Using a two-dimensional approach to model the short timescale zonal flow in Earth's core

Abstract:

Reconstructions of the flow in Earth's outer core, based on surface magnetic data, suggest
the existence of decadal accelerations in its mean zonal component. The presence of these zonal
accelerations is confirmed by oscillations observed in the length-of-day, reflecting the
accelerations' role in core-mantle angular momentum exchange. While they were once believed to
represent the free modes of torsional oscillations, recent investigations show that zonal
accelerations should have periods shorter than 10 years. Our understanding of which mechanisms are
responsible for creating and controlling decadal zonal accelerations is, therefore, incomplete.

To address this problem we construct a two-dimensional model of the dynamics in the fluid
core, with flows driven by thermal convection and coupled to a background magnetic field. Our
quasi-geostrophic approach is justified by the Taylor-Proudman theorem, which states that a rapidly
rotating fluid shows little variation parallel to its rotation axis. Using two spatial dimensions,
as opposed to three, allows us to reach higher resolutions and more realistic parameters with
comparable computational power.

We find that the convective dynamics in our model are capable of producing mean zonal
flows with characteristics similar to those inferred from geomagnetic reconstructions. We quantify
the contribution of each term in the force balance to address how zonal accelerations are driven in
our model and, from this, extrapolate our results to conditions in Earth's core.