Abstract Observational analyses suggest a significant positive correlation between the year-to-year convection over the South China Sea (SCS)/western Pacific (WP) and the SCS summer monsoon (SCSSM) onset date. An investigation shows that there is an asymmetric relationship between the area-mean outgoing longwave radiation and the SCSSM onset date index. The analysis found that the influence of the intraseasonal scale circulation is the main cause of this asymmetric relationship. On the interannual scale, circulation and sea surface temperature (SST) anomalies are distributed symmetrically. During the convection active (inactive) years, the SST anomaly field indicates a La Ni n ˜ a (El Ni n ˜ o)-like pattern. The SCS-WP low-level westerly (easterly) anomaly is enhanced, leading to an increase (decrease) in moisture and ascending (descending) motion. The enhanced convection results in weakening (enhancement) of the subtropical high and provides a favorable condition for early (late) SCSSM establishment. On the intraseasonal time scale, the convection intensity is stronger during the active years than during the inactive years. The northwestward propagation is significant during the active years. This feature is not observed during inactive years. This difference is attributed to the asymmetric meridional distribution of the atmospheric convective instability and moisture disturbance during active years, not during inactive years. In active years, the atmosphere is potentially more (less) unstable and the moisture is to the north (south) of the intraseasonal oscillation (ISO) convective center, which is conducive to generating a favorable unstable environment for the development of new convection north of the ISO convection center, leading to northward convection propagation. The moistening in the SCS-WP is primarily attributed to the phase leading horizontal advection term. An ISO moisture budget analysis reveals that the largest positive contribution is the vertical moisture advection term.

中文翻译：

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大气环流对南海夏季风爆发的不对称影响

摘要 观测分析表明，南海 (SCS)/西太平洋 (WP) 的逐年对流与南海夏季风 (SCSSM) 爆发日期之间存在显着正相关。一项调查表明，面积平均出射长波辐射与 SCSSM 开始日期指数之间存在不对称关系。分析发现，季节内尺度环流的影响是造成这种不对称关系的主要原因。在年际尺度上，环流和海面温度（SST）异常对称分布。在对流活跃（不活跃）年，海温异常场显示出类似La Ni n ∼ a (El Ni n ∼ o)的模式。SCS-WP低空西风（东风）异常增强，导致水分增加（减少）和上升（下降）运动。增强的对流导致副热带高压减弱（增强），为早期（晚期）SCSSM 建立提供了有利条件。在季节内时间尺度上，活跃年的对流强度强于不活跃年。在活跃年，向西北传播是显着的。在不活跃的年份中不会观察到此特征。这种差异归因于活跃年而非不活跃年期间大气对流不稳定和水分扰动的不对称子午分布。在活跃年份，大气可能更不稳定（更少），水分在季节内振荡（ISO）对流中心的北（南）处，这有利于为ISO对流中心以北的新对流发展产生有利的不稳定环境，导致对流向北传播。SCS-WP 中的润湿主要归因于相位领先的水平平流项。ISO 水分预算分析表明，最大的正贡献是垂直水分平流项。