Collaborative Research: Exploiting Geomagnetic Records to Describe Past and Present Ocean Variability

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The electrically conducting ocean generates electric currents and associated magnetic fields as it flows through the Earth’s magnetic field. These electric currents vary in time with changes in the ocean’s electrical conductivity and velocity, and can be detected remotely by land and satellite magnetometers. There is therefore an opportunity to recover ocean flow and conductivity variability from magnetometer data. Extracted signals are very useful, as they represent depth integrals of conductivity and conductivity transport, which can stand as proxies for heat content and heat transport over the measurement durations – quantities that have been hard to measure with other methods. Magnetometer data come from hundreds of land magnetic observatories (some with hourly data extending back a century) as well as from modern satellite magnetic surveys. The proposed work will extract past ocean variability from long, land geomagnetic observatory records, develop forward models for predicting the oceanic magnetic fields, and ultimately develop data assimilation strategies for both land and satellite observations. This project will examine both fundamental components of ocean electrodynamics and exploit a new data type for monitoring and understanding ocean variability and steric sea-level changes. The electromagnetic field modules in the model used here will be made publicly available. Workshops will be organized to introduce students and early career scientists to analysis of geomagnetic observatory data and its connection to oceanographic applications.Extracting oceanic signals from magnetic data is challenging because the signals are relatively weak and a priori knowledge of the signal is required. This proposal demonstrates method feasibility with preliminary work on a century of ocean tidal variability extracted from the Honolulu geomagnetic observatory data. These independent data confirm a trend toward increasing tidal amplitudes previously found in Honolulu tide gauge data that has been attributed to ocean warming. The Honolulu series will be thoroughly analyzed to optimize the extraction of ocean tidal signals. The methods will be extended to extract other predictable ocean signals there, such as inertial oscillations and interannual oscillations. Similar methods will be applied to geomagnetic observatory data from other locations. Extensions will include canonical-correlation multivariate analyses of data from multiple locations as well as from satellite data and geomagnetic field models. Additional tasks will involve the development and use of forward models to calculate the magnetic (and electric) fields given tidal and circulation ocean model flow. Extracted signals represent depth integrals of conductivity and conductivity transport, useful as proxies for heat content and heat transport over the measurement durations – quantities that have been hard to measure with other methods. Thus, in addition to addressing fundamental components of ocean electrodynamics the proposed work will be useful in exploiting a new data type for monitoring and understanding ocean variability and steric sea-level changes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.