  For the past few centuries, the temporal variation in the 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.
