Modulation of northern hemisphere wintertime stationary planetary wave activity: East Asian climate relationships by the Quasi-Biennial Oscillation

The modulation of the relationship between the tropospheric stationary planetary wave activity and the East Asian winter climate by the tropical quasi-biennial oscillation ( QBO) wind in the stratosphere is investigated. In the QBO easterly phase, a significant warming appears in northeastern Asia in the presence of high wave activities. This corresponds to a weakened East Asian trough at 500-hPa, which determines the extent to which cold waves penetrate into East Asia. However, in the QBO westerly phase, both the surface warming and the weakening of the East Asian trough become insignificant in response to high wave activities. The possible mechanism for this QBO modulation on the tropospheric wave activities may be attributed to the indirect influence of the QBO induced polar and extratropical stratospheric circulation changes. Under the QBO easterly phase conditions, the tropospheric wave activity flux divergence in the higher-latitude region is enhanced due to enhanced upward Eliassen-Palm ( EP) flux into the stratosphere, while the wave flux convergence in the subtropical troposphere is increased due to the decrease in the equatorward flux. This leads to an enhanced wave activity difference and thus the associated East Asian climate anomalies become larger and robust. An opposite effect ( i. e., reduced wave activity difference) appears in the QBO westerly phase. Further analyses reveal that the QBO significantly modulates both the zonal wave number 1 and 2 in the sea level pressure field but not for wave number 3. The combination of wave number 1 and 2 patterns leads to the strongest surface temperature anomalies in northern Asia.