# Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle

## Abstract

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.

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## Data availability

The datasets generated as part of this study, together with the GMT scripts and data files necessary to reproduce all figures, are freely accessible on UAL Dataverse at https://doi.org/10.7939/DVN/TL8BP6 (ref. 60).

## Code availability

All source codes used to generate the numerical simulations presented in this work are freely accessible on UAL Dataverse at https://doi.org/10.7939/DVN/TL8BP6 (ref. 60).

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## Acknowledgements

We thank N. Schaeffer for sharing his original numerical quasi-geostrophic code, which we extended over the course of this project, and N. Gillet for sharing his flow models. Figures were created using the GMT software59. Numerical simulations were performed on computing facilities provided by WestGrid and Compute/Calcul Canada. This work was supported by a Discovery grant from NSERC/CRSNG.

## Author information

Authors

### Contributions

M.D. designed the project and wrote the manuscript. The custom numerical codes were designed and written by both M.D. and C.M. The numerical experiments were carried out and analyzed by both M.D. and C.M.

### Corresponding author

Correspondence to Mathieu Dumberry.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information Primary Handling Editors: Rebecca Neely; Melissa Plail.

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## Extended data

### Extended Data Fig. 1 The lower magnetic field strength in the Pacific.

a, Br at the CMB over the time-period 1590-1990 from the gufm field model (ref. 4). The r.m.s. amplitude of Br in the Pacific (pink dashed circle) is 195.15 μT, the global average is 237.33 μT, for a ratio of 0.8223. b, Br at the CMB, in 2015 from the CHAOS-6 field model (ref. 8) truncated at spherical harmonic degree 16. The r.m.s. amplitude of Br over the Pacific (pink dashed circle) is 217.83 μT, the global average is 286.40 μT, for a ratio of 0.7606.

### Extended Data Fig. 2 Equatorial planforms of the secular variation in QG model.

Snapshots of $$\frac{\partial a}{\partial t}$$ (top row), $$\frac{\partial {b}_{s}}{\partial t}$$ (middle row) and $$\frac{\partial {b}_{\phi }}{\partial t}$$ (bottom row) from our quasi-geostrophic model for Ra = 5 × 108, Pm = 0.1 and different choices of χ. The time snapshots are the same as those for the vorticity planforms shown in Fig. 3 of the main text. The Pacific region is in the bottom section of each planform, delimited by a dashed green line in panels b, c and d. The color scales on the right are common to all 4 panels.

## Supplementary information

### Supplementary Information

Document containing Supplementary Information.

## Source data

### Source Data Fig. 4

Data points for Fig. 4.

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