AN OBSERVATIONAL STUDY ON THE BEHAVIOR OF THE SUBTROPICAL HIGH OVER THE WEST PACIFICIN SUMMER

The behavior of the subtropical high(SH) over the West Pacific in summer manifests marked inter-annual,seasonal,intra-seasonal and short period(5-10 days) variations.In strong summer monsoon years the ridge of SH extends fur ther northward than in weak monsoon years,and there are two separ ate anticy clone centers,one is over the West Pacific and the other over the Middle Pacific.In the weak monsoon years the behavior of SH mainly manifests a ridge extending west ward from the Middle Pacific.For seasonal variations the ridge of SH is situated to the south of 20°N before June,extends to 25°N when the Meiyu seaso n in the Yangt ze River Valley begins,and extends further northward to the north of 30°N in mid July,when the Meiyu season ceases in Septem ber ther idge of SH retreats rapidly southward.For intra-seasonal variations there are two modes,The firstmode is the extension of the ridge of SH west ward from the Middle Pacific with a period of 20-30 days.The second mode is that the ridge of SH extends west ward from the Middle Pacific,intensifies to form a separate high pressure center,and stag nates in the West Pacific.The extension or retreat of the ridge of SH has a close relation to the break or cease of the Meiyu in China.For short period (5-10 days)variations,when a trough in the westerly in the middle latitudes moves east ward,intensifies rapidly to the east o f Japan Islands,and extends southw ard into the subtropical,the ridge of SH to the west of the digging trough intensifies and extends into the China mainl and.