Inter-decadal Shift of East Asian Summer Monsoon in the Early 1990s

Using JRA-25 and NCEP/NCAR reanalysis data from 1979 to 2009, dominant modes of summer season (June—August) 850 hPa wind field variability over East Asia is revealed by means of the complex vector empirical orthogonal function method. The two reanalysis data are consistent with the description of the first East Asian summer monsoon (EASM) mode, whereas the first mode had been studied, showing that the first mode could not reflect the inter-decadal shift of Chinese summer precipitation in the early 1990s. Consequently, the inter-decadal shift feature of the second EASM mode is deeply analyzed, as well as its effect on summer precipitation in China. Moreover, the possible external forcing factors exerting effects on the inter-decadal shift of EASM are discussed.Results show that, EASM which is revealed by two sets of reanalysis data to have undergone one inter-decadal shift in the early 1990s. The inter-decadal shift time of EASM is consistent with the inter-decadal shift time of summer precipitation in China. EASM is closely related to the mid-high latitude atmospheric circulation anomalies. Corresponding anomalous 500 hPa geopotential height fields show an anomalous quasi-zonal teleconnection pattern in northern Eurasia, whereas the distribution of summer precipitation anomalies show a meridional dipole pattern. Accompanied by the inter-decadal shift of EASM, after the early 1990s, summer precipitation decreases in the majority of northern China, especially in north of the northeast and the area between the Yangtze River and the Yellow River in the vicinity of 105°E. While summer precipitation increases significantly in South China and the Huaihe River Basin. From the perspective of dynamic, the characteristics of inter-decadal shift of summer precipitation in China are described. The difference distribution of summer 500 hPa geopotential height fields between two periods (1993—2009 and 1979—1992) show northern Eurasian quasi-zonal teleconnection pattern, then the difference distribution of summer 850 hPa wind fields show the structure that there are two anomalous anti-cyclonic circulations in southeast of Lake Baikal and south of Japan, while there are two anomalous cyclonic circulations in southern China and Okhotsk Sea.The possible external forcing for the inter-decadal shift of EASM are various, including summer sea surface temperature (SST) in the northwestern Pacific, north Indian Ocean and the part of high latitude ocean (North Atlantic and North Pacific), as well as changes of spring Eurasia snow water equivalent in the early 1990s, inter-decadal shift of spring Arctic sea ice in the early 1990s, especially the high-latitude forcing factor. The role of these external forcing in inter-decadal shift of EASM is unclear and further study is essential.