Hydrologic Processes Associated with the First Transition of the Asian Summer Monsoon: A Pilot Satellite Study

Abstract

Results of a pilot study of the evolution of large-scale hydrologic processes associated with the first transition of the Asian summer monsoon in conjunction with the launching of the South China Sea Monsoon Experiment (SCSMEX) in May 1998 are presented. SCSMEX is a major international field experiment to study the water and energy cycles of the Asian monsoon region, with the aim toward better understanding and improved prediction of the onset, maintenance, and variability of the monsoon of southern China, Southeast Asia, and the western pacific region. In this paper, the utility of reliable satellite data in revealing characteristics of the South China Sea (SCS) monsoon is emphasized. Using a combination of satellite-estimated rainfall, moisture, surface wind, and sea surface temperature, the authors present some interesting and hitherto unknown features in large-scale atmospheric and oceanic hydrologic processes associated with the fluctuation of the SCS monsoon. Results show that, climatologically, the SCS monsoon occurs during mid-May when a major convection zone shifts from the eastern Indian Ocean?southern Indochina to the SCS. Simultaneous with the SCS monsoon onset is the development of a moist tongue and frontal rainband emanating from the northern SCS, across southern China and the East China Sea to southern Japan, as well as the enhancement of equatorial convection in the western Pacific ITCZ. Analysis of the satellite-derived moisture and rainfall shows that the onset of the SCS monsoon during 1997 was preceded by the development of eastward-propagating supercloud clusters over the Indian Ocean. The satellite data also reveal a strong onset vortex over the SCS and large-scale cooling and warming patterns over the Indian Ocean and western Pacific. These features signal a major shift of the large-scale hydrologic cycle in the ocean?atmosphere system, which underpins the SCS monsoon onset. The paper concludes with a brief discussion of the observational platform of SCSMEX and a call for the use of satellite data, field observations, and models for comprehensive studies of the Asian monsoon.