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Abstract.In this study, the connection between Arctic Oscillation (AO) and variability of East Asian winter mon-

The summer rainfall over the middle‐lower valley of the Yangtze River and over the whole eastern China experienced a notable regime shift in about 1979. This change is consistent with a simultaneous jump‐like change in the 500 hPa geopotential height (Φ500) over the northern Pacific. The rainfall over the Yangtze River valley is closely related to the Φ500 averaged over the area 20°–25°N, 125°–140°E, with a correlation coefficient of 0.66 for the period 1958–1999. Since 1980, the subtropical northwestern Pacific high (SNPH) has enlarged, intensified, and extended southwestward. The changes in the SNPH are strongly associated with the variations of the sea surface temperatures (SSTs) of the eastern tropical Pacific and tropical Indian Ocean. The anomalies of these SSTs, responsible primarily for the shift of the summer rainfall over the Yangtze River through the changes in SNPH, precede the Φ500 signals with different leading times.

[1] Long-term observational data reveal that both the frequency and amount of light rain have decreased in eastern China (EC) for 1956-2005 with high spatial coherency. This is different from the trend of total rainfall observed in EC, which decreases in northern EC and increases in southern EC. To examine the cause of the light rain trends, we analyzed the long-term variability of atmospheric water vapor and its correlation with light rain events. Results show very weak relationships between large-scale moisture transport and light rain in EC. Because of human activities, pollutant emission has increased dramatically in China for the last few decades, leading to a significant reduction in visibility between 1960 and 2000. Cloud-resolving model simulations over EC show that aerosols corresponding to polluted conditions can significantly increase the cloud droplet number concentration (CDNC) and reduce droplet sizes compared to pristine conditions. This can lead to a significant decline in raindrop concentration and delay raindrop formation because smaller cloud droplets are less efficient in the collision and coalescence processes. Together with weaker convection, the precipitation frequency and amount are significantly reduced in the polluted case in EC. Satellite data also reveal higher CDNC and smaller droplet size over polluted land in EC relative to pristine regions, which is consistent with the model results. Observational evidences and simulations results suggest that the significantly increased aerosol concentrations produced by air pollution are at least partly responsible for the decreased light rain events observed in China over the past 50 years.

East Asia is dominated by a typical monsoon climate. The East Asian summer monsoon (EASM) exhibits considerable variability on a wide range of time scales during the 20 th century. A substantial portion is the multi-decadal variability. Over the recent decades, the EASM has been weakening from the end of the 1970s which results in a "southern China flood and northern China drought" rainfall pattern. Understanding the mechanisms responsible for the weakening tendency has been a challenge for climate research community. Examinations on the long-term change of the EASM during the 20 th century find no significant trends, indicating the pronounced weakening tendency of the EASM in recent decades is unprecedented. After documenting the prominent features of the interdecadal climate transition, a review is presented in this paper on the proposed explanations to the observed changes. The proposed factors include the Indian Ocean and far western Pacific warming, the tropical central-eastern Pacific warming, the weakening sensible heat source over the Tibetan Plateau, and the aerosol forcing, as well as internal variability. While parts of the monsoon circulation changes can be explained in terms of the proposed mechanisms, it is still beyond the scope of our current knowledge to present a complete picture. Much remains to be learned about the mechanisms that produce such multi-decadal changes in the EASM, but it seems still unclear whether human activities and global warming are playing significant roles. ZusammenfassungOstasien wird von einem typischen Monsunklima beherrscht. Der ostasiatische Sommermonsun (EASM) zeigt während des 20. Jahrhunderts eine erhebliche Variabilitätüber ein breites Spektrum von Zeitskalen hinweg. Ein größerer Teil davon ist multidekadische Variabilität. Seit dem Ende der 1970er Jahre hat sich der EASM abgeschwächt, was zu dem "Südchina-Flut -Nordchina-Dürre" Muster geführt hat. Das Verständnis für die Ursachen dieser Abschwächungstendenz stellt eine Herausforderung für die Klimatologie dar. Untersuchungen der langfristigenÄnderungen des EASM während des gesamten 20. Jahrhunderts zeigen keine signifikanten Trends, was bedeutet, dass dieÄnderung in den letzten Jahrzehnten ohne Beispiel ist. Nach einer Dokumentation der wichtigsten Phänomene dieser interdekalen Klimaänderung bietet die vorliegende Arbeit einenÜberblicküber die möglichen Erklärungen für die beobachtetenÄnderungen. Diese beinhalten unter anderem eine Erwärmung des mittleren undöstlichen Pazifiks, eine Abschwächung der Wärmequelleüber dem tibetischen Plateau, einen Antrieb durch Aerosol wie auch interne Variabilitäten. Während Teile derÄnderung der Monsunzirkulation hierdurch erklärt werden können, liegt eine vollständige Erklärung des Phänomens noch jenseits unseres derzeitigen Wissensstands. Es muss noch vielüber die Mechanismen verstanden werden, die solche interdekalenÄnderungen des EASM hervorrufen, wobei es noch unklar ist ob menschliche Aktivitäten und die globale Erwärmung eine signifikante Rolle spielen.tropical and subtrop...

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[1] The present study examines the relationship between the Arctic Oscillation (AO) and the East Asian summer monsoon. Two rainfall data sets are used. One is obtained from 10 stations along the Yangtze River to the southern Japan and the other from gridded global land rainfall data for the period 1900-1998. All data are high-pass filtered before analyzing to highlight the interannual variability. Results show that the AO significantly influences on year-to-year variations in the East Asian summer monsoon rainfall. When AO leads by one month, the correlation between May-July AO and summer total rainfall is À0.44. When AO leads by two months, correlation becomes À0.32. Of all monthly, May AO shows the strongest connection to the summer monsoon rainfall. Correlation coefficient between them is À0.45. The large-scale atmospheric circulation patterns in East Asia in association with the AO are also evident. A positive phase of the AO in late spring is found to lead to a northward shift in the summertime upper tropospheric jet stream over East Asia. This northward shift of the jet stream is closely related to anomalous sinking motion in 20°-40°N and rising motion in surrounding regions. These changes give rise to a drier condition over the region extending from the Yangtze River valley to the southern Japan and a wetter condition in the southern China. Possible mechanisms connecting the late spring AO and summer monsoon rainfall are suggested.