盛夏两类东亚高空西风急流北跳的动力过程

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...

Dynamical Processes of Two Categories of Northward Jumps of the East Asian Upper\|Tropospheric Jet Stream in Mid Summer

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.