Modeling and Predicting the Short‐Term Evolution of the Geomagnetic Field. Issue 6 (19th June 2018)

• Record Type: Journal Article
• Authors: Bärenzung, Julien; Holschneider, Matthias; Wicht, Johannes; Sanchez, Sabrina; Lesur, Vincent
• Is Part Of: Journal of geophysical research. Volume 123:Issue 6(2018)
• Page Start: 4539
• Page End: 4560
• Publication Date: 2018-06-19
• Subjects: core flow -- assimilation -- prediction -- length of day; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2017JB015115
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: We propose a reduced dynamical system describing the coupled evolution of fluid flow and magnetic field at the top of the Earth's core between the years 1900 and 2014. The flow evolution is modeled with a first‐order autoregressive process, while the magnetic field obeys the classical frozen flux equation. An ensemble Kalman filter algorithm serves to constrain the dynamics with the geomagnetic field and its secular variation given by the COV‐OBS.x1 model. Using a large ensemble with 40, 000 members provides meaningful statistics including reliable error estimates. The model highlights two distinct flow scales. Slowly varying large‐scale elements include the already documented eccentric gyre. Localized short‐lived structures include distinctly ageostophic features like the high‐latitude polar jet on the Northern Hemisphere. Comparisons with independent observations of the length‐of‐day variations not only validate the flow estimates but also suggest an acceleration of the geostrophic flows over the last century. Hindcasting tests show that our model outperforms simpler predictions bases (linear extrapolation and stationary flow). The predictability limit, of about 2, 000 years for the magnetic dipole component, is mostly determined by the random fast varying dynamics of the flow and much less by the geomagnetic data quality or lack of small‐scale information. Key Points: The model we propose permits to estimate the reliability of certain patterns the flow at the Earth's core‐mantle boundary is exhibiting The model we present enables to predict the evolution of the geomagnetic field and to accurately estimate prediction errors We show that the discrepancies between the predicted trend in length of day and the observed one can be explained by our flow model