Dozens of East Asian summer monsoon(EASM)indices have been proposed to represent the main features of anomalous summer monsoon activity based on different physical consideration.In this study, sixteen representative EASM indices were calculated based on the NCEP/NCAR reanalysis data (1961—2010) and detailed comparison has been performed to detect the differences among the EASM indices.We also explored the basic physical features of various EASM indices by comparing their differences in the related anomalous patterns of atmospheric general circulation and summer rainfall.The possible linkages of EASM indices with the predominant modes of EASM were also discussed.
It is found that sixteen EASM indices can be generally classified into two categories (Category I and Category II).Category I mainly exhibits the decadal variation of EASM while Category II is featured by strong interannual variability of EASM.Further analysis shows that:1) Category I mainly reflects a consistent spatial variation of EASM and is closely linked to the first leading mode of EASM acquired by the extended empirical orthogonal function (EEOF) analysis of 850 hPa wind.Positive(negative)index of category shows intensified (weakened) southerly in the whole East Asian area, which is accompanied with anomalously intensified cyclonic (anti-cyclonic) circulation around the Lake Baikal and slightly northward (southward)western Pacific subtropical high (WPSH).There are also corresponding anomalous 500 hPa geopotential height fields exhibiting a "+-+"("-+-") anomalous zonal pattern in the mid-high latitudes, originating from the western Europe, across the West Siberia plain, and extending to northeast Asia.Meanwhile, there is significantly increased (reduced) rainfall in North and Northeast China but reduced (increased) rainfall in the Yangtze River and its southern area, which consists of a typical dipole of monsoon rainfall anomalies.Category I shows better description of summer precipitation anomalies in North and Northeast China.2)Category II of EASMI generally represents an out-of-phase variation of the summer wind between southern and northern China.Positive (negative) index corresponds to weaker (stronger) summer monsoon in southeast China but stronger (weaker) summer monsoon in northeast China, which is accompanied with anomalously intensified cyclonic (anti-cyclonic) circulation over the Northwest Pacific and northward (southward) shifting of WPSH.Correspondingly, 500 hPa geopotential height anomalies exhibit a "-+-"("+-+") meridional tripole pattern in East China, from the western North Pacific (WNP), across Japan to the Okhotsk Sea, which is similar to the typical East Asia-Pacific (EAP) teleconnection pattern.The summer precipitation decreases (increases) in the middle and lower reaches of the Yangtze River, while it increases (decreases) over the east of Northeast China and the southeast coast of China.Category II shows better description of summer precipitation over the Yangtze River basin.