the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Apr 2026
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.
Status: open (until 11 Jun 2026)
- RC1: 'Comment on egusphere-2026-1183', Anonymous Referee #1, 11 May 2026 reply
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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.