The Combined Effect of Preceding Winter El Niño-Southern Oscillation and Southern Hemisphere Annular Mode on the Spring Rainfall in South China

Understanding the variability of spring rainfall in South China (SCSR) is important for local agriculture and daily life. While preceding boreal winter El Niño-Southern Oscillation (ENSO) and Southern Hemisphere Annular Mode (SAM) are recognized to influence subsequent SCSR, their potential combined effect remains unclear. This study indicates that the co-occurrence of El Niño and negative SAM (SAMN) exerts a combined effect on SCSR, with larger anomalies than those associated with single El Niño or SAMN events. To investigate the underlying mechanism, differences in sea surface temperature anomalies (SSTA) between joint and single factor events are analyzed, given that SSTA may serve as a bridge linking the cross-seasonal influence of both pre-winter ENSO and SAM on SCSR. Specifically, the Southern Ocean Dipole (SOD)—an inverse SSTA pattern between around 37.5°S and 60°S—appears to play a role in the cross-seasonal influence of SAM on SCSR. Diagnostic analysis suggests that the SAMN signal stored in the negative phase of SOD may influence meridional circulation, which in turn induces westerly wind anomalies that affect tropical central-eastern Pacific surface wind. Therefore, during joint El Niño & SAMN events, the tropical central-eastern Pacific SST is warmer than in single El Niño or SAMN events, coinciding with an intensified anticyclone over the western North Pacific and enhanced southwesterly moisture transport over South China. Anomalous low-level convergence and upward motion are also observed over South China, consistent with the increased SCSR. These findings suggest a potential SOD‑mediated pathway linking tropical ENSO and extratropical SAM to SCSR variability.