A Unified Oscillator Model for the El Niño-Southern Oscillation
Abstract
The delayed oscillator, the western Pacific oscillator, the recharge-discharge oscillator, and the advective-reflective oscillator have been proposed to interpret the oscillatory nature of the El Niño-Southern Oscillation (ENSO). All of these oscillator models assume a positive ocean-atmosphere feedback in the equatorial eastern and central Pacific. The delayed oscillator assumes that the western Pacific is an inactive region and wave reflection at the western boundary provides a negative feedback for the coupled system to oscillate. The western Pacific oscillator emphasizes an active role of the western Pacific in ENSO. The recharge-discharge oscillator argues that discharge and recharge of equatorial heat content cause the coupled system to oscillate. The advective-reflective oscillator emphasizes the importance of zonal advection associated with wave reflection at both the western and eastern boundaries. Motivated by the existence of these different oscillator models, a unified oscillator model is formulated and derived from the dynamics and thermodynamics of the coupled ocean-atmosphere system. Consistent with ENSO anomaly patterns observed in the tropical Pacific, this oscillator model considers sea surface temperature anomalies in the equatorial eastern Pacific, zonal wind stress anomalies in both the equatorial central Pacific and the equatorial western Pacific, and thermocline depth anomalies in the off-equatorial western Pacific. If the western Pacific wind-forced response is neglected, thermocline and zonal wind stress anomalies in the western Pacific are decoupled from the coupled system, and the unified oscillator reduces to the delayed oscillator. If wave reflection at the western boundary is neglected, the unified oscillator reduces to the western Pacific oscillator. The mathematical form of the recharge-discharge oscillator can also be derived from this unified oscillator. Most of the physics of the advective-reflective oscillator are implicitly included in the unified oscillator, and the negative feedback of wave reflection at the eastern boundary is added to the unified oscillator. With appropriate model parameters chosen to be consistent with those of previous oscillator models, the unified oscillator model oscillates on interannual timescales.
... They even reported first insights into the existence of the extra glacial termination TIII.a, which reproduces patterns of events equivalent to other glacial terminations despite being amid MIS 7 (Cheng et al., 2009). Superimposed on a strong precessional δ 18 O signal forced by regional insolation and sea surface temperature gradients, these precisely dated speleothem records from Sanbao cave in China show positive δ 18 O anomalies interpreted to reflect changes in the position and timing of monsoon rainfall coincident with the weakening of the AMOC in the North Atlantic (Wang, 2001;Cheng et al., 2009;Cai et al., 2015;Barker and Knorr, 2021;Jian et al., 2022). If AMOC weakening is linked to glacial meltwater release in the North Atlantic, these records may, therefore, provide indirect indicators of the chronology of glacial terminations. ...
The full understanding of climate feedbacks responsible for the amplification of deglaciations requires robust chronologies for these climate transitions, but in the case of marine records, radiocarbon chronologies are possible only for the last glacial termination. Although the assumed relationships between the marine isotopic record and the orbital parameters provide a first-order chronology for glacial terminations, an independent chronological control allows the relationships between orbital forcing and the climate response to be evaluated over multiple previous terminations. To assess this, we present geochemical records from the western Mediterranean, including two speleothems and one marine sediment core. The most notable speleothem, the so-called RAT, established a new long terrestrial climate record for this region, spanning Marine Isotope Stages from MIS 11 to MIS 7. Its absolute U / Th dates provide an exceptional chronology for the glacial terminations IV, III, and III.a. The onset of these three glacial terminations was marked by rapid δ18O depletions, reflecting ocean freshening by ice melting, thus providing an excellent tie point for regional marine records also sensitive to such freshening. This is exemplified by new δ18O data of the Ocean Drilling Program (ODP) 977 site from the Alboran Sea, where the speleothem chronology was employed to adjust its age model. The new chronologies reveal an earlier onset of the deglacial melting for the TIV and TIII.a that is in contrast to the generally accepted marine chronologies and indicates that the duration of these deglaciations was variable, with TIV being particularly longer (∼ 20 kyr). This study also supports that the onset of deglacial melting always occurred during a declining precession index, while a nonunique relation occurred with the obliquity parameter.
... The El Niño-Southern Oscillation (ENSO), being the strongest air-sea coupled system in the tropical Pacific (TP) on an interannual scale (Bjerknes 1969;Chen et al. 2013Chen et al. , 2023aRen et al. 2022;Hu et al. 2024), exerts profound global impacts on weather and climatic variations through teleconnections (Luo et al. 2017;Song et al. 2017;Zhao et al. 2022;Xu et al. 2023;Gao et al. 2023;Huang et al. 2018Huang et al. , 2021Huang et al. , 2024. Early investigations centered on the emergence, evolution, and phase shifts of ENSO primarily within the TP (Schopf and Suarez 1988;Weisberg and Wang 1997;Picaut et al. 1997;Jin 1997;Wang 2001). Subsequent studies unveiled the pivotal role of extratropical forcings in shaping ENSO, including the Arctic Oscillation, the North Pacific Oscillation, Arctic sea ice, and the Aleutian Low (Vimont et al. 2001;Chen et al. 2014Chen et al. , 2020Ding et al. 2015;Borhara et al. 2023;Cheng et al. 2023Cheng et al. , 2024a. ...
The leading coupled mode of air-sea variability in the subtropical northeastern Pacific is recognized as the North Pacific Meridional Mode (NPMM). This mode acts as a pivotal conduit, connecting extratropical atmospheric shifts with the tropical El Niño-Southern Oscillation (ENSO), thereby carrying significant weight in enhancing the accuracy of ENSO forecasting. This study endeavors to evaluate the efficacy of the Community Integrated Earth System Model (CIESM) in replicating the NPMM and its influence on the evolution of ENSO. The results show that the CIESM can effectively simulate the air-sea coupling process of the NPMM and the NPMM-ENSO relationship, with the highest skill among 40 CMIP6 models. Furthermore, the CIESM successfully captures the thermodynamic and oceanic dynamical processes of the NPMM influences on the subsequent ENSO events, including the wind-evaporation-sea surface temperature feedback mechanism and the trade-wind charging mechanism. Additionally, it is found that the bias in the NPMM-ENSO relation across 40 CMIP6 models stems from the intensity of the WES feedback process, particularly the atmospheric response to sea surface temperature (SST) anomalies in the subtropical North Pacific, which is modulated by the meridional position of the Intertropical Convergence Zone (ITCZ). Consequently, the findings of this study hold significant potential for enhancing the accuracy of climate model simulations and ENSO predictions.
... Several low-order conceptual models have been developed, including the recharge-discharge oscillator (Jin 1997a), the delayed oscillator (Suarez and Schopf 1988;Battisti and Hirst 1989), the western Pacific oscillator (Weisberg and Wang 1997), and the advectivereflective oscillator (Picaut et al. 1997). Later, a unified ENSO oscillator motivated by the dynamics and thermodynamics of Zebiak and Cane's coupled ocean-atmosphere model has been built (Wang 2001). Some other conceptual models have also been constructed to emphasize the nonlinear dynamics of ENSO and explore the ENSO features in the decadal time scale (Timmermann et al. 2003;Roberts et al. 2016;Timmermann and Jin 2002). ...
Understanding El Niño–Southern Oscillation (ENSO) dynamics has tremendously improved over the past few decades. The ENSO diversity in spatial pattern, peak intensity, and temporal evolution is, however, still poorly represented in conceptual ENSO models. In this paper, a physics-informed auto-learning framework is applied to derive ENSO stochastic conceptual models with varying degrees of freedom. The framework is computationally efficient and easy to apply. Once the state vector of the target model is set, causal inference is exploited to build the right-hand side of the equations based on a mathematical function library. Fundamentally different from standard nonlinear regression, the auto-learning framework provides a parsimonious model by retaining only terms that improve the dynamical consistency with observations. It can also identify crucial latent variables and provide physical explanations. This methodology successfully reconstructs the equations of a realistic six-dimensional reference ENSO model based on the recharge oscillator theory from its data. A hierarchy of lower-dimensional models is derived, and their representation of ENSO (including its diversity) is systematically assessed. The minimum model that represents ENSO diversity is four-dimensional, with three interannual variables describing the western Pacific thermocline depth, the eastern and central Pacific sea surface temperatures (SSTs), and one intraseasonal variable for westerly wind events. Without the intraseasonal variable, the resulting three-dimensional model underestimates extreme events and is too regular. A limited number of weak nonlinearities in the model are essential in reproducing the observed extreme El Niño events and the observed nonlinear relationship between eastern and western Pacific SSTs.
Significance Statement
This study develops a physics-informed auto-learning approach to improve the modeling and understanding of El Niño–Southern Oscillation (ENSO), a major climate phenomenon influencing global weather and climate. The auto-learning framework explores the causality between key processes to systematically produce stochastic conceptual models with different climate factors that simulate the diversity of observed ENSO events. The key finding is that the minimal model for characterizing the ENSO diversity (with the least number of climate factors) is a four-variable model capturing thermocline depth, sea surface temperatures, and wind bursts that can reproduce intensity and spatial pattern variation. This advancement provides an interpretable tool to identify the minimum sufficient processes governing ENSO behavior for improved predictability.
Despite advances in climate modeling, simulating the El Niño-Southern Oscillation (ENSO) remains challenging due to its spatiotemporal diversity and complexity. To address this, we build upon existing model hierarchies to develop a new unified modeling platform, which provides practical, scalable, and accurate tools for advancing ENSO research. Within this framework, we introduce a dual-core ENSO model (DCM) that integrates two widely used ENSO modeling approaches: a linear stochastic model confined to the equator and a nonlinear intermediate model extending off-equator. The stochastic model ensures computational efficiency and statistical accuracy. It captures essential ENSO characteristics and reproduces the observed non-Gaussian statistics. Meanwhile, the nonlinear model assimilates pseudo-observations from the stochastic model while resolving key air-sea interactions, such as feedback balances and spatial patterns of sea surface temperature anomalies (SSTA) during El Niño peaks and improving western-central Pacific SSTA magnitudes and spatial accuracy. The DCM effectively captures the realistic dynamical and statistical features of the ENSO diversity and complexity. Notably, the computational efficiency of the DCM facilitates a rapid generation of extended ENSO datasets, overcoming observational limitations. The outcome facilitates the analysis of long-term variations, advancing our understanding of ENSO and many other climate phenomena.
The recharge oscillator (RO) is a simple mathematical model of the El Niño Southern Oscillation (ENSO). In its original form, it is based on two ordinary differential equations that describe the evolution of equatorial Pacific sea surface temperature and oceanic heat content. These equations make use of physical principles that operate in nature: (a) the air‐sea interaction loop known as the Bjerknes feedback, (b) a delayed oceanic feedback arising from the slow oceanic response to winds within the equatorial band, (c) state‐dependent stochastic forcing from fast wind variations known as westerly wind bursts (WWBs), and (d) nonlinearities such as those related to deep atmospheric convection and oceanic advection. These elements can be combined at different levels of RO complexity. The RO reproduces ENSO key properties in observations and climate models: its amplitude, dominant timescale, seasonality, and warm/cold phases amplitude asymmetry. We discuss the RO in the context of timely research questions. First, the RO can be extended to account for ENSO pattern diversity (with events that either peak in the central or eastern Pacific). Second, the core RO hypothesis that ENSO is governed by tropical Pacific dynamics is discussed from the perspective of influences from other basins. Finally, we discuss the RO relevance for studying ENSO response to climate change, and underline that accounting for ENSO diversity, nonlinearities, and better links of RO parameters to the long term mean state are important research avenues. We end by proposing important RO‐based research problems.
Since the 21st century, the El Niño and Southern Oscillation (ENSO) exhibits more pronounced signals in the central Pacific (CP) rather than the eastern Pacific (EP), but the prediction skill has waned, suggesting limited understanding of crucial dynamics within the prediction framework. The ocean mixing around the mixed layer base, which transfers heat downward in a diabatic manner, was considered a potential influencing factor; yet, its effect has not been adequately examined in either CP or EP regions due to insufficient data. Here, we propose an Argo profile data-based mixing estimation model, which yields abundant estimates of subsurface ocean mixing and turbulent heat flux. Consequently, we find significant positive feedback of the ocean mixing-induced diabatic warming/cooling on the CP ENSO, but not on the EP ENSO. Particularly, the diabatic effect dominates sea surface temperature change in the CP region, highlighting the necessity for diabatic CP ENSO positive feedback dynamics in prediction models.
The present study has investigated interannual variations in the tropical-subtropical Pacific Ocean by applying Helmholtz decomposition to wave energy fluxes to indicate the direction of group velocity. The main source of energy is located in the central equatorial Pacific Ocean, while the anticyclonic and cyclonic circulation of wave energy appears in the western and eastern tropical Pacific Ocean, respectively. In the western basin, the anticyclonic circulation of wave energy is overridden at low and mid-latitude regions by the westward transfer of wave energy associated with the energy flux potential. Regional averages of the energy flux streamfunction and energy-flux potential associated with El Niño and La Niña events are similar to each other and show a quadratic dependence on the observed Niño-3 index. For example, the regional average of the energy-flux potential associated with the wind input in the central Pacific Ocean may be regressed as 0.72 ∆T² + 0.31 (in giga watts), where ∆T is the sea surface temperature anomaly (in degrees Celsius) associated with the Niño-3 index. The symmetry between El Niño and La Niña events is owing to the fact that the wave energy quantities do not differentiate between upwelling and downwelling (Kelvin) waves.
El Niño is frequently followed by La Niña, while La Niña tends to sustain into the next year or even longer, exhibiting a notable phase transition asymmetry (TA) of El Niño-Southern Oscillation (ENSO). Here this study explores the potential influences of tropical Indian Ocean (TIO) decadal variability on TA, based on a comparative analysis of the relationship between the TIO sea surface temperature anomalies (SSTA) and ENSO during different periods. Generally, the TIO SSTA strengthened TA before the 1980s, corresponding to a highly positive relationship between the whole TIO SSTA and ENSO. However, the weakening effect was exhibited after the 1980s when the correlation diminished. After the late 1990s, ENSO was only positively correlated with western TIO, with the westerly exhibit of the SSTA center leading to smaller impacts on TA. Besides, TIO SSTA tends to weaken TA by promoting the transition efficiency of La Niña, while bringing little effect on that of El Niño. Physically, compared to the mid-1970s, TIO SSTA triggered westerly wind anomalies during the autumn and winter of the La Niña development phase in the central equatorial Pacific in the late 1990s, which sped up the decay of La Niña. It then regenerated westerly anomalies in the following winter, facilitating the development of El Niño. This study quantifies the impact of the TIO SSTA on TA in seasonal signals and investigates the decadal variability of such influence, aiming to further understand phase transition asymmetry and offer valuable insights for the prediction of multi-year La Niña.
Understanding the interactions between the El Niño-Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO) is essential to studying climate variabilities and predicting extreme weather events. Here, we develop a stochastic conceptual model for describing the coupled ENSO-MJO phenomenon. The model adopts a three-box representation of the inter-annual ocean component to characterize the ENSO diversity. For the intraseasonal atmospheric component, a low-order Fourier representation is used to describe the eastward propagation of the MJO. We incorporate decadal variability to account for modulations in the background state that influence the predominant types of El Niño events. In addition to dynamical coupling through wind forcing and latent heat, state-dependent noise is introduced to characterize the statistical interactions among these multiscale processes, improving the simulation of extreme events. The model successfully reproduces the observed non-Gaussian statistics of ENSO diversity and MJO spectra. It also captures the interactions between wind, MJO, and ENSO.
It is argued from SST observations for the period 1950-90 that the tropical Indo-Pacific ocean-atmosphere system may be described as a stable linear dynamical system driven by spatially coherent Gaussian white noise. Evidence is presented that the predictable component of SST anomaly growth is associated with the constructive interference of several damped normal modes after an optimal initial structure is set up by the white noise forcing. In particular, El Nino-Southern Oscillation (ENSO) growth is associated with an interplay of at least three damped normal modes, with periods longer than two years and decay times of 4 to 8 months, rather than the manifestation of a single unstable mode whose growth is arrested by nonlinearities. From the results of several tests based on statistical properties of linear and nonlinear dynamical systems, one may conclude that much of the ENSO cycle in nature is dominated by stable, forced dynamics. -from Authors
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 atmospheric heating and sea surface temperature (SST) anomalies during the mature phase of El Niño are observed to show both eastern and western Pacific anomaly patterns, with positive anomalies in the equatorial eastern/central Pacific and negative anomalies in the off-equatorial western Pacific. The detailed spatial patterns of the heating anomalies differ from the SST anomalies. The heating anomalies are more equatorially confined than the SST anomalies, and maxima of positive and negative heating anomalies are located farther to the west than the SST anomalies. The Gill-Zebiak atmospheric model assumes that the atmospheric initial heating has the same spatial patterns as the SST anomalies. This assumption results in some unrealistic model simulations for El Niño.When the model heating anomaly forcing is modified to resemble the observed heating anomalies during the mature phase of El Niño, the model simulations have been improved to 1) successfully simulate equatorial easterly wind anomalies in the western Pacific, 2) correctly simulate the position of maximum westerly wind anomalies, and 3) reduce unrealistic easterly wind anomalies in the off-equatorial eastern Pacific. This paper shows that off-equatorial western Pacific negative atmospheric heating (or cold SST) anomalies are important in producing equatorial easterly wind anomalies in the western Pacific. These off-equatorial cold SST anomalies in the western Pacific also contribute to equatorial westerly wind anomalies observed in the central Pacific during the mature phase of El Niño. Although off-equatorial cold SST anomalies in the western Pacific are smaller than equatorial positive SST anomalies in the eastern Pacific, they are enough to produce atmospheric responses of comparable magnitude to the equatorial eastern Pacific. This is because the atmospheric mean state is convergent in the western Pacific and divergent in the equatorial eastern Pacific. By either removing the atmospheric mean convergence or removing off-equatorial cold SST anomalies in the western Pacific, the atmospheric responses show no equatorial easterly wind anomalies in the western Pacific. In the Gill-Zebiak model, the mean wind divergence field is an important background state, whereas the mean SST is secondary.
We analyze the linearized version of an analytical model, which combines linear ocean dynamics with a simple version of the Bjerknes hypothesis for El Niño. The ocean is represented by linear shallow water equations on an equatorial beta-plane. It is driven by zonal wind stress, which is assumed to have a fixed spatial form. Stress amplitude is set to be proportional to the thermocline displacement at the eastern boundary.It is shown that, for physically plausible parameter values, the model system can sustain growing Oscillations. Both growth rate and period scale directly with the time that an oceanic Kelvin wave needs to crow the basin. They are quite sensitive to the coupling parameter between thermocline displacement and wind stress, and the zonal location and meridional width of the wind.The most important parameter determining this behavior of the system is the coupling constant. For strong coupling the system exhibits exponential growth without oscillation. As the coupling is decreased the growth rate decreases until a transition value is reached. For smaller values of the coupling the growing modes of the system oscillate, with a period which is infinite at the transition value and decreases for decreasing coupling. The inviscid system has growing modes for any positive feedback, no mater how weak, though the growth rate rapidly becomes very small. For very weak coupling the period approaches the first resonance period of the free ocean. The model can also be expressed as a nondifferential delay equation, The components of dfis equation are easy to interpret physically and allow some insights into the nature of the oscillations. The relation of our results to other recent work and its implications for El Niño and the Southern Oscillation are discussed.
Using 43 years of Comprehensive Ocean-Atmosphere Data Set and related data for the period 1950-1992, an examination is made into the regional dependence of ocean-atmosphere coupling in relation to the El Niño-Southern Oscillation (ENSO). The cross correlation between sea surface temperature (SST) and sea level pressure (SLP) anomalies over the global tropics shows two patterns of significant negative correlation consistent with a local hydrostatic response of SLP to SST: (1) the eastern Pacific, where the correlation is symmetric about and largest on the equator, and (2) the western Pacific, where symmetric regions of negative correlation are found off the equator, separated by a region of positive correlation on the equator. Anomalies within these two patterns vary out of phase with each other. While the SLP anomalies on both sides of the basin are of similar magnitude, the SST anomalies in the east are much larger than those in the west. Despite this disparity in the SST anomaly magnitudes between the eastern and western Pacific we argue that the ocean-atmosphere couplings in the western and west-central Pacific are important for ENSO. The off-equator SST anomalies in the west enhance the SLP anomalies there, and they appear to initiate easterly wind anomalies over the far western Pacific during the peak El Niño phase of ENSO. As these easterlies evolve, their effect upon the ocean tends to oppose that of the westerly wind anomalies found over the west-central Pacific. These competing effects suggest a mechanism that may contribute to coupled ocean-atmosphere system oscillations. The west-central equatorial Pacific (the region separating the eastern and western patterns), while exhibiting large momentum and heat flux exchanges, shows minimum correlation between SST and SLP. Thus neither the SST and SLP anomaly magnitudes nor the correlation between them is alone indicative of ocean-atmosphere coupling, and the regional dependence for such coupling in relation to ENSO appears to be more complicated than mechanistic interpretations of ENSO would suggest.
significant cross correlation of the two records is near zero lag. The 1963, 1965, and 1969 E1 Nifio events are characterized by a persistently deep pycnocline. The model pycnocline variability at Talara, Peru, leads the observed SST variability by 2 months. The lag structure of pycnocline variability cross correlations indicates that the variability at the equator is related to the excitation of internal Kelvin and Rossby waves. The onset of the 1965 and 1969 El Nifio events was triggered by a large amplitude downwelling Kelvin wave excited by relaxation of the easterlies west of the dateline. None of the El Nifio events of the 1960's were related to anomalous relaxations of the wind field over the central Pacific. In addition, the seasonal intensification of the southeast trades over the central Pacific was not as strong as during non-El Nifio years. The subsequent cessation of the remotely forced seasonal upwelling caused the pycnoline to be depressed throughout the El Nifio year. During the southern summer, reestablishment of the semiannual variability of the southeast trades over the central equatorial Pacific excited a seasonal downwelling Kelvin wave. This second major downwelling impulse resulted in the double peak downwelling signature observed in sea level records. The minor El Nifio of 1963 was soley due to the cessation of the semi-annual wind stress variability east of 180 ø. The absence of remotely forced upwelling Kelvin waves kept the pycnocline deeper than normal following the seasonal downwelling at the outset of the year. There was not a relaxation of the wind field west of the dateline prior to the 1963 El Nifio.
El Niño and La Niña are the two complementary phase of the Southern Oscillation. During E1 Niña, the area of high sea surface temperatures increases, while the atmospheric convection zones of the tropical Pacific expand and merge so that there is a tendency toward spatially homogeneous conditions. La Niña is associated with low sea surface temperatures near the equator, with atmospheric convergence zones that are isolated from each other, and with spatial wales smaller than those of El Niña. It is proposed that both phases of the Southern Oscillation can be attributed to unstable interactions between the tropical ocean and atmosphere. During El Niña, the increase release of latent heat to the atmosphere drives the instability. During La Niña, when the heating of the atmosphere decreases, the compression of the convection into smaller and smaller areas permits an instability that intensifies the trade winds and the oceanic currents. The unstable air-sea interactions are modulated by the seasonal movements of the atmospheric convergence zones, and this determines some of the characteristics of the perturbations that can be amplified. The zonal integral of winds along the equator, rather than winds over a relatively small part of the Pacific such as the region west of the date line, is identified as a useful indicator of subsequent developments in the Pacific.
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
A model is used to study ocean-atmosphere interaction in the tropics. The model ocean consists of the single baroclinic mode of a two-layer ocean. Thermodynamics in the upper layer is highly parameterized. If the interface is sufficiently shallow (deep), sea surface temperature is cool (warm). The model atmosphere consists of two wind states that interact with the ocean according to the ideas of Bjerknes. When the eastern ocean is cool, the trade winds expand equatorward in the central Pacific, simulating an enhanced Walker circulation (WC). When the eastern ocean is warm, the trade winds expand eastward, simulating an enhanced Walker circulation (WC) there. For reasonable choices of parameters, the model oscillates at all time scales associated with the Southern Oscillation.
The WC has positive feedback with the ocean. This interaction generates persistence, and thereby makes it possible for solutions to oscillate at long time scales. Interaction of the HC with the ocean prevents the model from ever reaching, an equilibrium state. Wind curl associated with the HC generates a Rossby wave in the subtropics. It is the travel time of this wave across the basin that sets the oscillation period of the model.