZ becomes more zonal. Precipitation in the equatorial Pacific also increases nonlinearly. The effect of increasing CO₂ levels and warming SSTs is also investigated. Global warming generally enhances the tropical Pacific precipitation response to El Niño. The precipitation response to El Niño is found to be dominated by changes in the atmospheric mean circulation dynamics, whereas the response to global warming is a balance between dynamic and thermodynamic changes. While the dependence of projected climate change impacts on seasonal variability is well-established, this study reveals that the impact of global warming on Pacific precipitation also depends strongly on the magnitude of the El Niño event. The magnitude and structure of the precipitation changes are also sensitive to the spatial structure of the global warming SST pattern.