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Ann. Geophys., 36, 167-180, 2018
https://doi.org/10.5194/angeo-36-167-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Regular paper
29 Jan 2018
Depth of origin of ocean-circulation-induced magnetic signals
Christopher Irrgang1, Jan Saynisch-Wagner1, and Maik Thomas1,2 1Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Section 1.3, Earth System Modelling, Potsdam, Germany
2Institute of Meteorology, Freie Universität Berlin, Berlin, Germany
Abstract. As the world ocean moves through the ambient geomagnetic core field, electric currents are generated in the entire ocean basin. These oceanic electric currents induce weak magnetic signals that are principally observable outside of the ocean and allow inferences about large-scale oceanic transports of water, heat, and salinity. The ocean-induced magnetic field is an integral quantity and, to first order, it is proportional to depth-integrated and conductivity-weighted ocean currents. However, the specific contribution of oceanic transports at different depths to the motional induction process remains unclear and is examined in this study. We show that large-scale motional induction due to the general ocean circulation is dominantly generated by ocean currents in the upper 2000 m of the ocean basin. In particular, our findings allow relating regional patterns of the oceanic magnetic field to corresponding oceanic transports at different depths. Ocean currents below 3000 m, in contrast, only contribute a small fraction to the ocean-induced magnetic signal strength with values up to 0.2 nT at sea surface and less than 0.1 nT at the Swarm satellite altitude. Thereby, potential satellite observations of ocean-circulation-induced magnetic signals are found to be likely insensitive to deep ocean currents. Furthermore, it is shown that annual temporal variations of the ocean-induced magnetic field in the region of the Antarctic Circumpolar Current contain information about sub-surface ocean currents below 1000 m with intra-annual periods. Specifically, ocean currents with sub-monthly periods dominate the annual temporal variability of the ocean-induced magnetic field.

Citation: Irrgang, C., Saynisch-Wagner, J., and Thomas, M.: Depth of origin of ocean-circulation-induced magnetic signals, Ann. Geophys., 36, 167-180, https://doi.org/10.5194/angeo-36-167-2018, 2018.
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