| Summary: | The western North Pacific monsoon in the Northern Hemisphere (NH) and the local Hadley circulation in the Southern Hemisphere (SH) are important components of the vertical meridional circulation over the western Pacific Ocean. Here, we define the SH meridional circulation (SHMC) and NH meridional circulation (NHMC) and investigate their relationship over the western Pacific Ocean. Although they are consistent integrally in the climatological circulation, the NHMC and SHMC do not have a positive change relationship but a significantly negative relationship. In addition, this negative correlation experiences an interdecadal change, with one closely related period during 1989–2003 (Period 2, P2) and two weakened related periods during 1979–1988 (Period 1, P1) and 2004–2017 (Period 3, P3). Through the analysis of the meridional wind field at lower and upper levels, we found that the SHMC moves northward during P2, which can result in a close relationship with the NHMC. The possible mechanism is as follows: The Walker circulation strengthened in P2, which strengthened the ascending motion over the western Pacific Ocean, thus inducing the SHMC to strengthen and move northward, resulting in a significantly stronger relationship with the NHMC.
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