Accurate forecasting of the South China Sea summer monsoon (SCSSM) onset is critical for water resource management in the rainy season across East Asia. The present work evaluates the skill of the ECMWF subseasonal-to-seasonal (S2S) forecasting system in predicting SCSSM onset dates using a hindcast experiment. The model provides accurate predictions of the SCSSM onset date 10 days in advance. The predictability mainly arises from the sea surface temperature anomaly (SSTA) in the northern Indian Ocean and central-eastern tropical Pacific, which usually coincides with moderate and extreme decaying El Niño, thus creating windows of opportunity for SCSSM onset forecasting. The model can reasonably reproduce the SCSSM-related SSTA and its induced atmospheric teleconnections. However, when anomalous Ural blocking influences the mid-high latitudes, the ECMWF model forecasts of the SCSSM onset are trapped in the synoptic timescale because of the limited ability of the model to consider blocking in forecasting.

中文翻译：

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ECMWF S2S预报系统对南海夏季风爆发的亚季节可预报性

准确预测南海夏季风 (SCSSM) 的爆发对于整个东亚雨季的水资源管理至关重要。目前的工作评估了 ECMWF 次季节到季节 (S2S) 预测系统在使用后报实验预测 SCSSM 开始日期方面的技能。该模型可提前 10 天准确预测 SCSSM 发病日期。可预测性主要来自印度洋北部和中东部热带太平洋的海面温度异常（SSTA），通常与中度和极端衰减的厄尔尼诺现象同时发生，从而为 SCSSM 爆发预报创造了机会窗口。该模型可以合理地再现 SCSSM 相关的 SSTA 及其诱发的大气遥相关。然而，当异常乌拉尔阻塞影响中高纬度地区时，