Decadal variability in core surface flows deduced from geomagnetic observatory monthly means

Whaler, Kathryn A.; Olsen, Nils; Finlay, Christopher C.

doi:10.1093/gji/ggw268

Cited by 11 publications

(19 citation statements)

References 40 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…9). To summarize, the azimuthal flow variations found by Whaler et al (2016) are generally consistent with our results, but our flows have greater power at small scales, and the amplitude of our time-varying flows are much larger. Given the restriction of our observations to mid and low latitudes, and because we base our flow parameterization on inertial modes in a full sphere that are not suitable for studying flow close to and inside the tangent cylinder, we are unable to comment on the acceleration of high latitude flows recently discussed by Livermore et al (2017) and Barrois et al (2018b).…”

Section: Relation To Previous Studiessupporting

confidence: 89%

“…The uncertainty estimates were taken to be the square root of the diagonal elements of these covariance matrices. A similar procedure has been used to determine error estimates for observatory data in recent revisions of the CHAOS model, and in the studies of core flow by Whaler et al (2016) and Barrois et al (2018b) that also used ground observatory monthly means.…”

Section: Ground Observatory Seriesmentioning

confidence: 99%

“…By making use of this equation at discrete times tp, p ∈ [1, P ] spanning the time interval for which observational data is provided, we build a multi-epoch matrix equation to simultaneously compute the SV at all tp (e.g. Whaler et al 2016). For our formulation of the core flow in terms of modes, the forward problem is completed by including a matrix which allows the computation of the toroidal-poloidal flow expansion from a linear combination of geostrophic polynomials and inertial modes.…”

Section: Forward Problemmentioning

confidence: 99%

See 2 more Smart Citations

Time-dependent low-latitude core flow and geomagnetic field acceleration pulses

Kloss

Finlay

2019

Geophysical Journal International

View full text Add to dashboard Cite

We present a new model of time-dependent flow at low latitudes in the Earth's core between 2000 and 2018, derived from magnetic field measurements made on board the Swarm and CHAMP satellites and at ground magnetic observatories. The model, called CoreFlo-LL.1, consists of a steady background flow without imposed symmetry plus a time-dependent flow that is dominated by geostrophic and quasi-geostrophic components but also allows weak departures from equatorial symmetry. Core flow mode amplitudes are determined at 4-month intervals by a robust least squares fit to ground and satellite secular variation data. The l 1 norm of the square root of geostrophic and inertial mode enstrophies, and the l 2 norm of the flow acceleration, are minimised during the inversion procedure. We find that the equatorial region beneath the core-mantle boundary is a place of vigorous, localised, fluid motions; time-dependent flow focused at low latitudes close to the core surface is able to reproduce rapid field variations observed at non-polar latitudes at and above Earth's surface. Magnetic field acceleration pulses are produced by alternating bursts of non-zonal azimuthal flow acceleration in this region. Such bursts are prominent in the longitudinal sectors from 80-130 • E and 60-100 • W throughout the period studied, but are also evident under the equatorial Pacific from 130 • E to 150 • W after 2012. We find a distinctive interannual alternation in the sign of the non-zonal azimuthal flow acceleration at some locations, involving a rapid cross-over between flow acceleration convergence and divergence. Such acceleration sign changes can occur within a year or less, and when the structures involved are of large spatial scale they can give rise to geomagnetic jerks at the Earth's surface. For example, in 2014, we find a change in the sign of the non-zonal azimuthal flow acceleration under the equatorial Pacific, as a region of flow acceleration divergence near 130 • E changes to a region of flow acceleration convergence. This occurs at a maximum in the amplitude of the time-varying azimuthal flow under the equatorial Pacific and corresponds to a geomagnetic jerk at the Earth's surface.

show abstract

Section: Relation To Previous Studiessupporting

confidence: 89%

Section: Ground Observatory Seriesmentioning

confidence: 99%

Section: Forward Problemmentioning

confidence: 99%

See 1 more Smart Citation

Time-dependent low-latitude core flow and geomagnetic field acceleration pulses

Kloss

Finlay

2019

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…Similarly, toroidal field components at SH degree l = 3 are also assumed to be unconstrained by surrounding velocity field scales. This choice is motivated by the particular low level of energy that these scales are exhibiting over recent epochs (see Baerenzung et al, ; Whaler et al, ). Finally, one spectral range is used to characterize the toroidal field spatial variance and memory effects between SH degrees l = 4 and l = 26.…”

Section: Geophysical Applicationmentioning

confidence: 99%

Modeling and Predicting the Short‐Term Evolution of the Geomagnetic Field

et al. 2018

View full text Add to dashboard Cite

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.

show abstract

“…They concluded that the frozen flux constraint remained compatible with ground-based and satellite magnetic records. Pursuing an alternative approach, and Whaler & Beggan (2015) obtained piecewise constant or linear flow models directly from magnetic data (see also Whaler et al 2016). C The Author(s) 2018.…”

Section: Introductionmentioning

confidence: 99%

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

Barrois

Hammer

Finlay

et al. 2018

Geophysical Journal International

View full text Add to dashboard Cite

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

show abstract

Decadal variability in core surface flows deduced from geomagnetic observatory monthly means

Cited by 11 publications

References 40 publications

Time-dependent low-latitude core flow and geomagnetic field acceleration pulses

Time-dependent low-latitude core flow and geomagnetic field acceleration pulses

Modeling and Predicting the Short‐Term Evolution of the Geomagnetic Field

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

Contact Info

Product

Resources

About