ALBERT

Description: Space geodetic determinations of a 6 μs length-of-day (LOD) anomaly at the diurnal S1 frequency are reconciled with excitation estimates from geophysical fluid models. Preference is given to a hybrid excitation scheme that combines atmospheric torques with oceanic angular momentum (OAM) terms from hydrodynamic forward modeling. A joint inversion of all data sets yields an LOD in-phase and quadrature estimate of (5.91, −0.22) μs, matching space geodetic S1 terms well within their formal uncertainties. Non-harmonic LOD excitations, while less than 30% of the time-averaged rotation rate contribution, are conclusively linked to El Niño–Southern Oscillation (ENSO) as the main perturbation of diurnal cycle characteristics in the troposphere. ENSO modulations of particular relevance are those in OAM, associated with the barotropic ocean response to regional modifications in the diurnal atmospheric pressure wave. The study thus highlights previously unexplored aspects of non-tidal mass-field variability in the Earth system. ©2017. American Geophysical Union. All Rights Reserved.

Description: 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.

Description: We present a benchmark in which magnetic induction codes are tested in a series of modeling studies. Ocean electric currents provide the forcing. We focus on the wind- and buoyancy-driven ocean-induced magnetic field; tides are not considered. The individual benchmarks start with the magnetostatic case of time-invariant forcing and homogeneous surface conductivity, and the benchmark complexity increases gradually from this starting point. We test a rich variety of induction codes. There are codes based on the full equation of electromagnetic induction, as well as codes that utilize certain approximations of the governing equation, e.g. Tyler (1997)’s thin layer approximation. We include both time-domain and frequency-domain codes. In order to assess the performance of the individual codes, we compare the spatial distributions and power spectra of the induced magnetic fields.

Description: We present a benchmark of magnetic induction codes driven by the ocean tidal flows. We concentrate on the principal lunar semi-diurnal tide M2, using the state-of-the-art TPXO8-atlas ocean flow model. We assess the effect of the two-dimensional thin-layer approximation, and the effect of self-induction. Large lateral variations of ocean electrical conductivity are assumed. For the underlaying Earth’s mantle we consider either a realistic one-dimensional conductivity profile, or use a perfect insulator approximation. Various numerical techniques are compared, including weak formulation in the spherical harmonic domain, contracting integral equations, and finite differences, both in the frequency and time domains. We compare the maps of magnetic field components at the ocean surface, as well as the spherical harmonic power spectra.