BAIU NEAR JAPAN AND ITS RELATION TO SUMMER MONSOONS OVER SOUTHEAST-ASIA AND THE WESTERN NORTH PACIFIC

The slowly evolving annual cycle (signified as ''L-mode'') was defined by the sum of the first three sinusoids in a series of climatological pentad mean data of infrared equivalent black body temperature, 850 h Pa wind, temperature, geopotential height and specific humidity, while the rapidly varying annual cycle (denoted by ''S-mode'') was determin ed by adding up the remaining sinusoids.During early summer, the L-mod e exhibited a blocking-type configuration with a ridge over the Kamcha tka-Okhotsk region, whereas a trough stretched eastward from northern China (center of a continental heat low) to Japan and beyond. The esta blishment of a locally independent L-mode anticyclonic cell over the S ea of Okhotsk enhanced low-level easterly anomalies from the vicinity of the Aleutian islands through northern Japan. The L-mode easterly an omalies merged with southwesterly anomalies along the southeastern per iphery of a continental-scale heat low, thus generating a pronounced c yclonic sheer zone around Japan accompanied by low-level moisture conv ergence. This L-mode lower tropospheric trough in intrinsically associ ation with the east-west temperature gradient between East Asia and th e western North Pacific plays a vital role in the formation of Baiu sy stem in early summer. The L-mode southwesterlies along the east coast of China, which are attributed to the prominence of the continental-sc ale heat low, served as a bridge that links the monsoon westerlies of tropical origin with the westerly jet of mid-latitude origin. This res ulted in producing a low-level westerly duct extending from the South China Sea to the central North Pacific. At the Baiu onset phase (mid-J une), the S-mode onset cyclone of convective origin developed over the South China Sea, and concurrently the S-mode onset anticyclone organi zed to the northeast of the onset cyclone. The consolidated effect of these onset vortexes amplified in the vicinity of the low-level wester ly duct was to cause the northward advection of warm and moist air fro m the tropics to southern Japan. By mid-July, the Asian continental he at low reached its peak and the summer monsoon over Southeast Asia bec ame fully established. At the Baiu withdrawal phase (late July), the c ontinental heat low began to decay because of land surface cooling; ne vertheless, the L-mode Pacific High still developed northward and was most intense at the beginning of August. As the east-west temperature gradient between the ocean and continent decreased, the L-mode lower t ropospheric trough near Japan dissipated, whereas L-mode WNPM (summer monsoon over the western North Pacific) trough prevailed in the subtro pics. The withdrawal of Baiu was also characterized by the dominance o f S-mode disturbances as well as the Baiu onset..