Multi-scale climate variations and mechanisms of the onset and withdrawal of the South China Sea summer monsoon

The onset of the South China Sea summer monsoon (SCSSM) heralds the establishment of the large-scale East Asian summer monsoon and the western North Pacific summer monsoon, as well as the beginning of the major rainy season. The occurrence of an early or late SCSSM onset is an important indicator for the summertime climate anomalies over East and Southeast Asia. Therefore, the onset of the SCSSM has received extensive research attention, and has been one of the key foci of monsoon research in recent decades. This paper reviews the multi-scale climate variations (from interdecadal to synoptic scales) and mechanisms of the SCSSM onset, with special attention paid to the progress in the past five years. On the interdecadal timescale, the SCSSM onset underwent a significant advancement in the mid-to-late 1990s, which may have been related to the background condition of the Pacific Ocean. On the interannual timescale, El Niño-Southern Oscillation (ENSO) is considered to be the most critical factor controlling the variability of the SCSSM onset. However, the linkage between ENSO and the SCSSM onset has become much weaker and insignificant in recent years. The intraseasonal oscillations (including the 30–60-day oscillation and quasi-biweekly oscillation) are important modulators of the SCSSM onset. The transition from an inactive phase to an active phase may be more worthy of attention than the phase itself. There appear to be close interactions between the SCSSM onset and synoptic-scale systems like tropical cyclones (TCs). TCs are important triggers for the SCSSM onset, while in return the SCSSM onset provides a favorable background condition for the formation and development of TCs. In addition, this paper also reviews a series of recent works on the withdrawal of the SCSSM. Finally, to close the paper, some important scientific issues worthy of further investigation are put forward.