For the past few centuries, the temporal variation in Earth’s magnetic field in the Pacific region has been anomalously low. The reason for this is tied to large-scale flows in the liquid outer core near the core–mantle boundary, which are weaker under the Pacific and feature a planetary-scale gyre that is eccentric and broadly avoids this region. However, what regulates this type of flow morphology is unknown. Here we present results from a numerical model of the dynamics in Earth’s core that includes electromagnetic coupling with a non-uniform conducting layer at the base of the mantle. We show that when the conductance of this layer is higher under the Pacific than elsewhere, the larger electromagnetic drag force weakens the local core flows and deflects the flow of the planetary gyre away from the Pacific. The nature of the lowermost mantle conductance remains unclear, but stratified core fluid trapped within topographic undulations of the core–mantle boundary is a possible explanation.

在过去的几个世纪中，太平洋地区地球磁场的时间变化异常低。其原因与岩心-地幔边界附近的液态外核中的大规模流动有关，后者在太平洋地区较弱，其特征是行星尺度的回旋体偏心并且广泛避开了该区域。但是，调节这种流动形态的因素尚不清楚。在这里，我们介绍了地球核心动力学数值模型的结果，该模型包括与地幔底部不均匀导电层的电磁耦合。我们显示出，当该层在太平洋下的电导率高于其他地方时，较大的电磁拖曳力会削弱本地磁心的流动，并使行星回转的流动偏离太平洋。