Gravitational torque drives multidecadal variations in length of day
Abstract
Fluctuations in the length of day (LOD) on decadal timescales are caused primarily by an exchange of angular momentum between the Earth’s mantle and core1,2,3. Several mechanisms have been proposed to explain this exchange, including electromagnetic4,5,6,7 and topographic8,9,10 coupling at the core–mantle boundary (CMB) and a gravitational torque by the inner core11,12. However, the precise nature of the core–mantle torque remains unknown. Here we show that the seismically reconstructed differential rotation of the inner core13,14,15 and core flows derived from magnetic field changes16,17 suggest that the multidecadal LOD changes are driven primarily by the gravitational torque and resisted by electromagnetic and topographic torques, consistent with results from Earth-like dynamo models18. Our reconstructed torque histories, although tied to the accuracy of the inner core rotation and core flow models, support a lowermost mantle that features near-neutrally buoyant thermochemical piles19,20,21, a post-perovskite (pPv) phase with a low viscosity22 and a highly conducting23,24 iron-enriched layer a few kilometres thick at its base25. Our results also suggest a low-viscosity inner core deforming in only a few years26,27,28 and yield an upper limit on the stratification at the top of the fluid core. Altogether, our study contributes to bringing into focus an emerging picture of the deepest regions of our planet.
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