Decadal change of the South China Sea summer monsoon onset

A significant decadal change in the timing of the South China Sea summer monsoon (SCSSM) onset is detected around 1993/1994: the climatological mean onset day is shifted from beginning of June to middle May after 1993. The relatively late onsets during the period of 1979-1993 are primarily associated with northward seasonal march of the Intertropical convergence zone (ITCZ), whereas the advanced onsets during 1994-2008 is affected by enhanced northwestward moving tropical disturbance from the equatorial western Pacific. During 1994-2008, enhanced intraseasonal variability (ISV) over the western Pacific is observed in April and May. The number of tropical cyclones (TC) which passed through the South China Sea and Philippine Sea in April and May during 1994-2008 is about doubled compared with those occurring during 1979-1993. Since there is no significant difference of surface pre-condition over the South China Sea and Philippines, the enhancements of the ISV activity and TC genesis after 1994 are major triggers for the advanced SCSSM onset. The SST over the western Pacific has significantly increased between 1980s and 2000s. It is suggested that it makes more favorable background condition for the TC genesis and the ISV activity.