Abnormally Delayed Onset of the South China Sea Summer Monsoon in 2025 Following a La Niña Event

The South China Sea summer monsoon (SCSSM) onset in 2025 was abnormally delayed by about one week, despite the occurrence of a preceding La Niña event that would conventionally favor an earlier onset. This study investigates the multi-timescale mechanisms underlying this abnormal delay using observations, reanalysis, and model simulations. The results show that the 2024/2025 La Niña featured unusually weak cold (pronounced warm) sea surface temperature anomalies (SSTA) in the equatorial central–eastern Pacific (western Pacific), shifting enhanced convection toward the eastern Maritime Continent. Linear baroclinic model experiments indicate that this equatorward movement of heating generated anomalous easterly winds over the South China Sea (SCS), inhibiting the establishment of monsoonal winds. On the intraseasonal timescale, active convection associated with the 30–60-day oscillation and quasi-biweekly oscillation did not propagate to the SCS until the end of May, which then triggered the monsoon onset. The delayed SCSSM onset also occurred under an interdecadal background of later monsoon onset after the late-2000s, possibly associated with warm SSTA in the equatorial western Indian Ocean. These findings highlight the evolving influence of El Niño–Southern Oscillation (ENSO), the key role of intraseasonal oscillations, and the importance of interdecadal background changes in modulating the SCSSM onset, offering insights for improved climate predictions.