Relationship Between The South China Sea Summer Monsoon Onset And The Tropical Cyclone Activity In The Western North Pacific

With the Joint Typhoon Warning Center (JTWC) tropical cyclone (TC) data and the NCEP/NCAR reanalysis data, the South China Sea summer monsoon (SCS SM) onset index Uses defined by Wang et al (2004) is selected from a variety of SCS SM onset indices for defining the onset pentad of individual years during 1965-2007. The relationship between SCS SM onset and the TC activity in the western North Pacific (WNP) is investigated by using the method of composite analysis.The genesis number and the occurrence frequency of TC over the WNP (especially the SCS) increases evidently associated with the SCS SM onset. In more than half years TC occur over the WNP (west of 150°E) during 1-2 pentads and the onset pentad before the SCS SM onset, implying a precursor for SCS SM onset prediction.During January to April of the early onset years, there is a evident intraseasonal oscillation (ISO) of easterly averaged over 5°-15°N in the WNP. The TC cluster in the ISO easterly reduced phase, and the genesis locations of most TC are in the western WNP. The 10°-20°N average convection and precipitation are stronger. During January to April of the late onset years, there is no regulative intraseasonal oscillation signal, the genesis locations of most TC are in the eastern WNP, and the 10°-20°N average convection and precipitation are weaker.In the early onset years, the SCS SM onset is accompanied by an evident 30-60d intraseasonal oscillation. The oscillations of westerly at 850hPa, OLR, precipitation are in phase from the eastern Indian Ocean to the western Pacific. The oscillation is relative consistent with the intraseasonal oscillation of TC activity. In the pentad before the SCS SM onset, the TC activity in WNP increases significantly. In the onset pentad, the TC activity further increases and the intensity of TC strengthens. In the first pentad after the onset, accompanied with the monsoon tough onset in the SCS, the TC activity in WNP reaches the peak. But in the third pentad after the onset, the monsoon tough gets weak and the TC activity in WNP decreases rapidly. In the seventh pentad after the onset, the TC activity decreases to the minimum. Since the eighth pentad after the onset, the TC activity increases again. While in the late onset years, there is no 30-60d intraseasonal oscillation accompanied by the SCS SM onset. Westerly and precipitation occur in the Bengal gulf (80°-100°E), then extend to the the SCS (110°-120°E) and the WNP east of the Philippines (120°-150°E) in sequence. Nearly no TC occurs during 1-2 pentads and the onset pentad before the SCS SM onset, but from the first pentad after the SCS SM onset, accompanied by the monsoon trough onset in the SCS, the TC activity in WNP starts to increase and the intensity of TC strengthens.The early summer monsoon onset may be affected by complex factors including the eastward movement of ISO from the eastern Indian Ocean, the westward movement of convection from the WNP, and the southward movement of convection from the middle latitude in the Northern Hemisphere. In most of the early onset years, the westward movement of convection from the WNP is at least one of the factors that affect the SCS SM onset. While in most of the late onset years, the SCS SM onset is not affected by any movement of convention from outside areas, only in a few of the late onset years, the SCS SM onset is affected by the eastward movement of ISO from the eastern Indian Ocean.