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A Western Pacific Oscillator Paradigm for the El Niño-Southern Oscillation
Abstract
A data-based hypothesis is presented on the mechanism of the El Niño-Southern Oscillation (ENSO), a major determinant of interannual global climate variability. The hypothesis emphasizes the importance of off-equator sea surface temperature and sea level pressure variations west of the dateline for initiating equatorial easterly winds over the far western Pacific. These winds compete with westerly winds over the equatorial central Pacific enabling the coupled ocean-atmosphere system to oscillate. Consistent with this hypothesis, an analogical oscillator model is constructed that produces ENSO-like oscillations. The proposed mechanism differs from the delayed oscillator paradigm in that wave reflection at the western boundary is not a necessary condition for the coupled ocean-atmosphere system to oscillate.
... The equatorial zonal wind (EZW) in the lower troposphere over the western Pacific may play a critical role in the ENSO phase transition (Weisberg and Wang, 1997;Huang et al., 1998;Wang et al., 1999;Kim and Lau, 2001;Huang et al., 2001;Kug and Kang, 2006;Chen et al., 2018), by triggering equatorial oceanic Kelvin waves and consequent thermocline adjustments. The equatorial easterly (westerly) wind anomalies over the western Pacific may trigger upwelling (downwelling) oceanic Kelvin waves, contributing to the rapid termination of El Niño (La Niña) (Wang et al., 1999;Kim and Lau, 2001). ...
... The EZW in the lower troposphere over the western Pacific may play a critical role in ENSO phase transition (Weisberg and Wang, 1997;Wang et al., 1999;Kim and Lau, 2001;Huang et al., 2001;Kug and Kang, 2006), by triggering equatorial oceanic Kelvin waves and thermocline adjustment. Significant positive correlations between the JJA western Pacific EZW index and the subsequent DJF Niño-3.4 index could be identified during both P1 (Fig. 10c) and P2 (Fig. 10d), indicating that the modulating effects from the western Pacific EZW on the following ENSO evolu-tion may be at work in both periods. ...
The western North Pacific summer monsoon (WNPSM) is an important subcomponent of the Asian summer monsoon. The equatorial zonal wind (EZW) in the lower troposphere over the western Pacific may play a critical role in the evolution of the El Niño-Southern Oscillation (ENSO). The possible linkage between the EZW over the western Pacific and the off-equatorial monsoonal winds associated with the WNPSM and its decadal changes have not been fully understood. Here, we find a non-stationary relationship between the WNPSM and the western Pacific EZW, with a significant intensification of the correlation between them around the late 1980s/early 1990s. This observed shifting WNPSM–EZW relationship could be explained by the changes in the related sea surface temperature (SST) configurations across the tropical oceans. The enhanced influences from the springtime tropical North Atlantic, summertime tropical central Pacific and maritime continent SST anomalies may work together to contribute to the recent intensified WNPSM–EZW co-variability. This observed recent strengthening WNPSM–EZW relationship may have profound impacts on the climate system, including prompting more effective feedback from the WNPSM on the subsequent ENSO evolution and boosting a stronger biennial tendency of the WNPSM–ENSO coupling system. The results obtained herein imply that the WNPSM, EZW, ENSO and tropical North Atlantic SST may be closely linked in a unified climate system with the quasi-biennial rhythm during recent decades, accompanied by the reinforcement of the WNPSM–ENSO interplay possibly triggered by the enhanced tropical Pacific–Atlantic cross-basin interactions.
... The recharge oscillator (RO) model (Jin, 1997) combines SST dynamics and oceanic adjustment dynamics into a coupled basin-wide RO that relies on the nonequilibrium between the zonal mean equatorial thermocline depth and wind stress. There are other types of theoretical models to highlight different physical processes, such as the western Pacific oscillator model (Weisberg and Wang, 1997), the advective-reflective oscillator model (Picaut et al., 1997), and the unified oscillator model (Wang, 2001). The recent review by Wang (2018) explicitly compared the similarities and differences among these oscillator models. ...
A spatiotemporal oscillator model for El Ni\~no/Southern Oscillation (ENSO) is constructed based on the sea surface temperature (SST) and thermocline depth dynamics. The model is enclosed by introducing a proportional relationship between the gradient in SST and the oceanic zonal current and can be transformed into a standard wave equation that can be decomposed into a series of eigenmodes by cosine series expansion. Each eigenmode shows a spatial mode that oscillates with a natural frequency. The first spatial mode, that highlights SST anomaly (SSTA) contrast in the eastern and western Pacific, the basic characteristics of the eastern Pacific (EP) El Ni\~no, oscillates with a natural period of around 4.3 years, consistent with the quasi-quadrennial (QQ) mode. The second spatial mode, that emphasizes SSTA contrast between the central and the eastern, western Pacific, the basic spatial structure of the central Pacific (CP) El Ni\~no, oscillates with a natural period of 2.3 years that is half of the first natural period, also consistent with the quasi-biennial (QB) modes. The combinations of the first two eigenmodes with different weights can feature complex SSTA patterns with complex temporal variations. In open ocean that is far away from the coastlines, the model can predict waves propagating both eastward and westward. Besides, the net surface heating further complicates the temporal variations by exerting forced frequencies. The model unifies the temporal and spatial variations and may provide a comprehensive viewpoint for understanding the complex spatiotemporal variations of ENSO.
... Although many international scientific experts have invested considerable research efforts in improving El Niño predictability and its potential theory [20][21][22][23][24][25][26][27][28][29][30][31], real-time El Niño pattern trends are difficult to solve completely. Therefore, large uncertainties still exist in the current ENSO prediction systems [32][33][34]. ...
In order to figure out the associated underlying dynamical processes of the 2014–2015 warming event, we used the ECCO (Estimating the Circulation and Climate of the Ocean) reanalysis from 1993 to 2016 and two combined scatterometers, QuikSCAT and ASCAT, to analysis hydrodynamic condition and ocean heat budget balance process in the equatorial tropical pacific. The spatiotemporal characteristics of that warming event were revealed by comparing the results with a composite El Niño. The results showed that the significant differences between the 2014 and 2015 warming periods were the magnitudes and positions of the equatorial easterly wind anomalies during the summer months. The abruptly easterly wind anomalies of 2014 that spread across the entire equatorial Pacific triggered the upwelling of the equatorial Kelvin waves and pushed the eastern edge of the warm pool back westward. These combined effects caused abrupt decreases in the sea surface temperatures (SST) and upper ocean heat content (OHC) and damped the 2014 warming process into an El Niño. In addition, the ocean budget of the upper 300 m of the El Niño 3.4 region showed that different dynamical processes were responsible for different warming phases. For example, at the beginning of 2014 and 2015, the U advection and subsurface processes played dominant roles in the positive ocean heat content tendency. During the easterly wind anomalies period of 2014, the U advection process mainly caused a negative tendency and halted the development of the warming phase. In regard to the easterly wind anomalies of 2015, the U advection and subsurface processes were weaker negatively when compared with that in 2014. However, the V advection processes were consistently positive, taking a leading role in the positive trends observed in the middle of 2015.
... Even the simplest oscillator models (Zebiak and Cane, 1987) recognize the importance of SST variation in the eastern and western Pacific but do not consider spatial gradients. For example, the west pacific oscillator (Weisberg and Wang, 1997), central and east Pacific oscillators (Zebiak and Cane, 1987) are somewhat different in conceptual formulation those describe the SST variations in the western and eastern side of the basin. ...
Dominant modes of SST in the west and east Pacific show strong but regionally different gradients caused by waves, internal dynamics, and anthropogenic warming, which drives air-sea interaction in the Pacific. The study discusses the relative contribution of SST gradients over the western and eastern Pacific to the prediction skill of SST in the central Pacific, where El-Nino, La-Nina, or El-Nino Modoki events project significantly. For this, the analysis develops a convolutional neural network (CNN) based prediction model to predict the Nino3.4 SST. CNN-based prediction models use a spatial filter at the initial stage, which is highly efficient in capturing the edges or gradients and hence are useful to understand the role of SST spatial gradients in the prediction skill. The study reports three CNN-based model experiments. The first one is a CTRL experiment that uses the whole equatorial Pacific domain SST pattern. The second and third models use the equatorial eastern and western Pacific domain SST only. Another novel feature of this study is that we have generated a large number of ensemble members (5000) through random initialization of CNN filters. It is found that random initialization affects the forecast skill, and the probability density function of the correlation skill of the 5000 models at each lead time shows a gaussian distribution. The model experiments suggest that the west Pacific SST model provides better Nino3.4 skills as compared to the east Pacific skill. The CNN-based model forecast based on the SST pattern, thus, shows the impact of the SST spatial pattern on the ENSO forecast.
... Conversely, the La Niña environment is characterized by a zonally steeper SST gradient in association with a stronger atmospheric circulation. Various conceptual models have been proposed and continuously improved for the evolution of the ENSO status [6][7][8][9][10][11][12] . Generally, the SST anomalies are understood as triggering ENSO phases through westerly wind bursts in the atmosphere 13 , propagating equatorial Kelvin waves to the east 14 and then forming Rossby waves in the north and the south away from the equator 15 . ...
Considering that the subtropical highs and tropical convections are observed as negative and positive vorticities respectively, the large-scale features of the atmospheric environment can be effectively represented using streamfunctions as defined by the Laplacian. By investigating the geographical patterns of streamfunctions from different modes of environmental variability, this study conceptualizes how the subtropical high expands and the region for tropical convections migrates in the western North Pacific. It is confirmed that, owing to the expansion of the subtropical high, the limited ocean area for tropical convections even bounded by the equator becomes narrower in the “La Niña mode” than that in the “El Niño mode”. This study finds that a warmer environment is likely to further expand the subtropical high to the west, and then the westernmost shift in the region for tropical convections appears in the “warmer La Niña mode”. A linear perspective suggests that every warmer La Niña environment could be one that people have scarcely experienced before.
... Third, because of the low computational cost, these conceptual models can also be used to predict the ENSO indices [25,26] and study the predictability from a statistical point of view [27] in light of ensemble forecast methods. Several low-order conceptual models have been independently developed, including the rechargedischarge oscillator [7,28], the delayed oscillator [29,16,30], the western-Pacific oscillator [31], and the advective-reflective oscillator [32]. Later, a unified ENSO oscillator motivated by the dynamics and thermodynamics of Zebiak and Cane's coupled ocean-atmosphere model has also been built [33]. ...
El Ni\~no-Southern Oscillation (ENSO) is the most predominant interannual variability in the tropics, significantly impacting global weather and climate. In this paper, a framework of low-order conceptual models for the ENSO is systematically derived from a spatially-extended stochastic dynamical system with full mathematical rigor. The spatially-extended stochastic dynamical system has a linear, deterministic, and stable dynamical core. It also exploits a simple stochastic process with multiplicative noise to parameterize the intraseasonal wind burst activities. A principal component analysis based on the eigenvalue decomposition method is applied to provide a low-order conceptual model that succeeds in characterizing the large-scale dynamical and non-Gaussian statistical features of the eastern Pacific El Ni\~no events. Despite the low dimensionality, the conceptual modeling framework contains outputs for all the atmosphere, ocean, and sea surface temperature components with detailed spatiotemporal patterns. This contrasts with many existing conceptual models focusing only on a small set of specified state variables. The stochastic versions of many state-of-the-art low-order models, such as the recharge-discharge and the delayed oscillators, become special cases within this framework. The rigorous derivation of such low-order models provides a unique way to connect models with different spatiotemporal complexities. The framework also facilitates understanding the instantaneous and memory effects of stochastic noise in contributing to the large-scale dynamics of the ENSO.
... Zebiak and Cane et al. [15,16] proposed the Zebiak-Cane model, which was the first dynamical model for ENSO prediction [135]. Subsequently, researchers have successively proposed a delayed action oscillator theory [17], charge-discharge oscillator theory [18,19], a western Pacific oscillator paradigm theory [20], advection feedback oscillator theory [21], and unified theory of oscillator [22]. So far, researchers have provided a complete explanation of the mechanism by which the ENSO cycle is generated. ...
El Niño/Southern Oscillation (ENSO) mainly occurs in the tropical Pacific Ocean every a few years. But it affects the climate around the world and has a dramatic impact on the development of ecology and agriculture. The analysis and prediction of ENSO become particularly important for meteorology and disaster management. However, due to insufficient data, spring predictability barrier (SPB), and model uncertainty, traditional analysis models face challenges. To address these issues, researchers begin to apply deep learning (DL) technologies to ENSO research, exploring the impact of ENSO on the world's extreme climate changes. In recent years, deep learning-based methods have obtained impressive progress with more accurate and effective predictions of ENSO. In this paper, we summarize the attempts of DL technologies in predicting ENSO. We first introduce the properties of ENSO, followed by the architecture introduction of DL technologies and their application to ENSO. We then investigate the potential of DL technologies for ENSO prediction from various aspects, including model evaluation metrics, prediction algorithms, overcoming SPB and prediction uncertainty. Finally, we provide discussions on the future trends and challenges of using DL technologies for ENSO prediction.
The most significant interannual variability phenomenon of our planet, namely the El Niño and the Southern Oscillation (ENSO), is discussed in this chapter starting with a definition to identify these events. The effort put to observe this phenomenon in the Equatorial Pacific along with the observed features of ENSO is discussed. The theories for the evolution of ENSO are also presented in this chapter.KeywordsEl Niño and the Southern Oscillation (ENSO)Southern Oscillation Index (SOI)BuoysExpendable bathythermographUpwellingBjerkness feedbackTeleconnectionLinear stochastic theoryDelayed oscillator theoryRecharge-discharge theoryAltimetryScatterometryRossby wavesKelvin wavesThermocline
Extratropical air–sea interactions have become increasingly involved in promoting the transition to El Niño– Southern Oscillation (ENSO) with climate change. In this study, we break down the effects of future warming on the 1-year lead relationship between a cold western North Pacific (WNP) phase and El Niño development the following winter. We apply a conditional probability approach and sea surface temperature (SST) budget analysis on historical and Shared Socioeconomic Pathway 3–7.0 (SSP370) model runs. With enhanced warming, cold WNP SST anomalies in the boreal winter further strengthen summer westerly anomalies in the western equatorial Pacific, which promote the intensification of surface convergence and anomalous Ekman and geostrophic advection, and positive SST anomalies in the central equatorial Pacific in the seasons prior to the El Niño. Downwelling equatorial Kelvin waves induced by the westerly wind stress facilitate entrainment of subsurface water into the mixed layer during the transition period to trigger stronger thermocline feedback in the central–eastern equatorial Pacific. As a result, the amplitude and frequency of El Niño and its tropical precursors are projected to increase with warming under the WNP influence. ENSO diversity modulated by this relationship depends on the relative strength of advective and thermocline feedbacks, as well as the background state at the time of the event. The intensification of positive Pacific Meridional Mode (PMM) southwesterlies during the WNP–ENSO transition suggests a strengthened three-way link between WNP, PMM and ENSO under enhanced warming that may promote stronger and/or more frequent El Niños.
Plain Language Summary
El Niño‐Southern Oscillation (ENSO) exerts pronounced climate impacts across the globe, but it is influenced by many factors. Previous studies have revealed that sea surface temperature anomalies (SSTAs) over the North Atlantic could stimulate ENSO events through two pathways, directly through regulating the tropical atmospheric circulation and indirectly via stimulating the mid‐latitude atmospheric teleconnection. In the indirect pathway, the physical mechanisms involve very complex interactions among multiple atmospheric and oceanic systems. Here, we reveal that the Tibetan Plateau (TP) plays an important bridging role in the indirect pathway. Both observations and model simulations show that winter‐spring positive North Atlantic tripole SSTAs can trigger downstream‐propagating Rossby wave train, which further cause an upper‐level anomalous cyclone (anticyclone) over the western TP (subtropical central‐western Pacific) during April and June. Consequently, upper‐level convergences and lower‐level divergences appear over the Maritime Continent, which induce surface westerly anomalies over the equatorial western Pacific, favoring the occurrence of subsequent autumn‐winter El Niño events. After flattening the TP in climate model, the overall responses of atmospheric and oceanic processes associated with El Niño development to the North Atlantic tripole SSTAs will be obviously weakened. Quantitatively, the TP's bridging effect accounts for about 38% contribution in the above process.
Six years of upper-ocean velocity, temperature, and surface wind data collected in the west-central Pacific at 08, 1708W reveal a slow ocean dynamical mode associated with the El Nino-Southern Oscillation (ENSO). Latent and sensible heat flux calculations using the basin-wide Tropical Atmosphere Ocean (TAO) array data show a coincident, slow ocean-atmosphere thermodynamical mode. Beginning with the La Nina conditions in 1988 through the peak El Nino conditions in 1992 the Equatorial Undercurrent (EUC) speed decreased along with the surface zonal wind stress and the zonal pressure gradient. Simultaneous with these were increasing trends in the Richardson number above the EUC core and in sea surface temperature (SST). After peak warming was achieved, the variations in all of these quantities reversed in a movement toward their previous La Nina conditions. As this evolved within the ocean the sensible and latent heat fluxes increased with large values emanating eastward from the western Pacific. The largest interannual perturbations, then, for both the surface momentum and heat flux quantities during this recent ENSO cycle were within the west-central Pacific, the transition region between the warmest waters found in the western Pacific warm pool and the coldest waters found in the eastern Pacific cold tongue. The observed ocean and atmosphere variability represents a positive feedback. This raises a question about the origin of negative feedback that is necessary for the coupled system to oscillate. Arguing from the standpoint of a Gill atmosphere and observed SST-sea level pressure correlation patterns, the paper draws a connection between condensation heating in the equatorial west-central Pacific and easterly winds over the equatorial western Pacific during the mature phase of El Nino. The formation of such easterlies by ocean-atmosphere coupling over the western Pacific is hypothesized as providing a negative feedback for reversing the sign of anomalous SST in the equatorial central Pacific. This mechanism may com- plement, but it is different from, the delayed oscillator mechanism for ENSO.
The stability, periodicity, and horizontal structure of equatorial modes in a coupled ocean-atmosphere model, simplified by the assumption that zonal wind stress anomalies are proportional to sea surface temperature anomalies lagged by a zonal phase difference, are examined analytically in an unbounded basin. The gravest coupled Rossby and Kelvin modes coexist with additional westward and eastward slow modes whose phase speeds are smaller than the former. Two of these four modes, one propagating westward and the other eastward, are destabilized in each case depending upon the model parameters. For some particular parameter choices. coupled Rossby and Kelvin modes merge with westward and eastward slow modes, respectively. For other parameters, however. they separate and remain distinct from the slow modes. For all of these modes the primary modifications by coupling relative to uncoupled oceanic equatorial waves are a decrease in phase speed and an increase in meridional scale.Among the model parameter effects, those of the zonal phase lag between the wind stress and SST anomalies and the coefficients of thermal and mechanical damping are the most interesting. Positive and negative phase lags represent the wind stress anomalies located to the west and east of the SST anomalies, respectively. The frequency of all modes is symmetric about zero phase lag, whereas the growth rate is antisymmetric about zero phase lag relative to the uncoupled damping rate. Wind stress anomalies to the west of SST anomalies favor slow mode growth and coupled Rossby and Kelvin mode decay. Dissipation for the slow modes and the coupled Rossby and Kelvin modes is controlled differently. For the slow modes the dissipation is mainly thermal, whereas for coupled Rossby and Kelvin modes the dissipation is mainly mechanical.
A simple nonlinear model is proposed for the El Nino/Southern Oscillation (ENSO) phenomenon. Its key feature is the inclusion of oceanic wave transit effects through a negative, delayed feedback. A linear stability analysis and numerical results are presented, showing that the period of the oscillation is typically several times the delay. It is suggested that such an effect can account for the long time scale of ENSO.
Approximations suggested by a scaling analysis are used to obtain analytic results for the eigenmodes of the system. A slow time scale, unstable eigenmode associated with the time derivative of the SST equation is suggested to be important in giving rise to interannual oscillations. This slow SST mode is not necessarily linked to conventional equatorial oceanic wave modes. A useful limit of this mode is explored in which the wave speed of uncoupled oceanic wave modes is fast compared to the time scales that arise from the coupling. This is referred to as the fast-wave limit. The dispersion relationship in this limit is used to present a number of coupled feedback mechanisms, which contribute simultaneously to the instability of the SST mode. -from Author
The effect of ocean boundaries on instability in coupled ocean-atmosphere models is determined. Growing modes in the bounded ocean case correspond in aspects of behavior and structure to particular modes of appropriate wavelength in the unbounded ocean case. The basic mechanism of instability is the same in both cases. Growing modes in the bounded ocean case feature wavelike disturbances that propagate slowly across the ocean basin. The direction of propagation and period of the oscillation are very sensitive to the values prescribed for oceanic thermodynamic coefficients. The period is set by the length of time that the coupled disturbance takes to propagate across the basin, and, for a 15 000km wide basin, ranges from about a year to many years. Instability occurs for a greater range of parameters, and growth rates are larger when the ocean basin is wide (eg 15 000km) than when it is narrow. The zonal width of the continent has little effect on model behavior. The Kelvin and symmetric low-n long Rossby components of the oceanic and atmospheric motion fields are of primary importance in model growth. -from Author
A simple nonlinear model is proposed for the El Nino/Southern Oscillation (ENSO) phenomenon. Its key feature is the inclusion of oceanic wave transit effects through a negative, delayed feedback. A linear stability analysis and numerical results are presented to show that the period of the oscillation is typically several times the delay. It is argued such an effect can account for the long time scale of ENSO.
Ship observations of sea surface temperature (SST), sea level pressure and surface wind, and satellite measurements of outgoing longwave radiation (OLR) (an indicator of deep tropical convection) are used to describe the large-scale atmospheric circulation over the tropical Pacific during composite warm and cold episodes. Results are based on linear regression analysis between the circulation parameters and an index of SST in the tropical Pacific during the period 1946–85 (1974–89 for OLR). Warm episodes along the Peru coast (i.e., El Nino events) and basin-wide warmings associated with the Southern Oscillation are examined separately. Charts of the total as well as anomalous fields of SST, sea level pressure, surface wind and OLR for both warm and cold episodes are presented. SST and surface wind anomalies associated with warm episodes are consistent with the results of Rasmusson and Carpenter (1982). El Nino events are characterized by strong positive SST anomalies along the coasts of Ecuador a...