Modelling of electromagnetic signatures of global ocean circulation: physical approximations and numerical issues

Abstract

The interactions of flowing electrically conductive seawater with Earth's magnetic field generate electric currents within the oceans, as well as secondary electric currents induced in the resistive solid Earth. The ocean-induced magnetic field (OIMF) is an observable signature of these currents. Ignoring tidally forced ocean flows, the global ocean circulation system is driven by wind forcing on the ocean surface and by the temperature-and salinity-dependent buoyancy force. Ocean circulation's magnetic signals contribute to the total magnetic field observed at the Earth's surface or by low-orbit satellite missions. In this paper, we concentrate on accurate numerical modelling of the OIMF employing various approaches. Using a series of numerical test cases in different scenarios of increasing complexity, we evaluate the applicability of the unimodal thin-sheet approximation, the importance of galvanic coupling between the oceans and the underlying mantle (i.e. the bimodal solution), the effects of vertical stratification of ocean flow as well as the effects of vertical stratification of both oceanic and underlying electrical conductivity, and the influence of electromagnetic self-induction. We find that the inclusion of galvanic ocean-mantle coupling has the largest effect on the predicted OIMF. Self-induction is important only on the largest spatial scales, influencing the lowest spherical harmonic coefficients of the OIMF spectrum. We find this conclusion important in light of the recent Swarm satellite mission which has the potential to observe the large-scale OIMF and its seasonal variations. The implementation of fully three-dimensional ocean flow and conductivity heterogeneity due to bathymetry, which substantially increases the computational demands of the calculations, can play some role for regional studies, or when a more accurate OIMF prediction is needed within the oceans, e.g. for comparison with seafloor observations. However, the large-scale signals at the sea surface or at satellite altitude are less affected.

Can seafloor voltage cables be used to study large-scale circulation? An investigation in the Pacific Ocean

“…We numerically predict the electromagnetic signals produced by ocean circulation using the time-domain numerical solver elmgTD of the electromagnetic induction equation (Velímský and Martinec, 2005;Velímský, 2013;Šachl et al, 2019;Velímský et al, 2019),…”

Analysis of Ocean Tide‐Induced Magnetic Fields Derived From Oceanic In Situ Observations: Climate Trends and the Remarkable Sensitivity of Shelf Regions

“…With the numerical 3-D induction solver x3dg of Kuvshinov (2008), we compute the electromagnetic response to interactions of ocean tide-induced currents and the heterogeneous electrical conducting environment; x3dg solves Maxwell's equations in the frequency domain using an iterative contracting volume integral equation approach (Pankratov et al, 1995;Singer and Fainberg, 1995). The solver has been shown to simulate electromagnetic fields of models with sharp contrasts and complicated geometries in an efficient and accurate manner (Kelbert et al, 2014;Sachl et al, 2019). The solver is forced with the computed monthly ocean tide-induced electric currents and conductance distributions (Velímský et al, 2018).…”

“…We also want to thank Alexey Kuvshinov for kindly providing his 3-D EM induction code and the model of mantle conductivity. The published data set Petereit et al (2019) provides the complete time series data of the computed global ocean tide-induced electromagnetic field signals. The data are permanently available at the public research data repository GFZ Data Services (DOI: http://doi.org/10.5880/ GFZ.1.3.2019.001).…”

Modeling magnetotelluric tippers on a sphere with particular emphasis on ocean induction effect and effect from oceanic sediments

“…In the past decade X3DG (which is also based on IE approach) has been used in numerous studies (Chen et al, 2020;Chulliat et al, 2016;Grayver et al, 2017;Guzavina et al, 2019;Honkonen et al, 2018;Irrgang et al, 2019;Khan et al, 2011;Manoj et al, 2006;Munch et al, 2020;Püthe et al, 2014b;Sabaka et al, 2020;Sachl et al, 2019;Saynisch et al, 2017;Schnepf et al, 2018;Telschow et al, 2018;Velimsky et al, 2018, among others) to compute EM field and responses in spherical 3-D conductivity models of the Earth induced by the sources of different (oceanic, ionospheric or/and magnetospheric) origin. However, X3DG, being developed around two decades ago, is viewed nowadays as a rather obsolete, in terms of computational efficiency, versatility and accuracy.…”