Stepwise Adjustments in Subseasonal Forecasts for the Anomalously Late Onset of the South China Sea Summer Monsoon in 2025: A Multiscale Perspective

The onset of the South China Sea summer monsoon (SCSSM) in 2025 was anomalously delayed until pentad 30 during the decay of a La Niña event, departing from the usual tendency toward an earlier onset under La Niña conditions. Observational analysis showed that a late-April 10–25-day cold-air intrusion, followed by the easterly phase of the 30–60-day boreal summer intraseasonal oscillation (BSISO) in May, acted sequentially to sustain pre-onset easterly anomalies over the South China Sea (SCS). This multiscale easterly-wind relay suppressed the establishment of low-level monsoon westerlies and delayed the SCSSM onset. Consistent with this evolution, real-time forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF) subseasonal-to-seasonal (S2S) system exhibited stepwise adjustments in the predicted onset date, progressing from an erroneous early-onset forecast in late April, through an intermediate adjustment in early May, to an accurate late-onset prediction in mid-May. The first adjustment arose from the model’s misrepresentation of the propagation and phasing of two 10–25-day disturbances in the tropical and subtropical regions surrounding the SCS, which produced an excessively strong and incorrectly phased easterly anomaly over the SCS in pentad 28. The subsequent adjustment occurred when the model successfully captured the convective amplitude and phase evolution of the BSISO and reproduced the persistent 30–60-day easterly anomalies preceding the observed onset. These results suggest that subseasonal prediction skill for SCSSM onset timing depends on the model’s ability to represent the relative contributions, phase relationships, and sequential interactions of multiscale intraseasonal signals.