Diagnostic Comparison of Wintertime East Asian Subtropical Jet and Polar-Front Jet: Large-Scale Characteristics and Transient Eddy Activities |
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Authors: | Xuejuan Ren Xiuqun Yang Tianjun Zhou Jiabei Fang |
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Institution: | (1) Department of Atmospheric Sciences, National Central University, Jhongli, Taoyuan, 32001, Taiwan |
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Abstract: | Diagnostic comparison of the East Asian subtropical jet (EASJ) and polar-front jet (EAPJ) in winter season is carried out
by using the ERA-40 dataset. The large-scale circulation characteristics and synopticscale transient eddy activities (STEAs)
associated with the EASJ and EAPJ are examined. The results show that the EASJ and EAPJ in the upper-level monthly mean data
have no clear geographical border, while the distribution of the numbers of jet cores from the daily data exhibits a distinct
boundary at the latitudes of the northern Tibetan Plateau. The two areas with large numbers of jet cores correspond to the
EASJ and EAPJ regions. The analysis of STEAs over the East Asian region shows a spatial match of STEAs with the EASJ and EAPJ
in winter: the strong EASJ is located within the weak southern branch of the STEA while the relatively weak EAPJ appears within
the active northern branch of the STEA, indicating that the EAPJ is the jet coexisting with the STEA. Further analysis shows
two anomalous modes of the winter EAPJ: the anomalous anticyclonic/cyclonic circulation and the weakened/strengthened local
westerly wind. The large-scale circulation anomalies in the Northern Hemisphere related to the first mode are concentrated
in the Eurasian mid to high latitudes, and are also influenced by the anomalous circulation in the upstream area. When the
local westerly wind over the EAPJ region is weakened/strengthened, the westerly jet in the eastern part of the EASJ and that
in the western Pacific region show opposite variations. The corresponding anomalous atmospheric circulation demonstrates the
Eurasian (EU) pattern. The EAPJ anomalies are also closely linked with the STEA anomalies over East Asia. The anomalies in
the northern branch of the STEA propagate as a wave train along its axis into the East Asian coastal waters, and then migrate
eastward to the oceanic region. However, the ones near the southern branch are trapped over the eastern part of East Asia
and its coastal waters at 200 hPa. |
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Keywords: | East Asian subtropical jet East Asian polar-front jet diagnostic comparison synoptic-scale transient eddy activities |
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