Impact of El Nino on Large-scale Circulation of Southeast Asian Summer Monsoon

Multi-year SST and NCEP/NCAR reanalyzed wind data were employed to study the impacts of El Nio on the Southeast Asian summer monsoon (SEASM). It was found that the impacts of El Nio on the SEASM differed distinctly from those on the East Asian summer monsoon (EASM) and the Indian summer monsoon (ISM). Composite analysis indicated that the "gear point" of coupling between the Indo-monsoon circulation and the Pacific-Walker circulation was located in the western margins of Southeast Asia when the developing stage of El Nio events covered the boreal summer. The anomalous circulations in the lower and upper troposphere and divergent circulation are all favorable for the strengthening of the SEASM during this period. Following the evolution of El Nio, the "gear point" of the two cells shifted eastward to the central Pacific when the mature or decaying period of El Nio events covered the boreal summer. The anomalous circulations are favorable for the weakening of the SEASM. The anomalous indexes of intensity of SEASM accord well with the above results. Additionally, the difference of SSTA patterns in the tropical Indo-Pacific Ocean between the two stages of the El Nio may play an important role.