Principal modes of climatological seasonal and intraseasonal variations ofthe Asian summer monsoon

Principal modes of climatological variation of the Asian summer monsoon are investigated. Data used in this study include the high cloud fraction prod uced by the International Satellite Cloud Climatology Project and sea level pressure, and 850- and 200-mb geopotential heights from ECMWF analysis for the five summers of 1985-89. It Is Shown that the seasonal evolution of me Asian summer monsoon is adequately described by a few leading EOFs, These EOFs capture the variations of regional rainbands over the East Asian and I ndian regions. The first mode is characterized by an increase in large-scale cloud over In dia and the subtropical western Pacific until mid-August. The second mode d epicts large-scale cloud variations associated with the East Asian rainband referred to as Mei Yu and Baiu. This mode is associated with the developme nt of summer monsoon circulation: a low pressure system over the Asian cont inent and a subtropical high over the Pacific. The third eigenmode is chara cterized by zonal cloud hands from northern India crossing the Korean penin sula to Japan, aad dryness over the oceans in the south of cloud bands. Thi s mode is related to the mature phase of Changma rainy season in Korea asso ciated with the northward movement of cloud bands and circulation systems f rom the subtropical western Pacific. This mode appears as a first principal mode of climatological intraseasonal oscillation (CISO) over the entire As ian monsoon region. The CISO mode has a timescale of about 2 months. The northward moving CISO also appears in the 850- and 200-mb geopotential height fields as a first mode of each. dataset. Based on the height variati ons of the CISO mode, it is suggested that the extratropical CISO during su mmer is related to a regional index cycle associated with the variation of north-south temperature gradient in East Asia.