Precessional modulation of ENSO strength and spatial structure in a transient CGCM simulation of the past 3 million years

A past 3-million-year transient simulation with Community Earth System Model version 1.2 reveals precessional dominance in El Niño–Southern Oscillation (ENSO) variability intensity and spatial structure. Under boreal winter-solstice perihelion (like late Holocene), basin-scale ENSO variability intensifies through strengthened thermocline and zonal advective feedbacks, driven by a broad El Niño-like mean surface warming and enhanced tropical Pacific rainbands. Under boreal vernal-equinox perihelion, central-Pacific ENSO variability shifts eastward moderately, primarily due to intensified zonal advective feedback, following a strengthened background South Pacific Convergence Zone (SPCZ) and interhemispherically opposing rainband changes. ENSO responses under boreal summer-solstice and autumn-equinox perihelion are contrastingly weaker. Consequently, both ENSO characteristics coevolve with the precession’s magnitude set by orbital eccentricity. Precessional modulation of ENSO progressively strengthened toward the present, mediated by a gradually intensified background SPCZ under greenhouse gas and ice-sheet forcings. We suggest that sea surface warming patterns and associated tropical rainbands are paramount in past ENSO changes.