Increasing Trend of Summertime Synoptic Wave Train Activity over the Western North Pacific since 1950

• 作者: Tim LI Xinyi ZHOU
• 来源期刊: 气象学报（英文版）
• 年，卷（期）: 2020,（5）
• 所属期刊栏目: ORIGINAL PAPER
• 页码范围: 1013-1024
• 页数: 12页
• 语种: 英文

摘要

The change of summertime synoptic-scale wave train (SWT) activity over the western North Pacific (WNP) since 1950 was investigated based on the NCEP–NCAR reanalysis data. It is found that the intensity of SWT has a rising trend, while its structure and phase propagation characteristics remain unchanged. Environmental factors responsible for the rising trend are investigated. By separating the whole period into three warming phases (P1: 1950–1958, P2:1978–1986, and P3: 2006–2014), we found that even though the vertical velocity shows a rising trend, the back-ground low-level vorticity over the monsoon trough region increases from P1 to P2 but decreases from P2 to P3, and so is the low-level barotropic energy conversion (CK). Thus, just the environmental dynamic factor could not ex-plain the continuous rising SWT trend. On the other hand, thermodynamic factor, such as the sea surface temperature (SST), moisture, and atmospheric instability, shows a clear step-by-step increasing trend. A non-dimensional synop-tic activity index (SAI) that combines the dynamic and thermodynamic factors is then proposed. This index well cap-tures the observed long-term trend of the SWT intensity.