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

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.