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Abstract
This paper examines the relationship between the East Asian summer
monsoon and the El Niņo 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 Niņo 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 Niņo event, suggesting that the devastating 1998 flood in
southern central China may be partially due to the delayed impact of the
’97/98 El Niņo. 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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