Core convection influenced by large lower mantle heterogeneity: Implications for the geomagnetic field

Abstract:

The high-latitude magnetic flux patches along the Canadian and Siberian longitudes in the geomagnetic field have been attributed to enhanced core convection near regions of high seismic shear wave velocity in the lower mantle. Observations of the secular variation show highly time-varying magnetic flux beneath Canada and relatively stable magnetic flux beneath Siberia. This variation is likely produced by a core flow regime that is substantially different in the Eastern and Western hemispheres. Here, we show that an east--west dichotomy in core convection can originate from a large lateral variation in heat flux at the core--mantle boundary. We examine the effect of a large lateral variation in outer boundary heat flux on convection in cylindrical annulus experiments as well as in spherical shell simulations. In either geometry, strongly driven states give rise to a coherent downwelling under the Siberian region and a cluster of time-varying vortices under the Canadian region. Spherical dynamo simulations indicate that this pattern of convection is entirely consistent with the observed time variation of the present-day field. In addition, the longitudinal homogenization of convection with large forcing may set a limit for the Rayleigh number in the core.