Convectively driven decadal zonal accelerations in Earth's fluid core

Abstract

Azimuthal accelerations of cylindrical surfaces co-axial with the rotation axis have been inferred to exist in Earth's fluid core on the basis of magnetic field observations and changes in the length-of-day. These accelerations have a typical timescale of decades. However, the physical mechanism causing the accelerations is not well understood. Scaling arguments suggest that the leading order torque averaged over cylindrical surfaces should arise from the Lorentz force. Decadal fluctuations in the magnetic field inside the core, driven by convective flows, could then force decadal changes in the Lorentz torque and generate zonal accelerations. We test this hypothesis by constructing a quasi-geostrophic model of magnetoconvection, with thermally-driven flows perturbing a steady, imposed background magnetic field. We show that when the Alfvén number in our model is similar to that in Earth's fluid core, temporal fluctuations in the torque balance are dominated by the Lorentz torque, with the latter generating mean zonal accelerations. Our model reproduces both fast, free Alfvén waves and slow, forced accelerations, with ratios of relative strength and relative timescale similar to those inferred for the Earth's core. The temporal changes in the magnetic field which drive the time-varying Lorentz torque are produced by the underlying convective flows, shearing and advecting the magnetic field on a timescale associated with convective eddies. Our results support the hypothesis that temporal changes in the magnetic field deep inside Earth's fluid core drive the observed decadal zonal accelerations of cylindrical surfaces through the Lorentz torque.

arXiv comment29 pages, 9 figures

References in corpus (9)

• Turbulent geodynamo simulations: a leap towards Earth's core
• Approaching the Asymptotic Regime of Rapidly Rotating Convection: Boundary Layers vs Interior Dynamics
• Spherical convective dynamos in the rapidly rotating asymptotic regime
• Large-scale vortices in rapidly rotating Rayleigh-Bénard convection
• Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth's core surface
• Axial invariance of rapidly varying diffusionless motions in the Earth's core interior
• Subcritical convection of liquid metals in a rotating sphere using a quasi-geostrophic model
• The dynamics and excitation of torsional waves in geodynamo simulations
• Can Core Flows inferred from Geomagnetic Field Models explain the Earth's Dynamo?

Cited by in corpus (2)

• Weak magnetic field changes over the Pacific due to high conductance in lowermost mantle
• pizza: An open-source pseudo-spectral code for spherical quasi-geostrophic convection