LARGE-SCALE CIRCULATIONS ASSOCIATED WITH THE EAST-ASIAN SUMMER MONSOON AND THE MEI-YU OVER SOUTH CHINA AND TAIWAN

Grid point data, visible and infrared cloud imageries, and cloud top temperatures during the period of May-June 1981-1986 were used to study the large-scale circulations in Asian monsoon region. Stream function, velocity potential, divergent part of the wind, convection index, cloud top temperature index, and moisture field were analyzed. The distribution of the half-monthly mean of these parameters is presented and discussed to reveal the characteristics of the evolution of large-scale circulations from the pre-Mei-Yu season (1-15 May) to the post-Mei-Yu season (16-30 June) in South China and Taiwan region. Also, convectively active and inactive Mei-Yu seasons and fronts were selected to study the interannual and intraseasonal variations of the large-scale circulation patterns. The results can be summarized as follows: (1) The Mei-Yu over South China and Taiwan occurred concurrently with the onset of the summer southwest monsoon over the South China Sea during the period of May 16-31 (Phase I). (2) The northward advance of the area of deep convection, ITCZ, and subtropical ridge at the post-Mei-Yu season (June 16-30) occurred concurrently with the establishment of the quasi-stationary position of Mei-Yu front over the Yangtze Valley and Japan, At the same time, a quasi-equivalent barotropic monsoon circulation system was well developed with the low-level cyclone under the upper-level anticyclone over northeastern Indian and Burma area. (3) The active Mei-Yu season was characterized by the further southward penetration of the northern (baroclinic) system and moisture flux convergence over the Mei-Yu region. The reversed situations were observed for the inactive Mei-Yu season. (4) The difference for the active and inactive Mei-Yu fronts was mainly controlled by the low-level flows. The active Mei-Yu front was accompanied by the southwest monsoonal flows originating from the Bay of Bengal and the tropical western Pacific, whereas the inactive front the southeasterly or easterly flows of the Pacific high circulation prevailed over the Mei-Yu region. Higher mixing ratio, stronger moisture flux, and flux convergence were observed over the Mei-Yu region for the active front as compared to those for the inactive one. (5) More frequent occurrence of the active fronts led to an active Mei-Yu season and the reverse was true for an inactive one.