A Study Of The Southeast Asian Summer Monsoon Onset, Evolution And Its Influence On The Weather And Climate Over The Southwest Of China

Using the NCEP/NCAR reanalysis data and the observational precipitation collected in China, it is explored of the India-Burma trough evolution during the first transition of the Asia summer monsoon and its impacts on the summer monsoon onset over tropical Asia. Statistical and dynamical diagnosis methods are employed in this study. The results reveal that the establishment of the India-Burma trough can promote the ascending motion and water vapor transport in the front of the trough, which may be one of the mechanisms causing the SEAA summer monsoon breaks out earlier than the SAA monsoon. In addition, the significant role of the interaction between the cold air from the mid-latitude and the Southeast Asian monsoon is emphasized. Finally, the strong rainfall events and the monsoon climate of the Southwest China are discussed associated with the Southeast Asian monsoon activities. The main conclusions are summarized as follow:(1) The onset of Asian summer monsoon over the tropical area exhibits distinct stages, with the onset date of SEAA 13 days before that of the SAA. During the SEAA onset, the low-level westerly wind intensifies abruptly, the India-Burma trough grows wider and deeper, positive vorticity and upward motion develops in the front of the trough, and the humidity increases at the low level. Meanwhile, there exists an evidence of active cold air over the mid- and high latitude. As for the SAA onset, similar circulation conditions emerge over the SAA. It is likely that the circulation which favors the convection moves westward.(2) The tropical Asian summer monsoon onset is closely related to the establishment of the India-Burma trough which is an important climate regime affecting the onset process. Firstly, the deepening of the low level trough can provoke the atmospheric upward motion over the front of the trough through the intense advection of positive vorticity, which would be one of the significant dynamic factors leading to the SEAA onset. Secondly, the development of the trough can augment the water vapor transport by the southwesterlies of the trough and form convective instability stratification in situ. This may be one of the significant mechanisms causing the SEAA summer monsoon breaks out earlier than the SAA(3) The Yunnan summer monsoon onset is also closely related to the evolution and westward expansion of the India-Burma trough associated with the upward motion field. On the other hand, the humidity over Yunnan area would increase rapidly as a result of the increasing westerly water vapor transport from the tropical Indian Ocean to the BOB, which provide a necessary moisture condition for the monsoon rainfall. And evidence shows the intrusion of cold air from the mid-latitude may be one of the factors triggering the monsoon onset. Concerning the interannual variation of precipitation, the circulation contributes to high precipitation in May is related to the abnormal deepening of the India-Burma trough along with intense vertical upward motion over the front of the trough. On the other hand, the large scaleabnormal water vapor transport plays a critical role in the drought and flood events of Yunnan province in the pre-summer period.(4) The interaction of the SEAA summer monsoon and the cold air as well as its influence to the rainfall is investigated through three severe rainfall events in the southwest part of China in July 2004. The results show that these three rainfall events are related to three cold surges from the mid- and high latitude.