The Decadal Change Of Relationship Between East Asian Winter Monsoon And East Asian Summer Monsoon And The Analysis Of Its Possible Causes

In terms of NCEP/NCAR reanalysis data, HadISST1 dataset and several representative monsoon indices which can represent the leading modes of the circulation and rainfall in East Asia, the variability of East Asian winter monsoon (EAWM) is divided into an ENSO-affected part and an ENSO-unaffected part. This study investigates not only the strengthening/weakening relationship between ENSO-affected/ENSO-unaffected EAWM and the following East Asian summer monsoon (EASM) by the methods of regression and moving-correlation., but also the different relationships between ENSO-affected EAWM and summertime circulation and rainfall in East Asia. The possible reasons of decadal change in relationships are discussed by analyzing the decadal changes of circulation anomalies and sea surface temperature anomalies(SSTA).The results are showed as blow.1) The negative relationships between EAWM indices and EASM indices were significant during 1960s but turned to insignificant after then. Also, ENSO-affected EAWM was negatively related to EAWM next year in early 1960s. But the negative relationship was weak during 1965—1979, and soon became strong and statistically significant during 1980—2004.2) During 1980—2004, a strong ENSO-affected EAWM lead to East Asia-Pacific teleconnection pattern both in lower-level and middle-level troposphere. The lower-level anomalous cyclonic circulation over the western North Pacific corresponding to the negative geopotential height anomalies in middle-level weakened the strength of West Pacific Subtropical High(WPSH). Alone the western edge of the lower-level anomalous cyclonic circulation, southwesterly of EASM was blocked by north wind in eastern China, causing a weak EASM. Whereas the circulation and rainfall anomalies in the above quantities were weak in ENSO-affected EAWM years during 1965—1979.3) The reasons why the dacadal change of relationship between ENSO-affected EAWM and EASM happened in late 1970s are investigated. Firstly, the phase characteristics of ENSO-affected EAWM and EASM are different before 1980 but turned to be same after 1980, leading to a strengthening relationship between them. Secondly, warm SSTA of Tropical western Pacific and cold SSTA of North Indian Ocean in strong ENSO-affected EAWM years persisting from winter to summer lead to a lower-level anomalous cyclonic circulation over the western North Pacific which maintains a so strong state from winter to summer that can be a superior connection of EAWM and EASM during 1980—2004. While SSTA in these key regions are too weak to support the lower-level anomalous cyclonic circulation during 1965—1979. Besides, the different phase of Pacific Decadal Oscillation(PDO) has different influence on the relationship of ENSO-affected EAWM and EASM. The positive phase of PDO benefit the strengthening negative relationship of them.