St abilization of summer monsoon in middle and lower reaches of the Changjiang river and seasonal transition of East-Asian circulation pattern in early summer

Abstract

Using data of 850 hPa pentadal meanθ _{s e} and composite wind fields in East China, characteristics of the mean circulation are examined with summer monsoon (SM) stabilized over the Changjiang middle and lower reaches (CMLR). The onset of steady SW monsoon consistent over the CMLR is marked by a considerable northward jump of the pentadal position of the 340 Kθ _{S e} isoline. This is considered as a seasonal change-over of South-China SM in its northward movement. Moreover, it is found that, when low-level SM becomes stabilized over the CMLR, an evident northward shift occurs of the high-level jet center along 115°E, indicating a certain relation between the seasonal variation of the East-Asian circulation pattern in early summer and the steadiness of SM in the CMLR.