East Asian Summer Monsoon Onset And Advance The Process Of Inter-annual Variation And Its Relationship With The Spring Thermal State Of The Tropical Western Pacific

The CMAP, GPI, OLR, TBB, NCEP/NCAR reanalysis data,the observed precipitation data of China, and the definition of the South China Sea Summer Monsoon (SCSM) onset proposed by Liang and Wu (2002) are used to analyze the interannual variation of the onset of the SCSM and the northward march of the East Asian summer monsoon (EASM), which is related to the thermal state of the tropical western Pacific. The main conclusions are as follows:1. The onset of the SCSM is the earliest in the Asia. There are the twin cyclone or the deep tough over the Bay of Bangle and the tropical east India Ocean before the onset of the SCSM in the climatological-mean. The strong westerly is over the Sumatra and Indo-China Peninsula ,and it extend eastward to the South China Sea (SCS). All of these lead the onset of the SCSM.After the onset of SCSM, the center of convective activity over the west of the Bay of Bengal to the east of the Indian Peninsula move northward. The strong westerly flow located at the west of the convective activity center. Those have close relationship with the onset of India monsoon.2. When the tropical western Pacific in a warming state in spring, the twin cyclones symmetric to the equator are early formed over the Bay of Bengal and the west of Sumatra before the SCSM onset. The strong southwesterly flow early appears over the equatorial eastern Indian Ocean, the Indo-China Peninsula and the SCS. These lead to the early onset of the SCSM. On the other hand, when the tropical western Pacific is in a cooling state in spring, since the twin anomaly anticyclones always appear over the equatorial eastern Indian Ocean and the Bay of Bengal, this leads to the weakening of the twin cyclones over these regions, which leads to the late appearance of the strong southwesterly flow over the Indo-China Peninsula, the northern part of the SCS and South China. This leads to late onset of the SCSM.3. The influence of the variation of thermal state of the tropical west Pacific on the 30-60-day oscillation over the SCS have close relationship with the onset date of the SCSM. The 10-20-day oscillation propagate westward from the SCS to the Indian Peninsula after the onset of the SCSM in the warm state of the tropical western Pacific.4. The physical mechanism effecting to the SCSM onset: When the tropical western Pacific in a warming state in spring, the Walker circulation is strong, the strong ascending branch of the Walker circulation is found over this region. The convergent flow at low level over this region makes for the westerly wind from the Bay of Bengal extending to the SCS. That leads the early onset of the SCSM. When the tropical western Pacific is in a cooling state in spring, the Walker circulation is weaken, the onset of the SCSM is late.5. A three-dimensional linearized model with the CISK mechanism is used to discuss the physical mechanism of the impact of the thermal states anomalies of the tropical western Pacific on the onset and evolution of the Asian summer monsoon. The results show that if the position of the heating due to convective activity is...