Large-Scale Circulations over East Asia during Baiu Period of 1979

Abstract

Large-scale situation over East Asia during the Baiu period of 1979 was studied on the basis of 10-day averaged fields. The development of warm ridge over the Mongol-Sibirsk Plateaus (Baiu ridge) and cold trough elongated from the Bering Sea cyclone to China (Baiu trough) were the major systems in Baiu season in middle-high latitudes. With the rapid temperature rise over the Continent and the slow temperature rise over the Pacific north of -35°N, the large temperature difference between the Continent and the Pacific was maintained. The circulations in the lower troposphere in the Baiu season were characterized by the northerly wind in the west side of the Baiu trough (Baiu-trough-northerly), the southerly wind over the South China Sea, low-level-jet like strong WSW wind along the northern periphery of the Pacific anticyclone, and the Indian monsoon westerly wind. While the Meiyu front was related with a shear line between the South China Sea southerly and the Baiu-trough-northerly wind (Baiu shear line), the Baiu front was located -1500km south of the Baiu trough and associated with convergence/confluence of SW wind to the strong WSW wind zone. Another feature of interest is the difference between the structure of the Meiyu and the Baiu fronts. The northward advance of the Pacific anticyclone and the Baiu front is the basic seasonal change. The quasi-periodic (-40-day period) intraseasonal variations of the Pacific anticyclone and the Baiu front were superposed upon this seasonal change. The intraseasonal variations of the Baiu front were related to the activity of ITCZ in the South China Sea as well as the variations of the monsoon precipitation. From the analysis, the role of the Tibetan Plateau, the Baiu-trough-northerly wind, cold advection associated with the Bering Sea cyclone over the cold northwestern Pacific in maintaining the Baiu/Meiyu front was inferred. This inference was supported by the results of prediction experiment by Nakamura, Hasegawa and Ninomiya (1985).