This study investigates the influence of early spring (March–April) soil moisture (SM) over the Greater Mekong Subregion (GMS) on the interannual variation of South China Sea summer monsoon (SCSSM) onset, using observational analyses and numerical experiments. It is found that when early spring SM over the GMS is wetter, westerly anomalies dominate the South China Sea, corresponding to an early onset of the SCSSM, and vice versa. The analyses of physical mechanism show that the positive anomalies of early spring SM decrease local surface air temperature by adjusting the surface latent heat flux and sensible heat flux. The persistence of anomalous ground cooling contributes to negative geopotential height anomalies in the middle troposphere, and further induces the eastward retreat of western North Pacific subtropical high in May. The anomalous cyclone over the South China Sea favors the early onset of SCSSM. The key physical processes linking the variability of early spring SM over the GMS and the following SCSSM onset are confirmed by the sensitivity experiments using a coupled atmosphere–land model (CAM6–CLM5). Specifically, the onset date of the SCSSM in the drier experiment is 11 days later than that in the wetter experiment. The present results have implications for the forecast of SCSSM onset.

中文翻译：

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大湄公河次区域早春土壤水分对南海夏季风发作年际变化的影响


本研究使用观测分析和数值实验，调查了大湄公河次区 （GMS） 上早春（3 月至 4 月）土壤水分 （SM） 对南海夏季季风 （SCSSM） 发作的年际变化的影响。研究发现，当 GMS 上早春 SM 较湿时，南海以西风异常为主，对应于 SCSSM 的提前开始，反之亦然。物理机制分析表明，早春 SM 的正异常通过调节表面潜热通量和感热通量来降低局部地表空气温度。异常地面冷却的持续导致对流层中部出现负位势高度异常，并进一步诱导北太平洋西部副热带高压在 5 月东退。南海的异常气旋有利于 SCSSM 的早期发作。使用耦合大气-陆地模型 （CAM6-CLM5） 的敏感性实验证实了将早春 SM 在 GMS 上的变化与随后的 SCSSM 发作联系起来的关键物理过程。具体来说，较干燥实验中 SCSSM 的开始日期比较湿实验晚 11 天。目前的结果对 SCSSM 发病的预测具有意义。