El Nino-Southern Oscillation Variability Research Articles

Future increase in extreme El Niño supported by past glacial changes

El Niño events, the warm phase of the El Niño–Southern Oscillation (ENSO) phenomenon, amplify climate variability throughout the world1. Uncertain climate model predictions limit our ability to assess whether these climatic events could become more extreme under anthropogenic greenhouse warming2. Palaeoclimate records provide estimates of past changes, but it is unclear if they can constrain mechanisms underlying future predictions3–5. Here we uncover a mechanism using numerical simulations that drives consistent changes in response to past and future forcings, allowing model validation against palaeoclimate data. The simulated mechanism is consistent with the dynamics of observed extreme El Niño events, which develop when western Pacific warm pool waters expand rapidly eastwards because of strongly coupled ocean currents and winds6,7. These coupled interactions weaken under glacial conditions because of a deeper mixed layer driven by a stronger Walker circulation. The resulting decrease in ENSO variability and extreme El Niño occurrence is supported by a series of tropical Pacific palaeoceanographic records showing reduced glacial temperature variability within key ENSO-sensitive oceanic regions, including new data from the central equatorial Pacific. The model–data agreement on past variability, together with the consistent mechanism across climatic states, supports the prediction of a shallower mixed layer and weaker Walker circulation driving more frequent extreme El Niño genesis under greenhouse warming.

Chile Niño/Niña in the coupled model intercomparison project phases 5 and 6

The north and central coast of Chile is influenced by El Niño-Southern Oscillation (ENSO) through oceanic and atmospheric teleconnections. However, it also experiences episodic oceanic warmings off central Chile (30°S) lasting a few months that are not necessarily associated with ENSO. These episodes, called “Chile Niño” events, besides their ecological and socio-economical impacts, have also the potential to influence tropical Pacific variability. Here, we investigate how realistically the models in the Coupled Model Intercomparison Project (CMIP, Phases 5 and 6) simulate Chile Niño/Niña (CN) events, and quantify their changes under anthropogenic forcing. Despite limitations of the global models in simulating realistically coastal upwelling dynamics, we show that they simulate reasonably well the observed spatial pattern, amplitude and seasonal evolution of CN events. They however fail to properly represent the positive skewness from observations. The analysis of a sub-group of models (36) that simulate ENSO realistically reveals that CN events increase in amplitude and variance in the future climate with no changes in their frequency of occurence. This is interpreted as resulting from compensating effects amongst changes in remote drivers and local feedbacks. In particular, ENSO variance increases while that of the South Pacific Oscillation decreases. Conversely, we found that while the Wind-Evaporation-SST feedback tends to increase and the coupling between mixed-layer depth and SST weakens, favoring the development of CN events, the thermocline and wind-SST feedbacks decrease. However, only the change in the thermocline feedback is correlated to changes in CN variance amongst the models, suggesting a dominant role of local oceanic stratification changes in constraining the sensitivity of CN to global warming.

Opposing Changes in Indian Summer Monsoon Rainfall Variability Produced by Orbital and Anthropogenic Forcing

AbstractFuture projections indicate that Indian Summer Monsoon Rainfall (ISMR) faces a “wetter and more variable” climate. However, the reasons remain uncertain. The Last Interglacial (LIG) climate provides a potential analog for future warming. Investigating ISMR responses to these two warming scenarios could help understand the causes of ISMR changes. Using PMIP4 simulations, we find that ISMR became “wetter and more stable” during the LIG, contrasting the future climate. The opposing changes in ISMR variability are related to divergent changes in the El Niño‐Southern Oscillation (ENSO) amplitudes, ENSO‐ISMR relationships, and ENSO‐induced large‐scale atmospheric circulation anomalies. During the LIG, orbital forcing weakened ENSO variability and its impacts on ISMR. A westward positioning of ENSO shifted the atmospheric circulation anomalies westward, suppressing extreme ISMR anomalies. These processes are supported by atmospheric model simulations. Our results suggest that different warming patterns (dynamic effects) are more critical than moisture‐increasing effects in controlling regional climate variability.

Linkage of El Niño-Southern Oscillation to astronomic forcing

Abstract Deciphering the causes of El Niño-Southern Oscillation (ENSO) represents one of the greatest scientific and societal challenges because ENSO impacts people’s safety, food, water, health, and economy. Traditionally, ENSO has been considered a phenomenon that is mostly influenced by the interactions between oceanic and atmospheric processes, i.e. by the internal variability in Earth’s ocean-atmosphere system. However, dendrochronological records of climate indices, ENSO among them, have statistically significant variance at periodicities related to solar activity (sunspots) and lunar precessions. Other studies indicate a modulation by the lunar nodal cycle of ocean’s mixing and its implications on water and air temperatures, steric sea levels, coastal flooding, rain, river discharge and heat transport. Those findings suggest that astronomic forces may influence ENSO. In fact, this study shows that a fit to well-established periodicities from lunar precessions, solar activity and their interactions explains 91% of the variance of an ENSO index smoothed at 5 years, and 67% of a 3 yr filtered ENSO index. Provided that the future Earth’s ocean-atmospheric system remains roughly within historical bounds under a changing climate in the next 2–3 decades, one can venture a projection into the future of 3- and 5 yr smoothed ENSO variability. Such projection is given here and might help in preparations for adaptation and mitigation measures caused by ENSO-related coastal hazards.

Different pacemakers of marine organic carbon burial in the North Yellow Sea from the early to late Holocene

Marginal seas play a crucial role in the global carbon cycle, contributing approximately 20% of the net CO2 uptake by the world's oceans. The North Yellow Sea (NYS), a semi-enclosed marginal sea in the western Pacific Ocean, serves as a significant carbon sink influenced by factors such as sea level changes, monsoon dynamics, ocean currents, and El Niño-Southern Oscillation (ENSO) activity. This study examines a 538 cm-long sediment core W03 from the NYS (10.3 kyr BP to present). We analyzed Organic Carbon (OC) proxies, including total organic carbon (TOC), the stable carbon isotope (δ13C) of TOC, total nitrogen (TN) and marine-produced lipid biomarkers. Our findings demonstrate a strong correlation between OC burial and sea-level fluctuations during the early Holocene, driven by the exposure of the continental shelf. During the mid-Holocene, the intrusion of the Yellow Sea Warm Current increased salinity and temperature conditions and nutrient levels of sea water, thereby enhancing marine OC burial. Over the last 3.3 kyr BP, ENSO variability and intensified East Asian Winter Monsoon (EAWM) strengthened the coastal currents, contributing to better OC preservation. Additionally, human activities on the surrounding mainland increased terrestrial input, altering the OC burial. The heightened activity of the Kuroshio Current also introduced more saline and warm water masses into the NYS, impacting phytoplankton productivity and community structure, as indicated by increased C37 alkenones. This study highlights the complex interplay of geological factors in OC burial processes within the NYS, providing valuable insights into OC dynamics in marginal seas under the impact of global change.

Observed Global Mean State Changes Modulating the Collective Influence of the Tropical Atlantic and Indian Oceans on ENSO

Abstract In the last decades, many efforts have been made to understand how different tropical oceanic basins are able to impact El Niño–Southern Oscillation (ENSO). However, the collective connectivity among the tropical oceans and their associated influence on ENSO are less understood. Using a complex network methodology, the degree of collective connectivity among the tropical oceans is analyzed focusing on the detection of periods when the tropical basins collectively interact and the Atlantic and Indian basins influence the equatorial Pacific sea surface temperatures (SSTs). The background state for the periods of strong collective connectivity is also investigated. Our results show a marked multidecadal variability in the tropical interbasin connection, with periods of stronger and weaker collective connectivity. These changes seem to be modulated by changes in the North Atlantic Ocean mean state a decade in advance. In particular, strong connectivity occurs in periods with colder than average tropical North Atlantic surface ocean. Associated with this cooling, an anomalous convergence of the vertical integral of total energy flux (VIEF) takes place over the tropical north–west Atlantic, associated with anomalous divergence of VIEF over the equatorial eastern Pacific. In turn, an anomalous zonal surface pressure gradient over the tropical Pacific weakens the trades over the western equatorial Pacific. Consequently, a shallower thermocline emerges over the western equatorial Pacific, which can enhance thermocline feedbacks, the triggering of ENSO events, and therefore, ENSO variability. By construction, our results put forward opposite conditions for periods of weak tropical basin connectivity. These results have important implications for seasonal to decadal predictions.

Comparison of machine learning models in forecasting different ENSO types

Accurate forecasting of the El Niño Southern Oscillation (ENSO) plays a critical role in mitigating the impacts of extreme weather conditions linked to ENSO variability on ecosystems. This study evaluates the performance of six machine learning models in forecasting two ENSO types: the Central Pacific El Niño (Niño 4 index) and the East Central Pacific El Niño (Niño 3.4 index). The models analyzed include the Feed Forward Neural Network (FFNN), Long Short-term Memory (LSTM) neural network, eXtreme Gradient Boosting Regressor, K-Nearest Neighbors Regressor, Gradient Boosting Regressor, and Support Vector Regressor, using the ENSO index lagged by six months as the predictor. The models were trained on the monthly ENSO indices from 1870 to 1992 and tested from 1993 to 2023. We also assess the relative predictability of the two ENSO types. Events were defined as when the ENSO index exceeded ±0.4. Our evaluation during the testing period reveals that for the analyzed models, the deep neural network models (LSTM and FFNN) demonstrated superior performance in forecasting ENSO at a 6-month lead time. Furthermore, all models achieved impressive all-season correlations ranging from 0.93 to 0.97 and threat score for the ENSO phases between 0.71 to 0.88 for Niño 3.4 events, and 0.72 to 0.93 for Niño 4 events. The predictability of the two ENSO types depended on the model and strength of the ENSO event. Considering both ENSO phases, La Niña events were forecasted with a higher accuracy relative to El Niño events, and all models, besides the deep learning models, notably fell short in capturing the extreme 2015/2016 Central Pacific El Niño event. These results highlight the potential of machine learning models, particularly the deep learning approaches, for skillful ENSO forecasting, by leveraging its historical data.

Contribution of El Niño Southern Oscillation (ENSO) Diversity to Low‐Frequency Changes in ENSO Variance

AbstractEl Niño Southern Oscillation (ENSO) diversity is characterized based on the longitudinal location of maximum sea surface temperature anomalies (SSTA) and amplitude in the tropical Pacific, as Central Pacific events are typically weaker than Eastern Pacific events. SSTA pattern and intensity undergo low‐frequency modulations, affecting ENSO prediction skill and remote impacts, and resulting in low‐frequency changes in ENSO variance. Yet, how different ENSO types contribute to these decadal variance changes remains unclear. Here, we decompose the low‐frequency changes of ENSO variance into contributions from ENSO diversity categories. We propose a fuzzy clustering of monthly SSTA to allow for non‐binary event category memberships, where each event can belong to different clusters. Our approach identifies two La Niña and three El Niño categories and shows that the major shift of ENSO variance in the mid‐1970s was associated with an increasing likelihood of strong La Niña and extreme El Niño events.

Decreased ENSO post-2100 in response to formation of a permanent El Niño-like state under greenhouse warming

Under transient greenhouse warming, El Niño-Southern Oscillation (ENSO) is projected to increase pre-2100, accompanied by an easier establishment of atmospheric convection in the equatorial eastern Pacific, where sea surface temperature (SST) warms faster than surrounding regions. After 2100, how ENSO variability may change remains unknown. Here we find that under a high emission scenario, ENSO variability post-2100 reverses from the initial increase to an amplitude far smaller than that of the 20th century. The fast eastern warming persists and shrinks the equatorial Pacific non-convective area, such that establishing convection in the non-convective area, as during an El Niño, requires smaller convective anomaly, inducing weaker wind anomalies leading to reduced ENSO SST variability. The nonlinear ENSO response is thus a symptom of the persistent El Niño-like warming pattern. Therefore, the oscillatory ENSO impact could be replaced by that from the permanent El Niño-like mean condition with cumulative influences on affected regions.

Subseasonal Variability of ENSO–East Asia Teleconnections Driven by Tropical Convection Over the Indian Ocean and Maritime Continent

AbstractThe El Niño–Southern Oscillation (ENSO) has a significant impact on the surface climate of East Asia by modulating the atmospheric circulation over the Kuroshio Extension. Here, we show that the ENSO–East Asia teleconnections are strongest in early winter due to the combined effects of the Indian Ocean and Maritime Continent convections, but weakest in mid‐winter as these tropical convections weaken. During the early El Niño winter, convection is enhanced over the Indian Ocean and suppressed over the Maritime Continent. The associated Rossby wave trains constructively interfere over the Kuroshio Extension, resulting in anticyclonic circulation anomalies. The equatorial central Pacific convection has a minimal impact on the East Asia. This result suggests that the Indian Ocean and the Maritime Continent convections, rather than the equatorial central Pacific convection, are the precursors of the early winter ENSO–East Asia teleconnections, and need to be considered for subseasonal‐to‐seasonal prediction in East Asia.

Multi‐Decadal Skill Variability in Predicting the Spatial Patterns of ENSO Events

AbstractSeasonal hindcasts have previously been demonstrated to show multi‐decadal variability in skill across the twentieth century in indices describing El‐Niño Southern Oscillation (ENSO), which drives global seasonal predictability. Here, we analyze the skill of predicting ENSO events' magnitude and spatial pattern, in the CSF‐20C coupled seasonal hindcasts in 1901–2010. We find minima in the skill of predicting the first (in 1930–1950) and second (in 1940–1960) principal components of sea‐surface temperature (SST) in the tropical Pacific. This minimum is also present in the spatial correlation of SSTs, in 1930–1960. The skill reduction is explained by lower ENSO magnitude and variance in 1930–1960, as well as decreased SST persistence. The SST skill minima project onto surface winds, leading to worse predictions in coupled hindcasts compared to hindcasts using prescribed SSTs. Questions remain about the offset between the first and second principal components' skill minima, and how the skill minima impact the extra‐tropics.

The South American precipitation trends under (or not) El Niño‐Southern Oscillation influences and relationship with large‐scale circulation

AbstractThe precipitation trend patterns in South America (SA) are determined using trend empirical orthogonal function analysis for the 1951–2016 period. The associated large‐scale tropical and extratropical anomalous circulation patterns are also examined. The words “total” and “residual” refer to the monthly anomalies and monthly anomalies without the El Niño–Southern Oscillation (ENSO) effects, respectively. The total precipitation features a positive trend in southeastern SA (SESA, southern Brazil, Uruguay, most of eastern Argentina) and northern Chile, and a negative trend over central‐eastern Brazil and central Amazonia. The residual precipitation shows an increased positive trend over most of the coastal extension of northern SA and Colombia; a weak positive trend over southern Brazil, northeastern Argentina, and northern Chile; and a negative trend over central‐eastern SA and western Amazonia. The differences between the total and residual precipitation trend patterns in tropical SA is explained as responses to total and residual zonally asymmetric anomalous sea surface temperature (SST) patterns, respectively. The total SST pattern along the equatorial Pacific configures the Pacific Decadal Oscillation, which impacts ENSO variability and as response intensifies the Walker circulation. Without the ENSO, the Walker cell is mainly driven by the tropical Indian and Atlantic Oceans, which configure residual asymmetric anomalous warming. Furthermore, the warming in the equatorial Indian and eastern Pacific Oceans, in the presence of ENSO, induces a Rossby wave train‐type anomalous pattern that extends across the South Pacific into SA and modulates the atmospheric anomalous circulation over SESA. In this region, an anomalous anticyclonic accompanied by an intensified South American Low‐Level Jet induces a moisture transport to SESA. This anticyclone is also observed in the absence of ENSO but is weaker. The results suggest the importance of ocean warming in the western Pacific‐Indian in the modulation of extratropical teleconnections to SESA in the tropical ocean warming scenario.

Sources of low-frequency variability in observed Antarctic sea ice

Abstract. Antarctic sea ice has exhibited significant variability over the satellite record, including a period of prolonged and gradual expansion, as well as a period of sudden decline. A number of mechanisms have been proposed to explain this variability, but how each mechanism manifests spatially and temporally remains poorly understood. Here, we use a statistical method called low-frequency component analysis to analyze the spatiotemporal structure of observed Antarctic sea ice concentration variability. The identified patterns reveal distinct modes of low-frequency sea ice variability. The leading mode, which accounts for the large-scale, gradual expansion of sea ice, is associated with the Interdecadal Pacific Oscillation and resembles the observed sea surface temperature trend pattern that climate models have trouble reproducing. The second mode is associated with the central Pacific El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode and accounts for most of the sea ice variability in the Ross Sea. The third mode is associated with the eastern Pacific ENSO and Amundsen Sea Low and accounts for most of the pan-Antarctic sea ice variability and almost all of the sea ice variability in the Weddell Sea. The third mode is also related to periods of abrupt Antarctic sea ice decline that are associated with a weakening of the circumpolar westerlies, which favors surface warming through a shoaling of the ocean mixed layer and decreased northward Ekman heat transport. Broadly, these results suggest that climate model biases in long-term Antarctic sea ice and large-scale sea surface temperature trends are related to each other and that eastern Pacific ENSO variability is a key ingredient for abrupt Antarctic sea ice changes.

ENSO weakens the co-variability between the spring persistent rains and Asian summer monsoon: Evidences from tree-ring data in southeastern China

The Spring Persistent Rains (SPR) and the Asian Summer Monsoon (ASM) are the two dominant rainfall systems in East Asia, providing together a majority of annual rainfall in southeastern China (SEC). Since observational data in SEC were mostly unavailable until the 1950s, proxy records that are capable of capturing the SPR and ASM variations are required to examine the long-term co-variability patterns between them. Tree-ring earlywood and latewood δ18O records in SEC were found to respond to relative humidity (RH) during the SPR and ASM seasons, respectively, allowing us, for the first time, to reconstruct the RH changes of SPR and ASM back to 1801. The two reconstructions can explain 44.9 % and 42.3 % of the instrumental variance. We observed a long-lasting wet epoch in the 1920s–60s for both the SPR and ASM, caused by a peak in the land–ocean thermal contrast. The El Niño-Southern Oscillation (ENSO) and the Intertropical Convergence Zone (ITCZ) were found to be the two leading tropical systems that modulated the SPR and ASM co-variability. During a period with weakened ENSO variance, the RH of SPR and ASM showed in-phase changes driven by the ITCZ. However, when the ENSO variance became strengthened, the co-variability collapsed since the ENSO can offset the influence of the ITCZ via teleconnections.

Potential impact of wintertime Arctic forcing on the subsequent sea surface temperature anomalies in the tropical eastern Pacific

Despite extratropical forcing being recognized as an important factor that can modulate El Niño-Southern oscillation (ENSO) properties on the interannual time scale, little is known about whether and how Arctic forcing changes the tropical sea surface temperature (SST). This current study reveals a significant link between the net surface sensible heat flux (SHF) in the Arctic and the SST anomalies in the tropical eastern Pacific (TEP). Specifically, anomalous upward SHF into the Arctic atmosphere in February leads to a warmer TEP in the subsequent spring and summer. A northeast-southwest-tilted North Pacific Oscillation-like atmospheric pattern associated with the upward Arctic SHF anomaly induces SST cooling in the subtropical North Pacific via positive Wind-Evaporation-SST feedback, which further promotes TEP SST warming via meridional heat advection, thermocline feedback, and nonlinear processes. The spring-to-summer TEP SST anomalies driven by the preceding anomalous Arctic SHF can potentially modulate the seasonal evolution of ENSO. Our findings imply that we should take into account the Arctic-tropics linkages when comprehensively understanding the ENSO variability and improving ENSO projection skills.

Detection of limit cycle signatures of El Niño in models and observations using reservoir computing

While the physics of the El Niño–Southern Oscillation (ENSO) phenomenon in the Tropical Pacific is quite well understood, there is still debate on several more fundamental aspects. The focus of this paper is on one of these issues that deals with whether ENSO variability, within the recharge-discharge oscillator theory arising from a stochastic Hopf bifurcation, is subcritical or supercritical. Using a Reservoir Computing method, we develop a criticality index as an indicator for the presence of a limit cycle in noisy time series. The utility of this index is shown in three members of a hierarchy of ENSO models: a conceptual box model, the classical Zebiak-Cane model and a state-of-the-art Global Climate Model. Finally, the criticality index is determined from observations, leading to the result that ENSO variability appears to be subcritical.

ENSO Impacts on Jamaican Rainfall Patterns: Insights from CHIRPS High-Resolution Data for Disaster Risk Management

This study examines the influence of the El Niño Southern Oscillation (ENSO) on Jamaica’s rainfall patterns, leveraging CHIRPS data from 1981 to 2021 in 370 locations. Our analysis reveals a distinct ENSO imprint on rainfall, with La Niña phases showing a consistently higher probability of exceeding various rainfall thresholds compared to El Niño. Notably, La Niña increases the likelihood of heavier rainfall, particularly in the wet seasons, with probabilities of exceeding 200 mm reaching up to 50% during wet season II. Spatially, the probability of total monthly rainfall (TMR) during La Niña is elevated in the northeastern regions, suggesting regional vulnerability to excess rainfall. Additionally, during El Niño, the correlation between TMR and the maximum air temperature (Tmax) is significantly stronger, indicating a positive and more pronounced relationship between higher temperatures and rainfall, with correlation coefficients ranging from 0.39 to 0.80. Wind speed and evapotranspiration show a negligible influence on TMR during both ENSO phases, maintaining stable correlation patterns with only slight variations. The results of this study underscore the necessity for differentiated regional strategies in water resource management and disaster preparedness, tailored to the unique climatic characteristics imposed by ENSO variability. These insights contribute to a refined understanding of climate impacts, essential for enhancing resilience and adaptive capacity in Jamaica and other small island developing states.

Diagnosing the Quasi-Equilibrium Response of ENSO Variability under a Range of CO2 Levels

Abstract Several recent studies have highlighted differences in simulated properties of El Niño–Southern Oscillation (ENSO) under transient and equilibrium responses to increasing CO2. However, the reasons behind these disparate responses and the extent to which they are robust to different scales of CO2 forcing remain unclear. In this study, we adopt a climate system model with reduced SST bias in the eastern tropical Pacific and incrementally apply abrupt increases in CO2, analyzing outputs after each simulation reaches quasi-equilibrium with the imposed forcing. The results suggest that ENSO activity under quasi-equilibrium first increases and then decreases with increasing CO2, peaking in simulations with CO2 concentrations similar to the present day. Bjerknes–Jin stability analysis indicates that changes in the ENSO growth rate result primarily from changes in the thermocline feedback and thermodynamic damping terms. While thermodynamic damping increases monotonically with increasing CO2, the positive thermocline feedback varies within the range of internal variability up to twice the preindustrial value of CO2 and then weakens sharply with further increases. The mechanisms behind these changes include weaker mean ocean upwelling and weaker dynamical coupling between the atmosphere and subsurface ocean associated with substantial near-surface freshening at higher levels of CO2. These changes steepen the thermodynamic barrier to mixing between the surface and subsurface, weakening the east–west temperature gradient in the mean state and suppressing variability in the cold tongue. Analysis of similar model simulations from the Coupled Model Intercomparison Project (CMIP6) archive indicates that changes in the Bjerknes–Jin stability index are robust but do not establish a consensus as to the mechanisms behind them. Significance Statement This study investigates how El Niño–Southern Oscillation changes with increasing CO2 forcing by using a global model with improved tropical Pacific climate. The simulations roughly correspond to scenarios in which emissions are reduced to maintain CO2 concentrations at near-constant values over a long period, with levels ranging from about 2/3 to more than 5 times the present-day concentration. Analysis of these simulations suggests that ENSO activity is strongest when CO2 concentrations are similar to the present-day and becomes substantially weaker when CO2 is more than double its present-day value. Reduced ENSO activity with further increases in CO2 is caused by weaker interactions between the atmosphere and the subsurface ocean. Both the amplitude of warm (El Niño) events and the occurrence frequency of warm and cold (La Niña) events decrease as ENSO events become first more difficult to grow and then more difficult to trigger with increasing CO2.

Effective reduced models from delay differential equations: Bifurcations, tipping solution paths, and ENSO variability

Conceptual delay models have played a key role in the analysis and understanding of El Niño-Southern Oscillation (ENSO) variability. Based on such delay models, we propose in this work a novel scenario for the fabric of ENSO variability resulting from the subtle interplay between stochastic disturbances and nonlinear invariant sets emerging from bifurcations of the unperturbed dynamics.To identify these invariant sets we adopt an approach combining Galerkin–Koornwinder (GK) approximations of delay differential equations and center-unstable manifold reduction techniques. In that respect, GK approximation formulas are reviewed and synthesized, as well as analytic approximation formulas of center-unstable manifolds. The reduced systems derived thereof enable us to conduct a thorough analysis of the bifurcations arising in a standard delay model of ENSO. We identify thereby a saddle–node bifurcation of periodic orbits co-existing with a subcritical Hopf bifurcation, and a homoclinic bifurcation for this model. We show furthermore that the computation of unstable periodic orbits (UPOs) unfolding through these bifurcations is considerably simplified from the reduced systems.These dynamical insights enable us in turn to design a stochastic model whose solutions – as the delay parameter drifts slowly through its critical values – produce a wealth of temporal patterns resembling ENSO events and exhibiting also decadal variability. Our analysis dissects the origin of this variability and shows how it is tied to certain transition paths between invariant sets of the unperturbed dynamics (for ENSO’s interannual variability) or simply due to the presence of UPOs close to the homoclinic orbit (for decadal variability). In short, this study points out the role of solution paths evolving through tipping “points” beyond equilibria, as possible mechanisms organizing the variability of certain climate phenomena.

Assessing the drift of fish aggregating devices in the tropical Pacific Ocean

Abstract. The tropical Pacific Ocean is characterized by its dominant zonal flow, strong climate dependence on the El Niño–Southern Oscillation (ENSO) and abundant tuna stocks. Tuna fisheries in the West and Central Pacific Ocean accounted for 55 % of the world-wide tuna catch in 2019 and are one of the main sources of income in many Pacific island nations. One of the dominant fishing methods in this region relies on the use of drifting fish aggregating devices (dFADs): rafts with long underwater appendages (on average 50 m deep) that aggregate fish. Although currents such as the North Equatorial Countercurrent (NECC) and South Equatorial Current (SEC) in the tropical Pacific Ocean vary strongly with ENSO, little is known about the impact of this variability in flow on dFAD dispersion. In this study, virtual Lagrangian particles are tracked for the period 2006 to 2021 over the domain in a 3D hydrodynamic model and are advected in simulations with only surface flow, as well as simulations using a depth-averaged horizontal flow over the upper 50 m, representing virtual dFADs. Zonal displacements, eddy-like behaviour and ENSO variability are then studied for both types of virtual particles. It was found that virtual particles advected by surface flow only are displaced up to 35 % farther than virtual dFADs subjected to a depth-averaged flow, but no other major differences were found in dispersion patterns. The strongest correlations between ENSO and virtual dFAD dispersion for the assessed variables were found in the West Pacific Ocean, with Pearson correlation coefficients of up to 0.59 for virtual dFAD displacement. Connections between ENSO and eddy-like behaviour were found in the western part of the SEC, indicating more circulation and meandering during El Niño. These findings may be useful for improving sustainable deployment strategies during ENSO events and understanding the ocean processes driving the distribution of dFADs.