Increasing Trend of Summertime Synoptic Wave Train Activity over the Western North Pacific since 1950
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Volume 34
OCTOBER 2020
Increasing Trend of Summertime Synoptic Wave Train Activity over the Western North Pacific since 1950 Xinyi ZHOU1 and Tim LI1,2* 1 Key Laboratory of Meteorological Disaster, Ministry of Education (KLME) / Joint International Research Laboratory of Climate and Environmental Change (ILCEC) / Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters (CIC-FEMD), Nanjing University of Information Science & Technology, Nanjing 210044, China 2 International Pacific Research Center and Department of Atmospheric Sciences, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA (Received January 27, 2020; in final form May 5, 2020)
ABSTRACT 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 background 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 explain 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 synoptic activity index (SAI) that combines the dynamic and thermodynamic factors is then proposed. This index well captures the observed long-term trend of the SWT intensity. Key words: global warming, synoptic wave train, barotropic energy conversion (CK), atmospheric instability parameter, synoptic activity index (SAI) Citation: Zhou, X. Y., and T. Li, 2020: Increasing trend of summertime synoptic wave train activity over the western North Pacific since 1950. J. Meteor. Res., 34(5), 1013–1024, doi: 10.1007/s13351-020-0013-y.
1.
Introduction
The dominant pattern of summertime synoptic-scale variability over the western North Pacific (WNP) is a northwest–southeast oriented synoptic wave train (SWT; Lau and Lau, 1990; Li and Hsu, 2017). The wave train has a typical period of 3–8 days and zonal wavelength of 2500–3000 km. It contains the alternating cyclonic and anticyclonic circulations, and its signal can be identified from either low-level vorticity or meridional wind fields (Lau and Lau, 1990; Tam and Li, 2006). The cause of frequent occurrence of SWT in the WNP is attributed to the instability of summer mean flow in the presence of a moisture–convection–circulation feedback (Li, 2006) or
energy conversion from the low-level background mean flow (Lau and Lau, 1992; Maloney and Hartmann, 2001).
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