Astroclimatic signatures in ENSO recurrence across equinoctial orbital asymmetries

Introduction This study examined the temporal organization of ENSO variability in relation to two Earth–Sun orbital-state descriptors, ΔLP and Fi, over the period 1948–2025. Methods Annual series were analyzed using Singular Spectrum Analysis (SSA), Information Theory, partial wavelet coherence (PWC), and multiple wavelet coherence (MWC) as complementary methods. ΔLP describes the evolving Earth–Sun orbital geometry through the combined effects of solar declination and Earth–Sun distance, whereas Fi is the Newtonian Earth–Sun gravitational-force descriptor proportional to the inverse square of Earth–Sun distance (1/d²). SSA was used to characterize recurrent quasi-periodic oscillatory components, while PWC and MWC assessed conditional and joint coherence structures. Results Several characteristic timescales were consistently recovered in both the BEST ENSO index and the orbital-state descriptors. Common coherence bands were identified around 2.5–3 years, near 4 years, and across a broader decadal-scale range of approximately 8–16 years. These bands are consistent with characteristic temporal scales independently recovered by SSA, particularly the approximately 2.5–3-year, 3.5–4-year, and 11–13-year reconstructed components, although their temporal expression remains non-stationary. The reconstructed low-frequency components also indicate convergence toward a common positive phase near the end of the observational record (c. 2025). Discussion Because the orbital-state descriptors are derived directly from astronomical ephemerides, their evolution beyond 2025 can be computed using the same methodology applied throughout the observational period. If the reconstructed low-frequency organization persists, the continued evolution of the orbital-state descriptors would remain within a positive low-frequency phase historically associated with El Niño-like intervals in the analyzed record. This provides an observationally testable hypothesis for future evaluation.