Based on observation analysis and numerical simulation, this study reveals that the spring land-sea thermal contrast between Yun-Gui Plateau and Beibu Gulf has a significant triggering impact on the variations of South China Sea summer monsoon onset dates (SCSSMOD) at the interannual timescale. The March–April thermal contrast with warm Yun-Gui Plateau and cold Beibu Gulf results in an anomalous low-level anticyclone over the western North Pacific, which in turn causes the active convection in the tropical western Pacific via the friction convergence. This active convection persists into May and moves northward towards the South China Sea. It subsequently excites an anomalous low-level cyclone in situ via the Matsuno-Gill mechanism. Then, the western Pacific subtropical high is tending to retreat eastward and the westerly wind anomalies occupy the South China Sea, promoting an early SCSSMOD. When the thermal contrast reverses, the SCSSMOD is inclined to be late. The effects mentioned above are independent of ENSO, indicating an effective indicator for monitoring and predicting the SCSSMOD at the interannual timescale.

云贵高原,北部湾,南海夏季风,季风爆发,年际变率