| Summary: | Previous studies have revealed the relationship between the Madden−Julian oscillation (MJO) and the Arctic Oscillation (AO). The MJO phase 2/3 is followed by the positive AO phase, and the MJO phase 6/7 is followed by the negative AO phase. This study reveals that the MJO phase 6/7−AO connection is modulated by the Quasi-Biennial Oscillation (QBO) through both tropospheric and stratospheric pathways during boreal winter. The MJO 2/3 phase and AO relationship is favored in both QBO easterly (QBOE) and westerly (QBOW) years because of the MJO-triggered tropospheric Rossby wave train from the tropics toward the polar region. The AO following the MJO 6/7 phase shifts to negative in QBOW years, but the MJO−AO connection diminishes in QBOE years. In QBOW years, the Asian-Pacific jet is enhanced, leading to more evident poleward propagation of tropospheric Rossby wave train, which contributes to the tropospheric pathway of the AO−MJO 6/7 connection. Besides, the enhanced Asian-Pacific jet in QBOW years is favorable for vertical propagation of planetary waves into the stratosphere in MJO phase 6/7, leading to negative AO, which indicates the stratospheric pathway of the AO−MJO 6/7 connection.
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