This paper examines the relationship between the East Asian summer monsoon and the El Niño Southern Oscillation (ENSO) during the period from 1958 to 1998, which is quite different from the link between the South Asian summer monsoon and ENSO.Major findings are: (1) A new index, called the East Asian Monsoon Index (EAMI), is defined for measuring the East Asian monsoon, which could extensively describe the south-north distribution of the East Asian summer monsoon’s activity. The interannual variability of the EAMI displays a significant negative correlation with the broad-scale Asian monsoon index proposed by Webster and Yang (1992) from 1976 to 1998. (2) A significant positive correlation between the summer 500 hPa geopotential height anomalies and the NINO-3 SST in the preceding fall and winter is found in the subtropical regions of East Asia and the western North Pacific, and in northeast Asia centered at 70°N, 137.5°E. A strong (weak) summer monsoon in the subtropical regions of East Asia tends to occur about two to three seasons after the NINO-3 SST anomalies exceed 1.5 °C (drop below −0.7 °C). (3) The above results suggest a delayed impact of the ENSO on the East Asian summer atmosphere circulation. During the summer after the El Niño reaches its mature phase, an anomalous blocking anticyclone tends to occur in northeast Asia. Meanwhile a subtropical high of the western North Pacific extends abnormally westward. This anomalous circulation pattern enhances the summer monsoon in subtropical East Asia. The abovementioned evolution of the circulation anomalies became more prominent in the unprecedented ’97/98 El Niño event, suggesting that the devastating 1998 flood in southern central China may be partially due to the delayed impact of the ’97/98 El Niño. The physical processes for the delayed impact besides the air-sea interaction in the tropical and subtropical western Pacific are discussed too.
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