The low geomagnetic secular variation and weak core flows in the Pacific

The temporal variation in the Earth's magnetic field is presently lower in the Pacific region, a geographic configuration that has persisted over the past few centuries. The reason for this is tied to the large scale core flows near the core-mantle boundary (CMB), which are weaker under the Pacific and feature a planetary gyre that is eccentric and broadly avoids this region. However, what regulates such a core flow morphology is less clear. Here, we show that this flow pattern can be explained if the conductance of the lowermost mantle under the Pacific region is greater by a factor of 6 to 9 than elsewhere on the CMB. Flows are then weaker in the Pacific, and the gyre avoids it, because of the larger electromagnetic drag force acting on flows in contact with this region of the CMB. The nature of the lowermost mantle conductance remains unclear, but we argue that core fluid trapped within topographic cavities of the CMB is the most likely explanation.