Day 1 - March 12, 2025 Poster Session (Makana Room)
Title: Nonlinear feedbacks from Tropical Instability Waves crucial to the tropical Pacific response to greenhouse warming
Abstract:
Tropical instability Waves (TIWs) dominate intraseasonal variability in the tropical Pacific Ocean, and strongly impact tropical ocean dynamics and marine ecosystems as well as modulating tropical climate variability. However, TIWs’ response to future greenhouse warming remains relatively unexplored because most current climate models cannot resolve TIWs well. Here we use a suite of TIW-resolving climate model simulations to reveal that there is a consistent decrease in TIW-driven temperature variability under CO2 increases, mainly due to the weakening of the meridional sea surface temperature gradient. Two distinct mechanisms affect TIW changes: north of the equator, sea surface height variability of Rossby mode increase due to intensified zonal current shear during boreal fall, creating a stronger oceanic momentum flux feedback on zonal current systems; Along the equator, TIW variability associated with the Yanai mode decreases, due to a combination of increased current shear and decreased temperature gradient. This results in a weaker heat flux feedback on mean temperature. On seasonal timescales, intensified TIW activity weakens the annual cycle of cold tongue temperature and equatorial zonal flow variability. These nonlinear feedbacks across different timescales are systematically absent in models which do not resolve TIWs. Improvement in TIW simulations is therefore a key pathway for reducing uncertainty in tropical Pacific climate projections.
Title: ENSO impacts on physical-ecological conditions in the California Current System
Abstract:
ENSO is recognized as one of the potentially predictable drivers of California Current System (CCS) ecosystem variability. In this study, we analyze a multi-decadal eddy-permitting ocean model hindcast, with multiple classes of phytoplankton and zooplankton, to assess the bottom-up physical-ecological response of the CCS during ENSO events. The biogeochemical and ecological response is represented by ENSO-composite anomalies, lag-correlations with an ENSO index, and histograms for ENSO years. The responses exhibit large-scale coherent relationships between physics and the ecosystem, including reduced ecological activity during El Niño, and increased ecological activity during La Nina. The response of the ocean temperature anomalies over the CCS is asymmetrical, with La Nina events being more consistently cold than El Niño events are consistently warm, which agrees with previous studies. The results demonstrate trophic level interactions during El Niño and La Nina events in which the larger components (diatoms, euphausiids, and copepods) are suppressed in the coastal upwelling zones during El Niño, while the smaller components (flagellates and ciliates) are enhanced. In addition, standing eddies of the CCS modulate the latitudinal structure of the ecological response to ENSO. These physical-biological analyses provide a view of some of the possible successes and limitations for predictable impacts of ENSO teleconnections on the CCS.
Title: The Three Dimensional Circulation of the The Wyrtki jets in the Indian Ocean
Abstract:
The Wyrtki jets are strong wind-forced zonal currents in the upper 100 m of the equatorial Indian Ocean that appear twice per year in the transition seasons between the Northeast and Southwest Monsoons. They play a critical role in the mass and heat balances of the Indian Ocean on seasonal to internannual time scales, particularly related to the development of Indian Ocean Dipole events. The westerly winds that force these jets are downwelling favorable, but vertical velocities associated with the Wyrtki jets have never been observed before because of their small magnitudes and lack of sufficient data to detect them. In this study, we use five years of velocity data from an array of acoustic Doppler current profilers (ADCPs) embedded in the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA) to estimate the three-dimensional circulation of the Wyrkti jets. Our estimates of vertical velocity are based on the continuity equation using horizontal velocities from the ADCP array. We compare these estimates to those from a high spatial resolution regional wind-force ocean circulation model of the Indian Ocean, which simulates the observed velocities with a high degree of fidelity. We then use the data and the model to define the structure, temporal variability and dynamics of the three-dimensional flow field associated with the Wyrtki jets on seasonal-to-interannual time scales.
Title: The diverse impacts of El Niño and La Niña events over the South Pacific and in French Polynesia with a focus on Tropical Cyclones Activity
Abstract:
The El Niño Southern Oscillation (ENSO) is the primary driver of global climate variability on an interannual scale, with El Niño and La Niña events disrupting atmospheric and oceanic conditions worldwide. These events form an "ENSO continuum," spanning from Central to Eastern Pacific occurrences, each producing distinct impacts. Understanding the diversity of these effects is especially important for the vast and under-studied region of south-central Pacific, including French Polynesia (FP), as it notably has a highly-variable tropical cyclone activity with some years seeing high cyclonic activity and others experiencing none. Using a multivariate cluster classification approach based on Relative Sea Surface Temperature (RSST), precipitation, and zonal wind, combined with an atmospheric model ensemble forced by observed SST, we show that ENSO intensity and spatial pattern diversity are the main factors driving interannual climate variability in FP. Notably, precipitation anomalies depend heavily on the strength and spatial configuration of ENSO, associated to distinct shifts in the South Pacific Convergence Zone (SPCZ) across the different ENSO clusters. This classification, specific to the region, also improves our understanding of ENSO influence on tropical cyclone activity. Combining it with best tracks archive data, environmental conditions from ERA5 and an augmented synthetic tropical cyclone dataset generated using a state-of-the-art TC downscaling model, we demonstrate that the probability of cyclones in FP depends heavily on the El Niño or La Niña flavor. Additionally, we highlight a stronger cyclonic activity in the South-Central Pacific during the 1981-2002 period compared to the 2002-2024 one, discussing possible explanations for this interdecadal modulation.
Title: A Statistically Accurate ENSO-MJO Coupled Conceptual Model
Abstract:
The El Niño-Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO) phenomena impact weather patterns in distinct areas along the equator and have broad effects beyond the tropics. ENSO exhibits diverse characteristics in spatial pattern, peak intensity, and temporal evolution. Observational evidence shows that the ENSO and MJO influence each other. Here, we develop a stochastically coupled conceptual model for ENSO and MJO. The model can reproduce not only the general properties of the observed ENSO events but also the complexity in patterns (e.g., Central Pacific vs. Eastern Pacific events), intensity (e.g., 10- or 20-year reoccurrence of extreme El Niños), and temporal evolution (e.g., more multi-year La Niñas than multi-year El Niños). The model can also reproduce the observed MJO propagation patterns and statistics. Such a statistically accurate dynamical-statistical model offers a valuable tool for understanding the coupled phenomenon. It helps predict the diverse features of the ENSO and the MJO. The model also has a unique ability to improve the imperfect output from more complicated models (e.g., GCMs) via effective data assimilation.
Title: Investigating the impacts of changes in ocean-atmosphere coupling and mean state on ENSO in a hybrid coupled model under global warming
Abstract:
Predicting the response of ENSO to global warming remains a challenging problem due to large differences across climate models. Overall, the climate change simulations of CMIP6 indicate a strengthening of ENSO sea surface temperature variability over the 21st century. Despite this emerging consensus, the underlying mechanisms by which climate change strengthens ENSO variability are not fully understood and large uncertainties in projections of ENSO persist. Previous experiments (Stuivenvolt-Allen et al. 2024) investigated how projected changes in the structure of ENSO wind-stress anomalies in the equatorial Pacific could impact ENSO magnitude and periodicity using a novel hybrid coupled model based on CESM2. The hybrid model uses the full ocean GCM coupled to a linear regression model in the tropical Pacific, simulating a realistic ENSO. By modifying the structure of wind stress anomalies, they isolate wind stress changes expected under global warming and investigate potential changes in ENSO characteristics in the absence of changes in the mean state. We build on their approach to explore a complementary question - how changes in the mean state associated with global warming can influence ENSO characteristics in the absence of changes in the structure of tropical Pacific wind stress anomalies. To that end, we are conducting numerical experiments that systematically explore the role of mean state changes in ENSO. Initially, we used a simple linear statistical atmosphere based on wind stress regression onto the Niño-3.4 index. Next, we will employ a nonlinear atmospheric model to help understand changes to extreme El Niño events.
Title: Uncrewed Surface Vehicles on the leading edge of a Tropical Instability Wave
Abstract:
As part of the broader effort to integrate Uncrewed Surface Vehicles (USVs) into ocean observing systems, NOAA PMEL has been conducting annual USV missions in the equatorial Pacific since 2017. These missions aim to develop adaptive sampling strategies for USVs, preparing for incorporation of USVs into the Tropical Pacific Observing System (TPOS) and for case studies in the upcoming T(POS)-Equatorial Pacific EXperiment (TEPEX). TEPEX seeks to address knowledge gaps in processes regulating sea surface temperatures (SSTs) in the eastern tropical Pacific, contributing to persistent biases in SST and precipitation climatologies, and reduced skill in El Niño Southern Oscillation (ENSO) predictions.
Tropical instability waves (TIWs) significantly influence SST variability in this region. This study presents findings from the 2019 TPOS Saildrone mission, which observed the passage of a TIW near 0°, 140°W. The 16-day case study uses the surface meteorology, shortwave and longwave radiation, sea surface temperature and salinity, as well as subsurface downward looking acoustic doppler current profilers (ADCPs) measurements from two drones. The high-resolution dataset provides insights into upper-ocean and air-sea interface variability during the TIW passage. Key findings include horizontal convergence consistent with downwelling on the TIW’s leading edge and a strong diurnal cycle in SST. These multiscale interactions highlight the challenges of observing and modeling such processes.
Title: From the Ocean Subsurface to the Upper Troposphere: A Machine-Learning Based Approach for Holistic ENSO Characterization
Abstract:
The El Niño-Southern Oscillation (ENSO) is a complex, interannual phenomenon in the tropical Pacific region. It is dominated by two modes, El Niño (warm state) and La Niña (cool state), which are often identified using average sea surface temperature anomalies across the Niño 3.4 region. Although the dynamics of ENSO span from the ocean subsurface through the upper troposphere, the common indices used to characterize ENSO only capture a portion of the phenomenon’s complexity through variables located at the air-sea interface. Additionally, the tightly bounded Niño 3.4 region associated with these indices does not capture the diversity. This study derives an index using an autoencoder, an unsupervised neural network that can learn nonlinear relationships without labeled data. The autoencoder is trained using ocean heat content, sea surface temperature, and outgoing longwave radiation from the Department of Energy (DOE) Energy Exascale Earth System Model Version 2 (E3SMv2) Large Ensemble. The region of interest encompasses much of the tropical Pacific Ocean, freeing the derived index from a fixed geographical domain. The skill of the index is assessed by investigating the associated spatial fields and teleconnections using composites. We discuss the notion of predictability, or more specifically, how we can determine ENSO’s predictability as a function of the index used to describe it. Reanalysis is subsequently used to fine-tune the new ENSO index using transfer learning, where the neuron updates can provide insight into E3SMv2 biases based on how much retraining is needed to reconstruct ENSO fields with the autoencoder.
Title: Why Do Oceanic Nonlinearities Contribute Only Weakly to Extreme El Niño Events?
Abstract:
Extreme El Niño events have outsized global impacts and control the El Niño Southern Oscillation (ENSO) warm/cold phases asymmetries. Yet, a consensus regarding the relative contributions of atmospheric and oceanic nonlinearities to their genesis remains elusive. Here, we isolate the contribution of oceanic nonlinearities by conducting paired experiments forced with opposite wind stress anomalies in an oceanic general circulation model, which realistically simulates extreme El Niño events and oceanic nonlinearities thought to contribute to ENSO skewness (Tropical Instability Waves (TIWs), Nonlinear Dynamical Heating (NDH)). Our findings indicate a weak contribution of oceanic nonlinearities to extreme El Niño events in the eastern Pacific, owing to compensatory effects between lateral (NDH and TIWs) and vertical processes. These results hold across different vertical mixing schemes and modifications of the upper‐ocean heat budget mixed layer criterion. Our study reinforces previous research underscoring the pivotal role of atmospheric nonlinearities in shaping extreme El Niño events.
Title: Observed Tropical Pacific Temperature Trends Outside Modelled Range When Computed in a Novel Temperature Coordinate
Abstract:
Tropical Pacific sea surface temperatures (SSTs) exert a substantial influence on regional and global climate, shaping large-scale atmospheric circulation and driving seasonal climate extremes. Atmosphere-ocean interactions in the tropical Pacific are tightly coupled, complex, and undergo large internal variability modulated by a strong background SST gradient across the Pacific basin. From 1982 to 2020, this gradient has intensified, but it remains unclear if this is due to internal variability or anthropogenic warming, and whether the observed trend lies within the envelope of climate model simulations. Here, we show that clarity on these questions can be achieved through judicious choices of gradient metrics that better isolate trends from internal variability. Past literature has examined long-term trends in the SST gradient using fixed east-west boxes—but the temperature within these boxes is strongly influenced by internal variability due to factors such as El Niño–Southern Oscillation. We introduce a semi-Lagrangian metric based on the difference between dynamic areas characterizing the warmest and coldest 50% of the tropical Pacific surface ocean. Previous studies using fixed east-west boxes have shown that the observed and modeled trends are unlikely to be consistent; our study shows that when utilizing a cold-warm semi-Lagrangian metric, the consistency is beyond unlikely, becoming virtually impossible.
Title: Skillful seasonal predictions of coastal risks from climate modes interactions and ENSO’s reorganized weather systems
Abstract:
Extreme weather and climate events result from complex interactions between physical processes at different scales. The convergence of multiple factors, including large-scale environmental conditions and local climate variability, can amplify the effects, resulting in significant societal impacts. Coastal regions are particularly vulnerable to sea level rise and changes in coastal water levels (CWL) due to climate variability, ocean circulation, and atmospheric conditions. The El Niño/Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are key drivers of interannual CWL variability in the Northern hemisphere, influencing storm activity, flooding, and erosion, with ENSO affecting the Pacific and NAO the Atlantic. While studies have extensively analyzed their independent effects, their combined influence on coastal hazards remain underexplored. This study uses diverse observational datasets to assess the modulation of extreme CWL and associated hazards by different phases of ENSO and NAO. We show that the frequent occurrence of La Niña conditions, although relatively weak in terms of severity, and the comparatively rare but exceptionally strong extreme El Niño events make the world's coastlines more vulnerable to flooding overall. However, the picture is different regionally, especially in the North Atlantic and Mediterranean regions, where the co-occurrence of El Niño events and different phases of the NAO tends to exacerbate extreme CWL compared to the local NAO variability alone due to the strengthening of the Pacific-Atlantic jet stream teleconnections either in the high or mid latitudes, depending on the ENSO type and the NAO phase. These results highlight the climate modes’ compounded risks to coastal populations and underscore the potential for improved seasonal forecasting of coastal hazards based on the consistent seasonal patterns associated with these oscillations.
Title: Biased ENSO Dynamic Feedback in the Atmosphere-Only Simulations of CMIP6 Climate Models
Abstract:
El Niño/Southern Oscillation (ENSO) is a key tropical Pacific atmosphere-ocean coupled phenomenon for modulating year-to-year climate worldwide, of which sea surface temperature (SST) variability is successfully simulated by the current generation of climate models. However, atmospheric feedback processes regarding the ENSO growth are systematically too weak, primarily due to the too-cold eastern equatorial Pacific SST in the atmosphere-ocean coupled models, potentially adding uncertainty to seasonal forecasts and future projections. Previous studies reported that atmospheric feedback in response to ENSO’s SST anomalies remains biased even in atmosphere-only simulations, but the reason is unclear. This study examined atmospheric internal processes to reveal ENSO feedback biases in the atmospheric part of CMIP6 climate models forced with observed SST. The net heat flux feedback is comparable to observations on average, but the central Pacific zonal wind feedback is underestimated in most models albeit a realistic equatorial precipitation-SST relation. The wind feedback bias is attributed to the wind responses to the equatorial precipitation anomalies that seasonally erroneously decline in boreal late winter. It is found that the model’s mean state with a higher equatorial Pacific vertical wind shear is unfavorable for enhancing the central Pacific zonal wind perturbation in response to ENSO’s precipitation anomalies and thus the atmospheric dynamic feedback.
Title: A Physics-Informed Auto-Learning Framework for Developing Stochastic Conceptual Models for ENSO Diversity
Abstract:
Understanding ENSO dynamics has tremendously improved over past 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. The limited number of weak nonlinearities in the model are essential in reproducing the observed extreme El Nino events and the observed nonlinear relationship between eastern and western Pacific SSTs.
Title: Drivers of Mean State-ENSO Amplitude Interactions in Large Ensemble Simulations
Abstract:
Understanding the response of the El Niño/Southern Oscillation (ENSO) to projected future climate change has long been a goal of the research community, but a comprehensive answer to this question has proved elusive to date. Here we evaluate a suite of large ensembles run with multiple climate models, under a variety of climate change scenarios, to assess the mechanisms for inter-model differences in ENSO projections. The majority of models project some degree of ENSO amplitude increase under 21st century scenarios, but many ensembles suggest that ENSO will peak or even decline by the end of the century. The relationship between ENSO and mean climate also appears to be altered substantially during the late 21st century, with some models exhibiting a decoupling between ENSO and the zonal sea surface temperature gradient. The connection between ENSO-mean state interactions and the behavior of atmospheric convection is examined, with a view towards identifying the role of convective ‘saturation’ in the late 21st century. Implications for model validation and constraining future projections based on historical climate are discussed.
Title: Stronger ENSO-induced global SST variability in a warming climate.
Abstract:
El Niño-Southern Oscillation (ENSO) is a prominent interannual climate variability that exerts a significant influence on remote regions through what is known as ENSO teleconnections. Understanding how ENSO teleconnections change under global warming is crucial for predicting future local subseasonal-to-seasonal climate variability. Sea surface temperatures (SSTs) are particularly well-known for their pronounced influence on local climates. However, despite recent progress, the response of SSTs to ENSO under global warming remains uncertain. In this study, we investigate the changes in ENSO impacts on global SSTs using data from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and the Community Earth System Model 1 (CESM1). These global climate models show a robust amplification of ENSO impacts on global SSTs, primarily driven by amplified El Niño-induced surface latent heat flux. This strengthened El Niño-induced surface latent heat flux is linked to the intensification of El Niño-driven atmospheric circulation and the greater global air-sea specific humidity difference, both of which are due to the Clausius-Clapeyron relationship. These results suggest that ENSO is likely to lead to more severe climate extremes and affect subseasonal-to-seasonal climate predictability.
Title: Using the Energy Exascale Earth System Model to Understand Changes in La Niña Characteristics in a Warming Climate
Abstract:
The El Niño-Southern Oscillation (ENSO), characterized by periodic shifts between warm and cold sea surface temperature anomalies (SSTAs) in the equatorial Pacific, is one of Earth’s most consequential modes of climate variability. However, the complexity of the physical processes driving ENSO’s response to anthropogenic climate change makes the net effects uncertain. Earth system models generally predict a shift towards an El Niño-like mean state and an increase in ENSO variability in the 21st century, including a potential increase in the frequency, duration, and intensity of La Niña events in the future. However, the atmospheric and oceanic feedback mechanisms responsible for these changes remain unclear. In this study, we analyze historical and future SSP370 simulations from the Energy Exascale Earth System Model version 1 Large Ensemble (E3SMv1-LE). Preliminary findings indicate that future ENSO events in E3SMv1-LE exhibit greater negative skewness, signifying an increased likelihood of extreme La Niña events compared to extreme El Niño events, which contrasts with prior studies suggesting an amplification of both ENSO phases. Additionally, La Niña events decay more slowly in the 21st century, leading to an increase in the number of multi-year La Ninas. To investigate the physical mechanisms driving these changes, we perform a mixed-layer heat budget analysis to identify what feedback mechanisms contribute to the simulated changes. We additionally consider the extent to which these findings are consistent across other Earth system model projections and can be validated against real-world observations, thereby enhancing our understanding of ENSO dynamics in a warming climate.
Title: The role of western Pacific salinity in El Niño development
Abstract:
The El Niño-Southern Oscillation (ENSO) is a major driver of interannual global climate variability, influencing various regions of the world. Ocean salinity is known to respond to El Niño events, but whether and how it may affect El Niño development is not fully understood. Through observational analysis and coupled model experiments with CESM2.1, we find that the ocean surface and subsurface salinity anomalies in the western Pacific can play an important role in El Niño development at its onset stage. The underlying physical mechanisms involve both ocean heat uptake and ocean zonal current changes in response to the salinity-induced density changes. Our CMIP6 multi-model analysis further suggests that a model that has a stronger sensitivity of this salinity mechanism tends to produce a stronger ENSO. Based on our large-ensemble coupled model experiments, the spring salinity anomalies can contribute to the El Niño amplitude by 0.3-0.5°C on average and significantly enhance the probability of extreme events. Our results support that ocean salinity can play an active role in El Niño dynamics.
Title: Improving El Niño conceptual model: an optimised recharge index and the Recharge Delayed Oscillator
Abstract:
The El Niño Southern Oscillation (ENSO) is the leading mode of climate interannual variability, with large socio-economical and environmental impacts, potentially increasing with climate change. Yet recharge indices and conceptual models of ENSO are still debated, limiting its understanding and predictability. Here we develop an improved recharge index by objectively optimizing the equations fit to observations and then revisit the two main conceptual models for explaining ENSO cyclic nature, namely, the Recharge Oscillator (RO) and the advective-reflective Delayed Oscillator (DO). Some previous studies have suggested that these two models capture similar physical processes, yet we show that they actually capture two dynamically-distinct feedbacks: the slow recharge/discharge process in the equatorial and southwest tropical Pacific mainly influencing the thermocline feedback and the 6-month delayed advective-reflective zonal feedback related to fast wave adjustment. We thus combine them in a hybrid Recharge Delayed Oscillator (RDO). We show how simple yet realistic it is. By exploring RDO eigenvalues dependency to parameters, we demonstrate that the RDO frequency and growth rate are highly sensitive to feedbacks relative strengths. The advective-reflective delayed and recharge feedbacks being larger in the western-central and eastern equatorial Pacific respectively, the RDO even captures some of ENSO spatial diversity. Adding seasonally-varying parameters and non-linearities further improves RDO realism. The great RDO sensitivity may explain the observed and simulated richness in ENSO's characteristics and predictability. Using this simple RDO framework, potentially adding complexity to it, could help us to investigate ENSO in observations and for climate models diagnostics and forecasts.
Title: ENSO predictability over the past 137 years based on a CESM ensemble prediction system
Abstract:
In this study, we conducted an ensemble retrospective prediction from 1881 to 2017 using the Community Earth System Model to evaluate El Niño–Southern Oscillation (ENSO) predictability and its variability on different timescales. To our knowledge, this is the first assessment of ENSO predictability using a long-term ensemble hindcast with a complicated coupled general circulation model (CGCM). Our results indicate that both the dispersion component (DC) and signal component (SC) contribute to the interannual variation of ENSO predictability (measured by relative entropy, RE). In detail, the SC is more important for ENSO events, whereas the DC is of comparable important for short lead times and in weak ENSO signal years. The SC dominates the seasonal variation of ENSO predictability, and an abrupt decrease in signal intensity results in the spring predictability barrier feature of ENSO. At the interdecadal scale, the SC controls the variability of ENSO predictability, while the magnitude of ENSO predictability is determined by the DC. The seasonal and interdecadal variations of ENSO predictability in the CGCM are generally consistent with results based on intermediate complexity and hybrid coupled models. However, the DC has a greater contribution in the CGCM than that in the intermediate complexity and hybrid coupled models.
Title: Generation mechanisms of SST anomalies associated with two flavors of ENSO focusing on vertical mixing
Abstract:
The El Niño/Southern Oscillation (ENSO) stands out as the most prominent interannual climate mode, profoundly affecting global climate. This phenomenon exhibits remarkable diversity and is commonly classified into two groups based on the location of maximum sea surface temperature anomalies. It has been suggested that positive sea surface temperature (SST) anomalies in the eastern equatorial Pacific associated with the canonical El Niño are mainly due to anomalous vertical advection, whereas anomalous zonal advection primarily contributes to the development of the positive SST anomalies in the central equatorial Pacific associated with El Niño Modoki. Using outputs from a realistic ocean model simulation, here we show that it is anomalous vertical mixing that predominantly contributes to the development of positive SST anomalies associated with both flavors of El Niño. More specifically, the anomalous warming by vertical mixing during the canonical El Niño may be partly explained by an anomalous deepening of the thermocline that leads to a decrease in the vertical temperature gradient, giving rise to suppression of the climatological cooling by vertical mixing. Also, an anomalously thick mixed layer reduces sensitivity to cooling by the mean vertical mixing and contributes to the anomalous SST warming. On the other hand, the positive SST anomalies associated with El Niño Modoki are due to a decrease in both the vertical temperature gradient and vertical diffusion coefficients.
Title: Dynamics of Spiciness Anomalies in the CESM2 Large Ensemble
Abstract:
The El Niño-Southern Oscillation (ENSO) exhibits decadal modulations in amplitude and pattern. Several hypotheses have been proposed to explain these low-frequency variations. One hypothesis is that slow variations in the Pacific Ocean mean state affect ENSO stability and consequently ENSO’s observed characteristics. Recent developments in large ensemble climate simulations now provide sufficient data to test this hypothesis. We investigate the role of spiciness anomalies emerging in the equatorial upwelling region as possible drivers of decadal ENSO modulations using 100 realizations of the Community Earth System Model Version 2 - Large Ensemble (CESM2-LE). Our results highlight the existence of a coupling between the subtropics and the equatorial Pacific by propagating spiciness anomalies at decadal time scales. Once these anomalies arrive in the equatorial upwelling region, their emergence rearranges the balance between sea surface temperature (SST), zonal wind stress, and ocean pressure gradients. This suggests that spiciness anomalies can impact ENSO via a modulation of the Bjerknes feedback strength. Our analysis indicates enhanced ENSO variance and a tendency towards more frequent and intense La Niña than El Niño events during time periods when cold and fresh spiciness anomalies arrive in the equatorial Pacific. The link is more pronounced for central Pacific than eastern Pacific ENSO events. A more comprehensive analysis is necessary to accurately quantify the effect of spiciness anomalies on ENSO, which will be addressed with targeted experiments in the future.
Title: The impacts of AMOC slowdown on ENSO in a warmer climate
Abstract:
This study examines how the Atlantic Meridional Overturning Circulation (AMOC) affects the El Niño-Southern Oscillation (ENSO) in response to anthropogenic warming. It compares climate change model simulations with declining and fixed AMOC strengths. After the 1980s, a weakened AMOC has been shown to reduce the strength of the annual cycle of sea surface temperature (SST) in the eastern equatorial Pacific and induce anomalous cross-equatorial northerly winds, thereby increasing ENSO variability by about 11%. According to an analysis of the Bjerknes stability index, the intensification of ENSO is primarily due to increased Ekman upwelling feedback caused by amplified atmospheric wind response to SST anomalies and oceanic upwelling response to equatorial wind stress anomalies. The weakened AMOC promotes Central Pacific El Niño events and reduces ENSO skewness. These AMOC effects on ENSO magnitude and complexity throughout the twenty-first century, however, are less than those expected from internal climate variability.
Title: Diverse El Niño onset timing and its associated dynamics
Abstract:
El Niño is typically phase-locked to the boreal winter, yet its onset timing exhibits considerable variability, leading to diverse and far-reaching climate impacts. The underlying physical mechanisms driving this variability remains inadequately understood. In this study, we show that El Niño onset can occur across a wide range of months, spanning from March to September. This variability is closely linked to ocean heat content and the occurrence of westerly wind bursts in the preceding spring. A stronger ocean heat content and more frequent westerly wind bursts facilitate an earlier onset by efficiently transporting warm water to the eastern equatorial Pacific. Evidence from MIROC6 simulations and a conceptual model further supports the curial roles of ocean heat content and westerly wind bursts in determining El Niño onset timing. These results enhance our understanding of El Niño dynamics and have important implications for seasonal climate prediction.
Title: A nonlinear full-field conceptual model for ENSO diversity
Abstract:
As the strongest year-to-year fluctuation of the global climate system, El Niño–Southern Oscillation (ENSO) exhibits spatial–temporal diversity, which challenges the classical ENSO theories that mainly focus on the canonical eastern Pacific (EP) type. Besides, the complicated interplay between the interannual anomaly fields and the decadally varying mean state is another difficulty in current ENSO theory. To better account for these issues, the nonlinear two-region recharge paradigm model is extended to a three-region full-field conceptual model to capture the physics in the western Pacific (WP), central Pacific (CP), and EP regions. The results show that the extended conceptual model displays a rich dynamical behavior as parameters setting the efficiencies of upwelling and zonal advection are varied. The model can not only generate El Niño bursting behavior but also simulate the statistical asymmetries between the two types of El Niños and the warm and cold phases of ENSO. Finally, since both the anomaly fields and mean states are simulated by the model, it provides a simple tool to investigate their interactions. The strengthening of the upwelling efficiency, which can be seen as an analogy to a cooling thermocline associated with the oceanic tunnel to the midlatitudes, will increase the zonal gradient of the mean state temperature between the WP and EP, i.e., resembling a negative Pacific decadal oscillation (PDO) pattern along the equatorial Pacific. The influence of the zonal advection efficiency is quite the opposite, i.e., its strengthening will reduce the zonal gradient of the mean state temperature along the equatorial Pacific.
Title: A Pacemaker Experiment Investigating the Independent Contribution of the El Niño-Southern Oscillation to Australian Rainfall in the Absence of the Indian Ocean Dipole
Abstract:
The El Niño-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD) are critical climate drivers that significantly influence Australia's interannual rainfall variability. However, their intertwined effects make it challenging to distinguish their individual impacts. This study employs the Conformal Cubic Atmospheric Model (CCAM), a variable-resolution atmospheric model, to investigate the separate influences of ENSO and IOD on Australian rainfall. We conduct a pacemaker experiment, which removes each climate mode from the input sea surface temperatures using empirical orthogonal function (EOF) filtering, while retaining all other non-ENSO/IOD-related interannual variabilities.
Our findings reveal that removing ENSO eliminates both ENSO- and IOD-rainfall correlations, whereas removing IOD only slightly reduces the ENSO-rainfall correlation and has a limited effect on the IOD-rainfall relationship. Notably, the elimination of IOD decreases total springtime rainfall by about 10%, while ENSO’s contribution to mean rainfall varies across different periods. This suggests that ENSO may primarily drive the observed ENSO/IOD-Australian rainfall variability, while IOD plays a crucial role in controlling long-term rainfall amounts. Additionally, we observed significant internal variability in ENSO/IOD-rainfall correlations, highlighting the importance of ensemble modelling in exploring ENSO/IOD-rainfall teleconnections.
Title: A Multiscale Stochastic Dynamical Coupled ENSO-MJO Model
Abstract:
Simulating the diversity of El Niño Southern Oscillation (ENSO) events and their interaction with the Madden-Julian oscillation (MJO) remains challenging for climate science. In this study, a simple dynamical stochastic model is developed to capture the key dynamics for ENSO, MJO, and the associated wind burst from intraseasonal to interannual processes. In the ocean process, thermocline feedback contributes to generating the eastern Pacific (EP) El Niño, and the zonal advection is for the central Pacific (CP) El Niño. In addition, a simple stochastic process is introduced to represent decadal variations in the background Walker circulation, modulating the occurrence of EP and CP events. In the atmosphere part, the state-dependent noises for intraseasonal components are applied to capture the characteristics of the wind burst, indirectly triggering the ENSO events. This unified framework allows for two-way feedback between ocean and atmosphere processes and successfully captures the non-Gaussian characteristics of SST anomalies and the eastward propagation of enhanced MJO activity during El Niño events. The model is also useful for helping to advance our understanding of the complex dynamics governing ENSO diversity and its multiscale interactions. It also facilitates multiscale data assimilation and forecast in practice.