Stronger and prolonged El Niño-Southern Oscillation in the Early Eocene warmth

S. Abhik (), Dietmar Dommenget, Shayne McGregor, David K. Hutchinson, Sebastian Steinig, Jiang Zhu, Fabio A. Capitanio, Daniel J. Lunt, Igor Niezgodzki, Gregor Knorr, Wing-Le Chan and Ayako Abe-Ouchi
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S. Abhik: Monash University
Dietmar Dommenget: Monash University
Shayne McGregor: Monash University
David K. Hutchinson: University of New South Wales
Sebastian Steinig: University of Bristol
Jiang Zhu: NSF National Center for Atmospheric Research
Fabio A. Capitanio: Monash University
Daniel J. Lunt: University of Bristol
Igor Niezgodzki: Polish Academy of Sciences
Gregor Knorr: Helmholtz Centre for Polar and Marine Research
Wing-Le Chan: University of Tokyo
Ayako Abe-Ouchi: University of Tokyo

Nature Communications, 2025, vol. 16, issue 1, 1-13

Abstract: Abstract The El Niño Southern Oscillation (ENSO) during the Early Eocene Climatic Optimum (EECO, 56–48 million years ago) is investigated using a multi-model ensemble of deep-time climate simulations. We reveal that ENSO sea surface temperature variability during the EECO had significantly longer periodicity and stronger amplitude than present-day conditions. These changes are attributed to intensified ocean-atmosphere feedback processes and enhanced in-phase tropical inter-basin interactions within a broader ocean basin compared to the present-day. Sensitivity experiments in coupled ocean-atmosphere models suggest that tectonic changes, particularly the expansion of the tropical ocean basin, play a dominant role in amplifying ENSO variability and extending its periodicity, while stronger inter-basin connections further enhance ENSO amplitude. Elevated atmospheric CO2 levels, though driving substantial mean-state changes, partially offset the tectonic influence on ENSO variability by modifying feedback processes. These findings underscore the role of tropical ocean basin geometry and atmospheric CO2 levels in shaping ENSO variability, offering insights into past climate dynamics and implications for future projections under sustained global warming.

Date: 2025
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DOI: 10.1038/s41467-025-59263-7

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