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https://doi.org/10.21203/rs.3.rs-7865695/v1
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.21203/rs.3.rs-7865695/v1
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
16 Apr 2026
| 16 Apr 2026
Status: this preprint is open for discussion and under review for Climate of the Past (CP).
Precession-forced asymmetric continental heating shapes ENSO variability
Abstract. Geologic and modelling evidence reveals that the ENSO is strongly affected by the precession of the Earth’s rotation axis, yet the mechanisms remain unclear due to interactions among multiple forcings. Using high-resolution model simulations reconstructing the ENSO activity across a full precessional cycle, we find that ENSO is strongest during austral summer perihelion, as today. This behavior arises from asymmetric continental heating: austral summer perihelion introduces strong warming on Australia, east of the Indo-Pacific ITCZ. Because deep convection favors the warmest areas, this causes the ITCZ and the Warm Pool to shift eastwards. As a result, the Pacific’s east-west thermal contrast is reduced, lowering the threshold for oscillations of convection and amplifying ENSO activity. In contrast, boreal summer perihelion warms Afro-Eurasia, shifts the ITCZ westward and weakens ENSO. Proxy records across the Indo-Pacific support this changes in climate state. Understanding asymmetric continental heating helps us link astronomical modulation to ENSO behavior and improve long-term predictions of tropical climate change.
How to cite. This is an external preprint publicly discussed on EGUsphere. Please find the correct citation on the original preprint server’s site at: https://doi.org/10.21203/rs.3.rs-7865695/v1
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Status: open (until 11 Jun 2026)
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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- RC1: 'Comment on egusphere-2026-1183', Anonymous Referee #1, 11 May 2026 reply
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RC2: 'Comment on egusphere-2026-1183', Anonymous Referee #2, 22 May 2026
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Liu and colleagues document results from 24 timeslice simulations with AWI-ESM over a full precession cycle, in which boundary conditions change the seasonal and hemispheric distribution of insolation. They focus on investigating how ENSO variability responds to global mean-state shifts driven by orbital forcing. I think that these simulations and investigations therein are valuable. However, several questions arise while reading this version of the manuscript, so I do not think the central claims are supported by the analyses presented. I detail my claims below:
Major Comments:
- The manuscript does not convincingly demonstrate a mechanistic link between asymmetric land heating and ENSO variability.
- The central claim in this paper is that precession-forced asymmetric continental heating shifts the Indo-Pacific ITCZ and warm pool, modifies the Pacific east-west thermal contrast, and thereby changes ENSO amplitude. However, the analyses presented do not mechanistically demonstrate this proposed causal chain. What is shown more clearly is that Northern Hemisphere and Southern Hemisphere moist static energy, land heating, and rainfall vary differently (and perhaps somewhat anti-phased acc to Fig. 4a–b) over a precession cycle. That result, while valuable in its own right,is not sufficient to establish that these changes mechanistically control interannual ENSO variability.
- The manuscript should first distinguish how orbital forcing reorganizes the seasonal-mean tropical climate and second, how such a mean-state reorganization changes the coupled ocean-atmosphere feedbacks that generate ENSO on interannual timescales (see e.g., DiNezio et al. 2013—J.Clim; Fedorov et al. 2020—Chapter 8, ENSO in a changing climate; Pontes et al. 2022—Nat.Geosci; Thirumalai & DiNezio et al. 2024—Nature; Molina et al. 2026—J Clim; such relevant papers need to be cited and discussed, especially in the context of the mechanism proposed in the manuscript). The current manuscript largely supports the first statement, but the second remains very underexplored and is a critical limitation of the analysis presented.
- A more compelling mechanistic demonstration would necessitate showing that the proposed heating asymmetry directly influences quantities involved in the time-transient and seasonally-phase-locked growth and decay of ENSO, such as Bjerknes feedback strength, surface heat-flux damping, mixed-layer depth, recharge/discharge behavior, westerly wind-burst statistics, or the seasonality of coupled instability (etc.) Recent ENSO literature has emphasized these specific process-level diagnostics, including the roles of upper-ocean stratification (Tuckman and Yang, 2026—arXiv), Walker circulation strength (Fedorov et al. 2020), warm-pool westward extent (Thirumalai, DiNezio et al. 2024—Nature), and ocean-atmosphere coupling (Okumura et al. 2010—J. Clim.) in modulating ENSO amplitude and the frequency of extreme El Niño events.
- The manuscript’s key point relies on Fig. 3, which shows a correlation between the east–west MSE “gap” and the Niño3 index. This is used as evidence that the zonal MSE contrast triggers a positive feedback, and essentially that this correlation outlines causation. During El Niño, the warm pool shifts eastward, increasing eastern-Pacific MSE and collapsing the MSE gap, making these two measures expressions of the same event. In order to showcase causality, what is the evolution across an interannual ENSO event acc. to the model and is there evidence of the gradient leading ENSO onset over time? What forcings affect such changes? Can this be isolated to mean state changes and then precessional changes? These questions remain unanswered.
- The proposed mechanism is not clearly separated from large-scale Indo-Pacific rainfall synchronizatoin.
- The manuscript suggests that continental heating alters convection and the warm pool, thereby amplifying ENSO. However, the analyses do not rule out a simpler explanation: both the MSE field and Niño3 variability could be responding to a larger-scale, precession-driven reorganization of rainfall in the Indo-Pacific, the ITCZ position, and the Walker circulation. In this scenario, MSE and ENSO variability would be related but not necessarily causally connected as proposed. To address this, the authors should demonstrate lead-lag relationships among MSE anomalies, convection, zonal wind stress, thermocline depth, and Niño3 SST on interannual timescales. If MSE anomalies are mechanistically important, they should precede or modulate the atmospheric wind response that initiates ENSO growth, rather than merely covary with mature ENSO SST anomalies. Composite analyses of the El Niño onset, development, peak, and decay phases would be far more convincing than correlations across broad spatial domains. For instance, composites of precipitation, MSE, surface winds, SST, and thermocline depth during El Niño onset would help test their proposed pathway in a more direct manner.
Minor Comments:
- It is unclear why Niño3 is the chosen index of focus, and not Niño3.4, as is standard for monitoring modern ENSO variability. The authors should show that this choice does not affect their conclusions.
- The authors cite Yang et al. 2024 for model “experimental settings”, but it would be helpful to include pCO2 and other greenhouse gas levels, alongside information related to obliquirt and other boundary conditions as well.
- The authors should specify the duration of each timeslice used for ENSO calculations, the number of independent ENSO events sampled, and include explicit uncertainty estimates for ENSO variance.
Citation: https://doi.org/10.5194/egusphere-2026-1183-RC2
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Latest update: 27 May 2026
Yufei Liu
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Hu Yang
CORRESPONDING AUTHOR
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Haowen Dang
State Key Laboratory of Marine Geology, Tongji University, Shanghai, China
Gerrit Lohmann
Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
Tao Lian
Second Institute of Oceanography, Hangzhou, China
Zhengyao Lu
Department of Physical Geography and Ecosystem Science, Lund University, Lund, Sweden
Xiaoxu Shi
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Jiping Liu
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Hang Wang
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Feng Jiang
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Xinyue Deng
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Xiaoming Hu
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Dake Chen
School of Atmospheric Sciences, Sun Yat-sen University, and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), China
Second Institute of Oceanography, Hangzhou, China
School of Oceanography, Shanghai Jiao Tong University, Shanghai, China
Short summary
Observations and reconstructions show that ENSO is influenced by astronomical precession in addition to anthropogenic warming. Using ~10,000-year simulations, we reconstruct a full tropical precession cycle and find that present ENSO activity is relatively strong within it. This results from asymmetric continental heating that alters the background climate and the threshold for ENSO development, linking astronomical forcing to tropical climate variability.
Observations and reconstructions show that ENSO is influenced by astronomical precession in...
Precession affects ENSO variability, which tends to be strong during periods of austral summer perihelion; however, the underlying mechanisms remain contentious. The authors propose that asymmetric continental heating between Afro–Eurasia and Australia at perihelion and aphelion drives the migration of the ITCZ, alters the Pacific east-west thermal contrast, and modulates the threshold for triggering ENSO events. For instance, summer perihelion induces strong warming over Australia (east of the Indo-Pacific ITCZ), shifting the ITCZ and the warm pool eastward, thereby facilitating the development of strong El Niño events. This mechanism bears resemblance to that of extreme El Niño under greenhouse warming, characterized by a reduced west-minus-east equatorial SST gradient.
Nevertheless, the proposed mechanism must be evaluated alongside other suggested processes. While these factors are not necessarily independent, they may be mutually inclusive, at least in part. Furthermore, the current results should be interpreted within the context of ongoing greenhouse warming, particularly regarding its impact on upper-ocean stratification (Cai et al., 2018, Nature). According to this study, we should be experiencing a high ENSO activity period, which aligns with observations. An attribution study (Cai et al., 2021, Nature Reviews Earth & Environment) indicates that ENSO amplitude has increased by over 30% since the 1960s, yet greenhouse warming accounts for only about 10% of this trend. This suggests that other processes likely play a significant role. The 10% increase is primarily attributed to enhanced upper-ocean stratification, despite the authors noting a La Niña-like mean state change. Under greenhouse warming, although rapid warming occurs over the Afro-Eurasian landmass, the effect of oceanic stratification appears to dominate.
Given these competing factors, I recommend that the authors investigate whether differences in upper-ocean stratification between periods of strong Australian warming and periods of strong Afro-Eurasian warming play a contributory role.