Abstract
The relationship between the enhanced convective regions in tropics and the change in geopotential height fields around Japan at the end of Baiu is investigated.
Based on outgoing longwave radiation (OLR) values around Japan, the 20 years when the withdrawal of Baiu was obvious are selected out of 24 years from 1979 to 2002, excluding 1982, 1993, 1998, and 2000, and end-days of the Baiu season are defined for each of the 20 year. Daily time series of OLR and geopotential heights at 850 hPa, 500 hPa, and 200 hPa are respectively composited with respect to the end-day of 20 years. A comparison between the composite maps of OLR before and after the end of Baiu reveals that the patterns of the changes in circulation are divided into four groups:1) enhanced convection in both the western North Pacific and the Philippines (type WP, 2 years）;2) enhanced convection only near the Philippines (type P, 7 years）;3) enhanced convection only in the western North Pacific (type W, 5 years）;and 4) no enhanced convection in either region (type N, 6 years).
Composite analyses of the geopotential height fields are performed for the pattern of types P, W, and N, respectively, and it is clarified that three main mechanisms described below cause the end of the rainy season:（1) the strengthening of the anticyclone around Japan as a result of the propagation of stationary Rossby waves from the tropical region and along the westerlies (type-P）;（2) the strengthening of the anticyclone around Japan as a result of the propagation of stationary Rossby wave along the westerlies (type-N）;（3) the eastward retreat of the Pacific High (type-W).
The changes of geopotential height fields before and after the end of Baiu in types P and N suggest that the propagation of stationary Rossby waves strengthen the Pacific anticyclone near Japan. It is suggested that the differences of the domains where convection was enhanced are related with the generation of stationary Rossby waves which act as the trigger for the end of Baiu.
