The East Asian Summer Monsoon Index And Its Changing Decadal Relationships With IOD And ENSO

The inter-annual variability of East Asian summer monsoon and its relationships with IOD, and that with ENSO are discussed through the contrast of many kinds of summer monsoon indexes using the NCEP/NCAR monthly reanalysis dataset . A new East Asian summer monsoon, East Asian Summer Monsoon Potential Vorticity Index (I_PV) is defined. Based on I_PV, the differences of atmospheric heat source andexternal forcing (i. e., SST) between strong and weak monsoon years are discussed. The relationships between East Asian summer monsoon with IOD, and that with ENSO are more discussed using I_PV .The results are summarized as follows:(1) The relationships between East Asian summer monsoon (EASM) and IOD, and that between EASM and ENSO display an inter-decadal variation feature. The association between EASM and IOD remains strong except during the eighties and beginning of the nineties. It is found that IOD and ENSO have opposite effect on EASM. The IOD and ENSO have complementarily affected the EASM. When the IOD-EASM correlation is high, the ENSO-EASM correlation is low. The obvious changes of distributions of sea surface temperature and periods of EASM, IOD and ENSO during 1972-1982 and 1983-1993, which may result in changes of periods of air-sea interaction and circulation, possibly lead to these inter-decadal variations ofrelationship.(2) East Asian Summer Monsoon Potential Vorticity Index (I_PV) have a clearphysical meaning and reveals the characters of monsoon circulation and the rainfall in eastern region of China. Especially it can indicate the anomaly of the summer rainfall in Yangtze River Valley.(3) The distributions of atmospheric heat source in between strong and weak monsoon years are different. The SSTA especially in the equatorial central and eastern Pacific, tropical Indian Ocean and South Sea of China have a relationship with the the annual variability of East Asian summer monsoon. The relationships between East Asian summer monsoon Potential Vorticity Index and IOD, and that between I_PV and ENSO display an inter-decadal variation feature, similar with those using indexes defined by Zhang Qingyun and Li Jianping etc.