Intraseasonal Oscillation and the Onset and Retreat Dates of the Summer Monsoon over East, Southeast Asia and the Western Pacific Region using GMS High Cloud Amount Data

Abstract

The seasonal cycle and the onset and retreat dates in the summer monsoon in East Asia, Southeast Asia and the Western Pacific region are analyzed using 12-year (April 1978-December 1989) means of the 5-day mean 1-degree latitude-longitude gridded GMS high-cloud-amount data. An analysis of these data showed the detailed seasonal cycles of two convective zones (ITCZ and the Baiu front) of the summer monsoon cloud defined by the regions with more than 30 (25 for Baiu front) percent of the mean high-cloud amount. The clouds associated with the ITCZ were observed to increase in Southeast Asia during May and subsequently spread to the South China Sea and the Western Pacific region in a series of sudden expansions during June and July.
The Baiu front branch is preceded by an increase in the high-cloud amount along the polar front located near 30°N just south of Japan around 26 April. During the middle of May, this cloud band rapidly moved south to 20-25°N. This period coincides with the onset of the Baiu season in Okinawa. Subsequently, this cloud band moves north to mainland Japan and the Yellow Sea with a series of sudden northward advances during June and July. These abrupt changes are associated with the phase-locking between the intraseasonal oscillation and the seasonal cycle of the monsoon clouds.
The onset and retreat dates derived from this study fill the major gaps in these dates over Southeast Asia and the adjacent Western Pacific region where the large area of the ocean prevented analysis of the onset (retreat) dates based on rainfall data.