Variation of the East Asian Summer Monsoon Rainfall

Abstract

Rainfall variation of the East Asian summer monsoon has long been believed to be caused by the transition of weather regimes in company with the evolution of monsoon circulation. However, this claim was neither comprehensively analyzed nor convincingly demonstrated. Four datasets [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, NCEP?NCAR reanalysis data, Japan Meteorological Agency (JMA) 6-h surface analysis maps, and Joint Typhoon Warning Center (JTWC) 6-h tropical cyclone tracks] were used to depict the climatology of the monsoon rainfall variation and to explore the cause of this variation, particularly through the relationship between the seasonal evolution of the monsoon circulation and the associated synoptic disturbance activity. The monsoon life cycle in the southern part of East Asia is basically developed by the sequential passages of the mei-yu rainband in early summer, the western Pacific subtropical high in midsummer, and the tropical cyclone activity in late summer. Although the monsoon onset in the northern part of East Asia, particularly Japan, is triggered by the mei-yu/baiu rainband, the monsoon break follows the rainband's dissipation caused by the northward-migrating subtropical high, and the monsoon revival accompanies the arrival of the midlatitude frontal activity. In view of the role played by the subtropical high in developing the monsoon break, the phase lag of the monsoon life cycle between the south and north is a result of this subtropical high's northward progression. Two rainfall maxima in the revival phase of the Japanese monsoon are generated by different mechanisms: the first is a result of the collaborative contribution of the frontal and tropical cyclone activity, but the second is primarily due to the midlatitude frontal activity. Despite the vertical phase reversal of monsoon circulation in the middle troposphere, two intraseasonal (10?24- and 30?60-day) monsoon modes propagating across East Asia do not undergo a vertical phase reversal until 200?250 mb. The vertical structure of these two intraseasonal modes causes monsoon rainfall to fluctuate intraseasonally between the low plains and the tallest East Asian mountain (Yu Shan, ?4 km above sea level) in Taiwan. In addition to these new findings, further research efforts are needed to explore the modulations of the frontal and tropical cyclone activity by intraseasonal modes and the monsoon revival in northeast Asia by the interannual climate mode of the summer monsoon circulation through the tropical cyclone and midlatitude frontal activity.