The Seasonal Cycle And Relationship Between Monsoon And Air-Sea Fluxes Over South China Sea

Based on the 1985-2004 US NCEP-NCAR daily reanalysis wind data and the objectively analyzed ocean-surface air-sea heat fluxes (OAFlux) daily data at the US Woods Hole Oceanographic Institution (WHOI), detailed investigations were conducted through the agency of composite and lag-correlation analysis on the South China Sea monsoon (SCSM) seasonal cycle and its relationship with the air-sea surface fluxes over area of 10°-20°N and 105°-120°E. Evidence shows that the monsoon and air-sea surface fluxes all exhibit substantial variance and interaction with each other whether before and after the onset of the summer SCSM or during earlier and later monsoon onset years. The primary conclusions are briefly summarized as follows:1. On the climatological basis, the SCSM and the air-sea surface fluxes all pass through a seasonal cycle of four phases:(1) Phase 1 (from pentads 66-72 and 1-14):within winter time, in which, with the strongest cold northeasterly (NE) prevailing over the South China Sea (SCS), there appears the maximum horizontal wind speed, air-sea contrast of specific humidity and air-sea contrast of temperature, and such that the maximum surface latent heat flux (LHF), sensible heat flux (SHF) and evaporation rate; Whereas, the intensity of the net surface longwave radiation flux (NLWRF) is relatively intermediate, while the net surface shortwave radiation flux (NSWRF), as well as the net surface heat flux (NHF) with net heat loose from sea to air, nevertheless, reaches its minimum at this phase.(2) Phase 2 (from pentads 15 to 27):corresponding to spring time, in which, with the southeasterly (SE) blowing from the southwestern part of the subtropical western Pacific high (SWPH), the horizontal wind speed and air-sea contrast of temperature are now weakest, the air-sea contrast of specific humidity is also relatively lower, therefore, the LHF, SHF and evaporation rate are all weakest; Whereas the gain of the NSWRF and then NHF is strongest for the sea is now largest, and so is the release of the NLWRF from sea to air, which may play a critical role in the later onset of the SW summer SCSM.(3) Phase 3 (from pentads 28 to 54):within summer time, in which, the vigorous southwesterly (SW) who may bring a large amount of water vapor and cloud is now occupying all over the SCS, in such a way that substantially suppresses the release of the NLWRF from sea to air and leads it to be its minimum, while the intensity of the other surface fluxes are relatively intermediate.(4) Phase 4 (from pentads 55 to 65):relative to autumn time, in which, both of the SCSM and the air-sea surface fluxes are being a transition stage from summer to winter.2. Within the whole season cycle, the SCSM and air-sea surface fluxes both vary with a stronger fluctuation in terms of magnitude as composed to the counterparts during earlier monsoon onset years. Averagely, the NE has stronger intensity in autumn and winter during later monsoon onset years, it is also the truth for the SE in spring and the SW in summer; As for the air-sea surface fluxes, the horizontal wind speed at 10 m, air-sea contrast of specific humidity, air-sea contrast of temperature, LHF, SHF and evaporation rate, are all increased in winter during later monsoon onset years, but decreased just several pentads before the climatologically summer monsoon onset time (pentad 28) and so are the NLWRF, NSWRF and NHF, whereas in other periods they all appear more fluctuating. It seems, therefore, that the former enhancement of the NE in winter and SW in spring as well as the variation of the air-sea surface fluxes just several pentads before the climatologically onset time may be responsible for the later onset of the summer SCSM.3. As composed to the counterparts during earlier monsoon onset years, during later monsoon onset years, the horizontal wind speed at 10 m, the specific humidity at 2 m, air-sea contrast of temperature, LHF, SHF and evaporation rate, are all reduced consistently over both of the north and south parts of the SCS if divided by 15°N (reduced in the north but enhanced in the south) within five pentads ahead (behind) of the climatologically summer monsoon onset time (pentad 28), whereas the NLWRF, NSWRF and NHF are all enhanced consistently (enhanced in the north but reduced in the south) before (after) the onset. The above variation of the air-sea surface fluxes may be in close relation to the transition and enhancement of the SE in the southwestern part of the SWPH to the vigorous SW of three different origins from before to after the summer monsoon onset during later monsoon onset years.