A simple model for mantle-driven flow at the top of Earth’s core

Abstract

We derive a model for the steady fluid flow at the top of Earth's core driven by thermal coupling with the heterogeneous lower mantle. The model uses a thermal wind balance for the core flow, and assumes a proportionality between the horizontal density gradients at the top of the core and horizontal gradients in seismic shear velocity in the lowermost mantle. It also assumes a proportionality between the core fluid velocity and its radial shear. This last assumption is validated by comparison with numerical models of mantle-driven core flow, including self-sustaining dynamo (supercritical) models and non-magnetic convection (subcritical) models. The numerical dynamo models show that thermal winds with correlated velocity and radial shear dominate the boundary-driven large-scale flow at the top of the core. We then compare the thermal wind flow predicted by mantle heterogeneity with the 150 year time-average flow obtained from inverting the historical geomagnetic secular variation, focusing on the non-zonal components of the flows because of their sensitivity to the boundary heterogeneity. Comparing magnitudes provides an estimate of the ratio of lower mantle seismic anomalies to core density anomalies. Comparing patterns shows that the thermal wind model and the time-average geomagnetic flow have comparable length scales and exhibit some important similarities, including an anticlockwise vortex below the southern Indian and Atlantic Oceans, and another anticlockwise vortex below Asia, suggesting these parts of the non-zonal core flow could be thermally controlled by the mantle. In other regions, however, the two flows do not match well, and some possible reasons for the dissimilarity between the predicted and observed core flow are identified. We propose that better agreement could be obtained using core flows derived from geomagnetic secular variation over longer time periods.

Seismic evidence for a change in the large‐scale tomographic pattern across the D′′ layer

“…It has already been shown in numerical models that the degree 2 tends to stabilize the LLSVPs [Forte et al, 2002], but does it still hold with weaker even degrees? Finally, several evidences from laboratory experiments [Forte et al, 2002] and numerical modeling [Bloxham, 2000;Amit et al, 2008;Olson et al, 2010;Gubbins et al, 2011;Driscoll, 2015] support the influence of the deepest mantle on the outer core fluid flow and thus on the topology of the geodynamo. For instance, Driscoll [2015] shows that applying a CMB heat flux that contains only degree and mode 2 generates a similar structure on the inner core boundary but shifted at 30 ∘ westward.…”

“…This layer thus plays a critical role not only in mantle convection but also in core convection, Earth's heat budget and geodynamo process. Therefore, any change in the lower mantle large-scale structure would have great implications in our understanding of the outer core dynamics [Amit et al, 2008;Bloxham, 2000;Olson et al, 2010;Gubbins et al, 2011;Driscoll, 2015]. Seismology has revealed that the D ′′ region is dominated by a degree 2 pattern of low-velocity heterogeneities [Dziewonski et al, 1977], with maximum amplitudes beneath Pacific and Africa, surrounded by a high-velocity circum-Pacific rim commonly interpreted as the imprint of Mesozoic subduction slabs [Ricard et al, 1993].…”

Accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation

“…A recent study by Aubert et al (2008) explains several geophysical observations of non‐axisymmetric core dynamics and properties using a numerical dynamo with heterogeneous outer boundary heat flux inferred from a lower mantle seismic shear velocity model (Masters et al 2000). One region where the flow comparison fails is below North America, where several core flow models find a clockwise vortex (Amit & Olson 2006; Pais & Jault 2008), but the model of Aubert et al (2008), as well as a simple mantle‐driven thermal wind prediction (Amit et al 2008) require an anticlockwise vortex. Moreover, to maintain the prominent intense magnetic flux patch below North America, convergence is necessary, and therefore a cyclone is expected.…”

The Magnetic Field of Planet Earth

“…Note that such assumptions also imply that the flow should then be symmetric with respect to the equator, an assumption that has the advantage of further reducing the non-uniqueness of the core surface flow determination, and which indeed seems to comply with the observations when considering fast changing flows (Pais and Jault 2008;Gillet et al 2009a). On longer time scales, however, the dynamics may very well be different, and it has indeed been pointed out that core surface flows averaged over centuries might rather reflect some influence of the asymmetric thermal boundary conditions imposed by the (very slowly) convecting mantle (Aubert et al 2007;Amit et al 2008, see also Sect. 4.3.1).…”