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  • 1
    Online Resource
    Online Resource
    An Asymptotic Expansion for the Recharge–Discharge Model of ENSO
    American Meteorological Society ; 2013
    In:  Journal of Physical Oceanography Vol. 43, No. 7 ( 2013-07-01), p. 1407-1416
    Details
    In: Journal of Physical Oceanography, American Meteorological Society, Vol. 43, No. 7 ( 2013-07-01), p. 1407-1416
    Abstract: The dynamics of El Niño–Southern Oscillation (ENSO) in the equatorial Pacific Ocean are largely associated with the slow thermocline adjustment at interannual and basin scales. This adjustment involves, among other things, the fast propagation and reflection of equatorial waves by wind stress forcing. A simple and straightforward asymptotic expansion of the long-wave equations is proposed using the low-frequency approximation. The asymptotic expansion is performed in Fourier space, retaining only the gravest equatorial long waves and baroclinic modes with the largest scale, and considering small dissipation by friction and boundary reflections. This leads to an asymptotic model for the thermocline response to wind stress forcing, which is in essence the ocean component of the recharge–discharge model of ENSO. The asymptotic model is nonheuristic and in broad agreement with some essential results scattered in previous studies. Thermocline variability is divided into a sloping “Tilt mode” that adjusts instantly to wind stress forcing and a zonal-mean “Warm Water Volume mode” that adjusts as a time integrator to wind stress curl. The model has a plausible energy budget and its solutions are in good agreement with observations. Results suggest that the net adjustment rather than the explicit delays of equatorial waves is essential for the slow thermocline adjustment, and this is best described by the recharge–discharge model.
    Type of Medium: Online Resource
    ISSN: 0022-3670 , 1520-0485
    URL: Article
    DOI: 10.1175/JPO-D-12-0161.1
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2013
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    detail.hit.zdb_id: 184162-2
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  • 2
    Online Resource
    Online Resource
    Absolute or convective instability in the equatorial Pacific and implications for ENSO
    Wiley ; 2013
    In:  Quarterly Journal of the Royal Meteorological Society Vol. 139, No. 672 ( 2013-04), p. 600-606
    Details
    In: Quarterly Journal of the Royal Meteorological Society, Wiley, Vol. 139, No. 672 ( 2013-04), p. 600-606
    Abstract: The El Niño–Southern Oscillation (ENSO) is driven by ocean–atmosphere interactions in the equatorial Pacific, and this variability is often attributed to coupled modes that are evidenced by the temporal stability analysis of anomaly models. Here, the further diagnostic of absolute/convective instability is considered, which assesses whether small perturbations localized in space and time (e.g. random wind disturbances) lead to instabilities that develop in‐place or propagate away from the perturbed region. It is shown that boundary conditions play a secondary role for this approach and that the development of large‐scale wave packets in the equatorial Pacific basin is possible, as in the case of an infinite domain. As an illustration, two simple coupled models are diagnosed that rely either on thermocline processes or zonal advective processes. The model with thermocline processes is ‘absolutely unstable’ and therefore develops intrinsic oscillations, while the model with zonal advective processes is ‘convectively unstable’ and therefore acts as a noise amplifier. The identification of the two instability regimes may characterize different ENSO formation mechanisms as a response to random wind disturbances. For the absolutely unstable regime, a standing ENSO‐like oscillation can develop in the equatorial Pacific without involving boundary reflections, while for the convectively unstable regime boundary reflections are essential. Copyright © 2012 Royal Meteorological Society
    Type of Medium: Online Resource
    ISSN: 0035-9009 , 1477-870X
    URL: Issue
    URL: Article
    DOI: 10.1002/qj.v139.672
    DOI: 10.1002/qj.1988
    RVK:
    UA 1000
    RVK:
    UA 7650
    Language: English
    Publisher: Wiley
    Publication Date: 2013
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    detail.hit.zdb_id: 2089168-4
    SSG: 14
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  • 3
    Online Resource
    Online Resource
    A theoretical model to analyze the Central to Eastern Pacific El Niño continuum
    Elsevier BV ; 2018
    In:  Ocean Modelling Vol. 130 ( 2018-10), p. 140-159
    Details
    In: Ocean Modelling, Elsevier BV, Vol. 130 ( 2018-10), p. 140-159
    Type of Medium: Online Resource
    ISSN: 1463-5003
    URL: Article
    DOI: 10.1016/j.ocemod.2018.07.006
    Language: English
    Publisher: Elsevier BV
    Publication Date: 2018
    detail.hit.zdb_id: 1126496-2
    detail.hit.zdb_id: 1498544-5
    SSG: 14
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  • 4
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    Online Resource
    A Tropical Stochastic Skeleton Model for the MJO, El Niño, and Dynamic Walker Circulation: A Simplified GCM
    American Meteorological Society ; 2018
    In:  Journal of Climate Vol. 31, No. 22 ( 2018-11-15), p. 9261-9282
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 31, No. 22 ( 2018-11-15), p. 9261-9282
    Abstract: A simple dynamical stochastic model for the tropical ocean atmosphere is proposed that captures qualitatively major intraseasonal to interannual processes altogether including El Niño–Southern Oscillation (ENSO), the Madden–Julian oscillation (MJO), the associated wind bursts, and the background dynamic Walker circulation. Such a model serves as a prototype “skeleton” for general circulation models (GCMs) that solve similar dynamical interactions across several spatiotemporal scales but usually show common and systematic biases in representing tropical variability as a whole. The most salient features of ENSO, the wind bursts, and the MJO are captured altogether including their overall structure, evolution, and fundamental interactions in addition to their intermittency, diversity, and energy distribution across scales. Importantly, the intraseasonal wind bursts and the MJO are here solved dynamically, which provides their upscale contribution to the interannual flow as well as their modulation in return in a more explicit way. This includes a realistic onset of El Niño events with increased wind bursts and MJO activity starting in the Indian Ocean to the western Pacific and expanding eastward toward the central Pacific, as well as significant interannual modulation of the characteristics of intraseasonal variability. A hierarchy of cruder model versions is also analyzed in order to highlight fundamental concepts related to the treatment of multiple time scales, main convective nonlinearities, and the associated stochastic convective parameterizations. The model developed here also should be useful to diagnose, analyze, and help eliminate the strong tropical biases that exist in current operational models.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-18-0263.1
    RVK:
    UA 4548
    Language: Unknown
    Publisher: American Meteorological Society
    Publication Date: 2018
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  • 5
    Online Resource
    Online Resource
    Influence of Recent Stratification Changes on ENSO Stability in a Conceptual Model of the Equatorial Pacific
    American Meteorological Society ; 2013
    In:  Journal of Climate Vol. 26, No. 13 ( 2013-07-01), p. 4790-4802
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 26, No. 13 ( 2013-07-01), p. 4790-4802
    Abstract: Changes in the mean circulation of the equatorial Pacific Ocean partly control the strong decadal modulation of El Niño–Southern Oscillation (ENSO). This relationship is considered from the linear stability of a conceptual recharge/discharge model with parameters tuned from the observed mean state. Whereas decadal changes in the mean thermocline depth alone are usually considered in conceptual ENSO models, here focus is given to decadal changes in the mean stratification of the entire upper ocean (e.g., the mean thermocline depth, intensity, and thickness). Those stratification changes modify the projection of wind stress forcing momentum onto the gravest ocean baroclinic modes. Their influence on the simulated frequency and growth rate is comparable in intensity to the one of usual thermodynamic and atmospheric feedbacks, while they have here a secondary effect on the spatial structure and propagation of SST anomalies. This sensitivity is evidenced in particular for the climate shift of the 1970s in the Simple Ocean Data Assimilation (SODA) dataset, as well as in a preindustrial simulation of the Geophysical Fluid Dynamics Laboratory (GFDL) model showing stratification changes similar to the ones after 2000. Despite limitations of the linear stability approach, conclusions on the sensitivity to stratification may be extended to interpret the modulation and diversity of ENSO in observations and in general circulation models.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/JCLI-D-12-00363.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2013
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    detail.hit.zdb_id: 2021723-7
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  • 6
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    Online Resource
    A Suite of Skeleton Models for the MJO with Refined Vertical Structure
    Portico ; 2015
    In:  Mathematics of Climate and Weather Forecasting Vol. 1, No. 1 ( 2015-01-30)
    Details
    In: Mathematics of Climate and Weather Forecasting, Portico, Vol. 1, No. 1 ( 2015-01-30)
    Abstract: 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 5 ms−1, (II) a peculiar dispersion relation with dw/dk ≈ 0, and (III) a horizontal quadrupole vortex structure. This model depicts the MJO as a neutrally-stable atmosphericwave that involves a simple multiscale interaction between planetary dry dynamics, planetary lower-tropospheric moisture and the planetary envelope of synoptic-scale activity. Here we propose and analyze a suite of skeleton models that qualitatively reproduce the refined vertical structure of the MJO in nature. This vertical structure consists of a planetary envelope of convective activity transitioning from the congestus to the deep to the stratiform type, in addition to a front-to-rear (i.e. tilted) structure of heating, moisture, winds and temperature. A first example of skeleton model achieving this goal has been considered recently in work by the authors. The construction of such a model satisfies an energy conservation principle, such that its solutions at the intraseasonal-planetary scale remain neutrally stable. Here, additional classes of skeleton models are constructed based on the same principle. In particular, those new models are more realistic then the former one as they consider fully coupled interactions between the planetary dry dynamics of the first and second baroclinic mode and the details of the vertical structure of moisture and convective activity. All models reproduce qualitatively the refined vertical structure of the MJO. In addition,when considered with a simple stochastic parametrization for the unresolved details of synopticscale activity, all models show intermittent initiation, propagation and shut down of MJO wave trains, as in previous studies.
    Type of Medium: Online Resource
    ISSN: 2353-6438
    URL: Article
    DOI: 10.1515/mcwf-2015-0004
    Language: Unknown
    Publisher: Portico
    Publication Date: 2015
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  • 7
    Online Resource
    Online Resource
    Sensitivity of ENSO to Stratification in a Recharge–Discharge Conceptual Model
    American Meteorological Society ; 2011
    In:  Journal of Climate Vol. 24, No. 16 ( 2011-08-15), p. 4332-4349
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 24, No. 16 ( 2011-08-15), p. 4332-4349
    Abstract: El Niño–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 further indicates that, in the weak coupling regime, the model allows for several ocean basin modes at low frequency. The latter can eventually merge into a low-frequency and unstable mode representative of ENSO as the coupling efficiency increases. Also, higher baroclinic modes project more energy onto the ocean dynamics for the same input of wind forcing. Therefore, in this study’s model, a shallower, yet more intense mean thermocline may still sustain a strong (i.e., unstable) and low-frequency ENSO mode. Sensitivity tests to the strength of the two dominant feedbacks (thermocline vs zonal advection) indicate that the presence of high-order baroclinic modes favors the bifurcation from a low-frequency regime to a higher-frequency regime when the zonal advective feedback is enhanced. It is suggested that the proposed formalism can be used to interpret and measure the sensitivity of coupled general circulation models to climate change.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    DOI: 10.1175/2011JCLI4148.1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2011
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 8
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    Online Resource
    Observations and Mechanisms of a Simple Stochastic Dynamical Model Capturing El Niño Diversity
    American Meteorological Society ; 2018
    In:  Journal of Climate Vol. 31, No. 1 ( 2018-01-01), p. 449-471
    Details
    In: Journal of Climate, American Meteorological Society, Vol. 31, No. 1 ( 2018-01-01), p. 449-471
    Abstract: El Niño–Southern Oscillation (ENSO) has significant impact on global climate and relevance for seasonal forecasts. Recently, a simple modeling framework was developed that captures the ENSO diversity, where state-dependent stochastic wind bursts and nonlinear advection of sea surface temperature are coupled to a simple ocean–atmosphere model that is otherwise deterministic, linear, and stable. In this article, the coupled model is compared with observations using reanalysis data over the last 34 yr, where the observed non-Gaussian statistics and the overall mechanisms of ENSO are both captured by the model. Then the formation mechanisms of both the central Pacific (CP) and the traditional El Niño in the model are systematically studied. First, ocean Rossby waves induced by easterly trade wind anomalies facilitate the heat content buildup. Then the reflected ocean Kelvin waves and the nonlinear advection lead to positive SST anomalies in the CP region and create a CP El Niño. Second, two formation mechanisms are revealed for the traditional El Niño, including the super (extreme) El Niño. The first mechanism indicates a preferred wind structure with easterly wind bursts (EWBs) leading westerly wind bursts (WWBs), where the EWBs build up heat content and then the WWBs trigger the El Niño. The second mechanism links the two types of El Niño, where a CP El Niño favors a heat content buildup and the advent of a traditional El Niño. This article also highlights the mechanisms of La Niña formation and El Niño termination.
    Type of Medium: Online Resource
    ISSN: 0894-8755 , 1520-0442
    URL: Article
    URL: Article
    DOI: 10.1175/JCLI-D-16-0880.1
    DOI: 10.1175/JCLI-D-16-0880.s1
    RVK:
    UA 4548
    Language: English
    Publisher: American Meteorological Society
    Publication Date: 2018
    detail.hit.zdb_id: 246750-1
    detail.hit.zdb_id: 2021723-7
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  • 9
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    Evaluating MJO event initiation and decay in the skeleton model using an RMM‐like index
    American Geophysical Union (AGU) ; 2015
    In:  Journal of Geophysical Research: Atmospheres Vol. 120, No. 22 ( 2015-11-27)
    Details
    In: Journal of Geophysical Research: Atmospheres, American Geophysical Union (AGU), Vol. 120, No. 22 ( 2015-11-27)
    Abstract: An RMM‐like index was created for the skeleton model that mimics observations Stochasticity helps improve MJO initiation and termination event statistics The skeleton model produces more realistic MJOs when forced with observed SSTs
    Type of Medium: Online Resource
    ISSN: 2169-897X , 2169-8996
    URL: Issue
    URL: Article
    DOI: 10.1002/jgrd.v120.22
    DOI: 10.1002/2015JD023916
    Language: English
    Publisher: American Geophysical Union (AGU)
    Publication Date: 2015
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    detail.hit.zdb_id: 2016800-7
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    SSG: 16,13
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  • 10
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    Online Resource
    Reinterpreting the thermocline feedback in the western-central equatorial Pacific and its relationship with the ENSO modulation
    Springer Science and Business Media LLC ; 2013
    In:  Climate Dynamics Vol. 41, No. 3-4 ( 2013-8), p. 819-830
    Details
    In: Climate Dynamics, Springer Science and Business Media LLC, Vol. 41, No. 3-4 ( 2013-8), p. 819-830
    Type of Medium: Online Resource
    ISSN: 0930-7575 , 1432-0894
    URL: Article
    DOI: 10.1007/s00382-012-1504-z
    Language: English
    Publisher: Springer Science and Business Media LLC
    Publication Date: 2013
    detail.hit.zdb_id: 382992-3
    detail.hit.zdb_id: 1471747-5
    SSG: 16,13
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