Planetary gyre, time-dependent eddies, torsional waves, and equatorial jets at the Earth's core surface

TL;DR

This study models Earth's core surface flows over 70 years, revealing large-scale gyres, eddies, jets, and torsional waves, and assesses their impact on Earth's rotation and magnetic interactions.

Contribution

It provides a detailed, time-dependent quasi-geostrophic model of core flows, incorporating uncertainties and magnetic interactions, with new insights into torsional waves and angular momentum exchanges.

Findings

Identification of time-dependent large-scale eddies and jets at the core surface.

Retrieval of torsional waves explaining length-of-day variations.

Revised understanding of electromagnetic core-mantle coupling effects.

Abstract

We report a calculation of time-dependent quasi-geostrophic core flows for 1940-2010. Inverting recursively for an ensemble of solutions, we evaluate the main source of uncertainties, namely the model errors arising from interactions between unresolved core surface motions and magnetic fields. Temporal correlations of these uncertainties are accounted for. The covariance matrix for the flow coefficients is also obtained recursively from the dispersion of an ensemble of solutions. Maps of the flow at the core surface show, upon a planetary-scale gyre, time-dependent large-scale eddies at mid-latitudes and vigorous azimuthal jets in the equatorial belt. The stationary part of the flow predominates on all the spatial scales that we can resolve. We retrieve torsional waves that explain the length-of-day changes at 4 to 9.5 years periods. These waves may be triggered by the nonlinear interaction between the magnetic field and sub-decadal non-zonal motions within the fluid outer core. Both the zonal and the more energetic non-zonal interannual motions were particularly intense close to the equator (below 10 degrees latitude) between 1995 and 2010. We revise down the amplitude of the decade fluctuations of the planetary scale circulation and find that electromagnetic core-mantle coupling is not the main mechanism for angular momentum exchanges on decadal time scales if mantle conductance is 3 10 8 S or lower.