Characteristics of the 1998 summer monsoon onset over the northern South China Sea

Observations from the May-June 1998 South China Sea Monsoon Experiment (SCSMEX) have been used to determine the characteristics of the onset of the summer monsoon over the northern South China Sea (SCS). The onset occurred in mid-May with a rapid increase in deep convection over a week-to-ten-day period, followed by a similar toten-day break, and then a resumption of deep convection for a week-to-ten-day period in early June. The temporal and spatial variabilities of rainfall rates from the atmospheric moisture budget, satellite, and model estimates generally agree well, although the mean rainfall rate from the models for the 47-day Intensive Observing Period (IOP) is about 25% greater than the other estimates. The SST gradually increased over the northern SCS during the IOP, interrupted by slight cooling following monsoon onset until early June when the warming resumed. Surface sensible and latent heat fluxes increased after onset, but then decreased in June as a result of warm, moist air advecting over cooler water near the south China coast. The mean apparent heat source Q(1) during convective periods resembled that determined for other tropical oceanic regions with a peak near 400 hPa. The apparent moisture sink Q(2) also resembled profiles for other tropical regions, except for larger values below 900 hPa. The heating and moistening rates and vertical eddy flux of total heat during the early-June active period were greater than those observed during the May monsoon onset active period, indicating more vigorous deep convection as the monsoon ensued, a finding supported by Tropical Rainfall Measuring Mission (TRMM) precipitation radar data.