Bimodal Astroclimatic Modulation of ENSO Dynamics by Net Shortwave Solar Radiation

The persistent Spring Predictability Barrier (SPB) suggests that current ENSO models may omit a crucial external forcing. This study identifies and quantifies a bimodal, season-dependent modulation of the El Niño–Southern Oscillation (ENSO) during the 1999–2024 period, driven by variations in net short-wave solar radiation. Using a SARIMAX framework combined with permutation tests, two distinct regimes were identified: a Short Cycle (March–May) showing a significant positive association with the Oceanic Niño Index (ONI), and a Long Cycle (June–February) exhibiting an inverse relationship. These empirical patterns demonstrate that the ENSO system responds differentially to solar radiation depending on the seasonal phase, providing a new astroclimatic perspective on its dynamics. The methodology rigorously accounts for intrinsic red noise and internal ONI autocorrelations, establishing a physically coherent mechanism that links seasonal orbital forcing to the ENSO energy balance. By integrating the SPB within an empirical external-forcing framework, these findings offer new insights that may enhance seasonal-to-annual predictability and improve the physical realism of ENSO models.