Climate Variability and Mechanisms of the East Asian Winter Monsoon and the Impact from the Stratosphere
东亚冬季风气候变异和机理以及平流层过程的影响

Recent advances in studies on the characteristics and mechanisms of the East Asian winter monsoon (EAWM) variability are reviewed in the paper. In particular, the possible effect of the stratospheric process on the EAWM and the climate anomalies are documented and investigated. It was found that the path of the EAWM, as well as the variation in its intensity, is important. The EAWM intensity is usually associated with uniform changes in wintertime temperature throughout China, whereas the EAWM path tends to induce a north-south temperature dipole pattern which is associated with two sub-systems that develop in the north and the south of China. Moreover, since the beginning of this century the forming of the EAWM was delayed while its intensity tended to decrease during the mature phase of the monsoon and intensify in early spring compared to earlier records. All these variations are shown to be closely related to the north-south temperature dipole pattern. Further analysis indicates that the north-south dipole pattern is one of the major modes of the EAWM variability and is closely associated with the intensity of the stratospheric Polar Vortex. When an anomalous polar vortex propagates downward, this may induce anomalous Arctic Oscillation (AO) and Siberian High in the lower troposphere, leading to the emergence of a north-south temperature dipole pattern in East Asia. Studies on the causes of EAWM variability suggest that the impact of the El Ni o-Southern Oscillation (ENSO) has weakened since the mid-late 1970s. In contrast, the impacts from the circulation anomalies in the mid-high latitudes such as the North Pacific Oscillation (NPO) and the Ural blocking have increased significantly. The mechanisms involved have also been investigated. In addition, the persistent weakening of the EAWM since 1987 may be related to the interdecadal change of the quasi-stationary planetary wave activity. Through wave-flow interaction the planetary wave activity may induce variations in the AO, the Siberian High, and the Aleutian Low, and result in anomalous EAWM. Finally, the possible effects of the 11-year solar cycle on the East Asian climate anomalies during the boreal winter and the associated processes are also discussed.