The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30–60- and 12–24-day monsoon modes. The former intraseasonal mode formed the northward-migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May–August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30–60-day monsoon trough and the 12–24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30–60-day monsoon ridge lines and the 12–24-day high center met near 15°–20°N in the northern South China Sea. The South China Sea monsoon wa... Abstract The onset and life cycle of the 1979 South China Sea summer monsoon were examined in the context of the 30–60- and 12–24-day monsoon modes. The former intraseasonal mode formed the northward-migrating monsoon trough/ridge, while the latter intraseasonal mode propagated westward in the South China Sea. The monsoon in this region exhibited three cycles over the summer (May–August), with the onset taking place about one cycle ahead of the onset of the Indian and Japanese monsoons. Climatologically, a summer trough line radiated out from the Indian monsoon trough region, across Indochina, to the northern South China Sea. The monsoon onset occurred when the 30–60-day monsoon trough and the 12–24-day low center arrived simultaneously at the northern South China Sea, close to the climatological summer trough line, in the middle of May. The breaks occurred when the 30–60-day monsoon ridge lines and the 12–24-day high center met near 15°–20°N in the northern South China Sea. The South China Sea monsoon wa...