The summer climate of China is mainly influenced by the East Asian Summer Monsoon (EASM). The onset of South China Sea Summer Monsoon (SCSSM) is the first stage of the EASM, indicating the arrival of the rainy season of eastern China. During the SCSSM onset, intraseasonal oscillation (ISO) is often active, and great attentions have been paid to its relationship with the onset of SCSSM. The previous studies took the positive phase (active convection and easterly wind anomalies) of the boreal summer ISO as the key condition for the SCSSM onset. 

In the latest study by SHAO Xie, HUANG Ping and HUNG Ronghui, however, the developing phase from inactive phase to active phase of the boreal summer ISO is found to be the most important mechanism to trigger the SCSSM onset. The SCSSM onset often occurs during the negative and developing phase of ISO in the late-onset years, while the monsoon onset occurs during the positive and developing phase in the early-onset years. 

Composite of the 30–60-day filtered U₈₅₀ of the onset-ISOs in (a) the early-onset years, (b) the normal-onset years and (c) the late-onset years. The thin circles denote the monsoon onset phases and the thick circles denote the averaged onset phases for each category. The shadings indicate the standard deviations of the 30–60-day filtered U₈₅₀ in each category. (d) Composite for filtered OLR for all years. (Figure plotted by IAP) 

The linear viewpoint is used to explain the phenomena that the developing ISO can trigger the SCSSM. In the SCSSM onset season, ISO dominates in the variation of all components, including the climate mean evolution. Therefore, only during the developing phase of ISO, the sum of ISO and climate mean can develop to break the threshold of the monsoon onset, and help the monsoon onset. Moreover, the sum of the two components can break the threshold only in the developing phase in view that the climate mean is below the threshold in early-onset years, while the monsoon can onset as long as ISO develops in the late-onset years because the climate mean is already positive. 

The discovery of the relationship of the SCSSM onset and the developing phase of ISO will help improve the forecast skill to the monsoon onset.   

Reference: 

Shao, X.，P. Huang*, and R.-H. Huang, Role of the phase transition of intraseasonal oscillation on the South China Sea summer monsoon onset, Climate Dynamics, 2015, 45:125-137