A possible mechanism effecting the earlier onset of southwesterly monsoon in the South China Sea compared to the Indian monsoon

A possible mechanism effecting the earlier onset of the South China Sea (SC S) southwesterly (summer) monsoon compared to the Indian monsoon is reveale d by use of the NCAR/NCEP reanalysis data during the 1998 South China Sea M onsoon Experiment (SCSMEX). It is found that the fundamental cause might be the different spatial distributions and time variations of the surface sen sible heat (SH) fluxes in the two areas. In the SCS area, positive surface sensible heating exists all the time in both the tropics and the subtropics with the latter being dominant after March. While in the India Peninsula ( IP) area, the tropical heating is the dominant one at the early period befo re May and the subtropical heating is smaller and even negative. It is furt her found that the west-east contrasts of the SH fluxes at middle latitudes are also favorable to the earlier onset of the SCS monsoon. The time evolutions of circulation expose the dynamic cause. A strong cyclo ne existing between the equator and 10 degrees N at the south tip of the IP on May 13 may be taken as an indicator of the monsoon developing process. The circulation of the cyclone and the blocking effect of the Tibetan Plate au (TP) combine together to make the southwesterly wind easily reach the SC S area. When and only when the cyclone moves towards the TP, becomes weaker and weaker and forms a shallow monsoon trough at last, the Indian monsoon bursts out. Therefore, it takes almost a month to occupy the entire IP area and is later than the SCS monsoon onset by about one month.