Baiu near Japan and Its Relation to Summer Monsoons over Southeast Asia and the Western North Pacific

[摘要] 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, 850hPa wind, temperature, geopotential height and specific humidity, while the rapidly varying annual cycle (denoted by “S-mode”) was determined by adding up the remaining sinusoids.During early summer, the L-mode exhibited a blocking-type configuration with a ridge over the Kamchatka-Okhotsk region, whereas a trough stretched eastward from northern China (center of a continental heat low) to Japan and beyond. The establishment of a locally independent L-mode anticyclonic cell over the Sea of Okhotsk enhanced low-level easterly anomalies from the vicinity of the Aleutian islands through northern Japan. The L-mode easterly anomalies merged with southwesterly anomalies along the southeastern periphery of a continental-scale heat low, thus generating a pronounced cyclonic shear zone around Japan accompanied by low-level moisture convergence. This L-mode lower tropospheric trough in intrinsically association with the east-west temperature gradient between East Asia and the western North Pacific plays a vital role in the formation of Baiu system in early summer.The L-mode southwesterlies along the east coast of China, which are attributed to the prominence of the continental-scale heat low, served as a bridge that links the monsoon westerlies of tropical origin with the westerly jet of mid-latitude origin. This resulted in producing a low-level westerly duct extending from the South China Sea to the central North Pacific. At the Baiu onset phase (mid-June), the S-mode onset cyclone of convective origin developed over the South China Sea, and concurrently the S-mode onset anticyclone organized to the northeast of the onset cyclone. The consolidated effect of these onset vortexes amplified in the vicinity of the low-level westerly duct was to cause the northward advection of warm and moist air from the tropics to southern Japan.By mid-July, the Asian continental heat low reached its peak and the summer monsoon over Southeast Asia became fully established. At the Baiu withdrawal phase (late July), the continental heat low began to decay because of land surface cooling; nevertheless, 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 tropospheric trough near Japan dissipated, whereas L-mode WNPM (summer monsoon over the western North Pacific) trough prevailed in the subtropics. The withdrawal of Baiu was also characterized by the dominance of S-mode disturbances as well as the Baiu onset.It is thus anticipated that the seasonal evolution of L-mode circulation field, associated with continent-ocean thermal contrast, strongly regulates the activities of S-mode perturbations in and around the low-level westerly duct. It seems likely that the westerly duct served as a wave guide for the barotropic Rossby wave dispersion of the S-mode disturbances. The presence of horizontally sheared L-mode flow may also contribute substantially to the development and persistence of S-mode vortexes via in situ barotropic interaction process between the two modes. At any rate, the combined contribution of the L-mode flow and S-mode disturbances is to make such local climatological events as the onset and withdrawal of Baiu so rapid and dramatic.

[学科分类] 大气科学