Decadal changes in the intraseasonal variability of intensity and location of East Asian polar-front jet around 2000 and associated mechanisms.

The East Asian jet stream is an important component of mid-high latitude circulation systems, and its intraseasonal oscillation plays a crucial role in the formation and development of persistent climate anomalies and extreme events over East Asia–Pacific region. This study examines the decadal changes in the intraseasonal variability (ISV) of the East Asian jet stream and investigates the possible reasons with reanalysis and observational datasets. It is found that the zonal winds over East Asia show notable ISV with a significant period of 10–25 days. The variance of 10–25-day filtered East Asian zonal winds exhibits a significant decrease around 2000, which is mainly contributed by the weakened ISV of westerlies within the key region of East Asian polar front jet (EAPJ). Further analysis reveals that the EAPJ experienced remarkable ISV changes in both intensity and location. The ISV of EAPJ intensity was weakened around 2000 due to decreased frequencies of both the extremely strong and weak cases, whereas the ISV of EAPJ location was evidently enhanced, with the southward shifts increased more than the northward ones. Along with the ISV increase (decrease) of EAPJ intensity (location), the dominant mode of EAPJ intraseasonal variation has changed from its intensity change to position shift since the year 2000. The decadal ISV changes and mode transition of EAPJ are closely associated with the modulation of lower-level large-scale circulation over Eurasia. The weakened ISV of meridional winds over Ural region after 2000 is favorable for an intraseasonal circulation pattern change from a continental-scale cyclonic anomalous pattern to a meridional dipole structure across the Eurasia in lower troposphere, resulting in regime shifts of East Asian temperature and its meridional gradient through temperature advection. The intraseasonal temperature gradient patterns display ISV changes parallel to those of EAPJ around 2000, indicating that the thermal anomalies are effective in leading to corresponding intensity and location variations of EAPJ via thermal wind relationship. Furthermore, the modulation of lower-level circulation is closely related to strengthened Arctic warming and weakened temperature variability over Barents sector after 2000, which may further be linked to sea ice reduction in the Barents Sea. [ABSTRACT FROM AUTHOR]