Diagnostic comparison of the East Asian subtropical jet and polarfront jet: Largescale characteri stics and transient eddy activities

A diagnostic comparison of the East Asian subtropical jet (EASJ) and polarfron t jet (EAPJ) in winter season is carried out by using ERA 40 monthly mean and da ily reanalysis dataset. The comparison is based on the largescale characterist ics and the transient eddy activities. It is demonstrated that the EASJ and the EAPJ shown by the monthly mean data have no clear geographical boundary at upper levels. The distribution of the numbers of jet core extracted from the daily da ta exhibits distinct boundary mark at the latitude of the northern part of the T ibetan Plateau. There exist two areas with large numbers of jet cores, correspon ding to the EASJ and EAPJ regions, respectively. The analysis of the synopticscale transient eddy activities (STEA) over the East Asian region shows a spatial match among STEA, EASJ, and EAPJ. The strong EASJ is accompanied by the weak south branch of the STEA, while the weak EAPJ is accompanied by the north branch with more active STEA, indicating that the EAPJ is a jet existing with the ST EA. Further analysis shows two anomalous modes of the winter EAPJ: One is the an ticyclonic/cyclonic anomalous circulation pattern, and the other is the weaken ed/strengthened local westerly wind. The largescale circulation anomalies rela ted with the first mode are mainly concentrated in the EuropeanAsian midhigh latitudes. It is influenced by the anomalous circulation in both the mid-high latitudes and the upstream area. When the local westerly wind over the EAPJ regi on is weakened/strengthened, the westerly jet from the east part of the EASJ to the western Pacific region shows an opposite variation. The atmospheric circulat ion anomaly shows the Eurasian (EU) pattern. The two major modes of the winter E APJ are also closely linked with the STEA anomaly over East Asia. The STEA anoma lies in the north branch of the STEA region propagate as wave train along the ax is of the north branch of the STEA and cross the East Asian coastal areas to the oceanic region. However, the ones near the south branch of the STEA region beco me trapped over the eastern part of East Asia and its coastal areas at 200 hPa.