Multi-spatial Modes Of East Asian Summer Monsoon Activity And Their Possible Associations With External Forcing

Based on ERA-40, ERA interim, JRA-25, and NCEP-DOE AMIP-Ⅱ reanalysis data (1979-2002), we compare the basic features of the multi-spatial modes, which reflect the anomalous activity of the East Asian Summer Monsoon (EASM). These results are explored by using statistical methods including correlation and empirical orthogonal function analysis of vector field. Then discusses the influence of previous winter tropical Pacific, tropical India Ocean (TIO) SSTA on the first mode of the EASM and its physical mechanism, and mainly analyzes the interdecadal variation of the second mode of the EASM and corresponding changes of the mid-high latitude atmospheric circulation. Moreover, the possible mechanisms of the decadal change are discussed. Finally, a brief analysis of the possible relationship between the spring Arctic sea ice and the third mode of the EASM was presented. Results suggest that:(1) The EASM exhibits three typical spatial modes with significant differences, which are independent of the dataset sources, with the exception of NCEP-2 in which slightly different features are shown.(2) The first mode represents an out-of-phase variation of the summer wind between southern and northern China. Correspondingly, the atmospheric circulation anomalies exhibit meridional tripole pattern in East China, from the western North Pacific, across Japan to the Okhotsk Sea. The summer precipitation tends to decrease in the middle and lower reaches of the Yangtze River. The second mode represents a consistency variation in the dominant mode of the EASM. There is an intensified anomalous anticyclonic circulation around Lake Baikal in the summer 850 hPa wind fields. And an anomalous zonal wave pattern over the midhigh latitudes appears in circulation fields, originating from western Europe, across the West Siberia plain, and extending to northeast Asia. Meanwhile, the summer precipitation mode of "flooding in the south and drought in the north" appears. The third mode reflects the westward and eastward movement of the EASM, southerly anomalies cover the areas east of 115°E, and northerly anomalies appear in the region west of 115°E. The SLP and the 500 hPa geopotential height show very similar correlation patterns, a wave-train-like pattern from Okhotsk to the ocean east to Japan and an anomalous quasi-zonal teleconnection pattern in northern Eurasia. Nevertheless, the distribution of summer precipitation anomalies in most areas of China is positive, with significant anomalies found only near the Huang-Huai basin.(3) The first mode of the EASM is mainly affected by the TIO and eastern equatorial Pacific SSTA in the previous winter. When the TIO and eastern Pacific are warmer (colder), the TIO SSTA persists through boreal summer to affect the northwest Pacific anomalous anticyclone (cyclone). And southerly (northerly) anomalies appear in the region south of the middle and lower reaches of the Yangtze River, which leads to the strengthening (weakening) of the EASM in the south and the weakening (strengthening) of the EASM in the north.(4) The second mode of the EASM begins to play a leading role in recent years and shows an interdecadal shift around 1993. After the early 1990s, the spring Arctic sea ice anomalies increased in Greenland and part of the Arctic Ocean and decreased in Kara Sea, Barents Sea etc. And soilmoisture around Lake Baikal tended to dry, sea land thermal difference in the low latitude decreased. These are all important factors to make northerly anomalies in most areas of China and lead to the interdecadal weakening of the second mode of the EASM. But the relative contribution and their relation and mutual effect of the three factors need to be further studied.(5) The third mode of the EASM is closely linked to the spring Arctic sea ice anomalies in the vicinity of the East Siberia Sea and Chukchi Sea. The positive (negative) anomalies there make the subsequent atmospheric circulation show northerly (southerly) anomalies cover the areas east of 115°E, and southerly (northerly) anomalies appear in the region west of 115°E.