El Niño–Southern Oscillation Asymmetry Research Articles

Equatorial Pacific Cold Tongue Bias degrades simulation of ENSO Asymmetry due to underestimation of strong Eastern Pacific El Niños

Abstract The El Niño-Southern Oscillation (ENSO) exhibits a considerable asymmetry in sea surface temperature anomalies (SSTa), as El Niño events tend to be stronger and centered further east than La Niña events. Here we analyze ENSO asymmetry in observations and preindustrial control integrations of 32 models participating in the Coupled Model Intercomparison Project phase 6 (CMIP6). Observations indicate a significant link between strong Eastern Pacific (EP) El Niño events, and the ENSO amplitude and asymmetry. The large CMIP6 database confirms this strong link. Most CMIP6 models suffer from an equatorial Pacific cold SST bias. This Cold Tongue bias hinders the southward migration of the ITCZ towards the equator over the eastern equatorial Pacific, which is characteristic of strong EP El Niño events in observations. Therefore, many models underestimate the eastern equatorial Pacific rainfall anomalies and simulate a wind stress feedback over the western Pacific that is too weak and too far west. As a result, the Cold Tongue bias exerts a strong control on the climate models’ ability to generate strong EP El Niño events, and therefore on the ENSO overall amplitude and asymmetry. We discuss the relevance of these results in view of a potential increase of strong EP El Niño events under global warming.

Redefined background state in the tropical Pacific resolves the entanglement between the background state and ENSO

Understanding the co-variability between the El Niño–Southern Oscillation (ENSO) and the background state in the tropical Pacific is critical for projecting future ENSO. The difficulty is rooted in a circular logic that the background state routinely defined by multi-decadal mean modulates, and is modulated by, ENSO. This circularity arises due to the asymmetry between El Niño and La Niña, resulting in a non-zero mean, referred to as the ENSO rectification effect. Here, we develop a method based on Box-Cox normalization to define the tropical Pacific background state and its associated anomalies, which removes the ENSO rectification effect and is referred to as the normalized mean state. The normalized mean state accurately quantifies ENSO-related anomalies, ENSO asymmetry, and the ENSO rectification effect. It is evident in both observations and model simulations that the normalized mean state has a clear asymmetric impact on the amplitude of ENSO. A warm background state weakens El Niño but strengthens La Niña through two key processes: the nonlinear response of precipitation to SST and oceanic zonal advection feedback. The normalized mean state successfully solves the circular reasoning fallacy resulting from ENSO asymmetry and offers a framework to study ENSO and tropical climate dynamics with far-reaching impacts on global climate.

Dominant contribution of atmospheric nonlinearities to ENSO asymmetry and extreme El Niño events

Extreme El Niño events have outsized impacts and strongly contribute to the El Niño Southern Oscillation (ENSO) warm/cold phase asymmetries. There is currently no consensus on the respective importance of oceanic and atmospheric nonlinearities for those asymmetries. Here, we use atmospheric and oceanic general circulation models that reproduce ENSO asymmetries well to quantify the atmospheric nonlinearities contribution. The linear and nonlinear components of the wind stress response to Sea Surface Temperature (SST) anomalies are isolated using ensemble atmospheric experiments, and used to force oceanic experiments. The wind stress-SST nonlinearity is dominated by the deep atmospheric convective response to SST. This wind-stress nonlinearity contributes to ~ 40% of the peak amplitude of extreme El Niño events and ~ 55% of the prolonged eastern Pacific warming they generate until the following summer. This large contribution arises because nonlinearities consistently drive equatorial westerly anomalies, while the larger linear component is made less efficient by easterly anomalies in the western Pacific during fall and winter. Overall, wind-stress nonlinearities fully account for the eastern Pacific positive ENSO skewness. Our findings underscore the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events and, more generally, ENSO asymmetry.

Efficient inference and learning of a generative model for ENSO predictions from large multi-model datasets

Historical simulations of global sea-surface temperature (SST) from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) are analyzed. A state-of-the-art deep learning approach is applied to provide a unified access to the diversity of simulations in the large multi-model dataset in order to go beyond the current technological paradigm of ensemble averaging. Based on the concept of a variational auto-encoder (VAE), a generative model of global SST is proposed in combination with an inference model that aims to solve the problem of determining a joint distribution over the data generating factors. With a focus on the El Niño Southern Oscillation (ENSO), the performance of the VAE-based approach in simulating various central features of observed ENSO dynamics is demonstrated. A combination of the VAE with a forecasting model is proposed to make predictions about the distribution of global SST and the corresponding future path of the Niño index from the learned latent factors. The proposed ENSO emulator is compared with historical observations and proves particularly skillful at reproducing various aspects of observed ENSO asymmetry between the two phases of warm El Niño and cold La Niña. A relationship between ENSO asymmetry and ENSO predictability is identified in the ENSO emulator, which improves the prediction of the simulated Niño index in a number of CMIP5 models.

El Niño and La Niña asymmetry in short-term predictability on springtime initial condition

El Niño-Southern Oscillation (ENSO) asymmetry in predictability on springtime initial condition remains unclear. From the perspective of the spring predictability barrier (SPB), this paper investigates the ENSO asymmetry in SPB and explores the potential factors that may lead to this asymmetry. Both the observation and 29 Coupled Model Intercomparison Project Phase 6 (CMIP6) models show that the spring sea surface temperature (SST) persistence is significantly higher in El Niño years than that in La Niña years, and the SPB intensity is stronger in La Niña years than that in El Niño years. Through the recharge oscillator model, observation and CMIP6 models, we demonstrate that the nonlinear wind stress response to SST anomalies in spring is the main cause of the asymmetric SPB intensity. By the mixed-layer heat budget of the tropical Pacific in the spring, we further identify that a stronger response of zonal wind stress in El Niño events can cause a stronger zonal advection feedback, which finally leads to a weaker SPB and enhances the predictability of El Niño. In contrast, the cooling SST in the spring only leads to weak easterly anomalies, the zonal advection feedback is relatively weaker, thus SPB is stronger and the predictability of La Niña is lower. From the perspective of SPB, we suggest that El Niño is more predictable than La Niña.

Changes in ENSO Characteristics in Model Simulations with Considerably Altered Background Climate States

Abstract Changes in the background climate are known to affect El Niño–Southern Oscillation (ENSO) by altering feedbacks that control ENSO’s characteristics. Here, the sensitivity of ENSO variability to the background climate is investigated by utilizing two Community Earth System Model, version 1 (CESM1), simulations in which the solar constant is altered by ±25 W m−2. The resulting stable warm and cold climate mean state simulations differ in terms of ENSO amplitude, frequency, diversity, asymmetry, and seasonality. In the warm run, ENSO reveals a larger amplitude and occurs at higher frequencies relative to the cold and control runs as well as observations. The warm run also features more eastern Pacific El Niños, an increased asymmetry, and a stronger seasonal phase locking. These changes are linked to changes in the mean state via the amplifying and damping feedbacks. In the warm run, a shallower mean thermocline results in a stronger subsurface–surface coupling, whereas the cold run reveals reduced ENSO variability due to a reduced Bjerknes feedback in accordance with a deeper mean thermocline and enhanced surface wind stress. A strong zonal advective and upwelling feedback further contribute to the large ENSO amplitude in the run with a warmer mean state. In the cold run, ENSO events are partly forced by anomalous shortwave radiation. However, in light of the large temperature contrast between the simulations of up to 6 K in the tropical Pacific, the relatively small changes in ENSO variability highlight the robustness of ENSO dynamics under vastly different climate mean states.

ENSO Asymmetry in CMIP6 Models

Abstract An interesting aspect of the El Niño–Southern Oscillation (ENSO) phenomenon is the asymmetry between its two phases. This paper evaluates the simulations of this property of ENSO by the Coupled Model Intercomparison Project phase 6 (CMIP6) models. Both the surface and subsurface signals of ENSO are examined for this purpose. The results show that the models still underestimate ENSO asymmetry as shown in the SST field, but do a better job in the subsurface. A much weaker negative feedback from the net surface heat flux during La Niña in the models is identified as a factor causing the degradation of the ENSO asymmetry at the surface. The simulated asymmetry in the subsurface is still weaker than the observations owing to a weaker dynamic coupling between the atmosphere and ocean. Consistent with the finding of a weaker dynamic coupling strength, the precipitation response to the SST changes is also found to be weaker in the models. The results underscore that a more objective assessment of the simulation of ENSO by climate models may have to involve the examination of the subsurface signals. Future improvements in simulating ENSO will likely require a better simulation of the surface heat flux feedback from the atmosphere as well as the dynamical coupling strength between the atmosphere and ocean. Significance Statement The ENSO phenomenon affects weather and climate worldwide. An interesting aspect of this phenomenon is the asymmetry between its two phases. Previous studies have reported a weaker asymmetry in the simulations by climate models. But these studies have focused on the ENSO asymmetry at the surface. Here by examining the ENSO asymmetry at the surface and the subsurface, we have found that ENSO asymmetry is better simulated in the subsurface than at the surface. We have also identified factors that are responsible for the degradation of the ENSO asymmetry at the surface as well as the remaining weakness in the subsurface, pointing out specific pathways to take to further improve ENSO simulations by coupled climate models.

Reduced ENSO Variability due to a Collapsed Atlantic Meridional Overturning Circulation

Abstract Atlantic meridional overturning circulation (AMOC) collapses have punctuated Earth’s climate in the past, and future projections suggest a weakening and potential collapse in response to global warming and high-latitude ocean freshening. Among its most important teleconnections, the AMOC has been shown to influence El Niño–Southern Oscillation (ENSO), although there is no clear consensus on the tendency of this influence or the mechanisms at play. In this study, we investigate the effect of an AMOC collapse on ENSO by adding freshwater in the North Atlantic in a global climate model. The tropical Pacific mean-state changes caused by the AMOC collapse are found to alter the governing ENSO feedbacks, damping the growth rate of ENSO. As a result, ENSO variability is found to decrease by ∼30% due to weaker air–sea coupling associated with a cooler tropical Pacific and an intensified Walker circulation. The decreased ENSO variability manifests in ∼95% less frequent extreme El Niño events and a shift toward more prevalent central Pacific El Niño than eastern Pacific El Niño events, marked by a reduced ENSO nonlinearity and asymmetry. These results provide mechanistic insights into the possible behavior of past and future ENSO in a scenario of a much weakened or collapsed AMOC. Significance Statement The Atlantic meridional overturning circulation (AMOC) has collapsed in the past and a future collapse due to greenhouse warming is a plausible scenario. An AMOC shutdown would have major ramifications for global climate, with extensive impacts on climate phenomena such as El Niño–Southern Oscillation (ENSO), which is the strongest source of year-to-year climate variability on the planet. Using numerical simulations, we show that an AMOC shutdown leads to weaker ENSO variability, manifesting in 95% reduction in extreme El Niño events, and a shift of the ENSO pattern toward the central Pacific. This study sheds light on the mechanisms behind these changes, with implications for interpreting past and future ENSO variability.

Will Increasing Climate Model Resolution Be Beneficial for ENSO Simulation?

AbstractIncreasing climate model resolution offers multifaceted benefits, such as improving modeled tropical cyclones. However, the extent to which it benefits El Niño‐Southern Oscillation (ENSO) simulation remains unknown. Here, comprehensive information on the sensitivity of ENSO performance to various resolutions is provided, based on a multi‐model and multi‐resolution ensemble of global coupled models. Overall, the reduced model biases of the equatorial sea surface temperature (SST) and precipitation mean state in higher‐resolution models may be attributed to increased oceanic resolution, and thus, better resolved eddy‐driven heat transport. ENSO spatial patterns were reproduced clearer in the eddy‐present models, likely due to the improved mean state and associated surface thermodynamic feedback. However, increasing atmospheric resolution alone deteriorates ENSO asymmetry, which may be due to the degradation of nonlinear atmospheric feedback. It remains challenging to alleviate the SST−shortwave‐flux feedback bias, which is a major source of too‐weak net heat flux feedback, irrespective of model resolution.

On the Air‐Sea Couplings Over Tropical Pacific: An Instantaneous Coupling Index Using Dynamical Systems Metrics

AbstractSea surface temperature (SST) anomalies over the tropical Pacific, such as El Niño‐Southern Oscillation (ENSO) events, can cause intensified air‐sea couplings and different climate impacts. Although there are quite a few indices describing the event magnitude, little attention is paid to directly measuring air‐sea coupling strengths. Using Dynamical Systems Metrics, we introduce an index to quantify air‐sea coupling strengths over the tropical Pacific. This coupling index explains the relatively weak impact of the super El Niño in 2015/2016, and characterizes the ENSO diversity and asymmetry. Particularly, this index can identify several unusual neutral days with prolonged strong air‐sea couplings, where SST anomaly distributions are not as homogeneous as the ENSO events, but which have caused intense climate impacts. Our results demonstrate the close relationship among air‐sea coupling strengths, SST anomalies, and coupled feedbacks, which may promote better understandings on the events diversity related to SST anomalies over the tropical Pacific.

Zonal Structure of Tropical Pacific Surface Salinity Anomalies Affects ENSO Intensity and Asymmetry

AbstractPrevious studies have revealed the potential importance of salinity in El Niño‐Southern Oscillation (ENSO) asymmetry, but the responsible mechanisms remain unclear. Here, we investigate how the zonal structure of surface salinity anomalies affects ENSO intensity and asymmetry by modifying freshwater flux in ocean general circulation model simulations. Results show that the amplitudes of El Niño and La Niña are both highly sensitive to zonal patterns of salinity anomalies, with the strongest sea surface temperature change occurring when maximum salinity anomalies are located near 170°W. Furthermore, the effect of salinity anomalies in the central Pacific on El Niño warming is larger than those in the western Pacific on La Niña cooling. Thus, the asymmetric salinity anomalies strengthen the temperature asymmetry between El Niño and La Niña. Temperature budget analysis shows that vertical mixing and entrainment due to salinity‐induced changes in stratification are a major response to different zonal patterns of freshwater flux forcing.

ENSO Amplitude Asymmetry in Met Office Hadley Centre Climate Models

Long climate simulations with the Met Office Hadley Centre General Circulation Model show weak El Niño-Southern Oscillation (ENSO) amplitude asymmetry between El Niño and La Niña phases compared with observations. This lack of asymmetry is explored through the framework of a perturbed parameter experiment. Two key hypotheses for the lack of asymmetry are tested. First, the possibility that westerly wind burst activity is biased is explored. It is found that the observed difference in wind burst activity during El Niño and La Niña tends to be underestimated by the model. Secondly, the warming due to subsurface non-linear advection is examined. While the model exhibits non-linear dynamic warming during both La Niña and El Niño, and thus a contribution to ENSO asymmetry, it is shown to be consistently underestimated in comparison with ocean reanalyses. The non-linear zonal advection term contributes most to the deficiency and the simulation of the anomalous zonal currents may be playing a key role in its underestimation. Compared with the ocean reanalyses, the anomalous zonal currents associated with ENSO are too weak in the vicinity of the equatorial undercurrent and the surface wind driven zonal currents extend too deep.

Impact of Westerly Wind Bursts on ENSO Based on a Hybrid Coupled Model: Part I – ENSO Simulation

ABSTRACT Westerly wind bursts (WWBs), usually occurring in the tropical Pacific region during the beginning and advancement of El Niño events, play a vital role in El Niño–Southern Oscillation (ENSO). In this study, we use a hybrid coupled model (HCM) for the tropical Pacific Ocean–atmosphere system to investigate the impact of WWBs on ENSO. To achieve this goal, we performed two experiments: first, the standard version of the HCM is integrated for years without prescribed WWBs events; and second, the WWBs are added to the HCM, which are constructed using observational data and WWB parameterization schemes. Results show that when WWBs are added to the HCM not only can a more realistic climatology of sea surface temperature (SST) in both spatial structure and temporal amplitudes be generated but also better ENSO features can be generated than with the HCM alone. The former is mainly reflected in the simulation of the SST seasonal cycle in the eastern Pacific and a reduction in the bias toward the cold tongue in the equatorial eastern Pacific, whereas the latter includes an improvement in a broad spectrum of ENSO features, such as ENSO amplitude, phase locking, asymmetry, and diversity. In particular, when WWBs are added to the HCM, the model can capture central Pacific ENSO events, which are absent in the original HCM. On the other hand, WWBs have little impact on cold events because very few WWBs occur during the cold phase of ENSO. Furthermore, the possible physical mechanisms responsible for these improvements as a result of adding WWBs are discussed.

Dynamics for El Ni\xf1o-La Ni\xf1a asymmetry constrain equatorial-Pacific warming pattern

The El Niño-Southern Oscillation (ENSO) results from the instability of and also modulates the strength of the tropical-Pacific cold tongue. While climate models reproduce observed ENSO amplitude relatively well, the majority still simulates its asymmetry between warm (El Niño) and cold (La Niña) phases very poorly. The causes of this major deficiency and consequences thereof are so far not well understood. Analysing both reanalyses and climate models, we here show that simulated ENSO asymmetry is largely proportional to subsurface nonlinear dynamical heating (NDH) along the equatorial Pacific thermocline. Most climate models suffer from too-weak NDH and too-weak linear dynamical ocean-atmosphere coupling. Nevertheless, a sizeable subset (about 1/3) having relatively realistic NDH shows that El Niño-likeness of the equatorial-Pacific warming pattern is linearly related to ENSO amplitude change in response to greenhouse warming. Therefore, better simulating the dynamics of ENSO asymmetry potentially reduces uncertainty in future projections.

Tropical Pacific Decadal Variability Induced by Nonlinear Rectification of El Niño–Southern Oscillation

AbstractOn the basis of 32 long-term simulations with state-of-the-art coupled GCMs, we investigate the relationship between tropical Pacific decadal variability (TPDV) and El Niño–Southern Oscillation (ENSO). The first empirical orthogonal function (EOF) mode for the 11-yr moving sea surface temperatures (SSTs) in the coupled models is commonly characterized by El Niño–like decadal variability with Bjerknes air–sea interaction. However, the second EOF mode can be separated into two groups, such that 1) some models have a zonal dipole SST pattern and 2) other models are characterized by a meridional dipole pattern. We found that models with the zonal dipole pattern in the second mode tend to simulate strong ENSO amplitude and asymmetry in comparison with those of the other models. Also, the residual patterns, which are defined as the summation of El Niño and La Niña SST composite anomalies, are very similar to the decadal dipole pattern, which suggests that ENSO residuals can cause the dipole decadal variability. It is found that decadal modulation of ENSO variability in these models strongly depends on the phase of the dipole decadal variability. The decadal changes in ENSO residual correspond well with the decadal changes in the dipole pattern, and the nonlinear dynamic heating terms by ENSO anomalies are well matched with the decadal dipole pattern.

Two Aspects of Decadal ENSO Variability Modulating the Long‐Term Global Carbon Cycle

AbstractThe El Niño–Southern Oscillation (ENSO) drives variations in terrestrial carbon fluxes by affecting the terrestrial ecosystem via atmospheric teleconnections and thus plays an important role in interannual variability of the global carbon cycle. However, we lack such knowledge on decadal time scales, that is, how the carbon cycle can be affected by decadal variations of ENSO characteristics. Here we examine how, and by how much, decadal ENSO variability affects decadal variability of the global carbon cycle by analyzing a 1,801‐year preindustrial control simulation. We identify two different aspects, together explaining ~36% of the decadal variations in the global carbon cycle. First, climate variations induced by decadal ENSO‐like variability regulate terrestrial carbon flux and hence atmospheric CO2 on decadal time scales. Second, decadal changes in the asymmetrical response of the terrestrial ecosystem, resulting from decadal modulation of ENSO amplitude and asymmetry, rectify the background mean state, thereby generating decadal variability of land carbon fluxes.

A brief review of ENSO theories and prediction

Although the El Nino-Southern Oscillation (ENSO) originates and develops in the equatorial Pacific, it has substantial climatic impacts around the globe. Thus, the ability to effectively simulate and predict ENSO one or more seasons in advance is of great societal importance, but this remains a challenging task. The main obstacles are the diversity, complexity, irregularity, and asymmetry of ENSO. The purpose of this article is to organically integrate the understanding of ENSO based on current progress on the physical mechanisms, prediction, and connections between the interannual ENSO phenomenon and physical processes on other time and space scales, and to provide guidance for future studies by extracting specific important questions.

A study of the effects of westerly wind bursts on ENSO based on CESM

Numerous works have indicated that westerly wind bursts (WWBs) have a significant contribution to the development of El Niño events. However, the simulation of WWBs commonly suffers from large biases in the current generation of coupled general circulation models (CGCMs), limiting our ability to predict El Niño events. In this study, we introduce a WWBs parameterization scheme into the global coupled Community Earth System Model (CESM) to improve the representation of WWBs and to study the impacts of WWBs on El Niño-Southern Oscillation (ENSO) characteristics. It is found that CESM with the WWBs parameterization scheme can generate more realistic characteristics of WWBs, in particular their location and seasonal variation of occurrence. With the parameterized WWBs, the skewness of the Niño 3 index is increased, in better agreement with observation. Eastern Pacific El Niño and central Pacific El Niño events could be successfully reproduced in the model run with WWBs parameterization. Further diagnoses show that the enhanced horizontal advection in the central Pacific and vertical advection in the eastern Pacific, both of which are triggered by WWBs, are crucial factors responsible for the improvements in ENSO simulation. Clearly, WWBs have important effects on ENSO asymmetry and ENSO diversity.

Mechanisms Reducing ENSO Amplitude and Asymmetry via an Enhanced Seasonal Cycle in the Mid-Holocene

Abstract Paleo proxy records have suggested that El Niño–Southern Oscillation (ENSO) variability during the mid-Holocene [8200 to 4200 years ago (8.2–4.2 ka)] was weaker than during the instrumental periods, but the mechanisms remain unclear. We examined processes of ENSO suppression using a coupled general circulation model (CGCM) that simulates ENSO amplitude and skewness under the present climate reasonably well. Two long simulations were performed: one using the preindustrial condition (CTRL) and the other using the 8-ka insolation having a greater seasonal cycle (MH8K). Consistent with proxy records and previous modeling studies, the ENSO amplitude weakened by 20% in MH8K compared to CTRL, mainly because of reduced thermocline feedback during the mature and decay phases. The weak thermocline feedback, likely a result of the loose equatorial thermocline in the mid-Holocene, suppresses the occurrence of extreme El Niño events and consequently explains the reduction in both ENSO amplitude and asymmetry. In MH8K, strengthened trade winds over the western-central Pacific Ocean act to cool the surface via evaporation while warmer water in the southern subtropical Pacific is transported beneath the equatorial thermocline, both contributing to diffuse the thermocline. Multimodel simulations for the mid-Holocene showed mean state changes and ENSO weakening similar to MH8K, but most models did not show reduced ENSO skewness, probably because of the failure in reproducing extreme El Niño events under the present climate.

The Collective Contribution of Atmospheric and Oceanic Components to ENSO Asymmetry

Four cross-coupled models were used to investigate the relative contributions of atmospheric and oceanic components to the asymmetry of the El Niño–Southern Oscillation (ENSO). Strong El Niño and La Niña events related to the negative heat flux feedbacks were found to be determined mainly by the atmospheric component, and the stronger sea surface temperature (SST) anomalies in the warm phase did not lead to an increased SST asymmetry. The skewness of the four models could be affected by both atmospheric and oceanic components; the atmospheric component determines the strength of positive and negative SST anomalies, and the oceanic component affects the strength of the negative SST anomalies in the cold phase under the same atmospheric component group. The Bjerknes stability index (BJ index) of warm and cold phases contributed to the El Niño–La Niña SSTA asymmetries in observation, but the BJ index did not necessarily explain the El Niño–La Niña SSTA asymmetries in climate model simulations. The SST asymmetries in these four models were closely associated with convective precipitation and wind stress asymmetries, which are also determined by both the atmospheric and oceanic components.