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 1^st, 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 (h_shift). The results, when considering h_shift, 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.

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.

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 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.