A cross-timescale oceanic bridge linking intraseasonal atmospheric forcing to ENSO predictability

Tropical western Pacific upper ocean circulation is traditionally regarded as a passive ENSO response, yet its active role in ENSO preconditioning remains unclear. Using observations and model experiments during 1993–2025, we show that pre-winter intraseasonal zonal current variability ( U ISV) systematically intensifies before El Niño and weakens before La Niña. Approximately 80% of ISV amplitude is explained by Indian Ocean Madden–Julian Oscillation (MJO) related wind forcing. Repeated MJO-driven U ISV generates a persistent low-frequency response through cumulative upper-ocean dynamical adjustment, enhancing Kelvin-wave-induced zonal heat advection and central-Pacific heat accumulation. U ISV intensifies approximately 8 months before ENSO onset, provides complementary long-lead upstream predictive information for ENSO preconditioning beyond monthly-mean circulation alone, and improves Niño-3.4 reconstruction ( r = 0.73). Our findings identify a cross-timescale ocean dynamical bridge through which repeated MJO-driven intraseasonal ocean variability contribute to seasonal-to-interannual climate variability and marine heat waves.