中文摘要：

在气候态上,7月底东亚高空西风急流中心突然从40°N北跳到45°N以北。逐年统计分析显示此次急流北跳存在两类典型方式：急流北侧西风强度增强引起的北跳（第一类）和急流中心西风强度的减弱引起的北跳（第二类）。本文基于1958年到2002年的NCEP/NCAR再分析资料,采用波活动通量诊断这两类典型北跳相应的动力过程,进一步探讨了其北跳的物理机制。在第一类急流北跳过程中,对流层高层远东高纬度地区形成的低压槽和中纬度南亚高压向东亚地区的伸展导致两者之间的鄂霍次克海上空位势高度经向梯度加强,从而西风增强,东亚高空西风急流北进;进一步的动力诊断分析结果表明,其中远东高纬度地区的低压槽是由源于西欧和北大西洋上空的波活动通量沿着欧亚大陆北部地区传播到远东地区形成负位势高度异常所致,而南亚高压向东亚地区的延伸主要和热带西北太平洋强降水激发的＂太平洋—日本（PJ）＂波列导致东亚地区形成正位势高度异常有关。第二类急流北跳主要受到沿着亚洲副热带西风急流传播的波活动影响：东传的波活动在东亚地区累积形成高压脊,导致东亚地区位势高度经向梯度减弱,东亚高空西风急流减弱北跳。

英文摘要：

The East Asian upper-tropospheric jet stream（EAJS） jumps northward abruptly in late July with its center shifting from 40°N to north of 45°N in climatology.Year-by-year statistics showed that there are two categories of typical northward jumps：The first is characterized by enhanced westerly to the north of the EAJS＇s axis and the second is by the weakened westerly in the EAJS＇s axis.In this study,the physical mechanisms for the two categories of northward jumps of the EAJS are investigated by using the NCEP/NCAR reanalysis data from 1958 to 2002 and the dynamical process is diagnosed based on the wave activity flux.The trough over the high-latitude Far East and the northeastward extension of the South Asia high in the upper troposphere,together leads to meridional gradient of geopotential height enhancing and then the westerly enhancing over the Okhotsk Sea between them,and results in the first category of northward jump of the EAJS.Actually,the high-latitude trough is contributed by the eastward propagating Rossby wave,which is originated from West Europe and the North Atlantic,along the north edge of Eurasian continent,and the extension of the South Asia high is mainly related to the ＂Pacific-Japan（PJ） ＂ teleconnection pattern induced by the positive rainfall anomaly over the tropical western North Pacific.The second category of northward jump,however,is mostly attributed to the Rossby wave along the Asian subtropical jet stream in the upper troposphere.The eastward propagating Rossby wave is blocked over East Asia and forms an in-situ ridge,reducing the meridional gradient of geopotential height and the westerly over East Asia and leading to northward jump of the EAJS.