April-June rainfall features in South China in relation to the establishment of summer monsoon in the South China Sea

摘要

In the context of daily rainfall from 58 stations in South China and day-to-day gridded reanalysis from NCEP/NCAR, study is performed of April-June precipitation features at different time intervals in the research area in relation to the establishment of summer monsoon in the South-China Sea (SCS). Results show that the rainfall consists of frontal and monsoon rainfall, the former occurring dominantly in April and the latter largely in June as its principal phase, indicating that the vapor transport, dynamic and thermal features vary greatly for both. Further analysis shows that the amount of the frontal precipitation bears an intimate relation to the time of monsoon onset. In the year of deficient frontal rainfall the Sri Lanka vortex appears earlier and spreads northwards, in combination with northward-advancing convective band from Sumatra to cause the Indo-Burma trough to be established, as well as the subtropical high-pressure belt to break and eastward retreat - all happen earlier than usual, a situation that favors the establishment of westerlies in the SCS, leading to earlier establishment of SCS summer monsoon. In years of plentiful frontal rainfall, however, the formation of the Bengal Bay trough depends mainly on the displacement of Sumatra convection into the Indo-China peninsula, a situation unfavorable for the earlier breaking of the subtropical high-pressure belt and its eastward movement, resulting in later establishment of west winds in the SCS, so that SCS summer monsoon has its establishment delayed.