Evolution of the Large Scale Circulation, Cloud Structure andRegional Water Cycle Associated with the South China Sea Monsoonduring May-June, 1998

摘要

This paper studies the evolution of the South China Sea (SCS) monsoon during May-June 1998, toelucidate relationships among the large scale circulation, organization of convection, cloud structures,and fluctuations of the regional water cycle of the SCS. Primary data used include field observationsfrom the South China Sea Monsoon Experiment (SCSMEX), and the satellite rain products from theTropical Rainfall Measuring Mission (TRMM). Prior to the onset of the SCS monsoon, enhanced convectiveactivities associated with the Madden and Julian Oscillation were detected over the equatorialIndian Ocean in early May while the SCS was under the influence of the West Pacific Anticyclone withprevailing low level easterlies and suppressed convection. Subsquently, a bifurcation of the MJO convectionnear 90°E led to the development of strong convection over the Bay of Bengal, which spawnedlow-level westerlies across Indo-China and contributed to the initial build-up of moisture and convectiveavailable potential energy over the northern SCS. The onset of the SCS monsoon occurred around May18-20, and appeared to be triggered by the equatorward penetration of extratropical frontal disturbances,originating from the continental regions of East Asia.Analysis of TRMM microwave and precipitation radar data revealed that during the onset phase,convection over the northern SCS consisted of squall-type rain cells embedded in meso-scale complexessimilar to extratropical systems. The radar Z-factor intensity indicated that SCS clouds possessed abimodal distribution, with a pronounced signal (>30 dBz) at a height of 2-3 km, and another one(>25 dBz) at the 8-10 km level, separated by a well-defined melting level signaled by a bright band ataround 5-km level. The most convectively active phase of the SCS monsoon, as measured by the abundanceof convective and stratiform hydrometeor types, inferred from the radar vertical profile, was foundto occur when the large scale vertical wind shear was weakest.The fluctuation of the water cycle over the northern SCS was found to be closely linked to the largescaledynamical and SST forcings. Before onset and during the break, the northern SCS was relativelywarm and served as a moisture source (E - P > 0) to the overlying atmosphere. During the active phase,the northern SCS was cooled, providing a strong sink (E - P « 0) for atmospheric moisture, with theprimary source of moisture coming from regions further west over Indo-China and the eastern IndianOcean. Vigorous water recycling by convective systems in the northern SCS occurred during the maturephase of the SCS monsoon, with precipitation efficiency (defined as the ratio of the surface precipitationto the sum of large scale moisture convergence and surface evaporation from the ocean) approaching96%. Westward transport of moisture from Indo-China into, and northward transport out of, the northernSCS provided the main source of moisture for the torrential rain over the YRV in mid-June 1998.The present results suggest that the SCS may play an important role in regulating the SCS monsoonactive and break cycles through charge and discharge of moisture, and convective available potentialenergy.