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Scholarly Works (9 results)

  • Article
  • Peer Reviewed
UC Irvine Previously Published Works (2020)
El Niño–Southern Oscillation (ENSO) transitions from one event to another in complex ways. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical-subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multiyear ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multiyear transitions can occur. A future warmer tropical Pacific is projected to decrease the frequency of occurrence of multiyear ENSO transitions but increase the occurrence of cyclic ENSO transitions.
    Cover page: A Control of ENSO Transition Complexity by Tropical Pacific Mean SSTs Through Tropical‐Subtropical InteractionCreative Commons 'BY' version 4.0 license
    • Thesis
    • Peer Reviewed
    UC Irvine Electronic Theses and Dissertations (2020)

    No two El Niño-Southern Oscillation (ENSO) events evolve the same way. Transitions from one ENSO event to another occur in various ways and constitute a key component of ENSO complexity. While ENSO complexity in amplitude, periodicity, and spatial patterns has been frequently studied, ENSO transition complexity has not yet been systematically explored in either the observations or climate model simulations. This dissertation uses statistical analyses and numerical modeling experiments to develop a dynamical framework that can explain how three key transition patterns of ENSO (i.e., episodic, cyclic, and multi-year ENSOs) are produced, and further applies this framework to examine these ENSO transitions in contemporary climate models and to project future changes of the transition complexity.

    This dissertation finds that the occurrence of the three ENSO transitions depends on two primary onset mechanisms of ENSO. The tropical Pacific onset (TP-onset) mechanism initiates sea surface temperature (SST) anomalies in the equatorial eastern Pacific through thermocline variations, while the subtropical Pacific onset (SP-onset) mechanism brings the subtropical SST anomalies into the equatorial central Pacific through a series of subtropical couplings. The TP-onset mechanism is found to mainly produce the cyclic ENSO transition and, as a result, contributes to reduced ENSO transition complexity. In contrast, the SP-onset mechanism is found to be capable of producing all three transitions and is a key source of the transition complexity. While the TP-onset mechanism has maintained its strength in the past six decades, the SP-onset mechanism became more important in producing ENSO events since the early-1990s. The intensified SP-onset mechanism increases ENSO complexity and can be a factor for the changing ENSO properties observed in the 21st century.

    By further focusing on the SP-onset mechanism, this dissertation finds that the tropical mean state of SSTs can control the occurrence of cyclic and multi-year ENSO transitions. Specifically, the mean SST in the eastern equatorial Pacific controls the frequency of cyclic transitions related to the SP-onset mechanism, while the mean SST in the central equatorial Pacific is responsible for the multi-year transitions. This control arises from the fact that the mean state SST determines how easily the anomalous warming/cooling from an ENSO event can excite deep convective heating in each region to activate the SP-onset mechanism and trigger another ENSO event in the following year. In a future warmer world, this mean state control is projected to increase the cyclic transition but decrease the multi-year transition of ENSO.

    In this dissertation, ENSO transition complexity is also compared between observations and contemporary climate model simulations. The El Niño transition complexity is found to be dominated in order by the episodic transition, cyclic transition, and multi-year transition. Interestingly, the reversed order is discovered for the La Niña transition complexity (multi-year, cyclic, and then episodic La Niña). This asymmetry between El Nino and La Niña transitions results from two reasons: 1) the SP-onset mechanism generates episodic El Niño more than the episodic La Niña due to the nonlinear growth of the equatorial wind anomalies it induces, 2) the nonlinear responses of the SP-onset mechanism to tropical Pacific mean SSTs favor more multi-year La Niña than multi-year El Niño. Contemporary models realistically produce the observed transition complexity for El Niño but fail to simulate the reversed order of La Niña transitions. These deficiencies arise from a weak subtropical onset mechanism in the models and a cold bias in tropical Pacific mean-state.

    Findings from this dissertation offer a novel perspective to understand and study ENSO complexity dynamics, which is a new area of ENSO research. The dynamical framework developed has the potential to branch a critical new direction of understanding ENSO dynamics, properties, and activities in present, past, and future climates.

      Cover page: Understanding ENSO Transition Complexity and its Underlying Dynamics
      • Article
      • Peer Reviewed
      UC Irvine Previously Published Works (2018)
      This study finds the seasonal footprinting (SF) mechanism to be a key source of El Niño–Southern Oscillation (ENSO) complexity, whereas the charged-discharged (CD) mechanism acts to reduce complexity. The CD mechanism forces El Niño and La Niña to follow each other, resulting in a more cyclic and less complex ENSO evolution, while the SF mechanism involves subtropical forcing and results in an ENSO evolution that is more episodic and irregular. The SF mechanism also has a tendency to produce multiyear La Niña events but not multiyear El Niño events, contributing to El Niño-La Niña asymmetries. The strength of CD mechanism has been steady, but SF mechanism has intensified during the past two decades, making ENSO more complicated. Most Climate Model Intercomparison Project version 5 models overestimate the strength of the CD mechanism but underestimate the strength of the SF mechanism, causing their simulated ENSOs to be too regular and symmetric.
        Cover page: The Distinct Contributions of the Seasonal Footprinting and Charged‐Discharged Mechanisms to ENSO ComplexityCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2020)
        The observed El Niño and La Niña exhibit different complexities in their event-to-event transition patterns. The El Niño is dominated in order by episodic, cyclic, and multiyear transitions, but the reversed order is found in the La Niña. A subtropical Pacific onset mechanism is used to explain this difference. This mechanism triggers El Niño/La Niña events via subtropical processes and is responsible for producing multiyear and episodic transitions. Its nonlinear responses to the tropical Pacific mean state result in more multiyear transitions for La Niña than El Niño and more episodic transitions for El Niño than La Niña. The CMIP5/6 models realistically simulate the observed transition complexity of El Niño but fail to simulate the transition complexity of La Niña. This deficiency in CMIP5 models arises from a weaker than observed subtropical onset mechanism and a cold bias in the tropical Pacific mean sea surface temperatures in the models.
          Cover page: Contrasting Transition Complexity Between El Niño and La Niña: Observations and CMIP5/6 ModelsCreative Commons 'BY' version 4.0 license
          • Peer Reviewed
          UC Irvine Previously Published Works (2020)
          An ENSO event can transition from one event to another in complex ways. An El Niño (La Niña) event can be preceded by a La Niña (El Niña) event to become a cyclic ENSO, by a neutral event to become an episodic ENSO, or by another El Niño (La Niña) event to become a multi-year ENSO. We find that the observed El Niño and La Niña exhibit different complexities in their event-to-event transition patterns. The El Niño is dominated in order by episodic, cyclic, and multi-year transitions, but the reversed order is found in the La Niña. This difference in the transition complexity arises from a subtropical Pacific forcing mechanism that triggers ENSO events. Using observational analyses and forced atmospheric model experiments, we show that a preceding ENSO event can activate a subtropical Pacific forcing mechanism to trigger another ENSO event during the following year. These tropical-subtropical Pacific interactions result in a cyclic ENSO transition if the two ENSO events are of opposite signs or a multi-year ENSO transition if they are of the same sign. The preceding ENSO event should excite deep convections in the tropical Pacific in order to activate the subtropical Pacific mechanism. This requirement enables mean temperatures in the cold tongue and warm pool to respectively control how easily the cyclic and multi-year transitions can occur. This mean state control enables the subtropical Pacific forcing mechanism to result in more multi-year transitions for La Niña than El Niño and more episodic transitions for El Niño than La Niña. Furthermore, a future warmer tropical Pacific is projected to decrease the frequency of occurrence of multi-year ENSO transitions but increase the occurrence of cyclic ENSO transitions.. We find that the CMIP5 and CMIP6 models can reproduce the transition complexity for El Niño but not for La Niña. The models tend to produce too many episodic La Niña events and too few multi-year La Niña events. We are able to link the former deficiency to a weaker than observed subtropical Pacific forcing mechanism in the CMIP5/6 models and the latter to a cold bias in mean state SSTs in the equatorial Pacific in the CMIP5/6 models. To achieve better simulations of ENSO transition complexity, further efforts are to improve the model deficiencies in simulating the SP-onset mechanism and mean SSTs in the equatorial Pacific.
            Cover page: Contrasting ENSO transition complexities between El Niña and La Niña: The underlying complexity dynamics and future changesCreative Commons 'BY' version 4.0 license
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2020)
            This study compares the impacts of the El Niño–Southern Oscillation (ENSO) on sea surface temperatures (SSTs) in the tropical North Atlantic Ocean and the tropical Indian Ocean during 1958–2004. It is found that the tropical atmospheric processes mediating the ENSO impacts are different between the two oceans for two reasons. First, the ENSO-induced anomalous Walker circulation is more extensive over the Atlantic than over the Indian Ocean. As a result, the atmospheric bridge (AB) mechanism is the major contributor to the differences in ENSO teleconnections between the two oceans. Secondly, SSTs in the tropical North Atlantic are under a greater control of the atmospheric thermal forcing than those in the tropical Indian Ocean. Due to these different controls, the tropospheric temperature (TT) mechanism also contributes to the different ENSO teleconnections. When compared with the observations, the mean of thirty-seven models from the Coupled Model Intercomparison Project Phase 5 overestimates the ENSO-induced SST response in the tropical Indian Ocean but underestimates the response in the tropical North Atlantic. The overestimation is brought about by a westward extension of ENSO SST anomalies in the models, which causes the AB mechanism to produce an overly strong impact on Indian Ocean SSTs. On the other hand, the underestimation is caused by a weaker-than-observed sensitivity in the simulated Atlantic SSTs to the thermal forcing produced by the TT mechanism.
              Cover page: ENSO’s impacts on the tropical Indian and Atlantic Oceans via tropical atmospheric processes: observations versus CMIP5 simulationsCreative Commons 'BY' version 4.0 license
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2020)
              During 1958–2017, the Central-Pacific (CP) El Niño evolved into La Niña less frequently (38%) than the Eastern-Pacific (EP) El Niño (75%). Composite analyses reveal that the reversal of zonal wind anomalies in the tropical western Pacific is the key mechanism for the EP El Niño's transitions to La Niña. This reversal induces oceanic Kelvin waves to promote the La Niña onset. This reversal mechanism is often triggered by the Indo-Pacific teleconnection produced by the EP El Niño. Consequently, the EP El Niño often experiences the transitional evolution. For the CP events, the southward shift of westerly anomalies in the tropical central Pacific is the key mechanism for transitions from El Niño to La Niña via local processes. However, the subtropical teleconnection of the CP El Niño prevents this mechanism from occurring and often causes the nontransitional evolution for the CP events.
                Cover page: Why Does the CP El Niño less Frequently Evolve Into La Niña than the EP El Niño?Creative Commons 'BY' version 4.0 license
                • Article
                • Peer Reviewed
                UC Irvine Previously Published Works (2021)
                This study examines a climate model hindcast of the responses of the western Pacific subtropical high (WPSH) to three types of El Niño events: the Eastern Pacific (EP) El Niño and the types I and II of Central Pacific (CP-I and CP-II) El Niño. These El Niño types differ from each other in the central locations and patterns of their sea surface temperature (SST) anomalies. They invoke four different mechanisms to impact the WPSH. Hindcasts produced by the Taiwan Central Weather Bureau (CWB) Climate Forecast System 1-tiered model (TCWB1T1) are analyzed. These hindcasts realistically simulate the impacts on the WPSH during the CP-I El Niño, but overestimate the impacts during the decaying summer of the EP El Niño and during the developing autumn and early winter of the CP-II El Niño. The overestimates are mainly the result of an overly strong Maritime Continent regional circulation mechanism during EP El Niños and an overly strong Indian Ocean capacitor mechanism during CP-II El Niños. Further analyses show that these model biases are related to biases in the simulations of El Niño SST anomalies and the Walker circulation. Both model deficiencies are common to contemporary coupled climate models.
                  Cover page: A study of climate model responses of the western Pacific subtropical high to El Niño diversityCreative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2018)
                  The ability to construct nitrate maps in the Southern Ocean (SO) from sparse observations is important for marine biogeochemistry research, as it offers a geographical estimate of biological productivity. The goal of this study is to infer the skill of constructed SO nitrate maps using varying data sampling strategies. The mapping method uses multivariate empirical orthogonal functions (MEOFs) constructed from nitrate, salinity, and potential temperature (N-S-T) fields from a biogeochemical general circulation model simulation Synthetic N-S-T datasets are created by sampling modeled N-S-T fields in specific regions, determined either by random selection or by selecting regions over a certain threshold of nitrate temporal variances. The first 500 MEOF modes, determined by their capability to reconstruct the original N-S-T fields, are projected onto these synthetic N-S-T data to construct time-varying nitrate maps. Normalized root-mean-square errors (NRMSEs) are calculated between the constructed nitrate maps and the original modeled fields for different sampling strategies. The sampling strategy according to nitrate variances is shown to yield maps with lower NRMSEs than mapping adopting random sampling. A k-means cluster method that considers the N-S-T combined variances to identify key regions to insert data is most effective in reducing the mapping errors. These findings are further quantified by a series of mapping error analyses that also address the significance of data sampling density. The results provide a sampling framework to prioritize the deployment of biogeochemical Argo floats for constructing nitrate maps.
                    Cover page: A Multi-variate Empirical Orthogonal Function Method to Construct Nitrate Maps in the Southern Ocean A Multi-variate Empirical Orthogonal Function Method to Construct Nitrate Maps in the Southern OceanCreative Commons 'BY' version 4.0 license
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