Gravitational torque drives multidecadal variations in length of day

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 core 1 , 2 , 3 . Several mechanisms have been proposed to explain this exchange, including electromagnetic 4 , 5 , 6 , 7 and topographic 8 , 9 , 10 coupling at the core–mantle boundary (CMB) and a gravitational torque by the inner core 11 , 12 . However, the precise nature of the core–mantle torque remains unknown. Here we show that the seismically reconstructed differential rotation of the inner core 13 , 14 , 15 and core flows derived from magnetic field changes 16 , 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 models 18 .