Definition of South China Sea monsoon onset and commencement of the East Asia summer monsoon

The climatological mean summer monsoon onset in the South China Sea (SCS) is a remarkably abrupt event. However, defining onset dates for individual years is noticeably controversial. The controversies and complications arise primarily from a number of factors: the intermittent southward intrusion of cold fronts into the northern SCS, the bogus onset in late April before the establishment of tropical monsoons over Indochina, and active intraseasonal oscillation. In this paper, a simple yet effective index, U-SCS, the 850-hPa zonal winds averaged over the central SCS (5degrees - 15degreesN and 110degrees - 120degreesE), is proposed for objectively defining the SCS monsoon onset. This onset index depicts not only the sudden establishment of the tropical southwesterly monsoon over the SCS but also the outbreak of the rainy season in the central-northern SCS. In this paper the East Asian summer monsoon (EASM) is defined as the broadscale summer monsoon over East Asia and the western North Pacific region (0degrees - 40degreesN, 100degrees - 140degreesE). It is shown that the seasonal transition of EASM can be objectively determined by the principal component of the dominant empirical orthogonal mode of the 850-hPa zonal winds, U-EOF1. It is found that the local index U-SCS represents U-EOF1 extremely well; thus, it can be used to determine both the SCS onset and the commencement of the broadscale EASM. The result suggests that the SCS summer monsoon onset indeed signifies the beginning of the summer monsoon over East Asia and the adjacent western Pacific Ocean. Evidence is also provided to show the linkage between the two salient phases of EASM: the local onset of the SCS monsoon and the local onset of the mei-yu ( the rainy season in the Yangtze River and Huai River basin and southern Japan).