Orals: Wed, 30 Apr | Room F1
According to the World Meteorological Organization, 2024 is the warmest year on record and the first year of the modern era that global mean surface temperatures have likely exceeded pre-industrial levels by 1.5°C. Temperatures in 2024 surpassed the record set in 2023, which is now the second warmest year of the modern era. The record warmth in 2023-2024 was accompanied by extraordinary weather and climate extremes around the globe including historic droughts and floods, widespread wild fires, and intense and prolonged marine heatwaves. Human-caused increases in heat trapping greenhouse gas concentrations are the fundamental underlying cause for these record high global mean surface temperatures, with atmospheric carbon dioxide levels reaching new highs in 2023 and 2024. Coincidentally, after nearly a decade of near-neutral or unusually cold conditions in the tropical Pacific, an El Niño that ranked among the strongest of the past 75 years emerged in the boreal spring of 2023 and peaked at the end of the year before decaying in the spring of 2024. This presentation will show that heat loss from the ocean to the atmosphere during the El Niño was primarily responsible for boosting global mean surface temperatures into record territory in 2023 and 2024, though elevated sea surface temperatures in other parts of the world ocean contributed to these global temperature extremes.
How to cite: McPhaden, M.: El Niño and Record Warm SSTs Boost Global Mean Surface Temperatures, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7450, https://doi.org/10.5194/egusphere-egu25-7450, 2025.
ENSO predictability relies largely on deterministic equatorial ocean dynamics, where wind variations during one phase trigger oceanic responses that favor a shift to the opposite phase. However, in observations, this deterministic response is obscured by air-sea coupled variations and stochastic Westerly Wind Bursts. Here, we present a method to isolate the ocean dynamics underpinning ENSO phase transitions using forced experiments with an Ocean General Circulation Model (OGCM). The control experiment is forced by interannually varying wind stresses, with thermal damping from air-sea heat fluxes computed interactively as relaxation to climatological Sea Surface Temperature (SST). This setup reproduces observed equatorial Pacific SST and heat content variations with high fidelity. To assess the role of ocean initial conditions, "memory" experiments branch from the control simulation every January 1st, replacing wind stresses with climatological values (i.e., no interannual wind anomalies). In these experiments, interannual anomalies arise solely from the evolution of equatorial planetary waves in the initial conditions. The ocean memory index (OMI) derived from these experiments demonstrates hindcast skill for 1-year lagged ENSO peaks comparable to or exceeding traditional precursors like Warm Water Volume or western Pacific heat content. This highlights the effectiveness of our methodology in isolating the ocean dynamics driving ENSO phase transitions. Our findings emphasize the central role of low-order equatorial Rossby waves (meridional modes 1-3) in ENSO's oceanic memory via reflections at the Pacific western boundary and indicate that widely used indices such as Warm Water Volume orwestern Pacific heat content do not optimally capture these processes.
How to cite: Liu, F., Vialard, J., Ethé, C., Person, R., Fedorov, A., Guilyardi, E., and Lengaigne, M.: Revealing Ocean Dynamics Driving ENSO Phase Transitions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10983, https://doi.org/10.5194/egusphere-egu25-10983, 2025.
The recharge oscillator (RO) model has been used to describe and understand different aspects of the El Niño Southern Oscillation (ENSO). One application involves fitting the RO model to observations or model output to identify if ENSO is a self-sustained or a damped oscillation driven by external weather noise such as westerly wind bursts. Fitting the linear recharge oscillator to observations and climate model simulations consistently yields an asymptotically stable system. This suggests that ENSO can be represented by a damped oscillator whose variability is sustained and made irregular by external stochastic forcing. We investigate the accuracy of methods that have been used to estimate the recharge oscillator parameters and their implied period and growth rate for ENSO using simulations of both linear and nonlinear recharge oscillators. Ultimately, we find that fitting the RO does not allow for robustly differentiating between a damped or a self-sustained regime. Specifically, we find that fitting a linear RO leads to parameters that imply a damped oscillator even when the fitted data were produced by a model that is self-sustained. As such, it seems challenging to conclude whether ENSO is a damped or a self-sustained system by fitting the recharge oscillator model to observations. It is therefore possible that ENSO could be described instead by a self-sustained oscillator.
How to cite: Weeks, E. and Tziperman, E.: Is ENSO a self-sustained or a damped oscillation? , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12670, https://doi.org/10.5194/egusphere-egu25-12670, 2025.
Recent observational and modeling studies have shown that coupled atmospheric and oceanic mesoscale processes exert a significant influence on ENSO dynamics. However, the spatial resolution of the latest generation of climate models is still insufficient to fully resolve mesoscale air-sea interactions. Furthermore, climate models exhibit significant biases in the simulation of the tropical Pacific mean state, which can affect the ability of the model to accurately reproduce ENSO variability. In particular, the Current Feedback to the Atmosphere (CFB) slows the large-scale mean circulation by reducing the mean energy input from the atmosphere to the ocean, while at the mesoscale it causes the "eddy killing" mechanism: a damping of eddies by ~30%, caused mainly by the transfer of energy from ocean currents to the atmosphere.
In this study, we perform a set of regional high-resolution oceanic (1/12°) and atmospheric (1/4°) coupled simulations, in which the CFB is considered (CTRL) or not (NOCFB), to quantify the impact of mesoscale air-sea interactions on the Pacific mean state and ENSO. The coupled simulations cover the entire Pacific basin (90°E-70°W) and the tropics (30°S-30°N) for the period 1980-2020. The impact of CFB on the oceanic mean state and ENSO is then assessed by comparing the two simulations. The CTRL simulation effectively reproduces the mean state and seasonal cycle of the tropical climate, specially the sea surface temperature (SST) pattern with an accurate representation of the warm pool and cold tongue extension. Additionally, the model properly simulates the equatorial current system, the equatorial thermocline, and key atmospheric characteristics of the tropics (e.g., ITCZ-SPCZ). These features enable the model to successfully represent the ENSO seasonal phase, including the onset, development, and termination of the most intense El Niño events (e.g., 1982/83 and 1997/98).
We show that, in addition to the slowdown of the equatorial current system from the surface to ~100 m depth and the reduction of equatorial eddy kinetic energy through decreased barotropic and baroclinic energy conversion, CFB enhances the equatorial zonal SST gradient by warming the western Pacific by up to 0.3°C and cooling the eastern Pacific by about 0.4°C. These effects directly impact the mean state of precipitation and net heat flux across the warm pool and cold tongue. Consequently, the ENSO asymmetry and nonlinearity are reduced by approximately 10% in the NOCFB simulation, thereby underscoring the rectifying effects of the CFB on large-scale SST patterns in the tropical Pacific basin.
How to cite: Conejero, C., Boucharel, J., Renault, L., and Menkes, C.: Rectified Effects of Regional Current Feedback on Large-Scale Air-Sea Interactions and ENSO, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9135, https://doi.org/10.5194/egusphere-egu25-9135, 2025.
Predicting ENSO diversity remains a fundamental challenge in seasonal-to-interannual climate forecasting, with Eastern (EP) and Central Pacific (CP) events arising from complex ocean-atmosphere interactions and remote forcing. This study highlights the significant role of the South American Monsoon System (SAMS) in modulating ENSO diversity.
Analysis of ERA5 reanalysis data (1940-2024) reveals distinct ENSO precursors during the monsoon onset phase in September-November (SON), five seasons before peak ENSO conditions. Enhanced SAMS precipitation, coupled with cold subtropical Southwestern Atlantic SST anomalies, precedes EP events. Conversely, an anomalous upper-level vortex over subtropical South America (VOSA) combined with positive sea-level pressure anomalies across the subequatorial and subtropical South Atlantic is more closely linked to CP events. These precursor patterns generate a stationary Rossby wave that initiates a cascade of processes in the South Eastern Tropical Pacific, including anomalous vertical motion, weaker trade winds, wind-evaporation-SST feedback, and Ekman coastal dynamics, culminating in ENSO development 12-15 months later.
To rigorously establish causation among these ENSO precursors and other known tropical Atlantic-Pacific basin interaction mechanisms, the Peter and Clark Momentary Conditional Independence (PCMCI+) algorithm was applied. The resulting causal networks largely validate the hypothesized physical mechanisms, also identifying key boreal spring mediators, such as SAMS precipitation, VOSA, and the South Pacific Oscillation. Sliding window analyses reveal a post-1980 intensification of the SAMS precipitation pathway, coinciding with shifts in the Atlantic-Pacific background state and satellite-based observational coverage. The PCMCI+ results challenge the causal significance of conventional precursors such as the Atlantic Niño and South Atlantic Subtropical Dipole, while emphasizing the atmospheric bridge connecting the subtropical South Atlantic and the tropical Pacific through monsoonal precipitation over South America.
Parallel analyses conducted on 20 historical-period simulations from the CESM2 Large Ensemble test these relationships under internal climate variability. Each member was subjected separately to the same method sequence applied to ERA5. While some members corroborate key findings—particularly the role of VOSA—the magnitude and exact timing of some causal links differ from reanalysis results. These discrepancies reflect both internal climate variability and model biases in representing tropical climate modes, including insufficient ENSO diversity, misrepresented teleconnections (e.g., an overemphasized role of the North Pacific Oscillation), and low SAMS variability.
These findings demonstrate that ENSO variability originates partly from cross‐basin processes initiated before the spring predictability barrier, highlighting the potential for enhanced early-season forecasts through incorporation of SAMS transition phase intensity. Despite remaining uncertainties regarding connection strength and multidecadal variability, this study establishes land-atmosphere interactions as significant contributors to pantropical climate interactions, warranting broader investigation of monsoon-ENSO pathways. Furthermore, it advocates for continued efforts to ensure that climate models accurately capture observed patterns and their underlying causal relationships.
How to cite: Bellacanzone, F. and Bordoni, S.: The South American Monsoon as an Atlantic-Pacific Bridge: Causal Pathways to ENSO Diversity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19689, https://doi.org/10.5194/egusphere-egu25-19689, 2025.
Over the past two decades, significant attention has been given to ENSO (El Niño Southern Oscillation) diversity, categorizing El Niño events as either central Pacific (CP) or eastern Pacific (EP) based on their dynamics, sea surface temperature (SST) and rainfall patterns, and global teleconnections. EP events include both moderate events and rare extreme events, such as those that peaked in December 1972, 1982, 1997, and 2015. Here, we will demonstrate that CP El Niño and moderate EP El Niño events are not clearly distinguishable in terms of SST, rainfall pattern, teleconnection, and driving mechanisms. In contrast, extreme EP events exhibit fundamentally different dynamics, driven by a massive reorganization of atmospheric convection across the Pacific. Drawing on our recent findings and the broader literature, we will highlight the distinctive attributes of extreme El Niño events. These events are expected to increase in frequency under global warming. They are linked to eastward-shifted teleconnection patterns, leading to specific and predictable impacts over North America. They also induce a much stronger and longer-lasting oceanic memory, resulting in a predictable transition to a two-year La Niña. Atmospheric nonlinearities, particularly those associated with the threshold for deep atmospheric convection, play a critical role in establishing those extreme El Niño events distinguishing features. In summary, CP and moderate EP events share many characteristics, while extreme El Niño events stand apart. These insights challenge the current approach to ENSO diversity and suggest that categorizing ENSO states as La Niña, neutral, moderate El Niño, and extreme El Niño is more relevant.
How to cite: Vialard, J., Beniche, M., Liu, F., Lengaigne, M., Guilyardi, E., and Fedorov, A.: Extreme El Niño events versus ENSO diversity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3054, https://doi.org/10.5194/egusphere-egu25-3054, 2025.
The mechanisms of La Niña onset diversity remain unclear. Here we identified three La Niña onset types using the K-means cluster analysis of equatorial SSTA evolutions from the preceding summer to the developing winter. The first onset type is characterized by a transition from a neutral year to La Niña (N2L). The second type is a transition from a central Pacific (CP) El Niño to La Niña (CE2L). The third type is a transition from a super El Niño to La Niña (SE2L). A key preceding signal for N2L is the warming in the tropical North Atlantic (TNA). During the autumn prior to N2L onset, positive SST and precipitation anomalies occurred in the TNA, and they induced anomalous easterlies in the equatorial western Pacific, which further triggered upwelling oceanic Kelvin waves, shallower equatorial thermocline and anomalous westward zonal currents, initiating a cooling at the equator through the zonal advective feedback. The onset of CE2L was caused by preceding anomalous easterlies in the equatorial eastern Pacific (EP), a direct response to the central Pacific heating associated with CP El Niño. The anomalous easterlies strengthened local surface latent heat flux and anomalous upwelling, leading to a cooling in EP. The SE2L onset was primarily attributed to a substantially shoaling of ocean thermocline associated with the discharge of the preceding super El Niño.
How to cite: Pan, X. and Li, T.: Diversity of La Niña Onset, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14760, https://doi.org/10.5194/egusphere-egu25-14760, 2025.
This study is focused on analysing the phase space dynamics of the El Niño-Southern Oscillation (ENSO) for two different types of ENSO, Eastern Pacific (EP) and Central Pacific (CP) in the Coupled Model Intercomparison Project (CMIP) 5 and 6 model simulations. The phase space is defined by a 2-dimensional representation of the eastern equatorial sea surface temperature anomaly (T) and the thermocline depth anomaly (h). We find that the dynamics of h in CMIP models appear to be regionally shifted to the east (hshift). The results, when considering hshift, suggest that CMIP models successfully capture the key aspects of observed ENSO diversity, including the asymmetries in the mean phase space, the extremes, and the phase speed, with distinct differences between EP and CP. We find significantly weaker interaction between CP and h as compared to EP and h, suggesting that the EP mode is more dynamically coupled to h than the CP mode, as it is observed. CMIP models reproduce the faster phase transition speed for EP, whereas, for CP, they replicate the weaker and slower observed phase transitions. However, CMIP model ensembles have substantial limitations and reproduce most observed characteristics with much weaker intensities. There is a large spread within the model ensemble, with only a few CMIP models accurately simulating the observed asymmetry of the ENSO phase space for CP and EP. Further, we found no significant improvement from CMIP 5 to CMIP 6 models in simulating the observed phase space dynamics of ENSO diversity.
How to cite: Priya, P. and Dommenget, D.: ENSO diversity in CMIP models within the recharge oscillator framework, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3902, https://doi.org/10.5194/egusphere-egu25-3902, 2025.
Global Climate Models (GCMs) are projecting future changes in different modes of variability influencing precipitation over South America (SA). Among them, El Niño-Southern Oscillation (ENSO) is the most important mode of inter-annual variability. However, the GCMs still present high inter-model variability. Therefore, it is still necessary to advance in the assessment of simulated ENSO impact before asserting future consequences. The aim of this study is to analyse inter-model variability in ENSO signal on precipitation simulated by GCM-CMIP6 over SA.
Daily precipitation and monthly sea surface temperature (SST) were obtained from 15 GCMs selected from CMIP6 and compared to ERA5 reanalysis, for the period 1981-2010. The ENSO was characterised through the Oceanic Niño Index (ONI) which was calculated based on SST anomalies over the Niño3.4 region. Total accumulated rainfall (PRCPTOT) was calculated in two trimesters October-December (OND) and December-February (DJF). These seasons were chosen because of the incidence of ENSO signal over SA.
Firstly, inter-model variability in the ONI values was assessed comparing the distributions with the index obtained from ERA5 and quantifying the number of cases under each ENSO phase: El Niño, La Niña and Neutral. The inter-quartil range is underestimated by 53% of the models and overestimated by one model, for both seasons. The rest of the models present similar distribution to ERA5. Consequently, the models that underestimate the inter-quartil range, overestimate the number of Neutral cases. Additionally, the extreme values of El Niño phase are more overestimated than the values of La Niña phase.
Secondly, the simulation of ENSO signal on PRCPTOT was assessed through Spearman correlation (5% significance level) and composite patterns. The analysis was focused on two main regions where ONI signal is stronger: Northern South America (NSA) and Southeastern of South America (SESA).
In general terms, for OND, the models are able to capture spatial patterns, in particular, with positive correlations over SESA and negative ones over NSA with 70% inter-model agreement. The rest of the models present higher spatial variability. The ensemble of the models also captures the spatial pattern correctly in almost all South America.
The ENSO signal in PRCPTOT for DJF is weaker, according to ERA5. The ensemble of the models captures the sign of the signal over the regions of interest, but fails over central Brazil, located among SESA and NSA. The level of agreement between the models is similar to OND over the regions with strong ENSO signal but, over transitional regions, the inter-model variability is higher.
Based on these results, composite analysis was carried out for the ensemble of the models. In general terms, the signal simulated by the GCMs is weaker than ERA5, but they adequately identify the regions and the sign of the signal.
The main result of this research is that ENSO signal on South America precipitation is well simulated by GCMs particularly over the regions where this signal is stronger. This study is a first step for a subsequent analysis of the future projections of the ensemble of the GCMs, considering other precipitation indices.
How to cite: Pantano, V. C., Iacovone, M. F., and Penalba, O. C.: Inter-model variability in the influence of El Niño-Southern Oscillation over the precipitation in South America, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-11263, https://doi.org/10.5194/egusphere-egu25-11263, 2025.
Recent studies have highlighted that extreme El Niño have distinct atmospheric winter teleconnections from those associated with other ENSO phases. While moderate El Niño and CP El Niño events display Pacific-North American (PNA)-like patterns opposite to La Niña teleconnections, only extreme El Niño events show a unique Tropical-Northern Hemisphere (TNH) pattern, driving warm anomalies over North America and increased rainfall in regions like California and Florida. However, despite projections of their increased frequency in warmer climates, future teleconnection changes for extreme El Niño remain under-explored. Using an extensive CMIP6 dataset spanning multiple Shared Socio-economic Pathways (SSPs) scenarios, models, and ensemble members, we perform a warming-level analysis of future changes in extreme El Niño winter teleconnections. Above +3°C warming, their specific TNH-like pattern weaken, and a signature similar to a negative North Atlantic Oscillation (NAO) pattern emerges. Wet anomalies over California and Florida weaken, dry anomalies over Northeast Brazil diminish, while dry anomalies over the Maritime Continent intensify during extreme El Niño. These changes appear to stem both from the eastward shift of tropical precipitation sources and changes in extratropical background circulation. Finally, we explore how changes in frequency of extreme El Niño and their associated teleconnection patterns drive changes in the broader ENSO teleconnection pattern.
How to cite: Beniche, M., Vialard, J., and Lengaigne, M.: How extreme El Niño teleconnections change in warmer climates, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16417, https://doi.org/10.5194/egusphere-egu25-16417, 2025.
Delayed warming of the Southern Ocean is one of the most robust features of CO2-driven climate response. This relative initial cooling in the Southern Ocean has recently been shown to have far-reaching impacts into the equatorial Pacific. The Southern Ocean-driven teleconnection mechanism has been examined in the atmospheric perspective, hence little is known about the role of ocean dynamics. In this study, we investigate the oceanic role in the teleconnection from the Southern Ocean to the tropical Pacific by applying a time-invariant zonally uniform surface heating between 40°S and 60°S, mimicking the effect of delayed warming of the Southern Ocean, using Community Earth System Model 1.2.2 (CESM1-CAM4). To better understand the oceanic role, we separate the contributions from buoyancy-driven and wind-driven ocean circulation by conducting two additional experiments in which the wind stress is prescribed to a repeating daily climatology, applied globally in one case and outside the tropical band between 10ºS and 10ºN in the other.
Consistent with previous studies, prescribed Southern Ocean warming leads to a weakening of the southern Hadley cell, inducing anomalous low pressure centered at 40°S and anomalous westerly wind from 20°S to 40°S. Anomalous westerly wind in the fully coupled experiment can cool the southwest Pacific by inducing northward Ekman transport, while both wind-stress prescribed cases do not experience anomalous Ekman transport and cooling effect. On the other hand, the southeast Pacific experiences more warming in the fully coupled experiment compared to other two cases, due to reduced coastal upwelling along the west coast of South America and weaker trade winds. In all experiments, reduced trade winds over the equator warm up the eastern Pacific, and smaller equatorial zonal temperature gradient induces weaker Walker circulation in fully coupled and tropical band coupled experiments, inducing further warming at the equatorial Pacific by reducing the Bjerknes feedback. However, even the global wind stress prescribed experiment experiences anomalous ocean heat release and warming, especially over the eastern equatorial Pacific, due to the shallow mean thermocline position and anomalously warmer thermocline. Ocean buoyancy change in the southern Pacific mid-latitudes can induce anomalous subtropical cell change in larger magnitude compared to the northern hemisphere, and this subtropical cell change interhemispheric asymmetry can induce heat convergence and warming of the equatorial thermocline.
These experiments reveal that a weakening of the equatorial upwelling motion is primarily driven by surface wind stress changes at low latitudes, while the equatorial sub-surface ocean heat convergence arises from the hemispherically asymmetric ocean subtropical cell changes driven by buoyancy changes. Not only the wind stress changes coupled to the southward ITCZ shift but also the buoyancy reduction in the southern Pacific mid-latitudes can contribute to the change in ocean circulation, contributing to the extratropics to tropics teleconnection effect. In conclusion, this research parsed out the importance of the buoyancy driven ocean circulation change for the extra-tropics to tropics teleconnection mechanism in the southern hemisphere.
How to cite: Lee, D., Shin, Y., Kim, H., and Kang, S.: Oceanic Role in the Teleconnection from Southern Ocean to Tropical Pacific, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-21795, https://doi.org/10.5194/egusphere-egu25-21795, 2025.
Atlantic Niño can influence ENSO by modulating the Pacific Walker circulation. This interbasin connection is dominated by central Atlantic Niño (CAN) events, which began to emerge around 2000. Our analysis of observational data and climate model simulations reveals that the influence of CAN on ENSO will strengthen in a warming climate due to an enhanced Pacific response. On one hand, increased variability of the eastern Pacific intertropical convergence zone leads to stronger subsidence anomalies induced by CAN; on the other hand, strengthened atmospheric variability over the North Indian Ocean enhances the region’s response to CAN-induced Kelvin waves, promoting easterly anomalies over the western tropical Pacific. These changes are further linked to the pronounced interhemispheric warming contrast projected by climate models. Our findings underscore the growing influence of Atlantic Niño on ENSO, with important implications for seasonal climate prediction and future climate change projections.
How to cite: Zhang, L., Wang, C., Han, W., Karnauskas, K., McPhaden, M., Hu, A., Xing, W., Chen, B., and Liu, H.: Strengthened Influence of Atlantic Niño on ENSO in a Warming Climate, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4946, https://doi.org/10.5194/egusphere-egu25-4946, 2025.
Large-scale interactions among the three tropical ocean basins—the Pacific, Atlantic, and Indian Oceans—can influence or modify climate variability. The international CLIVAR initiative on Tropical Basin Interactions (TBI) seeks to establish a unified understanding of the mechanisms driving these interactions and their role in climate predictability. This paper introduces the Tropical Basin Interaction Model Intercomparison Project (TBIMIP), which provides an experimental framework for investigating interactions among the tropical basins. Within the TBIMIP, a series of three-ocean "pacemaker" experiments were conducted using the CESM2 model, in line with the experimental protocol. Specifically, Tier 2 experiments, where sea surface temperatures (SSTs) were restored to observed full-field data, were carried out by the South China Sea Institute of Oceanology, Chinese Academy of Sciences. These experiments, by restoring both observed anomalies and climatological means, allow for the examination of how biases in the mean-state of one ocean might influence the mean state of others. Furthermore, they enable an exploration of how correcting these mean-state biases could affect the large-scale interactions between the three tropical ocean basins.
How to cite: Wang, C., Fan, H., and Chen, S.: Investigating Tropical Basin Interactions: Insights from the TBIMIP Pacemaker Experiments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5143, https://doi.org/10.5194/egusphere-egu25-5143, 2025.
In this contribution, we argue that other processes beyond the ocean dynamical thermostat mechanism can be key for initial equatorial Pacific cooling from a quadrupling of CO2 concentration, with those processes being influenced by internal climate variability. The thermostat mechanism, which is proposed to explain the cooling trend observed in recent decades, suggests that the cold upwelling in the eastern equatorial Pacific has delayed the warming of the equatorial Pacific from rising CO2. We run a large-ensemble of 250 simulations of an abrupt CO2 quadrupling with the Max Planck Institute - Earth System Model (MPI-ESM) climate model. While the ensemble mean shows weak initial warming in the equatorial Pacific in the first 2 years, compared to global changes, the individual ensemble members show a wide spread of responses, with 42 out of the 250 members simulating a cooling. We then separate between the 42 cooling members and the 46 members that warm the most and analyze the upper-ocean energy budget. The main driver distinguishing the two groups is the change in the meridional heat transport, with energy divergence driving cooling in the central-western Pacific and energy convergence driving warming in the eastern Pacific. This is mainly caused by the change in ocean meridional velocities and is amplified by the change in the meridional temperature gradient. Strengthened easterlies over the central and western Equatorial Pacific increase Ekman transport away from the Equator that drives cooling, while weakened easterlies decrease Ekman transport, warming the eastern equatorial Pacific. In contrast to the meridional heat transport, cooling due to the ocean dynamical thermostat mechanism from vertical heat transport is similar in all the cases and cannot explain the ensemble spread. Other contributions to the energy budget play a minor role, such as the shortwave surface radiation, or are a response to the temperature anomaly rather than a driver, such as the latent heat flux. Over longer, multidecadal timescales, both cooling and warming simulations converge to show amplified warming in the eastern equatorial Pacific, consistent with past studies. Our findings suggest that other mechanisms can be more important than the thermostat mechanisms for cooling the equatorial Pacific, with a large impact of internal variability. This highlights the need for large ensembles of simulations in studies of the initial response to increasing CO2.
How to cite: Moreno-Chamarro, E., Putrasahan, D., and M. Kang, S.: Initial equatorial Pacific cooling due to CO2 forcing shaped by internal variability, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3969, https://doi.org/10.5194/egusphere-egu25-3969, 2025.
Climate-model mean-state biases such as the double-Intertropical Convergence Zone (ITCZ) bias and cold-tongue bias are long known, but a definitive solution to them has remained elusive due to the highly coupled nature of the climate system, and they remain a problem in the current generation of climate models (Tian & Dong 2020). Recently, as anthropogenically forced trends have more clearly emerged from the noise of internal variability, systematic biases in simulated trends compared to observations have also been demonstrated (e.g., Wills et al. 2022), notably in the tropical Pacific sea-surface temperature (SST) pattern and Walker circulation strength. This has raised interest in understanding the influence of mean-state biases on climate trends. Here we show that there are a handful of CMIP6 models with a much-reduced double-ITCZ bias in the East Pacific, a region that plays an important role in two-way teleconnections between the tropical Pacific and Southern Ocean (Dong et al. 2022). These low-ITCZ-bias models show tropical Pacific SST trend patterns that are more like observations, with bands of relatively little warming extending from the southeastern subtropical Pacific to the equator. Moreover, these models show much larger amplitude decadal SST variability throughout the Indo-Pacific, which is more in line with observations. We provide evidence that the more patterned forced SST trends and larger decadal SST variability both arise from a stronger propagation of southeast subtropical Pacific SST anomalies towards the equator when this propagation is not blocked by a double ITCZ. Our findings suggest that reducing the double-ITCZ bias in climate models has the potential to substantially improve climate projections.
Tian, B. & Dong, X. The double-ITCZ bias in CMIP3, CMIP5, and CMIP6 models based on annual mean precipitation. Geophysical Research Letters 47, e2020GL087232 (2020).
Wills, R. C., Dong, Y., Proistosecu, C., Armour, K. C. & Battisti, D. S. Systematic climate model biases in the large-scale patterns of recent sea-surface temperature and sea-level pressure change. Geophysical Research Letters 49, e2022GL100011 (2022).
Dong, Y., Armour, K. C., Battisti, D. S. & Blanchard-Wrigglesworth, E. Two-way teleconnections between the Southern Ocean and the tropical Pacific via a dynamic feedback. Journal of Climate 35, 6267–6282 (2022).
How to cite: Jnglin Wills, R. and DiNezio, P.: Climate models without an East Pacific Double ITCZ better simulate tropical Pacific climate variability and change, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17292, https://doi.org/10.5194/egusphere-egu25-17292, 2025.
The El Niño-Southern Oscillation (ENSO) is the dominant mode of interannual variability, with global climate impacts that underscore the importance of its accurate representation in climate models. Building on the success of the CLIVAR ENSO metrics package, our work focuses on developing an advanced workflow for evaluating ENSO in the ACCESS family of models, extending its functionality to include diagnostics for the Indian Ocean Dipole (IOD).
Our approach leverages the IRIS-based pre-processors within ESMValTool, enabling a modular and flexible development of ENSO and IOD diagnostics. These are implemented as Python-based diagnostics that can seamlessly integrate into both exploratory Jupyter Notebooks and the traditional YAML-based ESMValTool recipes. The notebooks, part of the open-source ACCESS-ENSO-Recipes, are hosted on GitHub and serve as a powerful platform for designing diagnostics, visualisation, and interactive data exploration. Meanwhile, the YAML recipes facilitate the semi-automated evaluation of multi-model ensembles and large datasets, ensuring compatibility with established workflows for climate model evaluation.
Our current suite of diagnostics aims to reproduce the functionality of the CLIVAR ENSO metrics package, focusing on ENSO variability, teleconnections, and physical processes. By utilising notebooks, we create an agile environment for developing and refining diagnostic tools, enhancing collaboration between scientists and model developers. At the same time, the structured recipe format ensures reproducibility and scalability, enabling systematic analysis across models and ensembles.
We plan to extend this approach to evaluate broader oceanic and atmospheric processes in the ACCESS models, enabling more comprehensive assessments of model performance. The ACCESS-ENSO-Recipes and Python diagnostics will also be shared on the ESMValTool GitHub repository to encourage collaboration and wider use in the climate modelling community.
This workflow balances innovation and scalability by integrating flexible notebooks with structured legacy tools. Advancing ENSO and IOD evaluation will improve climate model accuracy and our understanding of their impacts on current and future climates.
How to cite: Beucher, R., Chun, F., Planton, Y., Sullivan, A., Chung, C., Rashid, H., Boschat, G., and Maher, N.: ACCESS-ENSO-Recipes: A Flexible Workflow for ENSO and IOD Evaluation Using ESMValTool and Jupyter Notebooks, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1273, https://doi.org/10.5194/egusphere-egu25-1273, 2025.
The El Niño/Southern Oscillation (ENSO) is a dominant mode of interannual climate variability, profoundly influencing global weather and climate systems. However, accurately simulating ENSO in climate models remains a major scientific challenge due to the complex coupled ocean-atmosphere interactions involved. Utilizing the ENSO Metrics Package, which evaluates tropical climatology, ENSO performance, and feedback biases, twenty-one atmospheric parameters related to cloud physics, microphysics, and turbulence schemes were tuned for ENSO simulations in the next-generation Max-Planck-Institute for Meteorology Earth System model, ICON XPP. Initial parameter perturbations were performed in AMIP simulations to estimate model sensitivities to each parameter. The optimal parameter combination for ENSO simulations was estimated based on the Nelder-Mead optimization scheme using the linear superposition of the parameter sensitivities. This approach effectively reduced the ENSO metrics cost function by 40% in the optimized run within AMIP experiments, including very good simulations of the Bjerknes and atmospheric net heat flux feedbacks. However, applying the optimized parameter sets to fully coupled ocean-atmosphere simulations resulted in very different parameter sensitivities and much less improved ENSO simulations. This discrepancy in the coupled model is largely related to very strong mean state changes in the Sea Surface Temperatures (SST) in the tropical. Direct tuning of parameters in coupled ICON XPP simulations will be explored in subsequent studies.
How to cite: Yu, D., Dommenget, D., Pohlmann, H., and Müller, W.: Improving ENSO Simulation through Optimization of Atmospheric Parameterizations in the ICON XPP Earth System Model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7893, https://doi.org/10.5194/egusphere-egu25-7893, 2025.
Posters on site: Wed, 30 Apr, 10:45–12:30 | Hall X5
Coastal upwelling, driven by alongshore wind that pushes surface waters away from the shore, brings nutrient-rich, cold, deep waters to the surface, fueling productivity and biodiversity in marine ecosystems. The Northern Humboldt Current System, as one of the most productive coastal upwelling systems, is renowned for its high fish catches due to persistent upwelling. Since upwelling cannot be directly measured, scientists rely on indices to estimate it through upward velocity, the Ekman transport, and temperature gradients. These indices are derived from winds data, nitrogen concentrations, and sea surface temperature. To identify which indices best correlate with upward velocity and local biogeochemistry in the system, we used a high-resolution regional model: CROCO-BioEBUS. Our results show that wind-based and nitrogen-based indices better correlate with the upward velocity and biogeochemical factors like nitrogen and chlorophyll concentrations. In contrast, temperature-based indices display different response compared to the wind- and nitrogen-based indices, particularly during extreme events like El Nino. During El Nino, the upwelling intensity increases at the base of the mixed layer depth. Nevertheless, this upwelling intensification did not lead to a major transport of nutrients, as the overall upwelled nitrogen decreased. Discrepancies between the sea-surface temperature-based and wind-based upwelling indices are highlighted during El Nino, suggesting the need to reconsider how upwelling is defined and measured in the Northern Humboldt Current System.
Key words: Upwelling Indices, Northern Humboldt Current System, El Niño
How to cite: Lizarbe, D., Xue, T., Juricke, S., and Zeller, M.: Interannual Variability in the Northern Humboldt Current System: Insights from various Upwelling Indices, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13860, https://doi.org/10.5194/egusphere-egu25-13860, 2025.
A recent study of a series of simulations with idealised geometries of the tropical ocean basins and land found remarkable changes in ENSO and tropical basin interactions that suggest that atmospheric dynamics are largely controlling ENSO dynamics. Here we will discuss shallow water atmosphere (Gill-type) model results to explore how simplified atmospheric dynamics can control growth rate (Bjerknes feedback) and period of ENSO. We find that for single tropical ocean basins larger than the Pacific the Bjerknes feedback becomes weaker due to the zonal length of the SST forcing and at the same time the meridional winds become stronger. This result suggest that basins larger than the Pacific will have weaker ENSO variability due to the atmospheric dynamics controlling the wind stress. Interactions with heat sources in remote tropical ocean basins have the ability to strongly enhance the Bjerknes feedback leading to stronger control on ENDO dynamics than the basin size. The changes in the winds stress also affect the period of ENSO by altering the wind stress curl, which for larger ocean basins gets closer to the equator and thereby increasing the Rossby wave speed supporting the finding that larger basins have short ENSO periods.
How to cite: Dommenget, D. and Wang, J.: Atmospheric dynamics controlling ENSO growth rate and period in a series of idealised worlds simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5289, https://doi.org/10.5194/egusphere-egu25-5289, 2025.
The decay pace of El Niño can significantly modify its impacts on the Asian climate during the post-El Niño summer. Hence, accurately reproducing the observed decay pace in state-of-art coupled models is essential for realistic climate simulations. In the CMIP6 models, El Niño decays slower than observed. This slower decay can be attributed to weaker-than-observed air-sea coupling in the models that causes a weaker atmospheric convective response and smaller westerly anomalies along the equatorial Pacific during the El Niño life cycle. The smaller westerly anomalies result in a slower discharge of equatorial ocean heat, weaker negative/positive thermocline anomalies along/off the equator and thus a weaker meridional gradient of the thermocline anomalies. This weakens the easterly current anomalies, diminishes the zonal advection feedback, and ultimately slows the decay pace of El Niño in the models.
How to cite: wei, Y.: On the Slow Decay of El Niño in CMIP6 Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1871, https://doi.org/10.5194/egusphere-egu25-1871, 2025.
Earth’s climate naturally fluctuates on timescales ranging from intraseasonal to centennial, even in the absence of changes in external forcings such as greenhouse gases or volcanic eruptions. The El Niño–Southern Oscillation (ENSO) exemplifies this internal (or unforced) variability within the climate system. Climate model simulations also exhibit this internal variability, as demonstrated by the diverse climate conditions observed in "initial-condition large ensembles" (LEs). These LEs enable to isolate the influence of internal variability and detect trends associated with climate change.
By examining the evolution of ENSO in LEs from the 6th Coupled Model Intercomparison Project (CMIP6), we find that ENSO variability has significantly increased during the historical period and is projected to continue increasing under future global warming in most models. This enhanced variability is often linked to changes in equatorial Pacific zonal or vertical temperature gradients. However, our analysis reveals complex, nonlinear relationships between ENSO characteristics and shifts in the mean climate state.
How to cite: Planton, Y., Lee, J., Wittenberg, A., Gleckler, P., Guilyardi, É., McGregor, S., and McPhaden, M.: ENSO Variability Changes in a Warming World, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19093, https://doi.org/10.5194/egusphere-egu25-19093, 2025.
The El Niño-Southern Oscillation (ENSO) is a dominant mode of climate variability, influencing not only the global climate but also atmospheric chemistry. The impact of ENSO on tropospheric ozone has been investigated in previous research, yet its influence on tropospheric ozone in a warmer world remains unclear. Here, we investigate changes in the impact of ENSO on tropospheric ozone in the Northern Hemisphere using Coupled Model Intercomparison Project Phase 6 (CMIP6) data under the SSP3-7.0 scenario. The results of linear regression indicate that the response of ozone to ENSO strengthens, with coefficients increasing significantly from the near future to the distant future. This enhancement might be associated with changes in atmospheric circulation. This study suggests that ENSO should be considered when predicting changes in Northern Hemisphere tropospheric ozone under accelerating global warming.
※ This work was supported by the Korea Environment Industry & Technology Institute (KEITI) through the “Climate Change R&D Project for New Climate Regime” funded by the Korea Ministry of Environment (MOE) (2022003560001), and by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2022R1A2C1008858).
How to cite: Yun, S., Jeong, J., and Moon, B.-K.: Global Warming Enhances the impact of ENSO on Northern Hemisphere Tropospheric Ozone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18652, https://doi.org/10.5194/egusphere-egu25-18652, 2025.
The El-Niño Southern Oscillation (ENSO) is the dominant mode of inter-annual climate variability, driven by ocean-atmosphere interactions in the tropical Pacific that alternate between the warm (El-Niño) and cold (La-Niña) phases over a 3-7 year cycle. With increasing greenhouse gases ENSO teleconnections are projected to change in future global warming scenarios. Land surface temperature and precipitation teleconnections are projected to change over more than 50% of global land regions by the end of the 21st century.Most land regions show a significant amplification of the teleconnection. This presentation will examine how the teleconnections are projected to change over the tropics and subtropics and how dynamic and thermodynamic components contribute to these changes.
This will lead into future work using a linear baroclinic model to improve the understanding of changes in dynamical ENSO teleconnection processes over the tropics and subtropics. For example, how does the projected eastward shift of the equatorial Pacific ENSO precipitation anomalies influence the changes in precipitation and temperature teleconnections over tropical and sub-tropical land regions?
How to cite: Goswami, D. J., Chadwick, D. R., Collins, P. M., Syktus, M. J., Trancoso, D. R., and ineson, S.: Future changes of ENSO precipitation and temperature teleconnections over tropics and subtropics., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8475, https://doi.org/10.5194/egusphere-egu25-8475, 2025.
This study examines the projected change in rainfall and temperature anomalies across Mainland Southeast Asia, focusing on the teleconnection of El Niño Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Five GCM’s from the CMIP6 project (GFDL-ESM4, IPSL-CM6A-LR, MPI-ESM1-2-HR, MRI-ESM2-0, and UKESM1-0-LL) are used to investigate the historical (1985-2014) and future periods based on two Shared Socioeconomic Pathways (SSPs), SSP3-7.0 and SSP5-8.5, divided into three periods: near-future (2015-2044), mid-future (2041-2070), and far-future (2071-2100). The impact of ENSO and IOD on climate anomalies is analyzed using partial correlation coefficient (PCOR) calculated between Niño 3.4 and DMI index. PCOR allows us to examine the influence of ENSO while excluding the effect of IOD, and vice versa. We divided the study into three seasons: March-April-May (MAM), June-July-August-September (JJAS), and October-November-December (OND). Generally, ENSO and IOD show positive correlations with temperature, which means the positive phase of each results in higher temperature, whereas their correlations with rainfall can be positive as well as negative. Negative correlations between ENSO and rainfall predominate in most MSEA areas leading to drier conditions during El Niño events, except during June-July-August-September during which ambiguous patterns occur with both negative and positive influences from ENSO. Meanwhile, IOD presents significant positive influences on rainfall over large areas. Future correlations are generally higher than historic ones, suggesting a potential for better predictability of seasonal forecasts.
How to cite: Wanthanaporn, U., Supit, I., Hutjes, R., and Chaowiwat, W.: Future ENSO and IOD associated seasonal rainfall and temperature anomalies over Mainland Southeast Asia., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3508, https://doi.org/10.5194/egusphere-egu25-3508, 2025.
The simulation of teleconnections in climate models can be hindered by biases. This study investigates the impact of model systematic errors on the teleconnections between the El Niño-Southern Oscillation (ENSO) and the North Atlantic-European (NAE) region during early winter (December), using historical simulations from phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP). Generally, a positive Indian Ocean Dipole (IOD) event concurs with the onset of an El Niño event. In December, a positive IOD can initiate a Rossby wave-train that propagates from the subtropical South Asian JET (SAJET) sector toward the NAE region, where it forces an atmospheric response that resembles the positive phase of the North Atlantic Oscillation (NAO). Models that fail to simulate the early-winter teleconnection projecting onto the NAO positive phase pattern consistently exhibit a weak Rossby wave source in the SAJET region. Additionally, these models simulate an overly strong subtropical Pacific jet stream, which favors meridionally bent Rossby wave-trains. This waveguide bias is likely due to a cold northwestern Pacific, a mean state bias common to many climate models. These findings suggest that an uncertainty factor regarding the ENSO teleconnection with the NAE may stem from both a degraded Indo-Pacific inter-basin coupling and an overly strong Pacific waveguide.
How to cite: Sabatani, D. and Gualdi, S.: ENSO teleconnections with the NAE sector during boreal early-winter in CMIP5/CMIP6 models: impacts of the atmospheric mean state, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4624, https://doi.org/10.5194/egusphere-egu25-4624, 2025.
This study examines the relationship between Indian Summer Monsoon Rainfall (ISMR: June- September Season) and climate drivers across the three tropical basins, namely the Pacific Ocean, Indian Ocean, and Atlantic Ocean, using observation and reanalysis data for the period 1961-2023. Although Sea Surface temperature (SST) conditions over the tropical Pacific Ocean have a major influence on the interannual variability of ISMR, their interactions with the Indian Ocean and the Atlantic Ocean also play a role. The interannual relationship between El Nino Southern Oscillation (ENSO) and ISMR is well-established, but its epochal variations and impact across different homogeneous regions of India (the north-west, north-east, central, and south peninsular India) is not yet fully understood. To understand this epochal variation in the ENSO-ISMR relationship and the role of the tropical basin interaction, this study considered three different periods (P-I: 1961-1980, P-II: 1981-2000, P-III: 2001-2023). The lead-lag correlation analysis showed that the SST conditions over the north Indian Ocean (NIO) and the south tropical Atlantic Ocean (STAO) during the preceding winter (October-January) season has a significant role on ISMR. The analysis showed a significant (insignificant) negative simultaneous correlation between ENSO and ISMR for P-I and P-III (P-II). Regression analysis reveals that in P-I, the SST conditions over the NIO as well as STAO had insignificant influence on the ISMR. In P-II, the role of the NIO became significant, particularly over the south peninsular India. In P-III, the influence from the NIO has reduced (although still significant), at the same time the role of STAO became significant. The thermodynamical analysis is performed to understand the mechanism relating the role of NIO and STAO in modifying the ENSO-ISMR teleconnection in the three periods. This study highlights the importance of the changes in the large-scale patterns of the oceanic as well as atmospheric fields and the interactions between the tropical Indian-Pacific-Atlantic Oceans, to the performance of ISMR.
How to cite: Sharma, T., Ratna, S. B., Richter, I., and Pai, D. S.: Influence of Tropical Ocean Basins on the Interannual Variability of Indian Summer Monsoon Rainfall during three recent epochs, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-651, https://doi.org/10.5194/egusphere-egu25-651, 2025.
The ENSO event is partially affected by tropical Atlantic El Nino, according to past studies. In this study, pacemaker experiments are conducted with NorESM in four configurations, totaling 20 members, in total during 1980-2020. The results show there is a correlation between tropical JJA ATL3 and the later DJF NINO3.4. This connection is strong before 2000, and disappears afterward. Which is consistant with observation. Zonal wind correlation indicated the path of this connection.
However, it was also found that the SST west of ATL3 has a stronger relationship to NINO3.4 in both periods. This implies that ATL3 is useful for detecting Atlantic ENSO, but the interaction is more robust in the closer region.
How to cite: Chiu, P.-G. and Keenlyside, N.: Influence of tropical Atlantic SST on the El Nino region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16024, https://doi.org/10.5194/egusphere-egu25-16024, 2025.
Year-to-year variability in Indian summer monsoon rainfall (ISMR) can greatly impact the densely populated Indian subcontinent. While it has been demonstrated previously that tropical Pacific and Indian Ocean SSTAs can influence ISMR variability, the relative contribution from each basin has been difficult to determine due to the complicated inter-basin interactions. Using observational data and the historical simulations (1950-2014) from seven CMIP6 models with large ensemble sizes, we apply a cyclo-stationary linear inverse model (CS-LIM) to assess the isolated contributions from tropical Pacific SSTAs, Indian Ocean SSTAs, and their interaction to ISMR interannual variability. Observational results indicate that Pacific SSTAs enhance precipitation variability over northeastern and southern India, while Indian Ocean SSTAs and the Indo-Pacific interaction reduce the variability, with the Indo-Pacific interaction strongly damping the precipitation variability over central India. In CMIP6 models, Pacific SSTAs typically increase ISMR variability, but their spatial patterns largely differ from observations. For the impacts from Indian Ocean SSTA and the Indo-Pacific interaction, all models capture the observed reduction in precipitation variability, but the magnitude and spatial patterns vary considerably, with most models failing to simulate the stronger damping effect due to the Indo-Pacific interaction.
How to cite: Guderian, E. and Han, W.: Isolated tropical Indo-Pacific SSTA Impacts on ISMR Variability in Observations and CMIP6 Historical Simulations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2895, https://doi.org/10.5194/egusphere-egu25-2895, 2025.
The tropics are a pivotal amplifier of global climate variability and change, largely through interannual phenomena such as the El Niño-Southern Oscillation (ENSO) and other tropical basin interactions, which underscores the importance of accurately modeling their behavior under changing climate forcings. Here, we investigate how mean state, seasonality, and interannual variability of the tropical Pacific respond to altered boundary conditions in past warm climates. We also examine how these changes affect teleconnections with the tropical Atlantic, specifically the Caribbean Sea.
Motivated by past warm climate time-windows covered by monthly-resolved proxy records of sea-surface temperature (SST) and hydrology derived from fossil corals, we use the fully coupled water and carbon isotope-enabled Community Earth System Model (iCESM) to perform time-slice simulations for three key climate intervals: Pre-Industrial (PI), Mid-Holocene (6 ka), and Last Interglacial (124 ka) at a nominal horizontal resolution of 1° in the atmosphere, land, ocean and sea-ice components. Mid-Holocene coral records from the Line Islands (central tropical Pacific) and Bonaire (southern Caribbean), as well as Last Interglacial coral records from Bonaire, are used for model–data comparisons.
In response to the changes in orbital and greenhouse-gas boundary conditions, both 6 ka and 124 ka simulations show distinct climate anomalies. The tropical eastern Pacific exhibits La Niña-like conditions in SST with a cooling by 0.4°C and 0.5°C during boreal winter for 6 ka and 124 ka, respectively, which is also evident from an increased zonal sea-level pressure gradient as compared to PI. Contrasting anomalies north and south of the equator over the tropical Pacific result in a statistically significant increase of the meridional SST asymmetry by 0.3°C for 6 ka and 0.6°C for 124 ka as compared to PI. These point to a reorganization of the tropical Pacific mean state. Concurrently, both of our simulations reveal a significant reduction in the interannual variability of SST in the Central Pacific and a significant increase in the Eastern Pacific, with 124 ka showing larger amplitudes of the anomalies by up to 20% relative to PI. Taken together, these patterns indicate a response of the tropical Pacific to warmer boundary conditions, altering large-scale atmospheric circulation and affecting teleconnections into neighboring basins.
Moreover, previous research points to relationships between Pacific SST interannual variability and southern Caribbean SST seasonality under modern climate conditions. Both our 6 ka and 124 ka simulations show increased SST seasonality in the southern Caribbean, which is consistent with evidence from coral records. In an ongoing analysis combining our simulations and available coral records from the Atlantic and Pacific, we further explore the characteristics of potential connections between seasonality and variability during past warm intervals in order to get deeper insights into Atlantic–Pacific climate dynamics and teleconnections under warm climates.
How to cite: Li, D., Prange, M., Schulz, M., Felis, T., and Merkel, U.: Tropical Pacific climate variability and Atlantic–Pacific teleconnections under Holocene and Last Interglacial forcings, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12537, https://doi.org/10.5194/egusphere-egu25-12537, 2025.
The Pacific Decadal Variability (PDV) is the dominant mode of Earth System variability on multi-decadal timescales. Depending on the phase of the PDV, it can either accelerate or mitigate the global warming trend. Additionally, PDV has significant societal impacts. It influences hazards across the Pacific region, including floods, droughts, and bushfires, as well as the two main ice sheets, resulting in substantial coastal impacts. Most studies investigating the drivers of PDV have focused on the instrumental period, concluding that PDV is primarily driven by atmospheric processes, with a relatively minor contribution from oceanic processes. However, the instrumental period may be too short to fully capture the low-frequency climate variability in the Pacific, particularly its associated oceanic processes. Moreover, this period is marked by large changes in external forcings, especially those resulting from anthropogenic greenhouse gas and aerosol concentrations. Here we present a novel, fully coupled paleoclimate reanalysis spanning the past 400 years. This reanalysis utilizes the Norwegian Climate Prediction Model (NorCPM), equipped with an Ensemble Kalman Filter data assimilation method. Originally developed for producing oceanic reanalysis with skills comparable to top-performing ocean reanalysis systems, we adapted NorCPM to incorporate hundreds of paleoclimate records, including coral, tree-ring, and ice-core observations, extending back four centuries. When compared with state-of-the-art atmospheric reanalyses and surface oceanic observations, our reanalysis demonstrates high skill across the Pacific domain. As a coupled model, it enables a detailed quantification of multi-decadal, two-way interactions between the ocean and atmosphere that drive the PDV. By comparing this fully coupled reanalysis with standalone simulations (i.e., those without ocean-atmosphere coupling), we quantify the contribution of these coupled interactions to the PDV. Finally, we present the impacts of long-lasting extreme PDV states on hydroclimate variability across the Pacific basin, providing new insights into the effects of PDV at regional and global scales.
How to cite: Dalaiden, Q., Counillon, F., Svendsen, L., Abram, N. J., Lyu, A., Wang, Y., and Keenlyside, N.: Towards an improved understanding of Pacific Decadal Variability using paleoclimate reanalysis, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6468, https://doi.org/10.5194/egusphere-egu25-6468, 2025.
This study investigates why observed decadal‐scale climate variability is predominantly pronounced in the Niño4 region compared to other equatorial Pacific areas using both observation and model sensitivity experiments. The initial shift to the negative phase of Tropical Pacific Decadal Variability (TPDV) is primarily driven by the upward and eastward migration of isopycnal negative temperature anomalies along the equator. Subsequently, the wind fields associated with the negative phase of the Pacific Meridional Mode (PMM) induce anomalous vertical currents in the equatorial Pacific. This leads to anomalous upwelling and downwelling of mean temperature in the Niño4 and Niño3 regions, respectively, thereby strengthening and weakening the corresponding subsurface‐produced sea surface temperature anomalies. Our findings clarify the roles of subsurface temperature anomalies in the phase reversal of TPDV and PMM in amplifying decadal variance, specifically in the equatorial central Pacific. Plain Language Summary: Observations have consistently highlighted prominent decadal climate variability in the Niño4 region, yet the underlying cause of this distinct pattern remains largely elusive. In this study, we use composite analysis during the phase transition of Tropical Pacific Decadal Variability (TPDV) and modeling experiments to elucidate the mechanisms governing the observed decadal climate variability in the Niño4 region compared to other equatorial areas. Our findings reveal that the eastward and upward propagation of negative subsurface temperature anomalies primarily drives the phase reversal of TPDV. Following this transition from positive to negative phase, the Pacific Meridional Mode (PMM) plays a crucial role. Specifically, PMM‐associated wind forcing induces anomalous upwelling and downwelling in the Niño4 and Niño3 regions, respectively. This results in anomalous vertical advection of mean temperature, contributing to the strengthening and weakening of decadal variances in these regions. Key Points: Subsurface temperature anomalies initiate the phase reversal of TPDV while PMM plays a key role in equatorial SSTAs post‐transitionVertical heat advection is crucial in reinforcing/weakening decadal variance in the Niño4/Niño3 regionPMM‐associated wind fields induce anomalous vertical advection after the TPDV phase transition
How to cite: Tseng, Y., San, S.-C., Ding, R., and Di Lorenzo, E.: Why is tropical Pacific decadal variability predominantly observed in the Nino4 region?, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2370, https://doi.org/10.5194/egusphere-egu25-2370, 2025.
