Sulian Thual

A Stochastic Skeleton Model for the MJO

AbstractThe Madden–Julian oscillation (MJO) is the dominant mode of variability in the tropical atmosphere on intraseasonal time scales and planetary spatial scales. Despite the primary importance of the MJO and the decades of research progress since its original discovery, a generally accepted theory for its essential mechanisms has remained elusive. In recent work by two of the authors, a minimal dynamical model has been proposed that recovers robustly the most fundamental MJO features of (i) a slow eastward speed of roughly 5 m s−1, (ii) a peculiar dispersion relation with dω/dk ≈ 0, and (iii) a horizontal quadrupole vortex structure. This model, the skeleton model, depicts the MJO as a neutrally stable atmospheric wave that involves a simple multiscale interaction between planetary dry dynamics, planetary lower-tropospheric moisture, and the planetary envelope of synoptic-scale activity. In this article, it is shown that the skeleton model can further account for (iv) the intermittent generation of MJ...

Vertical structure variability and equatorial waves during central Pacific and eastern Pacific El Ninos in a coupled general circulation model

Recent studies report that two types of El Niño events have been observed. One is the cold tongue El Niño or Eastern Pacific El Niño (EP El Niño), which is characterized by relatively large sea surface temperature (SST) anomalies in the eastern Pacific, and the other is the warm pool El Niño (a.k.a. ‘Central Pacific El Niño’ (CP El Niño) or ‘El Niño Modoki’), in which SST anomalies are confined to the central Pacific. Here the vertical structure variability of the periods during EP and CP is investigated based on the GFDL_CM2.1 model in order to explain the difference in equatorial wave dynamics and associated negative feedback mechanisms. It is shown that the mean stratification in the vicinity of the thermocline of the central Pacific is reduced during CP El Niño, which favours the contribution of the gravest baroclinic mode relatively to the higher-order slower baroclinic mode. Energetic Kelvin and first-meridional Rossby wave are evidenced during the CP El Niño with distinctive amplitude and propagating characteristics according to their vertical structure (mostly first and second baroclinic modes). In particular, the first baroclinic mode during CP El Niño is associated to the ocean basin mode and participates to the recharge process during the whole El Niño cycle, whereas the second baroclinic mode is mostly driving the discharge process through the delayed oscillator mechanism. This may explain that the phase transition from warm to neutral/cold conditions during the CP El Niño is delayed and/or disrupted compared to the EP El Niño. Our results have implications for the interpretation of the variability during periods of high CP El Niño occurrence like the last decade.

Simple stochastic model for El Niño with westerly wind bursts

Significance Understanding the role that atmospheric wind bursts play in the initiation and maintenance of the El Niño Southern Oscillation (ENSO) in the tropics is a crucial problem in ocean–atmosphere sciences. We provide insight into the problem by proposing a simple ENSO model, amenable to detailed analysis, where wind burst activity is driven by a simple stochastic jump process that depends on the strength of the western Pacific warm pool. The model captures key features of the observational record, such as the probability density function and power spectrum of eastern Pacific sea surface temperatures. In addition, the varying intensity and strength of El Niño events from the model are also consistent with real observations including super El Niño episodes. Atmospheric wind bursts in the tropics play a key role in the dynamics of the El Niño Southern Oscillation (ENSO). A simple modeling framework is proposed that summarizes this relationship and captures major features of the observational record while remaining physically consistent and amenable to detailed analysis. Within this simple framework, wind burst activity evolves according to a stochastic two-state Markov switching–diffusion process that depends on the strength of the western Pacific warm pool, and is coupled to simple ocean–atmosphere processes that are otherwise deterministic, stable, and linear. A simple model with this parameterization and no additional nonlinearities reproduces a realistic ENSO cycle with intermittent El Niño and La Niña events of varying intensity and strength as well as realistic buildup and shutdown of wind burst activity in the western Pacific. The wind burst activity has a direct causal effect on the ENSO variability: in particular, it intermittently triggers regular El Niño or La Niña events, super El Niño events, or no events at all, which enables the model to capture observed ENSO statistics such as the probability density function and power spectrum of eastern Pacific sea surface temperatures. The present framework provides further theoretical and practical insight on the relationship between wind burst activity and the ENSO.

Low-Frequency Variability of Temperature in the Vicinity of the Equatorial Pacific Thermocline in SODA: Role of Equatorial Wave Dynamics and ENSO Asymmetry

Abstract The Simple Ocean Data Assimilation (SODA) reanalysis (1958–2001) is used to investigate the decadal variability in the equatorial thermocline in the Pacific. Whereas the thermocline depth exhibits weak variation at decadal time scales, the temperature change in the vicinity of the thermocline in the western Pacific is significant and has a vertical scale of ∼150 m. Based on a modal decomposition of the model variability, it is shown that such temperature change can be interpreted to a large extent as vertical displacements of the isotherms associated with the Kelvin and first meridional Rossby waves of the first three baroclinic modes. This indicates that decadal change at the subsurface in the warm pool region may be forced by the winds, consistent with the results of a multimode linear model simulation. The decadal mode of vertical temperature can be described by the first two dominant statistical modes (EOFs): the first mode is associated with changes in the slope of the thermocline (swallowin...

Sensitivity of ENSO to Stratification in a Recharge-Discharge Conceptual Model

AbstractEl Nino–Southern Oscillation (ENSO) is driven by large-scale ocean–atmosphere interactions in the equatorial Pacific and is sensitive to change in the mean state. Whereas conceptual models of ENSO usually consider the depth of the thermocline to be influential on the stability of ENSO, the observed changes in the depth of the 20°C isotherm are rather weak, on the order of approximately 5 m over the last decades. Conversely, change in stratification that affects both the intensity and sharpness of the thermocline can be pronounced. Here, the two-strip conceptual model of An and Jin is extended to include three parameters (i.e., the contribution of the first three baroclinic modes) that account for the main characteristics of the mean thermocline vertical structure.A stability analysis of the model is carried out that indicates that the model sustains a lower ENSO mode when the high-order baroclinic modes (M2 and M3) are considered. The sensitivity of the model solution to the coupling efficiency fu...

Asymmetric intraseasonal events in the stochastic skeleton MJO model with seasonal cycle

Abstract The stochastic skeleton model is a simplified model for the Madden–Julian oscillation (MJO) and intraseasonal-planetary variability in general involving coupling of planetary-scale dry dynamics, moisture, and a stochastic parametrization for the unresolved details of synoptic-scale activity. The model captures the fundamental features of the MJO such as the intermittent growth and demise of MJO wave trains, the MJO propagation speed, peculiar dispersion relation, quadrupole vortex structure, etc. We analyze here the solutions of a stochastic skeleton model with an idealized seasonal cycle, namely a background warm pool state of heating/moistening displacing meridionally during the year. The present model considers both equatorial and off-equatorial components of the envelope of synoptic scale convective activity, which allows for a large diversity of meridionally symmetric and asymmetric intraseasonal events found in nature. These include examples of symmetric events with MJO quadrupole vortex structure, half-quadrupole events with off-equatorial convective heating structure, as well as tilted events with convective heating structure oriented north-westward and associated northward propagation that is reminiscent of the summer monsoon intraseasonal oscillation. The model also reproduces qualitatively the meridional migration of intraseasonal variability during the year, that approximatively follows the meridional migration of the background warm pool.

A skeleton model for the MJO with refined vertical structure

The Madden–Julian oscillation (MJO) is the dominant mode of variability in the tropical atmosphere on intraseasonal timescales and planetary spatial scales. The skeleton model is a minimal dynamical model that recovers robustly the most fundamental MJO features of (I) a slow eastward speed of roughly

An Asymptotic Expansion for the Recharge-Discharge Model of ENSO

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