Impacts of Stratospheric Polar Vortex Shift on the East Asian Trough

Impacts of stratospheric polar vortex shift on the wintertime East Asian trough (EAT) on intraseasonal time scales are investigated using a reanalysis dataset and a climate model. The result based on composite analysis shows that the shift of the stratospheric polar vortex toward eastern Siberia (ES-shift event) is associated with higher geopotential height at 500 hPa than normal over East Asia, corresponding to the weakened EAT. Furthermore, the simulated EAT is also weakened when nudging the stratospheric state toward that during the ES-shift events. This study further found that there is no significant difference in the stratospheric polar vortex intensity between the ES-shift events and nonshift events, implying that the polar vortex strength change may have little influence on the possible connection between the polar vortex shift and the EAT change. The underlying mechanisms are listed as follows: First, the positive potential vorticity (PV) anomalies in the lower stratosphere associated with ES-shift events could explain approximately 40% of the local westerly anomalies in the upper troposphere to the north of East Asia via PV conservation, leading to the rise of the geopotential height over East Asia and the weakening of the EAT. Second, the shift of stratospheric polar vortex could modulate the synoptic-scale Rossby wave activity in the upper troposphere, favorable for the southward propagation of synoptic-scale waves and divergence of extended Eliassen-Palm flux in the upper troposphere. Finally, the transient wave feedback could enhance the tropospheric westerly anomalies in the north of East Asia and induce positive height anomalies to its south, further weakening the EAT. Our results revealed that the stratospheric polar vortex shift leads the EAT intensity variation by around 2-5 days, implying that the stratospheric polar vortex shift could be applicable to the prediction of the EAT intensity.